Plant and Soil Responses to Concrete and Basalt Amendments Under Elevated CO₂: Implications for Plant Growth, Enhanced Weathering and Carbon Sequestration

Abstract

The rise in greenhouse gases underscores the urgency of carbon dioxide removal (CDR) as a complement to emission reductions. Enhanced rock weathering (ERW) holds promise by coupling geochemical carbon sequestration with agronomic benefits, although integrative experimental evidence remains limited. This study evaluated two amendments (recycled concrete in wheat, C₃, and basalt in maize, C₄) under ambient and elevated CO₂ conditions (~1000 ppm). Conducted in a greenhouse over 21 weeks using loam soils, the experiment evaluated four treatments comprising three different particle-size ranges (<2 mm, 2–6 mm, and 6–15 mm) and a control. Plant growth (height, total and partitioned biomass), grain quality (N and protein), and soil properties (pH, electrical conductivity, and carbonates) were measured. Elevated CO₂ enhanced biomass, particularly vegetative biomass in wheat (+42.6%) and root biomass in maize (+55%), without significantly increasing yield. In wheat, particle size was decisive: intermediate fractions (2–6 mm) yielded the best results. In maize, basalt effects were less consistent. Concrete amendments increased soil pH and carbonate content, especially with coarse particles and elevated CO₂, whereas basalt-induced responses were slower and more variable. These findings confirm the potential of ERW as a dual climate–agronomic strategy while highlighting the need for long-term, field-scale validation.

1. Introduction

Human activities, primarily through greenhouse gas emissions, have caused unequivocal global warming, with a mean increase of 1.1 °C during 2011–2020 compared to 1850–1900 [1]. Carbon dioxide (CO₂) is the main gas involved, reaching its highest concentration in 2019 over the past two million years. Meanwhile, methane (CH₄) and nitrous oxide (N₂O) reached their highest levels in 800,000 years [2].

To meet the objectives of the Paris Agreement, drastic emission reductions are indispensable. However, they are insufficient on their own, making carbon dioxide removal (CDR) an essential complement.

CDR encompasses practices and technologies that remove and durably store CO₂ and features in all scenarios consistent with limiting warming to 2 °C or below [3,4]. It consists of both nature-based solutions (NbS), such as reforestation and soil management, and technological alternatives, such as direct air capture and geological storage, although the latter remain in early stages of development and require rapid scale-up [5,6,7].

Within this portfolio of strategies, enhanced rock weathering (ERW) stands out for its triple potential to capture atmospheric carbon, improve soil fertility, and mitigate acidification. According to the IPCC [8], ERW involves the intentional acceleration of the natural weathering of silicate and carbonate rocks to remove CO₂ from the atmosphere. This is achieved by applying finely ground minerals to soils or other environments. This approach can result in higher crop yields in the first year, increased soil pH, and greater nutrient uptake [9,10].

Enhanced rock weathering is a geochemical carbon storage pathway in which atmospheric CO₂ is converted into carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) through reactions with minerals rich in alkaline earth metal oxides [11,12]. Its central principle lies in accelerating natural geochemical processes by applying finely ground minerals, thereby increasing the reactive surface area and enhancing the release of basic cations such as Ca²⁺ and Mg²⁺ [13]. These cations react with dissolved CO₂ in soil water to form bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻). The resulting compounds can precipitate as stable carbonates within soils or remain dissolved and be transported to aquatic systems, both contributing to long-term carbon sequestration [14].

Among naturally occurring minerals, silicates are regarded as the most suitable candidates for enhanced rock weathering due to their high Ca and Mg content, wide availability, favorable reactivity, and the chemical stability of the resulting siliceous residues [15]. In addition, recent studies have highlighted the potential of industrial byproducts such as recycled concrete and steel slags, which offer abundant and reactive materials that could serve as viable alternatives for carbon sequestration [16,17].

When applied to agricultural land, ERW functions not only as a geochemical mitigation technology but also as part of the broader framework of nature-based solutions (NbS). It combines carbon sequestration with additional benefits for crop productivity and soil management [9,18,19].

Basalt is considered one of the most attractive amendments for ERW due to its global availability, low cost, and provision of essential nutrients such as Ca, Mg, Si, and K [10]. Nevertheless, its dissolution kinetics are relatively slow and strongly influenced by physicochemical factors. This may limit its short-term impact on carbonate formation [20,21,22].

In contrast, industrial byproducts such as recycled concrete contain more soluble phases that rapidly release basic cations and promote carbonate precipitation. Crushed concrete has been reported to achieve CO₂ removal rates of up to 0.55 t CO₂ ha⁻¹ [23], with dissolution rates markedly faster than those of silicate minerals like basalt [24]. Furthermore, between 1% and 4% of global concrete production remains unused, equivalent to 250–1000 million tons annually, making it an abundant and widely available resource for carbon dioxide removal strategies [23].

While ERW provides agronomic benefits, plants are not passive recipients but active agents shaping soil geochemistry through rhizosphere processes. Root and microbial exudates, including organic acids and chelating compounds, lower soil pH, enhance mineral solubility, and accelerate the release of nutrients [25,26,27]. These mechanisms, together with nutrient cycling, sustain dynamic fluxes that improve soil fertility [28,29,30]. Vegetation also influences hydrological regimes. Deep rooting systems increase contact between soil-respired CO₂ and minerals, potentially enhancing weathering rates by up to 200% compared with shallow-rooted systems [31,32,33,34].

Beyond the type of amendment and crop species, atmospheric CO₂ concentration is a central regulator of enhanced rock weathering. Elevated CO₂ increases plant productivity and rhizosphere CO₂ partial pressure. This, in turn, intensifies mineral dissolution through carbonic acid formation and enhanced root–microbial activity [35,36,37,38]. This process promotes nutrient release, as evidenced by greater phosphorus availability and higher root-to-shoot ratios observed in plants grown under elevated CO₂ [36,37].

In addition to accelerating weathering, elevated CO₂ enhances the flux of dissolved inorganic carbon to groundwater and oceans. This contributes to long-term sequestration [35,38,39,40]. Forest and soil studies report increased cation concentrations, alkalinity, and bicarbonate export, particularly under conditions of high nutrient availability [35,38,40]. However, the magnitude of these effects varies with soil type, vegetation, and local heterogeneity [41].

Although numerous studies have described the geochemistry of ERW and its theoretical potential for carbon removal, interdisciplinary experimental evidence remains scarce in two main areas: (i) most research has assessed agronomic effects (e.g., biomass, yield) and geochemical processes (e.g., soil pH, inorganic carbon) separately, without integrating both under elevated CO₂ [42,43]; and (ii) many trials have been conducted in tropical climates or simplified laboratory conditions, whereas temperate agricultural systems under controlled atmospheric environments remain underrepresented [44].

The use of industrial materials such as recycled concrete, in addition to silicate amendments, represents an emerging research avenue with significant agroecological and climate co-benefits [17]. Yet, experiments that simultaneously integrate mineral amendments, crops, and CO₂ enrichment remain extremely rare.

This study provides novelty and relevance by combining wheat (C₃) with recycled concrete and maize (C₄) with basalt under controlled conditions. This approach enables comparison of distinct physiological pathways and mineral reactivities, while simultaneously assessing vegetative and reproductive productivity, grain quality, and soil carbonate formation using sealed modules that allow direct comparison between ambient and elevated CO₂ conditions.

Accordingly, the specific objectives of this research were (1) to evaluate the effects of elevated CO₂ and amendment particle size on crop growth and quality in wheat and maize; (2) to analyze soil carbonate formation under different atmospheric and amendment conditions; and (3) to explore the differential role of C₃ and C₄ species in ERW processes, emphasizing the triadic interaction between plant, substrate, and atmospheric CO₂.

2. Materials and Methods

2.1. Soil and Plant Selection

The soil used in this study was collected from an industrial area in the municipality of Getafe (40°17′18.9″ N, 3°40′51.8″ W). The area consists of a soil association dominated by Haplic Luvisols, developed on arkosic sediments rich in feldspar and quartz, and characterized by a medium texture. The main physicochemical properties of the soil are summarized in Table 1.

The plant species selected were wheat (Triticum durum) and maize (Zea mays). Seeds were provided by the Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA-CSIC, Madrid, Spain). Wheat seeds were directly sown into pots, whereas maize seeds were pre-germinated in trays and transplanted into the experimental substrate once seedlings had developed.

Although the experiment was not designed to maximize crop yield but rather to assess plant–soil responses to amendments, the selected soil, while not optimal, was suitable for the chosen crops.

Its alkaline and calcareous nature may restrict the availability of phosphorus and micronutrients, particularly affecting maize during early growth and grain filling. Wheat generally tolerates such conditions better due to its adaptation to Mediterranean alkaline soils.

Despite its low organic matter content and limited biological activity, the soil exhibited a high cation exchange capacity. This indicates a substantial potential for nutrient retention associated with calcium- and magnesium-dominated exchange complexes. The low electrical conductivity confirms the absence of salinity. Overall, the soil represents a realistic, low-fertility Mediterranean environment suitable for testing the potential of amendments to improve fertility and enhance carbon sequestration under elevated CO₂ conditions [45,46].

Table 1. Summary of the main chemical and textural properties of the experimental soil.

Parameter	Unit	Value
pH (1:2.5 H2O) 1	–	8.7
Electrical conductivity 1	dS·m−1	0.15
Organic matter 1	%	0.77
Organic carbon 1	%	0.45
Total nitrogen 1	%	0.064
C/N ratio 1	–	7.0
Cation exchange capacity (CEC) 1	cmol(+)·kg−1	65.3
Base saturation Ca2+	%	70.8
Base saturation Mg2+	%	25.3
Base saturation K+	%	2.0
Base saturation Na+	%	1.7
Available phosphorus (Olsen) 1	mg·kg−1	13.1
Total carbonates 2	%CaCO3	9.6
Active lime 3	%CaCO3	2.1
Soil Texture (Bouyoucos Method) 1
Sand	%	47.4
Silt	%	43.5
Clay	%	9.1

Note: Soil analysis performed by the Laboratory of Agricultural Chemistry and Instrumental Analytical Techniques, Universidad Politécnica de Madrid (9 January 2025). ¹ According to Official Method of the Spanish Ministry of Agriculture (MAPA, 1994) [47]. ² According to UNE Standard 103-200-93. ³ According to UNE Standard 103-200-93.

Table 1. Summary of the main chemical and textural properties of the experimental soil.

Parameter	Unit	Value
pH (1:2.5 H2O) 1	–	8.7
Electrical conductivity 1	dS·m−1	0.15
Organic matter 1	%	0.77
Organic carbon 1	%	0.45
Total nitrogen 1	%	0.064
C/N ratio 1	–	7.0
Cation exchange capacity (CEC) 1	cmol(+)·kg−1	65.3
Base saturation Ca2+	%	70.8
Base saturation Mg2+	%	25.3
Base saturation K+	%	2.0
Base saturation Na+	%	1.7
Available phosphorus (Olsen) 1	mg·kg−1	13.1
Total carbonates 2	%CaCO3	9.6
Active lime 3	%CaCO3	2.1
Soil Texture (Bouyoucos Method) 1
Sand	%	47.4
Silt	%	43.5
Clay	%	9.1

Note: Soil analysis performed by the Laboratory of Agricultural Chemistry and Instrumental Analytical Techniques, Universidad Politécnica de Madrid (9 January 2025). ¹ According to Official Method of the Spanish Ministry of Agriculture (MAPA, 1994) [47]. ² According to UNE Standard 103-200-93. ³ According to UNE Standard 103-200-93.

The amendments consisted of recycled concrete applied to wheat and basalt applied to maize. The basalt used in this experiment was sourced from the Can Saboia quarry (http://www.pedreracansaboia.com, accessed on 7 November 2025), and the recycled concrete aggregates were obtained from a local concrete-recycling facility (https://www.descomcasar.es, accessed on 7 November 2025). Both materials were first fragmented using a hydraulic hammer and subsequently crushed with a jaw crusher to obtain particles in three defined size fractions. The elemental composition of both silicate materials was provided directly by the suppliers and is reported in

Table 2. Chemical composition of the basalt and recycled concrete amendments.

Recycled Concrete		Basalt
Element	%	Element	%
SiO2	52.18	SiO2	42.60
CaO	24.27	Al2O3	14.18
Al2O3	8.23	CaO	10.39
Fe2O3	3.14	MgO	8.79
MgO	2.13	FeO	6.40
K2O	1.07	Fe2O3	5.00
Na2O	0.47	Na2O	3.80
SO3	0.63	TiO2	2.80
TiO2	0.52	K2O	0.96
		MnO	0.19
		P2O5	0.19

.

The experiment was conducted in pots placed within two independent CO₂-controlled chambers inside a greenhouse located at the same School. Each chamber covered an area of 37 m². One chamber was maintained under ambient CO₂ conditions, while the other operated under elevated CO₂ conditions, achieved via controlled propane combustion in a burner.

Four experimental treatments were established for each crop within each chamber:

• soil without amendment (control, C),
• soil + amendment with particles < 2 mm (<2),
• soil + amendment with particles 2–6 mm (2–6),
• soil + amendment with particles 6–15 mm (6–15).

Although enhanced rock weathering experiments often employ finely ground materials (<1–2 mm) to maximize reactive surface area, this study intentionally included larger fractions (up to 15 mm) to evaluate their agronomic and geochemical potential under controlled conditions. This approach was supported by previous studies demonstrating the feasibility of coarser particles for weathering and carbon capture processes [16,48,49,50].

For maize (basalt amendment), four replicates were used per treatment, whereas for wheat (concrete amendment), three replicates were used. In total, considering both modules, 56 experimental pots were set up. This design followed a previous pilot phase (2023–2024) in which concrete amendments were extensively tested with wheat using five replicates per treatment, yielding consistent and statistically robust results. Therefore, replication in the present experiment was optimized to focus on the evaluation of basalt under elevated CO₂ conditions. This difference in the number of replicates between crops does not affect the statistical validity of the analyses, as each crop-amendment system was evaluated independently.

Figure 1 shows the experimental setup and the arrangement of pots within the two experimental chambers. The elevated CO₂ chamber was equipped with a propane burner, an air quality monitoring system (Kunak AIR Pro), and a CO₂ regulator. These, together with the forced ventilation system, ensured stable atmospheric conditions throughout the experiment. The experimental configuration for both crops, including treatments and particle-size ranges, is summarized in

Table 3. Experimental setup of treatments applied to Zea mays and Triticum durum.

Species	Treatment	Description	Replicates
Triticum durum	C (elevated CO2)	Substrate + T. durum + CO2 (elevated)	r1, r2, r3
	C (ambient CO2)	Substrate + T. durum + CO2 (ambient)	r1, r2, r3
	<2 mm (elevated CO2)	Substrate + T. durum + Concrete (<2 mm) + CO2 (elevated)	r1, r2, r3
	<2 mm (ambient CO2)	Substrate + T. durum + Concrete (<2 mm) + CO2 (ambient)	r1, r2, r3
	2–6 mm (elevated CO2)	Substrate + T. durum + Concrete (2–6 mm) + CO2 (elevated)	r1, r2, r3
	2–6 mm (ambient CO2)	Substrate + T. durum + Concrete (2–6 mm) + CO2 (ambient)	r1, r2, r3
	6–15 mm (elevated CO2)	Substrate + T. durum + Concrete (6–15 mm) + CO2 (elevated)	r1, r2, r3
	6–15 mm (ambient CO2)	Substrate + T. durum + Concrete (6–15 mm) + CO2 (ambient)	r1, r2, r3
Zea mays	C (elevated CO2)	Substrate + Z. mays + CO2 (elevated)	r1, r2, r3, r4
	C (ambient CO2)	Substrate + Z. mays + CO2 (ambient)	r1, r2, r3, r4
	<2 mm (elevated CO2)	Substrate + Z. mays + Basalt (<2 mm) + CO2 (elevated)	r1, r2, r3, r4
	<2 mm (ambient CO2)	Substrate + Z. mays + Basalt (<2 mm) + CO2 (ambient)	r1, r2, r3, r4
	2–6 mm (elevated CO2)	Substrate + Z. mays + Basalt (2–6 mm) + CO2 (elevated)	r1, r2, r3, r4
	2–6 mm (ambient CO2)	Substrate + Z. mays + Basalt (2–6 mm) + CO2 (ambient)	r1, r2, r3, r4
	6–15 mm (elevated CO2)	Substrate + Z. mays + Basalt (6–15 mm) + CO2 (elevated)	r1, r2, r3, r4
	6–15 mm (ambient CO2)	Substrate + Z. mays + Basalt (6–15 mm) + CO2 (ambient)	r1, r2, r3, r4

.

The polypropylene pots had a capacity of 12 L, with a top diameter of 31 cm and a height of 24 cm. Maize seedlings were transplanted into the pots, while wheat was directly sown on 19 December 2024. The experiment lasted for 21 weeks and concluded in mid-May 2025, when irrigation and data recording for maize were completed.

2.2. Plant Growth

Plant growth and development were monitored throughout their phenological stages, recording the vegetative phases of both maize and wheat and measuring plant height weekly. Soil moisture was monitored using a USB soil sensor (GemHo, model CHTR-8IN1-01, Shandong, China) connected to an Arduino-based system. These values were used as a reference to adjust irrigation in 200 mL increments.

At the end of the experiment, after 140 days for wheat and 153 days for maize, plants were harvested by cutting them at soil level. They were then air-dried for several weeks. The growth parameters analyzed included plant height and dry biomass, which was partitioned into stems, roots, wheat and maize grains, wheat husks, and maize cobs. Cob dry weight was determined after oven drying (Thermo Scientific, Langenselbold, Germany) at 60 °C for 24 h.

Seeds were subsequently analyzed at the INIA-CSIC laboratory to determine nitrogen and protein content using the Kjeldahl method. The analyses followed the official procedures AOAC 979.09 (Protein in Grains) and AACC 46.11A [51]. Due to limited seed availability, samples were pooled by amendment treatment within each CO₂ chamber before nitrogen and protein analyses. This approach was necessary because the minimum sample weight required for the Kjeldahl procedure exceeded the mass of seeds obtained per individual pot. Consequently, the results represent composite values rather than means of independent replicates.

2.3. Chemical Analyses

Carbonate content was determined using a titrimetric method according to the FAO Standard Operating Procedure for Soil Calcium Carbonate Equivalent [52]. Soil pH and electrical conductivity (EC) were measured using a pH meter (Crison Micro pH 2000, Crison Instruments, Barcelona, Spain) and a conductivity meter (WTW LF 90, Wissenschaftlich-Technische Werkstätten, Weilheim, Germany). For these measurements, 20 g of <2 mm soil were mixed with 50 mL of water, and the samples were shaken for 20 min prior to testing.

2.4. Statistical Analysis

Statistical analyses were performed using IBM SPSS Statistics software (version 29.0.2.0). The choice of tests depended on the nature of each variable. Assumptions of normality were verified through residual inspection and the Shapiro–Wilk test, and homogeneity of variances was assessed using Levene’s test.

To evaluate the effects of elevated CO₂ and amendment particle size on biomass fractions (stem, root, grain, husk/cob, and total biomass), as well as on the harvest index and root-to-shoot ratio, a multivariate analysis of variance (MANOVA) was applied. This was complemented by univariate ANOVAs and post hoc comparisons (Tukey’s test under homogeneity of variances, and Games–Howell when heterogeneity was detected [53]). In addition, simple main effects for the CO₂ × treatment interaction were examined using Bonferroni-adjusted pairwise comparisons.

For carbonate formation, initial and final values, as well as percentage increase, were analyzed using a two-way ANOVA. The fixed factors were CO₂ conditions and amendment particle size. Post hoc comparisons were conducted using Tukey’s test, and simple main effects were further examined with Bonferroni-adjusted pairwise comparisons. Additionally, non-parametric Spearman correlations were calculated to explore relationships between carbonate increase, pH, and electrical conductivity (EC).

3. Results

3.1. Impact of Amendments and CO₂ Condition on Plant Development

Growth and biomass responses to elevated CO₂ conditions differed markedly between wheat and maize. In T. durum, plants grown under elevated CO₂ showed a significantly reduced final height but greater total biomass compared to those under ambient CO₂ conditions.

This response was mainly driven by increases in aboveground, root, and husk fractions, with no significant effect on grain yield. In Z. mays, elevated CO₂ also led to a moderate reduction in plant height, while total biomass increased, largely due to enhanced root and cob development. However, aboveground and grain biomass remained statistically unchanged (p < 0.05).

These results indicate a differential allocation of carbon under elevated CO₂ conditions, with wheat exhibiting a stronger biomass redistribution than maize.

Amendment particle size also had a clear influence on wheat performance. In T. durum, soils amended with finer particle-size fractions (<2 mm and 2–6 mm) produced plants that tended to be taller and yield more grain and total biomass. However, these differences were not statistically significant compared with the control.

Conversely, the coarser fraction (6–15 mm) significantly reduced these parameters under both elevated and ambient CO₂ conditions (Table 2). In Z. mays, no significant differences were found among particle-size treatments, and no consistent trend was observed across the analyzed variables.

Table 4. Mean and standard deviation of plant height and biomass fractions (aboveground, grain, husk/cob, root, and total biomass) measured in Triticum durum and Zea mays grown under elevated and ambient CO₂ conditions.

Variable	Species	Chamber	C	<2	2–6	6–15
Plant height (cm)	T. durum	Enriched	56.67 ± 1.89 a	55.67 ± 1.69 a	57.23 ± 0.32 a	46.33 ± 2.57 b
		Ambient	68.03 ± 2.0 a	72.13 ± 1.59 a	66.87 ± 5.59 a	58.5 ± 3.91 b
	Z. mays	Enriched	142.63 ± 10.54	137.70 ± 10.94	134.00 ± 12.33	144.18 ± 7.31
		Ambient	178.20 ± 11.73	190.10 ± 8.60	190.00 ± 6.94	172.20 ± 4.33
Aboveground biomass (g)	T. durum	Enriched	29.57 ± 4.03	26.73 ± 1.1	30.53 ± 3.52	22.53 ± 7.51
		Ambient	19.07 ± 3.4	21.33 ± 3.05	22.27 ± 3.72	14.03 ± 2.15
	Z. mays	Enriched	22.57 ± 5.24	22.98 ± 5.76	20.57 ± 3.12	26.00 ± 4.82
		Ambient	22.43 ± 4.27	21.45 ± 3.90	21.37 ± 0.74	24.20 ± 4,33
Grain husk (g)/Corncob (g)	T. durum	Enriched	8.23 ± 1.71 ab	9.87 ± 1.25 a	9.53 ± 0.81 a	5.67 ± 2.19 b
		Ambient	3.1 ± 0.0	3.67 ± 0.35	3.23 ± 0.64	1.73 ± 1.25
	Z. mays	Enriched	4.37 ± 1.78	9.58 ± 5.60	4.23 ± 0.72	5.90 ± 0.50
		Ambient	5.57 ± 1.22	3.18 ± 2.64	4.66 ± 4.23	1.88 ± 0.87
Grain (g)	T. durum	Enriched	6.83 ± 2.63 ab	9.0 ± 1.22 a	7.67 ± 1.08 a	3.0 ± 1.83 b
		Ambient	8.93 ± 1.01 a	8.3 ± 0.46 a	10.57 ± 1.81 a	3.1 ± 1.61 b
	Z. mays	Enriched	0.73 ± 1.47	4.69 ± 6.99	0.15 ± 0.17	0.11 ± 0.22
		Ambient	0.20 ± 0.40	0.00 ± 0.00	3.28 ± 3.85	0.35 ± 0.42
Roots (g)	T. durum	Enriched	0.57 ± 0.31	0.83 ± 0.15	1.07 ± 0.38	0.8 ± 0.35
		Ambient	0.2 ± 0.0	0.43 ± 0.12	0.57 ± 0.15	0.4 ± 0.2
	Z. mays	Enriched	17.23 ± 3.65	17.53 ± 6.31	15.45 ± 2.30	22.45 ± 4.80
		Ambient	7.28 ± 1.56	5.75 ± 1.32	5.83 ± 1.81	6.55 ± 0.70
Total biomass (g)	T. durum	Enriched	45.2 ± 7.67 ab	46.43 ± 0.91 ab	48.8 ± 4.56 a	32.0 ± 10.5 b
		Ambient	31.3 ± 4.36 ab	33.73 ± 3.43 ab	36.63 ± 6.01 a	19.27 ± 4.58 b
	Z. mays	Enriched	44.91 ± 10.44	54.76 ± 15.16	40.40 ± 4.73	54.46 ± 9.65
		Ambient	35.66 ± 5.05	30.27 ± 3.09	35.14 ± 2.26	32.98 ± 3.63

Note: Amendment treatments correspond to the control (C) and three amendment particle-size fractions: <2 mm, 2–6 mm, and 6–15 mm. Superscript letters denote groups that are statistically significant from each other within each variable measured.

summarizes the effects of ambient and elevated CO₂ conditions and amendment particle size on plant growth and biomass allocation.

Growth monitoring over time showed a rapid increase during the early weeks, followed by gradual stabilization in wheat. Alternatively, maize exhibited continuous growth up to approximately 120 days (Figure 2). Overall, plants grown under ambient conditions reached greater heights than those under elevated CO₂ conditions. The largest differences were observed in the extreme particle-size treatments (<2 mm and 6–15 mm).

The harvest index (HI) of wheat was significantly affected by both CO₂ conditions and the amendment treatment, with no significant interaction between factors. As shown in

Table 5. Mean and standard deviation of the harvest index (HI) and root-to-shoot ratio measured in Triticum durum and Zea mays grown under elevated and ambient CO₂ conditions.

Variable	Species	Chamber	C	<2	2–6	6–15
Harvest Index (HI)	T. durum	Enriched	0.15 ± 0.04 ab	0.19 ± 0.03 a	0.16 ± 0.02 ab	0.09 ± 0.04 b
		Ambient	0.29 ± 0.01 a	0.25 ± 0.02 a	0.29 ± 0.01 a	0.15 ± 0.06 b
	Z. mays	Enriched	0.01 ± 0.02	0.08 ± 0.10	0.00 ± 0.00	0.00 ± 0.00
		Ambient	0.00 ± 0.01	0.00 ± 0.00	0.08 ± 0.09	0.01 ± 0.01
Root-to-shoot ratio	T. durum	Enriched	0.02 ± 0.01	0.03 ± 0.01	0.03 ± 0.01	0.04 ± 0.02
		Ambient	0.01 ± 0.00	0.02 ± 0.01	0.03 ± 0.01	0.03 ± 0.01
	Z. mays	Enriched	0.78 ± 0.18	0.75 ± 0.15	0.75 ± 0.01	0.86 ± 0.09
		Ambient	0.33 ± 0.06	0.28 ± 0.08	0.27 ± 0.07	0.28 ± 0.08

Note: Amendment treatments correspond to the control (C) and three amendment particle-size fractions: <2 mm, 2–6 mm, and 6–15 mm. Superscript letters denote groups that are statistically significant from each other within each variable measured.

, HI values were generally higher under ambient CO₂ conditions, particularly in the control and intermediate particle-size treatments. In maize, HI was low across all treatments but showed a significant CO₂ conditions × amendment treatment interaction, indicating a combined effect of both factors. For the root-to-shoot ratio, no significant effects of CO₂ conditions, amendment treatment, or their interaction were found in wheat. Conversely, the elevated CO₂ conditions significantly increased the root-to-shoot ratio in maize, suggesting a greater biomass allocation to the root system in response to CO₂ enrichment. Detailed mean values and standard deviations for both parameters are provided in Table 5.

Grain nitrogen and protein contents varied according to the amendment treatment, with no clear differences between elevated and ambient CO₂ conditions (Figure 3). In wheat, the highest values were observed in the intermediate particle-size fraction (2–6 mm), reaching up to 2.6% N and approximately 16% protein, whereas the coarser fraction (6–15 mm) showed the lowest values. In maize, nitrogen content ranged between 1.7% and 2.3%, and protein between 10% and 14%, with the coarser fractions (2–6 mm and 6–15 mm) showing higher values compared with the control and the finest fraction (<2 mm). Overall, no consistent differences between CO₂ conditions were observed for either species, although values tended to be slightly higher under ambient CO₂ conditions. Because seed samples were pooled within treatments for analysis, variability within treatments could not be fully assessed. Therefore, these results should be interpreted as indicative trends rather than statistically independent means.

3.2. Soil Carbonate Formation and Physico-Chemical Properties

Soil carbonate formation exhibited contrasting patterns between crops and amendment treatments (Figure 4). In wheat, both concrete particle size and its interaction with the CO₂ conditions had significant effects on the percentage increase in carbonates. Coarse particles (6–15 mm) under elevated CO₂ conditions produced the highest increases, whereas no significant differences among amendment treatments were observed under ambient CO₂ conditions.

In the maize system, significant effects of amendment treatment and CO₂ conditions were also detected, along with a notable interaction between both factors. Fine fractions (<2 mm) showed the lowest increases under ambient CO₂ conditions, while coarse particles (6–15 mm) displayed a more favorable trend under elevated CO₂ conditions. However, differences among amendment particle sizes were not statistically significant.

Taken together, these results indicate that the carbonate formation potential of the substrate depends on both amendment particle size and CO₂ conditions. The response was more pronounced in concrete than in basalt. Figure 4 and

Table 6. Mean and standard deviation of carbonate increase (ΔCaCO₃%), pH, and electrical conductivity (EC) in soils amended with concrete (Triticum durum) and basalt (Zea mays), grown under elevated and ambient CO₂ conditions.

Variable	Species	Chamber	C	<2	2–6	6–15
Carbonate increase (%)	T. durum	Enriched	−0.25 ± 0.47 a	1.24 ± 1.80 a	0.40 ± 1.96 a	4.63 ± 1.03 b
		Ambient	0.76 ± 0.61	1.52 ± 0.72	1.02 ± 0.28	1.21 ± 0.72
	Z. mays	Enriched	−0.30 ± 1.39	0.22 ± 0.48	0.99 ± 0.87	1.82 ± 1.39
		Ambient	0.27 ± 0.30 a	−2.55 ± 0.25 b	0.85 ± 1.91 a	0.29 ± 1.25 a
pH	T. durum	Enriched	7.59 ± 0.13 b	7.93 ± 0.026 a	7.89 ± 0.011 a	7.96 ± 0.14 a
		Ambient	7.48 ± 0.20 b	7.75 ± 0.066 ab	7.92 ± 0.092 a	7.78 ± 0.045 a
	Z. mays	Enriched	7.80 ± 0.054	7.75 ± 0.086	7.74 ± 0.085	7.77 ± 0.056
		Ambient	7.71 ± 0.062	7.79 ± 0.18	7.82 ± 0.045	7.77 ± 0.12
EC (dS·m−1)	T. durum	Enriched	3.07 ± 0.87 b	1.27 ± 0.26 a	1.26 ± 0.20 a	0.93 ± 0.75 a
		Ambient	2.23 ± 0.52 b	1.63 ± 0.20 ab	0.81 ± 0.24 a	1.48 ± 0.34 ab
	Z. mays	Enriched	1.15 ± 0.33	1.51 ± 0.61	1.58 ± 0.52	1.44 ± 0.35
		Ambient	1.84 ± 0.43	1.51 ± 0.67	1.04 ± 0.15	1.37 ± 0.48

Note: Amendment treatments correspond to the control (C) and three amendment particle-size fractions: <2 mm, 2–6 mm, and 6–15 mm. Superscript letters denote groups that are statistically significant from each other within each variable measured.

summarize the main differences between CO₂ conditions and amendment treatments, as well as the key measurements of carbonate increase and geochemical parameters.

Soil pH exhibited contrasting responses to amendments treatment across crops. In the concrete amendment (wheat), significant effects were observed for amendment treatment (p < 0.001) and CO₂ conditions (p = 0.025), with no interaction between factors. All concrete treatments increased soil pH compared with the control, with more pronounced rises under elevated CO₂ conditions.

Alternatively, in the basalt amendment (maize), soil pH was not significantly affected by amendment treatment, CO₂ conditions, or their interaction (p > 0.05). This result indicates a more stable soil response to this amendment under both atmospheric conditions.

Electrical conductivity (EC) exhibited contrasting patterns between crops. In wheat, EC depended exclusively on amendment treatment (p = 0.001): the control displayed the highest EC values, whereas amended soils, particularly those with intermediate particle-size fractions (2–6 mm), showed significantly lower values. In maize, neither amendment treatment nor CO₂ conditions had significant effects (p > 0.05), although a slight tendency toward lower EC values was observed with basalt application. Figure 5 illustrates the main patterns observed for soil pH and EC, while the corresponding values are provided in Table 6.

Correlation analyses revealed distinct patterns between crops. In wheat, the carbonate increase was positively correlated with soil pH (r = 0.457; p = 0.025) and negatively correlated with electrical conductivity (EC) (r = −0.516; p = 0.010). Additionally, pH and EC were strongly inversely related (r = −0.854; p < 0.001). In maize, no significant correlations were found between carbonate increase and either pH (r = −0.034; p = 0.858) or EC (r = −0.008; p = 0.965). However, a consistent negative relationship was observed between pH and EC (r = −0.839; p < 0.001).

These findings collectively demonstrate that concrete amendments in wheat soils promote higher pH and enhanced carbonate formation while reducing electrical conductivity (EC). The effects were stronger for coarse particle-size fractions and under elevated CO₂ conditions. By contrast, basalt amendments in maize soils produced a more variable and environment-dependent response, with limited effects on soil chemistry and less consistent carbonate formation.

4. Discussion

4.1. Agronomic Responses Under Elevated CO₂

The results indicate that elevated CO₂ conditions significantly affected plant growth dynamics and biomass allocation in both crops, showing distinctive responses between C₃ and C₄ species.

Free-Air CO₂ Enrichment (FACE) experiments show that CO₂ enrichment enhances photosynthetic assimilation in C₃ plants by increasing chloroplastic CO₂ concentration and suppressing photorespiration. It also improves the efficiency of Rubisco carboxylation, while reduced stomatal conductance further enhances intrinsic water-use efficiency [54,55]. Together, these processes sustain biomass accumulation when water and nutrients are not limited. Under nutrient constraints, photosynthetic acclimation and nitrogen dilution may occur. In such cases, carbon allocation tends to shift toward roots to support nutrient uptake [56,57].

Alternatively, C₄ plants possess an internal CO₂-concentrating mechanism that already saturates Rubisco under current atmospheric conditions. As a result, biomass or yield responses to elevated CO₂ are generally limited when resources are sufficient. However, FACE experiments indicate that elevated CO₂ can still improve water-use efficiency, delay stress onset, and promote greater carbon allocation to roots under heat or episodic water stress. This physiological adjustment supports belowground carbon investment and contributes to potential soil carbon sequestration under stress-prone environments [54,58,59,60]

The physiological framework described above is reflected in our results, which reveal contrasting biomass allocation patterns between the two species under elevated CO₂ conditions.

In wheat (C₃), total biomass increased by 42.6% under elevated CO₂ conditions. This response was mainly due to higher vegetative and husk biomass, while grain yield remained unchanged and plant height decreased. These results are consistent with FACE studies reporting biomass increases of approximately 20% in C₃ crops, driven by enhanced photosynthesis, reduced stomatal conductance, and greater resource-use efficiency, provided that water and nutrients are not limiting [54,55,61].

In maize (C₄), total biomass increased by 55%, primarily due to enhanced root development, with minimal changes in aboveground biomass (+3%) and no consistent response in grain yield. This pattern aligns with evidence indicating that C₄ crops exhibit limited biomass or yield gains under elevated CO₂ conditions, as their CO₂-concentrating mechanism already saturates Rubisco, except under water-stress conditions [54,58].

Under such conditions, elevated CO₂ conditions can improve water-use efficiency by promoting lignin synthesis and reduced stomatal conductance. These changes alleviate water stress, indirectly enhancing photosynthesis and yield [59,60]. Although water stress was avoided in this experiment, chamber temperatures reached up to 40 °C during the final growth stages. Under these conditions, maize plants grown under elevated CO₂ displayed greater stress tolerance.

Furthermore, the observed shift toward greater root biomass allocation in maize aligns with previous reports for C₄ crops. In these species, the main effect of elevated CO₂ conditions manifests as changes in the root-to-shoot ratio rather than increases in aboveground biomass. Several studies have shown that plants exposed to elevated CO₂ develop larger and morphologically altered root systems, enhancing water uptake capacity under drought conditions [59,62]. This expansion of the root system in C₄ species represents an adaptive strategy to water limitation and contributes to soil carbon sequestration [63].

Ultimately, these findings reinforce that elevated CO₂ conditions enhance total biomass accumulation in C₃ crops, whereas in C₄ species, the effects are more closely associated with resource reallocation, particularly toward root biomass.

4.2. Effects of Amendments and Particle Size

Amendment particle size is a key factor influencing both rock weathering kinetics and plant physiological responses. Focusing on the latter, finer particles increase the reactive surface area and nutrient release. However, they can also alter soil structure and restrict root aeration, whereas coarser fractions enhance porosity but reduce geochemical reactivity. This balance between surface reactivity and physical structure strongly affects water retention, aggregate stability, and oxygen diffusion in the root zone, as previously demonstrated in substrate studies evaluating particle composition and plant establishment [64,65,66]. The interplay between these physicochemical properties and plant functional traits appears to drive the differential growth responses observed among species and amendment types.

In wheat, growth tended to be higher in the intermediate particle-size fraction (2–6 mm), whereas coarse particles (6–15 mm) significantly reduced plant biomass. This suggests that intermediate fractions provide an optimal balance between chemical reactivity and substrate structure. Similar behavior has been reported for concrete grinding residue (CGR), where intermediate particle sizes improve water retention and aeration [67]. Collectively, both current and previous results suggest that intermediate granulometries offer the most favorable physical–chemical conditions for plant development. However, the lack of strong statistical differences emphasizes the need for field-scale validation [68,69].

In maize, total and organ biomass did not differ statistically among treatments, indicating a lower sensitivity to basalt particle size. Slight trends toward greater aboveground biomass were observed with fine particles (<2 mm). This stability may relate to the robust and adaptive root system of maize. Such a system is capable of anatomical and mechanical adjustments that maintain growth under variable substrate textures [70,71,72].

In essence, these findings demonstrate that amendment particle size modulates crop response in a material- and species-dependent manner. The effects were clearer in wheat with concrete than in maize with basalt, underscoring the need to optimize granulometry to balance geochemical efficiency with agronomic performance.

4.3. Grain Quality and Nutrient Dynamics

Grain quality, assessed through nitrogen and protein content, showed contrasting responses to elevated CO₂ conditions, revealing a decoupling between yield and nutritional quality under controlled environments.

Elevated CO₂ conditions did not produce significant increases in grain biomass in either crop. In wheat, biomass gains were concentrated in vegetative organs (stems, roots, and husks). Grain production remained stable or slightly higher under ambient conditions, resulting in higher harvest index values compared with plants grown under CO₂ enrichment.

Nitrogen and protein concentrations in wheat were consistently higher under ambient conditions, confirming that elevated CO₂ conditions compromise nutritional quality. This finding aligns with meta-analyses reporting 5–15% reductions in nitrogen and protein levels in cereals grown under elevated CO₂. These decreases are attributed to the nitrogen dilution effect, in which biomass accumulation is not accompanied by proportional nutrient uptake [56,57,73].

In maize, differences were less pronounced: nitrogen and protein concentrations remained relatively stable, with only slight decreases under elevated CO₂ conditions, while grain weight was equal to or even higher than under ambient conditions. This agrees with previous evidence showing that C₄ crops are less sensitive to nitrogen and protein dilution under elevated CO₂ [73,74,75].

Regarding amendment particle size f, in wheat, finer and intermediate sizes (<2 mm and 2–6 mm) produced higher nitrogen and protein contents, whereas coarse particles (6–15 mm) reduced both parameters. This suggests that larger fragments limit the effectiveness of concrete amendments due to their lower reactive surface area and reduced rhizosphere interaction. In maize, however, no consistent differences among amendment particle size were observed. This indicates a limited sensitivity to granulometry, likely compensated by its more robust and adaptive root system [70,71].

Ultimately, these findings demonstrate that elevated CO₂ conditions promote vegetative growth without enhancing grain yield. They also reduce nitrogen and protein concentrations, particularly in C₃ crops, thereby underscoring the nutritional challenges associated with future climate scenarios.

4.4. Soil Carbonate Formation and Physico-Chemical Responses

The results indicate that amendment treatments significantly influenced soil geochemical dynamics, affecting carbonate formation and related edaphic properties such as pH and electrical conductivity (EC). In soils cultivated with wheat, concrete amendments promoted stronger alkalinization and carbonate accumulation, particularly under elevated CO₂ conditions. In a divergent manner, in maize soils, basalt amendments produced more moderate effects that were mainly dependent on amendment particle size.

Concrete is a highly heterogeneous material composed of multiple mineral phases (portlandite, C–S–H, aluminates, silicates, among others) that exhibit contrasting dissolution kinetics and distinct pH dependencies [16]. The differences observed in carbonate accumulation between concrete-amended and control soils confirm that, in the presence of this material, geochemical reactions occur that are absent in unamended soils. These reactions primarily involve the dissolution of calcium-bearing phases followed by carbonate precipitation.

Previous studies have shown that both the magnitude and rate of concrete weathering are controlled by multiple factors. These include the material’s chemical composition and particle size distribution [76,77], the concentration and partial pressure of CO₂ [78,79,80], as well as porosity and microstructure, which regulate gas diffusion and access to reactive phases [77]. These factors explain the differences observed between amendment treatments and CO₂ conditions in this experiment. They also highlight the importance of considering both the intrinsic properties of concrete and the environmental conditions when assessing its carbonation potential in agricultural soils.

In this experiment, treatment with coarse concrete particles (6–15 mm) resulted in the highest carbonate formation. This size range provides an optimal balance for accelerated weathering under soil conditions. Unlike previous studies that used much finer particles and consequently incurred high energy costs for grinding, the coarser fractions in this study maintained strong reactivity for carbonate formation. At the same time, they substantially reduced energy demand.

Life-cycle assessments identify rock grinding as the main contributor to energy consumption and CO₂ emissions in enhanced rock weathering [42]. As target particle size decreases, grinding energy requirements rise sharply. Emissions range from 5 to 35 kg CO₂ per tonne of rock for particle sizes between 10 and 100 µm, depending on the regional electricity mix [81]. Energy models also show that power consumption increases nonlinearly as materials approach ultrafine dimensions. Although dissolution rates improve, rising from about 16% for particles smaller than 100 µm to nearly 55% for those below 10 µm over a ten-year period, the production of ultrafine particles greatly increases energy use. In some energy scenarios, this additional consumption can counteract the expected carbon removal benefits, as emissions from grinding and transport may offset part of the gained sequestration potential [82,83]. These findings suggest that intermediate to coarse granulometries may maximize overall carbon removal efficiency by minimizing energy inputs while preserving sufficient reactivity and permeability for sustained carbonation.

Moreover, in agricultural soils, coarse fractions react more slowly at first but provide a prolonged release of alkalinity. Over time, this leads to a convergence of pH effects [84]. This represents not only a practical advantage by reducing energy requirements for grinding but also an agronomic benefit, as it supports long-term improvement of soil chemical properties.

In the case of basalt, which is mainly composed of calcium- and magnesium-silicate minerals, its dissolution kinetics are considerably slower than those of carbonate-rich materials. This is reflected in the lower increases in carbonate content, with even negative values under certain conditions.

These results indicate that carbonate loss processes, such as leaching or CO₂ degassing, predominated over precipitation during early weathering stages. This difference arises because calcite dissolves and reacts much faster than silicate minerals.

Consequently, concrete, which is rich in carbonate phases, generates immediate increases in dissolved inorganic carbon (DIC). By contrast, basalt, lacking calcite, depends on the gradual dissolution of silicates to release cations and sustain carbonation, an inherently slower process [17].

Several enhanced rock weathering experiments using basalt support this observation, indicating that increases in carbonate content are generally low or undetectable over short incubation periods.

Vienne et al. [22] reported, after 99 days with Solanum tuberosum, a clear increase in alkalinity but no significant change in total inorganic carbon (TIC), suggesting that solid carbon fixation requires longer timescales. Similarly, Kelland et al. [21] found no detectable increases in TIC in mesocosm experiments, attributing this to slow carbonate precipitation kinetics. Conversely, Haque et al. [15] showed that with more reactive minerals such as wollastonite, measurable increases in TIC occurred within just 55 days. These results highlight that the mineralogical composition of the amendment governs the rate of dissolved carbon precipitation.

Regarding soil chemistry, pH and EC responses differed markedly between amendments. In concrete-amended soils, a significant pH increase was observed across all treatments compared with the control. This behavior is consistent with the high intrinsic alkalinity of concrete and the release of basic cations from calcium-bearing phases. The pH rise, in turn, promotes carbonate dissolution and CO₂ capture [23]. In parallel, EC decreased relative to the controls. This pattern aligns with the increase in pH, the immobilization of cations in solid carbonate phases, and the resulting reduction of ions in solution [85].

By comparison, basalt-amended soils showed no significant effects on either pH or EC compared with the controls. This again reflects the slow kinetics of cation release from silicate minerals.

In concrete-amended soils, the increase in carbonate content was positively correlated with pH and negatively correlated with EC. These relationships highlight the connection between alkalinization, carbonate precipitation, and the consumption of soluble ions.

Alternatively, no significant correlations were found between carbonate content and chemical variables in basalt-amended soils. The only consistent relationship was a negative correlation between pH and EC, which is characteristic of an early stage of silicate weathering. At this point, cation release has not yet produced substantial carbonate accumulation or noticeable changes in soil chemistry.

In addition to the intrinsic properties of the amendments, the CO₂ environment proved to be a key modulating factor in carbonation processes and soil geochemical responses.

Elevated CO₂ conditions (~1000 ppm) influenced carbonation differently depending on the amendment type. In concrete-amended soils, the main effect of CO₂ was not significant. However, the interaction between amendment treatment and CO₂ conditions was significant. This indicates that elevated CO₂ conditions enhanced carbonation only within specific particle-size fractions, particularly in the 6–15 mm range, which showed the highest increases under elevated CO₂.

In contrast, in basalt-amended soils, both the main effect of CO₂ and its interaction with particle size were significant. This shows that higher CO₂ partial pressure increased overall carbonation and modified size-dependent patterns. This behavior aligns with geochemical theory, as CO₂ enrichment raises soil respiration and pore CO₂ concentrations, enhancing acidification and mineral weathering [35]. Previous accelerated weathering experiments under CO₂-rich atmospheres have reported similar trends, with greater cation release and pedogenic carbonate formation compared with ambient conditions [36,37].

In conclusion, these results demonstrate that elevated CO₂ conditions acts as a catalyst for inorganic carbon sequestration. However, their effectiveness depends strongly on the mineralogical composition and particle size of the amendment.

4.5. Limitations and Future Research Directions

This study provides an initial experimental insight into the interactions between amendments and elevated CO₂ conditions under controlled greenhouse conditions. While the results offer valuable evidence on plant growth responses and inorganic carbon dynamics in both C₃ and C₄ systems, several limitations must be acknowledged.

First, the relatively short duration of the experiment (approximately five months) and the use of pot-based systems constrain the evaluation of mineral dissolution and carbonate precipitation processes. These reactions typically occur over much longer timescales under field conditions. The 21–week experiment therefore captures only short-term responses, reflecting early stages of soil alkalinization and potential carbonate formation rather than long-term carbon sequestration. These findings should thus be interpreted as indicative of the system’s potential for carbon stabilization, not as verified CO₂ removal.

Other enhanced rock weathering studies, either through field trials of greater duration or through model-based extrapolations, likewise indicate that several years of weathering and monitoring are required before substantial carbonate accumulation can be detected. Measurable net CO₂ drawdown can only be demonstrated after extended observation periods, typically spanning multiple years [9,22,86]. This reinforces the need for longer-term, field-based deployments to validate CDR performance under agricultural conditions.

Likewise, greenhouse experiments, although ideal for isolating the effects of elevated CO₂ conditions and amendment particle size, have inherent limitations. Their controlled nature restricts the extrapolation of results to real agricultural settings, which are characterized by high spatial and temporal variability in soil, climate, and biotic interactions.

Another limitation concerns the relatively small number of replicates. This reduces the statistical power and the ability to detect subtle but potentially meaningful effects, especially in soil parameters. Furthermore, the study focused on only two crop species (wheat and maize) and two amendment materials (recycled concrete and basalt). This restricts the generalization of the findings to other mineral compositions or crop types.

Additionally, the lack of direct quantification of dissolved inorganic carbon (DIC) fluxes and net soil–atmosphere CO₂ exchanges prevented a comprehensive assessment of the overall carbon balance associated with enhanced rock weathering. Consequently, the observed increase in soil carbonate content should be regarded as potential rather than verified carbon dioxide removal.

Future research should therefore prioritize long-term field experiments that include continuous monitoring of dissolved inorganic carbon (DIC) and CO₂ fluxes. Incorporating isotope tracing would further clarify the fate of carbon within the system and distinguish between biotic and abiotic pathways of carbonate formation. Integrating these measurements with life-cycle assessment frameworks will enable evaluation of net energy use and emissions, providing a more robust quantification of the overall climate benefits of enhanced rock weathering.

Expanding the range of mineral and industrial materials tested is essential to capture the diversity of possible reactions and outcomes. It will also be important to integrate variable moisture, temperature, and microbiological conditions to better understand how these factors influence geochemical and biological processes. These processes ultimately drive mineral weathering and carbonate formation. In addition, incorporating empirical data from long-term monitoring into regional- and global-scale predictive models will strengthen the connection between experimental evidence and practical applications. Such modeling efforts could provide valuable insights into the long-term feasibility of using recycled minerals as part of sustainable carbon dioxide removal and soil restoration strategies.

5. Conclusions

This study demonstrates that ERW, when integrated with crop cultivation, represents a promising strategy for CDR with additional agronomic co-benefits. The results show that elevated CO₂ conditions promoted biomass accumulation in both wheat (C₃) and maize (C₄), though with species-specific responses. In wheat, the increase occurred mainly in vegetative organs and had limited effects on grain yield. In maize, however, elevated CO₂ conditions primarily stimulated root biomass, with little impact on aboveground productivity.

Amendment particle size emerged as a key factor influencing both plant performance and soil chemistry. In wheat, recycled concrete intermediate particle-size fractions enhanced growth and grain quality. Meanwhile, coarse fragments promoted carbonate formation under elevated CO₂ conditions. In contrast, basalt particle size had a more limited influence on maize growth, although it still modulated soil carbonate dynamics.

From a soil physico-chemical perspective, recycled concrete exhibited higher reactivity, increasing pH and favoring carbonate precipitation. In comparison, basalt showed slower and more CO₂-dependent responses under elevated CO₂ conditions. These findings highlight the importance of jointly considering mineralogical composition, amendment particle-size distribution, and crop physiology when optimizing enhanced rock weathering strategies in agricultural systems.

In summary, this experiment provides a proof of concept integrating C₃ and C₄ crops, industrial by-products, and natural silicate minerals under elevated CO₂ conditions. The results establish a foundation for future large-scale and long-term field trials that incorporate comprehensive carbon accounting, multifactorial designs, and diverse cropping systems. Such efforts will be essential to robustly evaluate the potential of enhanced rock weathering as both a climate mitigation strategy and a driver of agroecological co-benefits.