Biochar Is Superior to Organic Substitution for Vegetable Production—A Revised Approach for Net Ecosystem Economic Benefit

Abstract

Biochar amendment and substituting chemical fertilizers with organic manure (organic substitution) have been widely reported to improve intensive vegetable production. However, considering its high potential for reducing carbon and reactive nitrogen (Nr) footprints, very few comprehensive evaluations have been performed on the environmental and economic aspects of biochar amendment or organic substitution. In this study, the comprehensive environmental damage costs from carbon and Nr footprints, measured using the life cycle assessment (LCA) methodology, followed a cradle-to-gate approach, and the carbon storage benefits were incorporated into the newly constructed net ecosystem economic benefit (NEEB) assessment frame in addition to the conventional product income–input cost-benefit methods. One kilogram of harvested vegetables for carbon/Nr footprints and one hectare of cultivated land per crop for cost and benefit were adopted as functional units considering the multi-cropping characteristics for intensive vegetable production. Five fertilization treatments were included: no fertilizer (CK); synthetic fertilizer application (SN); biochar amendment (NB); organic substitution (NM); and a combination of biochar and organic substitution (NMB). These were investigated for five consecutive years of vegetable crop rotations in a typically intensified vegetable production region in China. Adopting the revised NEEB methodology, NB significantly reduced carbon footprint by 73.0% compared to no biochar addition treatment. Meanwhile, NB significantly increased the total benefits by 9.7% and reduced the environmental damages by 52.7% compared to NM, generating the highest NEEB, making it the most effective fertilization strategy among all treatments. It was 4.3% higher compared to NM, which was not significant, but significantly higher than SN and NMB, by 23.0% and 13.6%, respectively. This finding highlights the importance of considering carbon storage benefit for properly assessing NEEB, which is important for developing effective agricultural management strategies and promoting intensive vegetable production with a more sustainable approach.

1. Introduction

The incorporation of sustainable production methods into intensive vegetable farming represents a pivotal advancement in contemporary agriculture. This endeavor seeks to achieve a delicate equilibrium between the escalating need for food and the imperative of environmental preservation [1]. Conventional intensive farming practices, although proficient in bolstering crop yields, frequently engender severe environmental degradation. This degradation encompasses emissions of greenhouse gases (GHGs), nitrogen (N) runoff culminating in eutrophication, the depletion of natural resources, and soil acidification and degradation [2]. In pursuit of agricultural methodologies that bolster food security while curtailing ecological footprints, organic alternatives and the application of biochar emerge as prominent candidates. Their potential to enhance soil quality and alleviate environmental challenges underscores their significance [3,4].

The organic substitution strategy is widely regarded as one of the most effective methods for mitigating negative environmental impacts. Not only does it enhance the environmental quality of ecosystems, but it also bolsters product competitiveness in intensive vegetable production [5]. While organic substitution garners attention, there is growing acknowledgment of the indispensable role of biochar application in mitigating the environmental impact of agriculture [6]. Biochar is a stabilized form of carbon produced by the pyrolysis of biomass, and its high porosity and large surface area improves soil structure, water retention, and nutrient availability, in addition to its ability to enhance crop resistance, and thus crop yields, by boosting the concentration of available cations [7,8]. The inherent resistance of biochar to decomposition renders it a potent vehicle for carbon storage, thereby contributing to diminished atmospheric carbon dioxide levels [9]. Moreover, biochar augments soil N fixation capacity, thereby curbing N loss and GHG emissions [10].

The primary objectives of agricultural production are to achieve the lowest environmental damage costs (EDCs) and the highest net economic benefits (NEBs). NEB indicates crop production avenues minus the purchase cost of agrochemical inputs. EDC refers to the costs associated with the reduced quality of environmental service functions due to Nr losses, including N footprints and GHGs from natural or anthropogenic activities [11]. From a life cycle assessment perspective, in addition to the significant emissions from on-farm applications, the production and transportation of agrochemical inputs, as well as the associated energy consumption, indirectly generate Nr and GHGs [12,13]. Net ecosystem economic benefit (NEEB), defined as the difference between NEB and EDC from crop production, has thus become widely used to assess the sustainability of agricultural practices [14].

However, the comprehensive assessment of the impact of organic substitution and biochar addition on NEEBs on the ecosystem, integrating production costs, crop yields, and environmental damages in intensive vegetable production, remains relatively unexplored. Our previous preliminary 5-year field observations and empirical modeling results indicated that organic substitution and biochar addition increased NEEB by increasing crop yields and reducing reactive N losses compared to synthetic N fertilizer alone [3].

In view of the role of biochar in carbon storage, particularly its capacity to sequester organic carbon over long time, this presents a significant opportunity for climate change mitigation and the enhancement of soil fertility and structure [15]. While existing studies have demonstrated the environmental and economic benefits of biochar in agricultural production, the reassessment of the quantitative NEEB of carbon storage benefits is still deficient. The long-term contribution of carbon sequestration has not been adequately considered in the calculation of NEEB, which may underestimate the role of biochar in agricultural sustainability [16]. In addition, biochar application costs should be considered for the term of its lifespan, even for a one-time application. Recognizing these unique characteristics of biochar, our current research is dedicated to reassessing the carbon footprint and NEEB of intensive vegetable production systems adopting our constructed NEEB frames, by introducing the dual benefits of biochar-improving agricultural productivity and sequestration gains through carbon sequestration on the one hand, and reducing the cost of environmental damage by lowering GHG emissions on the other. Based on this approach, the sustainability of vegetable production can be more thoroughly evaluated and explored. Therefore, we hypothesize that biochar application may be superior to organic substitution in terms of NEEB by incorporating new components for carbon storage benefit for vegetable production.

2. Materials and Methods

2.1. Experimental Site and Treatments

The field experiment was conducted in a long-term experimental vegetable field in Nanjing, Jiangsu Province, China (32°01′ N, 118°52′ E), with an average annual temperature and precipitation of 17.5 °C and 1107 mm, respectively. The test soil pH was 7.9, the bulk density was 1.3 g cm⁻³, the organic carbon was 18.8 g C kg⁻¹, the total nitrogen was 2.3 g N kg⁻¹, the cation exchange capacity (CEC) was 25.3 cmol kg⁻¹, and it contained 3.1% of clay particles, 66.4% of silt particles, and 30.5% of sand particles. Soil physicochemical properties were measured using the methods described in a previous study [3].

Five experimental treatments were established, which were no fertilizer or biochar addition (CK); synthetic fertilizer application (SN); SN plus 20 t ha⁻¹ biochar amendment (NB); substituting 50% of chemical N fertilizer with organic manure (NM); and NM plus 20 t ha⁻¹ biochar amendment (NMB). The N application rate was 240 kg N ha⁻¹ crop⁻¹ as urea; the phosphate (P) application rate was 120 kg P ha⁻¹ crop⁻¹ as superphosphate; the potassium (K) application rate was 240 kg K ha⁻¹ crop⁻¹ as potassium chloride. The organic manure was applied at 120 kg N ha⁻¹ crop⁻¹ containing 1.1% TN, 0.13% P, 0.83% K and 28.3% organic matter. The organic manure purchased from Nanjing Mingzhu Fertilizer Co., Ltd. (Nanjing, China). Biochar purchased from Henan Sanli New Energy Co. (Shangqiu, China), which was produced by pyrolyzing wheat straw at 500 °C under a continuous carbonization furnace for 4 h. Usually, biochar produced at low temperatures (e.g., 400 °C) is more appropriate for agricultural soil improvement, while at high temperatures (e.g., 600 °C), it is more suitable for the remediation of organic and inorganic polluted environments [17]. Therefore, choosing 500 °C as the pyrolysis temperature can take into account the product characteristics and application needs. The usually recommended pyrolysis time for biochar is 2–6 h. Choosing 4 h as the pyrolysis time can ensure that the biomass is fully decomposed, and, at the same time, avoid the degradation of product quality that may be caused by pyrolysis for too long. The properties were as follows: carbon content 467 g C kg⁻¹; nitrogen content 5.9 g N kg⁻¹; pH 9.4, CEC 24.1 cmol kg⁻¹; and ash content 20.8%. Crop rotations, species, tillage, and pest controls followed local practices.

2.2. System Boundaries

In this study, we estimated the reactive N (Nr) and carbon footprints of intensive vegetable production under different fertilizer treatments using the life cycle assessment (LCA) method. The system boundary (Figure 1) extends from the production of intensive vegetables to the required energy for producing input agrochemicals, and the harvesting, and can therefore be divided into the foreground and field interfaces. The Nr footprint includes nitrous oxide (N₂O), NO_X emissions, N runoff and leaching, and ammonia (NH₃₎ volatilization losses released at the field interface after planting, as well as those released at the foreground interfaces. The carbon footprint consists of GHG emissions at the foreground interface and soil N₂O emissions and organic carbon fixation at the field interface. In this study, we did not consider the emissions of carbon dioxide (CO₂) and methane (CH₄) from straw handling, transportation, and production during the biochar production process due to its negligible amounts. In addition, the CO₂ absorbed by biomass during growth is theoretically balanced with that released when it is burned or decomposed, and therefore can be considered carbon neutral according to the IPCC definition [18,19].

Figure 1. System boundaries for the life cycle assessment of the C footprint, Nr footprint and the net ecosystem economic benefit (NEEB) of intensive vegetable production.

2.3. Soil Organic Carbon Fixation Rate and NECB

The accumulation and transformation of organic carbon in soil is a long-term and slow process, which can be determined by measuring changes in soil organic carbon (SOC) for long-term experiments. However, for short-term experiments, it cannot be measured directly, and so the net ecosystem carbon balance (NECB) or model predictions are often used to estimate it indirectly [4]. Based on the short experimental period in this study, this method of estimating the NECB was used to determine the δSOC. If the value of NECB is positive, it indicates net organic carbon accumulation in the ecosystem, and, vice versa, a negative value indicates net organic carbon loss in the ecosystem [20]. The relationship between δSOC and NECB was converted from the coefficient of 0.21 as reported in previous studies [21] and 1.00 under biochar amendment, since biochar decomposition and associated CO₂ emissions were ignored due to its relative stability [22,23]. δSOC and NECB were calculated as follows:

NPP = NPP_harvest + NPP_residue + NPP_exudate

(1)

GPP = NPP/0.52

(2)

NECB = GPP − Re − H − CH₄ + Manure-C

(3)

δSOC = 0.21 × NECB + 1.00 × Biochar-C

(4)

where GPP (gross primary production) denotes gross primary productivity, which is estimated from net primary productivity (NPP) [24]. In this study, all the crops were leafy vegetables, so NPP_harvest refers to the sum of the carbon content of the harvested part of the vegetable crop, NPP_residue refers to the sum of the carbon content of the above-ground part and the root system left in the field after harvesting, and as the vegetables were harvested once, including the root system, which was all removed, the value was 0. NPP_exudate refers to the sum of the carbon content of the root secretion, which was estimated to account for 7% of the total biological carbon content [25]. Ecosystem respiration (Re) refers to the cumulative CO₂ emissions from the ecosystem as measured by the static dark box method, while H refers to the amount of carbon taken away by the harvested vegetables, which was determined to be 393 and 354 mg kg⁻¹ dry weight for baby bok choy and spinach, respectively. The specific results are shown in Table 1.

Table 1. Components of NECB and δSOC (t C ha⁻¹ crop⁻¹) under different strategies in intensive vegetable production.

Treatment	NECB	δSOC	Re	Harvest	CH4	Carbon	GPP	NPP	Yield
CK	0.15	0.032	0.19	0.33	−0.00044	0	0.67	0.35	16.35
SN	0.28	0.058	0.28	0.53	−0.00030	0	1.09	0.57	24.17
NB	9.73	9.73	0.33	0.67	−0.00038	9.34	1.39	0.73	29.78
NM	1.35	0.28	0.25	0.58	0.00016	0.98	1.20	0.62	27.89
NMB	10.59	10.59	0.27	0.51	0.0015	10.32	1.05	0.54	27.50

Notes: NECB: net ecosystem carbon balance; δSOC: the SOC sequestration rate; Re: Ecosystem respiration; GPP: Gross primary production; NPP: net primary production. CK: no fertilizer or biochar addition; SN: synthetic fertilizer application; NB: SN plus 20 t ha⁻¹ biochar amendment; NM: substituting 50% of chemical N fertilizer with organic manure; NMB: NM plus 20 t ha⁻¹ biochar amendment.

The carbon footprint of vegetable production (kg CO²-eq kg⁻¹ vegetable) for each treatment was calculated using the following equation [26]:

$$\mathrm{CF}=({\mathit{FI}}_{\mathrm{iCO}2} +{\mathrm{N}}_2\mathrm{O} \times 273 \times {\displaystyle \frac{44}{28}}-\delta \mathrm{SOC} \times {\displaystyle \frac{44}{12}})/\mathrm{Y}$$

(5)

where CF represents the carbon footprint; FI_iCO2 denotes the GHGs emitted at the foreground interface, which is obtained by multiplying their respective dosages by their carbon emission factors (Table 2); N₂O (kg N ha⁻¹) and soil organic carbon storage (δSOC) (kg C ha ⁻¹ crop⁻¹) represent the N₂O emission and soil organic carbon storage of each treatment at the field interface, respectively. Y denotes the total vegetable yield (kg ha⁻¹) of each treatment.

Table 2. Carbon emissions (kg CO₂-eq ha⁻¹ crop⁻¹) and coefficients (kg CO₂-eq unit⁻¹ input) under different strategies in intensive vegetable production.

Components	Items	Unit	Coefficients	CK	SN	NB	NM	NMB
Chemical input	Urea	kg N	8.30	0	1992	1992	996	996
	Organic fertilizer	kg N	11.30	0	0	0	1356	1356
	Biochar	Kg C	0	0	0	0	0	0
	Phosphorus	kg P2O5	1.50	360	360	360	331	331
	Potassium	kg K2O	0.98	235	235	235	169	169
	Plastic Film	kg	19.00	699	699	699	699	699
	Pesticide	kg	18.00	119	119	119	119	119
Farm operation	Fuel	kg	3.94	376	376	376	376	376
	Electricity	kW·h	0.95	269	269	269	269	269
Soil GHG emission	N2O	kg N	273.00 a	101	1279	1132	929	1037
	δSOC	kg C	-	118	213	35,680	1038	38,829

^a Coefficients for GHG emission were adopted from report of IPCC 2021 [18]. Notes: N₂O: nitrous oxide; δSOC: the SOC sequestration rate. CK: no fertilizer or biochar addition; SN: synthetic fertilizer application); NB: SN plus 20 t ha⁻¹ biochar amendment; NM: substituting 50% of chemical N fertilizer with organic manure; NMB: NM plus 20 t ha⁻¹ biochar amendment.

2.4. EDC and NEEB

The carbon and Nr footprints were assessed along with the EDC, NEB and NEEB of each treatment. The EDC caused from Nr and GHGs was calculated as follows [26]:

EDC = Nr_iA × P_i + CO₂A × P_CO2

(6)

where Nr_i A (kg N ha⁻¹) denotes the total emission of Nr “i”; P_i (¥ kg⁻¹ N) denotes the EDC per unit mass of Nr “i”; CO₂ A (t CO₂-eq ha⁻¹) represents the total emission of GHGs; and P_CO2 (¥ t⁻¹ CO₂-eq) represents the greenhouse effect caused by the CO₂ emission (expressed by the international carbon trading price). The specific EDC are shown in Table 3, where the updated EDC per unit of GHG emissions is 103.7 ¥ t⁻¹ CO₂-eq.

$$\mathrm{NEB}={\sum }_{a=1}^b(\mathrm{Y}a\times \mathrm{P}a)-{\sum }_{m=1}^n(\mathrm{R}m\times \mathrm{P}m)$$

(7)

NEEB = NEB + Carbon benefit − EDC

(8)

Table 3. Components of NEB (×10³ CNY ha⁻¹ crop⁻¹) and EDC (×10³ CNY ha⁻¹ crop⁻¹) under different strategies in intensive vegetable production.

Components	Items	Unit	(¥ Unit−1)	CK	SN	NB	NM	NMB
Cost a	Urea	kg N	3.6	0.00	0.86	0.86	0.43	0.43
	Manure	kg N	1.9	0.00	0.00	0.00	0.23	0.23
	Phosphorus	kg P2O5	2.9	0.70	0.70	0.70	0.64	0.64
	Potassium	kg K2O	3.8	0.91	0.91	0.91	0.66	0.66
	Biochar	t	2600	0.00	0.00	5.20	0.00	5.20
	Plastic Film	kg	13	1.20	1.20	1.20	1.20	1.20
	Pesticide	kg	25	0.17	0.17	0.17	0.17	0.17
Income a	spinach gains	kg	2.9	18.01	19.31	21.38	24.41	21.91
	baby bok choy gains	kg	2.0	7.28	10.30	12.90	13.21	14.39
	baby bok choy gains	kg	2.0	7.14	11.37	17.61	15.98	13.87
	baby bok choy gains	kg	2.0	5.96	13.01	16.43	12.08	13.04
	spinach gains	kg	2.9	3.44	7.01	7.05	6.26	4.79
EDC b	N2O emission	kg N	2.1	0.0004	0.0062	0.0056	0.0046	0.0050
	NO emission	kg N	29.6	0.0014	0.013	0.0098	0.0080	0.0084
	NH3 volatilization	kg N	37.5	0.00	0.29	0.26	0.23	0.24
	N leaching	kg N	9.3	0.00	0.18	0.12	0.13	0.12
	N runoff	kg N	9.3	0.00	0.090	0.056	0.064	0.060
	Foreground Nr loss	kg N	36.7	0.10	0.24	0.39	1.92	2.07
	GHG emission	kg CO2-eq	103.7	0.22	0.55	0.54	0.54	0.56
Carbon benefit	SOC	kg CO2-eq	103.7	0.012	0.022	3.70	0.11	4.03

^a Coefficients for cost and income were adopted according to the current local market price. ^b Coefficients for damage cost were adopted from report of Xia and Yan., Xia et al., Zhou et al. [4,26,27]. Notes: NO: nitric oxide; NH₃: ammonia. CK: no fertilizer or biochar addition; SN: synthetic fertilizer application; NB: SN plus 20 t ha⁻¹ biochar amendment; NM: substituting 50% of chemical N fertilizer with organic manure; NMB: NM plus 20 t ha⁻¹ biochar amendment.

The NEB is the difference between the vegetable yield benefit and the cost of agrochemical inputs, where Ya (kg ha⁻¹) and Pa (¥ kg⁻¹) represent the yield of vegetable and the market price of the vegetable, respectively; Rm (kg ha⁻¹) and Pm (kg⁻¹) represent the use and the actual purchase prices of agrochemical inputs and energy, respectively. The detailed price is shown in Table 3. The redefined NEEB is the difference between the NEB, carbon benefit and EDC as mentioned above and in Figure 1. Carbon benefit refers to carbon storage, as expressed by the international carbon trading price similar to CO₂ emission, and the updated price is 103.7 ¥ t⁻¹ CO₂-eq.

2.5. Statistical Analyses

The data were calculated and graphed using Microsoft Excel software (2013) and Origin software (Version 9.0, Northampton, MA, USA), and the test of the significance of the differences between treatments was analyzed using SPSS 20.0 software (Chicago, IL, USA) (p < 0.05), and multiple comparisons were made using the Duncan method.

3. Results

3.1. Carbon Footprints of Biochar Amendment and Organic Substitutions

In the present study, the range of NECB for each treatment was 0.77–52.95 t C ha⁻¹ (Table 1), which showed a net accumulation. The range of δSOC for each treatment during the experimental period was 0.16–52.95 t C ha⁻¹ (Table 1). The δSOC was significantly higher in the biochar addition treatments (NB and NMB), by 33.37–166.09% and 36.41–180.83%, respectively, compared to the NM and SN, and the difference between the biochar addition treatments was not significant.

Carbon footprint, which is the ratio of net GHG emissions to vegetable yield, ranged from 1.32–2.21 kg CO₂-eq kg⁻¹ vegetable for all treatments except organic carbon fixation during the experimental period. The SN treatment had the highest carbon footprint and the CK treatment had the lowest (Figure 2), as the production of N fertilizers was the primary carbon emission factor, accounting for 37.38–44.85% of the carbon footprint range, excluding organic carbon fixation. Biochar addition and organic substitution alone reduced carbon footprint by 21.09%, 10.61%,14.74%, and 3.41% compared to SN and NMB treatments, respectively, but there was no significant difference between the two treatments. Notably, the total carbon footprint considering the soil carbon pool ranges from 0.53–2.20 kg CO₂-eq kg⁻¹ per vegetable. Biochar addition significantly increased the soil carbon pool, which ultimately reduced the total carbon footprint of NB and NMB treatments by 75.33%, 70.60% and 75.67%, 71.00% (p < 0.05) compared to SN and NM treatments, respectively.

Figure 2. Carbon footprint of different fertilization treatments in intensive vegetable production. N fertilizer refers to the manufacture and transport of inorganic and organic fertilizers. Farm operations refer to fuel and electrical energy consumption. Other refers to the manufacture and transport of potassium, phosphorus, pesticides, and plastic film. The lowercase letters represent significant differences according to Tukey’s multiple range test (p < 0.05). CK: no fertilizer or biochar addition; SN: synthetic fertilizer application; NB: SN plus 20 t ha⁻¹ biochar amendment; NM: substituting 50% of chemical N fertilizer with organic manure; NMB: NM plus 20 t ha⁻¹ biochar amendment.

3.2. EDC of Biochar Amendment and Organic Substitutions

Based on actual measurements and empirical model estimates, we present Nr losses at the foreground and field interfaces separately (Table 4), where the foreground includes the production of fertilizers (synthetic fertilizers, manure, and biochar), pesticide and plastic film production, agrochemical transport, and energy consumption. To avoid repetition of the results, the field interface is the total Nr losses (N₂O, NO, NH₃, N leaching, N runoff), using specific data from a previous study [3], which showed that organic substitution significantly reduced N₂O and NO emissions by 18.9–27.4% and 35.9–39.5%, respectively, compared with SN. NB increased NH₃ volatilization by 13.3% and 6.3%, but decreased N leaching and N runoff by 5.9–11.2% compared to organic substitution. Based on the above results and with reference to the respective EDC factors reported in the literature, the list of EDC for each treatment is presented in Table 3. The EDC ranged from 0.33–3.06 thousand CNY in all treatments. In the treatments where organic fertilizers was applied, foreground interface Nr loss was the main contributor to the EDC, with an average percentage of 66.87%, in addition to which GHG emissions were the main contributor, with percentages ranging from 39.23–67.92% (Figure 3). Thus, organic substitution significantly worsened the EDC. However, the NB showed no difference in EDC with the SN treatments, though greater than CK. Among the losses of each Nr at the field interface, although N leaching was the hotpot emission link of Nr, ammonia volatilization exceeded the environmental damage caused by N leaching due to the highest cost of environmental damage, which accounted for more than the environmental damage caused by ammonia volatilization in the EDC of all treatments.

Table 4. Nr losses (kg N ha⁻¹ crop⁻¹) at the foreground and field interfaces under different strategies in intensive vegetable production.

Treatment	Synthetic Fertilizers	Manure	Biochar	Energy	Others	Field Nr Losses
CK	0	0	0	0.8	2.04	0.28
SN	3.8	0	0	0.8	2.04	40.4
NB	3.8	0	4	0.8	2.04	28.4
NM	1.9	47.8	0	0.8	1.81	29.4
NMB	1.9	47.8	4	0.8	1.81	29.0

Notes: Energy refers to fuel consumption, and Others refer to the manufacture and transport of potassium, phosphorus, insecticides and plastic film. CK: no fertilizer or biochar addition; SN: synthetic fertilizer application); NB: SN plus 20 t ha⁻¹ biochar amendment; NM: substituting 50% of chemical N fertilizer with organic manure; NMB: NM plus 20 t ha⁻¹ biochar amendment.

Figure 3. Contributions of different sources to environmental damage costs (EDC) under different treatments in intensive vegetable production. The lowercase letters represent significant differences according to Tukey’s multiple range test (p < 0.05). CK: no fertilizer or biochar addition; SN: synthetic fertilizer application); NB: SN plus 20 t ha⁻¹ biochar amendment; NM: substituting 50% of chemical N fertilizer with organic manure; NMB: NM plus 20 t ha⁻¹ biochar amendment.

3.3. NEEB of Biochar Amendment and Organic Substitutions

Adopting the newly revised NEEB frames, the NB treatment had the highest NEEB with 68.65 thousand CNY per crop season as the most effective management strategy (Figure 4), which was significantly higher than the SN and NMB treatments, by 23.02% and 13.57%, respectively. It is worth noting that the carbon benefits of the treatments differed significantly, with the NMB treatment being 8.92% higher than the NB treatment, and at the same time about 200 times higher than the SN treatment, which is the primary reason explaining why the NEEB of the NMB was higher than that of the SN. However, the difference between NB and organic substitution alone was not significant, due to the high cost of biochar. On the one hand, the strong stability of biochar can be sequestered in agro-ecosystems for a long period of time, thus reducing the cost of biochar agricultural inputs in this experimental cycle; on the other hand, biochar has a strong carbon sequestration potential. This result emphasizes the importance of considering soil carbon sequestration potential when assessing agro-environmental impacts and demonstrates the advantages of biochar in improving eco-economic benefits.

Figure 4. Costs, gains and NEEB under different treatments in intensive vegetable production. Different lowercase letters represent significant differences in total costs and total gains, and different uppercase letters represent NEEB (p < 0.05). CK: no fertilizer or biochar addition; SN: synthetic fertilizer application); NB: SN plus 20 t ha⁻¹ biochar amendment; NM: substituting 50% of chemical N fertilizer with organic manure; NMB: NM plus 20 t ha⁻¹ biochar amendment.

4. Discussion

Our newly revised framework incorporates the storage of organic carbon and spans the lifetime of biochar incorporation in the field. Biochar, as a multifunctional soil amendment, not only improves soil quality and crop yields, but also possesses a unique carbon storage function, which give it a huge potential, making it essential for combating climate change and achieving sustainable agriculture [28,29]. Considering the multiple benefits of biochar, we conclude that biochar is superior to organic substitution in terms of improved benefits and reduced EDC, emerging as the most beneficial practice for NEEB in intensive vegetable production. This finding is consistent with the findings of Lehmann and Joseph [30], who emphasized the potential of biochar for improving the economic viability and environmental desirability in agricultural systems. However, this is contrary to our previous preliminary results that we have found that NEEB was significantly better with organic substitution than with biochar addition and that the NEEB was lower with combined application compared to N fertilizer alone. The primary reason for this contradiction is that the study’s inclusion of carbon benefits in the NEEB assessment framework resulted in significantly higher carbon benefits for the NMB treatment compared to the SN treatment. Thus, NEEB was superior to the SN treatment, which suggests that biochar’s ability to sequester carbon makes it uniquely suited for agricultural production. When added to the soil, it is able to conserve carbon stably over a long period of time, thus creating a “carbon sink” in the farmland [31]. Unlike conventional organic substitutes, biochar can be stabilized in the soil for decades or even centuries, meaning that it can effectively convert atmospheric CO₂ into stable carbon in the soil. This long-term carbon sequestration not only helps mitigate climate change, but also creates additional financial gains for farmers in the carbon trading market [16]. In contrast, while organic matter can increase the SOC content of the soil in the short term, it decomposes more rapidly, and the carbon it fixes is quickly re-released into the atmosphere. Therefore, from the perspective of long-term carbon sequestration benefits, organic alternatives are less beneficial than biochar.

In addition, the multiple benefits of biochar include its positive impact on soil microbial communities. Studies have demonstrated that biochar can provide a favorable habitat for beneficial microorganisms in the soil, promoting their activity and diversity [32]. These microorganisms play a crucial role in breaking down organic matter, recycling nutrients, and enhancing crop disease resistance, thus indirectly increasing agricultural productivity and soil carbon sequestration rates [33]. In contrast, while organic alternatives can also improve the soil microbial environment, their effectiveness is limited by the rapid and unstable decomposition of organic matter compared to biochar. Therefore, we believe that biochar is most beneficial to NEEB in intensive vegetable production because of its long-term stability and multifunctionality, making it a more promising strategy for sustainable agricultural management.

For the ultimate NEEB, biochar was higher than the organic substitution, but the difference was not significant. This lack of significance can primarily be attributed to the relatively elevated input costs associated with biochar application, which poses a significant barrier to its widespread adoption [3]. Therefore, we believe that, through exploring alternative feedstock sources to develop cost-effective production methods, and to achieve an excess of supply over demand for biochar production to reduce purchasing costs, biochar remains the most economically viable strategy. In addition, the combination of organic substitution and biochar exhibited the expected synergistic benefits, which may involve their effects on soil microbial activity and possible changes in the rate of nutrient release from organic fertilizers by biochar [34,35]. Therefore, further research should explore how to optimize the combined use of these two substances to maximize their contribution to agricultural sustainability and climate change mitigation. In particular, biochar additions significantly enhanced NEEB, suggesting that advantages in soil carbon storage capacity can significantly influence the net economic benefits across the ecosystem. This finding supports the potential for biochar to be used as a sustainable fertilizer treatment option in agricultural practices, particularly in the quest to reduce N footprints and enhance soil health, while providing a more solid scientific basis for its promotion in agricultural production [36].

While biochar offers significant advantages in terms of net emission reductions, it is important to recognize the variability and environmental dependence of these results. Their effectiveness can vary depending on soil type, climatic conditions, crop type, and biochar characteristics [37]. This variability is also reflected in the findings of Jeffery et al. [38], who note that the impact of biochar on crop yields and GHG emissions is highly context dependent. Thus, while our study makes a compelling case for a wider adoption of these sustainable practices, it also highlights the need for site-specific assessments to fully realize their potential benefits. For its practical application, site-specific biochar application strategies need to be developed, based on specific agroecosystem characteristics, to maximize its net ecosystem economic benefits. In addition, new research on biochar application should be adopted for considering the emissions resulting from the removal of straw, drying, and transportation to the pyrolysis plant.

In summary, our study emphasizes the need to integrate sustainable practices into agriculture. By recognizing the multifaceted benefits of biochar, including its role in carbon storage, policymakers and farmers can make informed decisions to optimize the ecological and economic outcomes of intensive vegetable production [39]. Future research should continue to explore the dynamic interactions between biochar, soil health, and crop productivity, further elucidating pathways that might contribute to more resilient and sustainable agricultural systems.

5. Conclusions

This study systematically assessed the superiority of biochar in vegetable production by revising the NEEB framework incorporating new component for its carbon storage benefits. The addition of biochar significantly reduced ecosystem carbon footprints, by promoting carbon storage compared to organic substitution. Meanwhile, it offset part of the input costs by increasing carbon storage benefits, resulting in the highest NEEB, which was significantly higher than that of the N fertilizer alone, but this difference was not significant when compared to the organic substitution. In conclusion, NB is a win-win approach that not only brings considerable economic benefits to farmers, but also promotes sustainable soil development. Given the high purchase cost of biochar, future research and practice should focus more on promoting the production of biochar in abundant supply to reduce the purchase cost.