Enhancing Carbon Sequestration in Barley via Silicon-Induced Phytolith Accumulation for Climate Change Mitigation

Abstract

Background: Phytolith-occluded carbon (PhytOC) is highly stable and constitutes an important long-term carbon pool in agroecosystems, particularly in nutrient-poor, sandy soils. Silicon (Si) uptake by plants is strongly associated with phytolith formation, with Si accounting for up to 90% of phytolith composition. However, the role of Si fertilization in enhancing PhytOC sequestration under field conditions remains insufficiently quantified. Integrated fertilization strategies supporting sustainable development in climate-resilient agriculture can enhance biological carbon sequestration by increasing phytolith formation and phytolith-occluded carbon accumulation, thereby improving the carbon sink potential of cereal-based agroecosystems. Methods: A field experiment was conducted to assess phytolith and PhytOC accumulation in barley biomass under different fertilization regimes, including foliar silicon application using the liquid immune stimulant Optysil and compost fertilization. Phytolith content was determined separately for grain and straw, and PhytOC stocks were converted into CO₂ equivalents to estimate annual sequestration potential. Results: Barley produced substantial amounts of phytoliths, with consistently higher concentrations in straw than in grain. Phytolith content ranged from 18.46 to 21.28 mg g⁻¹ DM in grain and from 27.89 to 38.97 mg g⁻¹ DM in straw. Depending on fertilization treatment, annual carbon sequestration through PhytOC ranged from 16.86 to 55.17 kg CO₂ equivalents ha⁻¹. Foliar silicon application increased PhytOC accumulation in barley biomass by up to threefold compared with treatments without Si. Conclusions: The results demonstrate that optimizing silicon fertilization can substantially enhance carbon sequestration in cropping systems via phytolith formation and PhytOC stabilization. Given the dominant role of cereals in crop rotations and their high phytolith-producing capacity as monocotyledonous plants, Si-mediated PhytOC sequestration represents a promising pathway for strengthening soil carbon storage and contributing to climate change mitigation.

1. Introduction

Climate change is the primary challenge faced by humanity in the 21st century. It is estimated that global CO₂ emission will have reached 36.1 Gt CO₂ by 2022 [1]. Carbon dioxide emission shows high temporal variability. Before the industrial revolution, the emission was very low. A slow increase was recorded until the mid-20th century. In 1950, the world emitted 6 billion tonnes of CO₂. Over only 40 years, the amount increased four times, exceeding 22 billion tonnes. We are currently emitting more than 34 billion tonnes of CO₂ annually [2]. The share of individual countries in global emissions is variable. China takes the lead with a share of as much as 58.4%, followed by the USA with a share of 16.5%, and Europe with a share of 14.3% [3]. The contribution of agriculture in global CO₂ emissions currently accounts for approximately 20% [4], and half of that emission is directly related to the use of agricultural land. Carbon dioxide emitted from agriculture is produced as a result of cultivation operations such as plowing, harrowing, and cultivating. When performing these treatments, we supply oxygen to the soil, which oxidizes carbon compounds according to the reaction C₆H₁₂O₆ + 6O₂ = 6CO₂ + 6H₂O. Easily decomposable buds that are part of the so-called “food humus” are oxidized [5,6]. This process is very fast, especially on well-aerated sandy soils. Therefore, to reduce CO₂ emissions into the atmosphere, cultivation systems that limit mechanical tillage are increasingly being introduced. Agriculture contributes to the emission of greenhouse gases, including CO₂, but it is also a major absorber of this gas [5,6,7].

Plants assimilate CO₂ through the process of photosynthesis to build their biomass, thereby playing a key role in the carbon cycle and in mitigating climate change. An element that may play a crucial role in increasing plant productivity, and thus in enhancing carbon dioxide absorption from the atmosphere, is silicon [8,9]. Plants absorb silicon from the soil in the form of monosilicic acid (H₄SiO₄), and then, through the process of polymerization, transform it into SiO₂·nH₂O, which accumulates in cellular structures as amorphous particles called phytoliths [8]. Phytoliths are microscopic bodies of amorphous hydrated silica formed within plant tissues during silica deposition. They are characteristic of specific plant groups, and their content in above-ground parts ranges from 0.5% dry matter in dicotyledonous plants to more than 15% dry matter in grasses [9,10,11]. The variable content of phytoliths between monocotyledonous and dicotyledonous plants results from different mechanisms of silicon uptake and accumulation [12]. Plants of the same species may differ in the amount of phytoliths produced, depending on climate and soil conditions [13]. During phytolith formation, between 0.1% and 3% of organic carbon can be incorporated into their structure as phytolith-occluded carbon (PhytOC) [14,15] but according to de Tombeur et al. [16] the changes are greater and range within the limits of 8.3–34.7%.

In terrestrial ecosystems, the total sequestration of organic carbon in the form of PhytOC is estimated at 156.7 ± 91.6 Tg CO₂ per year [17]. Therefore, the formation of PhytOC by plants can be considered a method of carbon sequestration, with carbon retained in this form in soils for several thousands of years as phytoliths are released during the decomposition of dead plant material [15,18,19]. The literature includes publications indicating that carbon sequestration through the use of phytoliths is significantly lower than previously estimated [16,20]. Although silicon is a commonly occurring element in soil, its availability to plants is low and depends on many biotic and abiotic factors [21,22,23]. Therefore, exogenous supplementation of silicon in the form of soil or foliar fertilizers may stimulate phytolith formation and carbon sequestration in the form of PhytOC [19,24]. Silicon fertilization plays a significant role in enhancing crop productivity, increasing stress tolerance, and strengthening long-term carbon sequestration through phytolith-occluded carbon (PhytOC). Silicon improves nutrient-use efficiency, reinforces cell walls, and modulates stress-responsive metabolic pathways, thereby enhancing photosynthetic performance and ultimately leading to greater biomass accumulation [25].

Field and greenhouse fertilization experiments consistently confirm that silicon application promotes improvements in crop yield and physiological resilience [26]. Vegetation experiments on rice, in which combined silicon and phosphorus fertilization was applied, demonstrated a substantial increase in shoot biomass and phytolith production. At the same time, it was found that higher silicon supply increased phytolith stocks in above-ground tissues [27]. The key climate-related impact of silicon, however, lies in its capacity to promote the formation of highly durable phytoliths that encapsulate organic carbon [13,25]. Studies conducted in forest ecosystems clearly show that the addition of silicon—either alone or in combination with biochar—significantly increases PhytOC content in both plants and soil, thereby enhancing long-term soil carbon storage. It should be noted, however, that the long-term stability of carbon trapped within phytoliths depends strongly on the physicochemical environment, particularly soil temperature and CO₂ concentration. These factors determine the dissolution rate of phytoliths and may influence the duration for which carbon remains sequestered [8,13,16,28].

Due to their widespread cultivation, cereals can play a role in carbon sequestration [10,13,29]. However, despite the recognized role of silicon in plant physiology and soil carbon dynamics, the combined application of Si fertilizer and compost in barley cultivation remains poorly explored, particularly under temperate climate conditions. Investigating this interaction is therefore novel and significant, as temperate regions represent major cereal-producing areas where optimized nutrient management could simultaneously improve crop productivity and strengthen long-term carbon stabilization in agroecosystems [15,30].

In the context of increasing pressure to develop climate-resilient and sustainable agricultural systems, identifying practices that enhance biological carbon sequestration has become a key research priority. Agricultural soils are increasingly recognized not only as sources of greenhouse gas emissions but also as important potential carbon sinks [15,17]. Therefore, understanding the agronomic factors that influence phytolith formation and phytolith-occluded carbon (PhytOC) accumulation in crop biomass is essential for improving carbon management in agroecosystems. In particular, integrated fertilization strategies combining mineral nutrients, organic amendments, and silicon may play a significant role in strengthening the climate mitigation potential of cereal-based production systems [15,17,30].

This study examined the effect of mineral, organic, and silicon fertilization on the concentration of phytoliths and PhytOC in the biomass of spring barley, as well as the potential for carbon dioxide biosequestration by barley under different fertilization conditions. Moreover, the authors sought to demonstrate, from a practical and policy-oriented perspective, how cereal-based cropping systems can be effectively leveraged to support the gradual and long-term accumulation of organic carbon in soils. The findings provide an evidence base for the development of agricultural and climate policies aimed at enhancing soil carbon sequestration through crop and residue management strategies.

2. Materials and Methods

2.1. Study Area and Site Descriptions

The experiments (Figure 1A,B) were conducted in the years 2021–2022 at the Experimental Station in Skierniewice (51°57′535 N, 20°9′254 E).

It was established on Luvisol soil [31] with each experimental plot being 15 m² in three replicates. The initial soil properties are presented in

Table 1. Characteristics of soil physicochemical properties—original research.

Properties *	Unit	Content
Soil texture 1	%	sand 76, silt 8, clay 16
pH 2	-	4.75
Total nitrogen (TN) 3	g kg−1	0.76
Total organic carbon (TOC) 4	g kg−1	8.40
P available 5	mg kg−1	38.57
K available 5	mg kg−1 mg	70.60
Mg available 6	kg−1	26.10

* according to: ¹ IUSS Working Group WRB [31], ² ISO 10390 [32], ³ ISO 11261 [33], ⁴ ISO 10694 [34], ⁵ PN-R-04023 [35], ⁶ PN-R-04020 [36].

.

The soil pH was determined in 1 M KCl by the potentiometric method on an automatic pH meter [32]. Total soil N was determined by the Kjeldahl method [33]. Total soil organic carbon was determined by means of the Vario Max analyser CHNS [34]. Available forms of phosphorus and potassium were determined by the Egner–Riehm method [35] and magnesium by the Schachtschabel method [36]. Both extraction procedures were selected due to their widespread applicability in European agronomic diagnostics and their suitability for soils of moderate to high organic matter content.

The atmospheric conditions were collected from the weather station localized in the experimental field in Skierniewice (Figure 2). The average annual temperature was 8.3 °C and the average rainfall was 685 mm. These data were further validated against long-term climatic normals (1991–2020) to confirm that experimental years exhibited typical regional conditions. The meteorological conditions indicate that the experimental years were neither cold nor exceptionally warm, but rather represented the midpoint of the climatic norm. Precipitation levels reflected moderately moist conditions, as the typical range for the region is 550–700 mm. Overall, it can be concluded that the meteorological conditions did not exert a confounding influence on the conducted experiment.

2.2. Experimental Design

Spring barley cv. Fantex were sown in Poland. The experiment covered the following fertilizer combinations (

Table 2. The flowchart for one replicate including all treatments of field experiment.

Control	Compost170	NPK + Si	Compost170 + Si	NPK	Compost120 + Nmin	Compost120	Compost120 + Nmin + Si
Compost120	Compost120 + Nmin	Control	NPK	Compost170 + Si	NPK + Si	Compost120 + Nmin + Si	Compost170
Compost170	Compost170 + Si	Compost120	NPK + Si	Compost120 + Nmin + Si	Control	NPK	Compost120 + Nmin

):

-

Control (fertilization 0 kg ha⁻¹);

-

NPK (100 kg N_min ha⁻¹—CO(NH₂)₂, 35 kg P ha⁻¹—Ca(H₂PO₄)₂, and 100 kg K ha⁻¹—KCl);

-

NPK + Si (100 kg N_min ha⁻¹—CO(NH₂)₂, 35 kg P∙ha⁻¹—Ca(H₂PO₄)₂, and 100 kg K ha⁻¹—KCl and 139.8 g Si ha⁻¹);

-

Compost170 (170 kg N_org ha⁻¹, Table 2);

-

Compost170 + Si (170 kg N_org ha⁻¹ and 139.8 g Si ha⁻¹);

-

Compost120 + N_min (120 kg N_org ha⁻¹ and 30 kg N_min ha⁻¹);

-

Compost120 + N_min + Si (120 kg N_org ha⁻¹ and 30 kg N_min ha⁻¹ and 139.8 g Si ha⁻¹).

The maximum dose of N in organic fertilizers was 170 kg ha⁻¹ (the permissible dose of N by Polish and EU legislation).

The dose of compost (Compost120) was reduced to 120 kg of N_org ha⁻¹ and 30 kg of N_min ha⁻¹ in the form of urea and was additionally applied to check whether the accumulation of phytolites is greater with organic fertilization alone or with organic–mineral fertilization.

Mineral fertilization was applied at the following doses: 100 kg N_min ha⁻¹—CO(NH₂)₂, 35 kg P ha⁻¹—Ca(H₂PO₄)₂, and 100 kg K ha⁻¹—KCl. The compost was applied at a dose corresponding to 170 kg N_org ha⁻¹ and 120 kg N_org ha⁻¹. The N_org is the dose of organic nitrogen. The N_min is the dose of mineral nitrogen in the form of urea [CO(NH₂)₂]. In the experiment, compost and mineral fertilization were applied before sowing silicon was applied as foliar fertilization at a dose 139.8 g Si ha⁻¹ as “Optysil” (200 mg SiO₂ L⁻¹) at three plant development stages: I: phase 3 of leaves (BBCH 18), II: beginning of stem shooting (BBCH 35), III: end of the flowering phase (BBCH 72). The experiment was conducted with three replicates (Table 2). Foliar application was performed under low-wind conditions (<2 m s⁻¹) to ensure adequate adherence of the Si formulation to leaf surfaces and to minimize spray drift. The compost used in field trials was produced through composting four organic waste substances, namely municipal sewage sludge, municipal green waste, sawdust, and spent mushroom substrate. The composting process was carried out in aerobic conditions for 12 weeks and included periodic mechanical turning to maintain optimal temperature (55–65 °C) and moisture levels (45–55%), ensuring pathogen reduction and stable humification. The chemical composition of compost pellets is presented in

Table 3. Chemical composition of compost *—original research.

pH	Corg	Si	N	S	P	Na	Ca	Mg	K
6.7	158.7	40.2	12.8	1.49	11.05	1.02	84.6	5.4	10.7

C_org—organic carbon, and macroelements—g kg⁻¹ dry matter. * Chemical analyses were performed according to standard procedures described in Datnoff et al. [37], Klute [38] and Sparks [39].

.

2.3. Analyses

After harvest, grain and straw samples were oven-dried at 70 °C and then ground using a laboratory mill (Retsch GmbH, Haan, Germany) at 5000 rpm. According to [40,41,42], phytolith content was determined following microwave-assisted acid digestion. Briefly, 0.25 g of plant material was placed in a Teflon vessel and treated with 3 cm³ HNO₃, 2.5 cm³ H₂O₂, and 0.5 cm³ HCl, followed by digestion for 50 min. The samples were rinsed with distilled water, centrifuged four times at 3500 rpm for 5 min, and the isolated phytoliths were dried at 70 °C for 24 h. Residual organic matter (peripheral organic carbon) was removed using the Walkley–Black method [43] with H₂SO₄ at 102–135 °C. Purified phytoliths were dried at 60 °C for 48 h and analyzed for carbon content using an Vario CNS (Elementar Analysensysteme GmbH, Langenselbold, Germany), with quality control performed using certified reference material GBW07405 (precision >5%).

Silicon content in plant tissues was determined via microwave digestion in HNO₃ and HF [21]. For each sample, 0.3 g of plant material was digested with 5 cm³ HNO₃ and 0.1 cm³ HF for 60 min. Silicon concentrations were measured using a ThermoElementar ICP-MS (Thermo Elemental Ltd., Winsford, UK), with reference to certified Corn Gluten Organic Analytical Standard 502-272, and a limit of Si detection (LOD) of 0.28 mg dm⁻³.

2.4. Calculations

The calculations were performed according to the methodology described by Song et al. [30].

(1)

The concentration of phytolith-occluded carbon (mg g⁻¹ DM) was determined as the product of phytolith concentration and carbon concentration within phytoliths:

PhytOC content of organ = phytolith content × carbon concentration in phytoliths

(1)

where phytolith content refers to the mass of phytoliths per unit dry matter of plant material (mg g⁻¹ DM), while carbon concentration in phytoliths indicates the amount of carbon contained in phytoliths per unit dry matter.

(2)

Annual phytolith accumulation in aboveground crop biomass, expressed as phytolith production flux (kg ha⁻¹ year⁻¹), was estimated by multiplying phytolith concentration by aboveground net primary productivity (ANPP):

Phytolith production flux = phytolith content × ANPP

(2)

where phytolith content represents phytolith concentration in aboveground dry biomass (mg g⁻¹ DM), and ANPP corresponds to crop aboveground biomass production per hectare per year (kg ha⁻¹ year⁻¹).

(3)

Annual sequestration of carbon dioxide through PhytOC in aboveground biomass (kg CO₂ ha⁻¹ year⁻¹) was calculated from phytolith production flux using the carbon-to-carbon dioxide conversion coefficient:

PhytOC production flux = phytolith production flux × 44/12

(3)

where the phytolith production flux (kg ha⁻¹ year⁻¹) was derived using Equation (2), whereas PhytOC content refers to the amount of occluded carbon contained within phytoliths, expressed on a dry matter basis (mg g⁻¹ DM); 44/12 is the molecular weight ratio of CO₂/C to C used for conversion.

2.5. Analysis of Experimental Data

All statistical analyses were performed using Statistica 14.0 software. The one-way analysis of variance (ANOVA) was conducted to evaluate the effect of fertilization treatments on the studied quantitative and qualitative characteristics of barley. When the ANOVA indicated significant treatment effects (p < 0.05, n = 3), post hoc multiple comparison procedures were performed using Tukey’s honestly significant difference (HSD) test at a significance level of p = 0.05 to identify pairwise differences between means. Linear regression analysis was applied to determine the relationships between (1) silicon concentration and phytolith content in grain and straw, and (2) phytolith content and PhytOC concentration. Correlation strength and significance were assessed using Pearson correlation coefficients. All results are presented in tables and figures as average values, and differences between treatments are considered statistically significant at p < 0.05.

3. Results

Barley responded positively to silicon fertilization (Figure 3).

The highest grain yield was obtained in the NPK + Si treatment (4.18 Mg·ha⁻¹), whereas the lowest yield occurred in the Compost170 treatment (3.06 Mg·ha⁻¹). The application of silicon in combination with mineral fertilization (NPK + Si) significantly increased crop yield compared to all other fertilization treatments (19%), except for the treatment with mineral fertilization alone (NPK). Combined mineral–organic fertilization (Compost120 + Nmin) as well as mineral–organic fertilization supplemented with silicon (Pelet120 + Nmin + Si) also significantly enhanced barley grain yield relative to the control (20%). At the same time, no significant differences in grain yield were observed between these two combinations. Straw yield increased (39%) significantly in all fertilization treatments compared with the control (0.88 Mgha⁻¹). Similar to grain yield, the highest straw yield was recorded in the NPK + Si treatment (1.59 Mg·ha⁻¹). The introduction of organic matter into the fertilization dose (Compost170 + Si) resulted in a significant decrease in straw yield (1.10 Mg·ha⁻¹) relative to the NPK + Si combination (31%). Phytolith content in barley grain varied from 20.16 to 22.79 mg g⁻¹ DM (

Table 4. Phytolith content and phytolith-occluded carbon (PhytOC) in barley grain and straw from the field experiment in Skierniewice, and estimated PhytOC yield expressed as CO₂ equivalents.

Treatment	GRAIN—Phytoliths	GRAIN— C Occluded in Phytoliths			GRAIN— PhytOC Yield	STRAW— Phytoliths	STRAW— C Occluded in Phytoliths			STRAW— PhytOC Yield	GRAIN + STRAW Total PhytOC Yield
	mg g−1 DM	mg g−1 *	mg g−1 DM	G **	kg CO2 eq ha−1	mg g−1 DM	mg g−1 *	mg g−1 DM	G **	kg CO2 eq ha−1	kg CO2 eq ha−1
1. Control	22.38 b	8.97 d	0.201 c	0.576 b	2.11 ab	63.72 b	48.83 a	3.11 b	2.74 a	10.04 a	12.15 a
2. NPK	20.16 a	5.96 a	0.120 a	0.476 a	1.74 a	73.01 d	46.97 a	3.43 c	4.49 d	16.47 d	18.22 d
3. NPK + Si	20.45 a	7.42 c	0.152 b	0.635 c	2.33 b	66.45 c	52.04 a	3.46 c	5.50 e	20.16 e	22.49 e
4. Compost170	20.75 a	10.21 e	0.222 d	0.680 c	2.49 b	52.53 a	46.30 a	2.68 a	2.44 a	8.95 a	11.44 a
5. Compost170 + Si	21.73 ab	7.85 c	0.163 bc	0.558 b	2.05 a	57.70 a	51.06 a	2.67 a	2.94 a	10.77 b	12.82 a
6. Compost120 + Nmin	21.51 ab	6.97 b	0.150 b	0.534 b	1.96 a	62.71 b	48.46 a	3.30 b	3.70 b	13.56 c	15.51 b
7. Compost120 + Nmin + Si	22.79 b	6.01 a	0.137 a	0.502 a	1.84 a	64.43 bc	52.65 a	3.12 b	4.00 c	14.66 d	16.49 c
Mean	21.40 ± 0.92 #	7.63 ± 1.43	0.164 ± 0.03	0.566 ± 0.07	2.07 ± 0.24	62.94 ± 6.05	49.47 ± 2.29	3.11 ± 0.30	3.69 ± 1.02	13.52 ± 3.68	15.59 ± 3.63

* mg g⁻¹ phytolith; ** in yield g; ^a–e The significance of differences between treatment means was assessed using a post hoc multiple comparison procedure based on Tukey’s honestly significant difference (HSD) test at the 5% probability level; ^# SD (standard deviation).

).

Phytolith content in straw was more than twice higher, and ranged from 52.53 to 73.01 mg g⁻¹ DM (Table 4). The phytolith content in grains under the NPK, NPK + Si, and Compost170 treatments was lower than in the control (9%). The other fertilization treatments did not have a significant effect on phytolith content compared to the control. In the case of straw, a significant increase in phytolith content compared to the control was observed under the NPK and NPK + Si treatments (7%). This is confirmed by the calculated correlation coefficients between the use of silicon and the content of phytolites in barley. With the increase in silicon content in the plant, there was a significant increase in the content of phytoliths in both the grain (r = 0.49*) and the straw (r = 0.36*) (Figure 4).

In the case of foliar Si application, phytolith content was usually significantly higher in comparison to the same treatment without silicon fertilization. The highest phytolith content in grain was recorded for the Compost120 + Nmin + Si treatment. In straw, it was determined for the NPK treatment.

The smallest PhytOC content in grain was determined in the NPK object (5.96 mg C g⁻¹ phytolith), and the highest (10.21 mg C g⁻¹ phytolith) in the case of excluded mineral fertilization (NPK170). In straw, the lowest value occurred in the case of the Compost170 + Si treatment (46.30 mg phytolith C g⁻¹), and the highest for the Compost120 + Nmin treatment (52.65 mg C g⁻¹ phytolith), as shown in Table 4. This is confirmed by the calculated correlation coefficients between the content of phytolith and the content of PhytOC in barley. In our studies we obtained a positive correlation between phytolith content with PhytOC content both in grain (r = 0.51*) and a stronger correlation in straw (r = 0.55*) (Figure 5).

This PhytOC content was used for the calculation of its equivalent in kg e-CO₂ ha⁻¹. For grain, the amount corresponded to 1.74–2.49 kg e- CO₂ ha⁻¹, and for straw from 8.95 to 20.16 kg e-CO₂ ha⁻¹, respectively. The total carbon equivalent accumulated in permanent bonds (grain + straw) was variable depending on treatments, and ranged from 11.44 to 22.49 kg e-CO₂ ha⁻¹ (Table 4).

4. Discussion

This study demonstrated that silicon (Si) amendment positively affected spring barley, increasing yield (Figure 3) and enhancing phytolith and phytolith-occluded carbon (PhytOC) content in biomass. Similar responses of monocotyledonous crops to Si fertilization have been widely reported. For example, Prabha et al. [29] found that the form of applied silicon did not influence yield, while Stephano et al. [44] showed that Si fertilization increased maize straw and grain yields by 10.6% and 4.8%, respectively.

Our results further confirm that both foliar and soil Si applications stimulate phytolith formation and PhytOC accumulation, thereby increasing the potential of barley for long-term CO₂ sequestration. These findings are consistent with recent studies indicating that Si fertilization enhances plant Si uptake, promotes phytolith production, and expands the phytolith-bound carbon pool in cereals and other species [45,46]. Silicon increases the concentration of plant-available monosilicic acid, which is polymerized and deposited in plant tissues as amorphous silica. During this process, small fractions of organic carbon become occluded and protected from microbial decomposition [13,24,47]. In the present study, positive correlations between tissue Si content, phytolith concentration, and PhytOC were observed, in agreement with previous reports [11,48]. Similar relationships have been documented in wheat and maize, where Si fertilization substantially increased phytolith and PhytOC contents [13,24].

The interaction between Si fertilization and organic amendments appears particularly important. In our study, compost combined with Si increased phytolith content in barley grain relative to NPK-based treatments, although responses varied depending on compost dose. Recent research suggests that combining Si with organic materials such as compost or biochar may enhance phytolith production and influence PhytOC stability by modifying soil Si availability, organic matter dynamics, and aggregate formation [28,49].

Higher phytolith concentrations in straw than in grain observed in this study are consistent with the preferential silica accumulation in vegetative tissues. Similar patterns have been reported in rice, where Si fertilization increased phytolith content mainly in vegetative organs [12,50]. This has practical implications, as straw retention may increase phytolith inputs to soil and promote long-term carbon stabilization [28,45,49,51].

Species and genotype also play a key role in Si uptake and PhytOC accumulation. Previous studies have demonstrated significant variability among grasses and sedges across habitats [52], and barley has been identified as a crop with considerable potential for phytolith and PhytOC accumulation alongside rice, maize, sorghum, sugarcane, and wheat [53]. Consequently, Si management strategies should be crop- and cultivar-specific, considering soil properties, climate, and agronomic objectives. Optimization of Si form, dose, and application timing also remains essential [11,26]. Phytolith formation represents a plant-mediated pathway of long-term CO₂ sequestration. In the present study, PhytOC was strongly correlated with phytolith content, supporting earlier findings [54]. Previous estimates indicate substantial sequestration potential in cereal systems [10,55], with particularly high values reported for sugarcane, maize, and bamboo [24,55,56].

Overall, our results indicate that silicon fertilization, especially when combined with organic amendments and crop residue retention, may enhance cereal productivity while contributing to climate mitigation through stable carbon storage in phytoliths [14,25,42,57]. The estimated CO₂ sequestration in spring barley ranged from 12.15 to 22.49 kg CO₂ eq ha⁻¹, suggesting a significant cumulative effect at a regional scale. In Poland, this could represent a meaningful contribution to climate-smart agriculture.

Building on previous modeling studies showing that arable soils can act as long-term carbon sinks under appropriate management [58], our findings highlight PhytOC as a complementary, plant-driven sequestration mechanism. Management-induced increases in phytolith production may therefore link crop physiology with long-term soil carbon dynamics, particularly in sandy soils, and reinforce the role of nutrient management as a key strategy for climate mitigation in croplands [59].

5. Conclusions

This study demonstrates that cereals cultivated under temperate conditions, such as spring barley, have a substantial capacity to produce phytoliths and sequester carbon in the form of phytolith-occluded carbon (PhytOC). The results indicate that barley strongly responds to diversified fertilization strategies, with silicon (Si) playing a particularly influential role in promoting phytolith formation. Foliar Si applications increased silica uptake and phytolith content in plant tissues, while mineral–organic fertilization further enhanced PhytOC accumulation. Across all treatments, straw consistently served as the primary phytolith reservoir, emphasizing its key role in carbon storage. Notably, carbon retained in phytoliths—expressed as CO₂ equivalents—was nearly three times higher in the NPK + Si treatment compared with the control, highlighting the potential of targeted Si management to strengthen the phytolith carbon sink in croplands. The findings suggest that both soil and foliar Si applications can be integrated into existing fertilization programs without major alterations to production routines. Retaining straw residues on the field, rather than removing or burning them, facilitates the gradual accumulation of phytoliths in the soil, enabling long-term stabilization of occluded carbon. Optimizing Si fertilization, in combination with organic matter management and conservation-oriented cultivation practices, can therefore substantially enhance the carbon sequestration potential of cereal-based agroecosystems. Beyond their scientific implications, the results provide practical insights for farmers and policymakers. Si fertilization represents a promising, readily adoptable strategy for improving crop resilience and productivity while simultaneously contributing to climate mitigation through soil carbon storage. Moreover, understanding the dynamics of PhytOC in cereal straw can inform strategies and recommendations aimed at promoting long-term carbon sequestration in agricultural soils. Overall, these findings highlight the potential of integrating Si management and sustainable residue practices to support climate-smart, sustainable agriculture in Central Europe.