Effects of a Single Biochar Application on Soil Carbon Storage and Water and Fertilizer Productivity of Drip-Irrigated, Film-Mulched Maize Production

Abstract

Biochar is a widely recognized soil amendment used to improve soil physicochemical properties and crop productivity. However, its compositive effects on soil water, fertilizer, and carbon in cropping systems are not extensively understood. A two-year field study was conducted to investigate the influence of integrating maize-residue-derived biochar on evapotranspiration, water and fertilizer utilization efficiency, soil organic carbon (SOC) content, and soil carbon emissions in maize farming, employing drip irrigation in conjunction with film mulching. The treatments included the following three biochar amendments: 15 (B15); 30 (B30); and 45 (B45) t ha⁻¹. Biochar was exclusively incorporated prior to sowing during the initial year of the experiment, and no biochar was used as a control (CK). After two years, the biochar amendments, especially B30, improved crop water productivity and the partial factor productivities of nitrogen, phosphorus, and potassium fertilizer. Relative to CK, the biochar amendments significantly reduced soil CO₂ and CH₄ emissions during crop growth by 17.6–40.6% and −1.21–268.4%, respectively, averaged across two years. The best treatment effect was achieved by adding 15 t ha⁻¹ of biochar. The biochar needed replenishing in the third year for B30 and in the fourth year for B45 to increase the SOC content by 20 g kg⁻¹. An application rate of 22 metric tons per hectare of biochar is proposed to optimize water and fertilizer utilization efficiency, alongside augmenting the soil organic matter content, within maize farming under drip irrigation and mulching practices on sandy loam soil. However, the biochar must be added at 20 t ha⁻¹ in the third year to counterbalance soil carbon losses and improve SOC.

1. Introduction

Recently, the frequency of extreme weather events worldwide has increased, owing to increasing atmospheric greenhouse gas concentrations. Globally, the carbon pool within the soil is widely acknowledged as the most substantial reservoir of carbon in the ecosystem [1,2]; in the arid and semi-arid regions of northern China, farmland ecosystems constitute a primary carbon source [3]. These arid districts, characterized by water shortages and organic matter limitations, grapple with substantial agricultural water consumption and large soluble fertilizer losses due to leaching below the root zone [4]. In addition to these phenomena, unsustainable farming practices have contributed to substantial soil carbon losses [5].

Despite being a major cereal crop producer in northwest China, the Hetao Irrigation District (HID) contends with escalating adversities linked to water scarcity. Furthermore, the region faces excessive synthetic nitrogen fertilizer inputs and high soil carbon losses [6]. Therefore, appropriate agronomic management measures are needed to reduce carbon losses and improve crop water and fertilizer productivity. Maize (Zea mays L.) is the main cash crop grown worldwide, with China being the third largest maize-producing and consuming country, producing, and consuming approximately 2.6 × 10¹¹ kg and 2.3 × 10¹¹ kg, respectively, in 2017. Maize plantings in the HID accounted for one-third of its irrigated area (approx. 7.3 × 10⁵ ha) in 2017, producing approximately 3.0 × 10⁹ kg maize yield [7] and large amounts of maize residues. The direct burning of these residues causes environmental pollution, but biochar offers a solution to this problem.

Biochar is an effective soil amendment and can enhance arid farmland fertility [8]. Notably abundant in fused aromatic carbons, biochar improves soil pore structure and chemical stability [9], reducing soil water losses and increasing fertilizer use efficiency, ultimately reducing irrigation water and fertilizer amounts and increasing yield [10]. Biochar can enhance soil pore structure, aggregate stability, and organic matter content, reducing soil bulk density and improving soil water retention [11]. According to one report, applying biochar does not significantly improve the hydrological characteristics of sandy soil [12]. Biochar undergoes a proper pyrolysis process and the large negative charge on biochar surfaces improves NH⁴⁺-N, NO³⁻-N, and ${\mathrm{P}\mathrm{O}}_4^{3-}$-P adsorption, reducing nitrogen leaching and increasing nitrogen utilization [13]. Furthermore, biochar can increase soil nutrient concentrations [14], promoting crop growth [13,15] and potentially reduce greenhouse gas emissions [16]. Suitable biochar application rates can increase crop yields and reduce greenhouse gas emissions [7].

Despite extensive investigations into the impact of biochar on mitigating greenhouse gas emissions [15,17] and enhancing soil physicochemical [18,19] characteristics in the HID, its role in soil carbon storage and water–fertilizer productivity under plastic-mulched and drip-irrigated maize production remains elusive. Thus, this study aimed to: (1) ascertain the effects of biochar application rates on water–fertilizer use efficiency; (2) determine SOC and soil greenhouse gas emissions (GHG) changes in response to biochar addition; and (3) propose reasonable biochar application and management measures for local mulched and drip-irrigation maize systems.

2. Materials and Methods

2.1. Study Site

Field trials were carried out at the Jiuzhuang Experimental Cooperative Center (Figure 1a) in Linhe City, Hetao Irrigation District, Inner Mongolia, China (107°18′ E, 40°41′ N; 1042 m) during the maize cultivation periods of 2015 and 2016. The experimental site chosen for this study is long-standing farmland cultivated by local farmers. Before the experiment commenced, the experimental site encompassed a rotational cropping system involving corn and sunflower crops. The area is characterized by a mid-temperate semi-arid continental climate with low cover and precipitation. The mean yearly rainfall and atmospheric temperature are roughly 140 mm and 6.8 °C, respectively, accompanied by an annual sunshine duration of 3230 h and a frost-free span lasting approximately 130 days. The terrain is high in the east and low in the west, with a ground slope of 1/6000. According to the U.S. textural classification triangle (IUSS Working Group WRB, 2006, https://wrb.isric.org/, accessed on 4 September 2024), sandy loam is the main soil texture, between 0 and 60 cm of depth.

The biochar was prepared from maize residue via slow pyrolysis in a steel carbonization furnace at 400–500 °C (biochar production accounts for about 50% of maize stover). The biochar had an organic carbon content of 925.7 mg kg⁻¹, a carbon-to-nitrogen ratio of ~67, and available nitrogen, phosphorus, and potassium contents of 159.2, 394.2, and 784.0 mg kg⁻¹, respectively (the determination method is shown in

Table 1. Main physical and chemical properties of sandy loam soil and maize residue biochar used in this study.

	Unit	Sandy Loam	Biochar
Bulk density	(g cm−3)	1.39	0.59
pH	/	8.5	9.1
EC	(μS cm−1)	318.5	NA
Organic carbon	(g kg−1)	14.47	925.74
Alkali-hydrolysable N	(mg kg−1)	14.47	159.15
Available Phosphorus	(mg kg−1)	5.3	394.18
Available Potassium	(mg kg−1)	184	783.98
C mass fraction	(g kg−1)	NA	47.17
N mass fraction	(g kg−1)	NA	0.71
H mass fraction	(g kg−1)	NA	3.83
C/N	%	NA	67.03

Note: NA, not available. Bulk density was determined using the drying method; pH was determined using pH indicator; EC was determined using conductivity meter; Organic carbon was determined using potassium dichromate oxidation spectrophotometric method; Alkali-hydrolysable N, available phosphorus, and available potassium were determined using atomic absorption spectrophotometer; and C, N and H mass fraction were determined using elemental analysis instrument.

). Table 1 provides information on the physical and chemical characteristics of the soil and biochar utilized in this research.

2.2. Experimental Design

The field experiment was conducted using a randomized complete block design, incorporating three replicates to mitigate the impact of spatial heterogeneity, i.e., twelve plots were laid out using a randomized complete block design. The size of every plot measured 90 m² (15 m × 6 m). The treatments included three biochar application rates [15 t ha⁻¹ (B15), 30 t ha⁻¹ (B30), and 45 t ha⁻¹ (B45)] and a control (CK) with no biochar. In April 2015, biochar was evenly spread over uncultivated soil and incorporated to a 30 cm depth using a rotary tiller. No further biochar was applied for the experiment’s duration.

Maize cultivar Simon No.6, widely grown in the HID, was sown in wide–narrow rows with spacings of 50 and 60 cm. The wide rows were mulched with 90 cm-wide polypropylene film. The seeds were planted near the edge of the spacious rows, with a distance of 30 cm between each plant and a planting density of 56,667 plants per ha⁻¹. Sowing and harvesting occurred on 22 May and 26 September 2015, and 23 May and 1 October 2016, respectively.

Drip irrigation was initiated by a soil moisture sensor placed at a depth of 25 cm, with a matric potential of −25 kPa and an irrigation amount of 22.5 mm. Irrigation was triggered 17 times in 2015 (382.5 mm total) and 13 times in 2016 (292.5 mm total). The detailed irrigation schedule for each treatment is depicted in Figure 1. To apply plastic film mulching for drip irrigation, a 16 mm diameter pipe was used to embed the drip irrigation tape, which had a flow rate of 1.38 L h⁻¹. The space between two drip emitters was 30 cm. Drip irrigation tape was laid in the center of the mulch, with each drip zone irrigating two rows of maize.

Irrigation, fertilization, and field management practices (e.g., weed control, spraying.) were the same in different treatments. We applied 339 kg ha⁻¹ of N, 192 kg ha⁻¹ of P, and 17 kg ha⁻¹ of K as fertilizers (urea, diammonium phosphate and compound fertilizer) in both study years, with 164 kg ha⁻¹ of N (48%); all P and K fertilizers were mechanically broadcast onto the soil surface at sowing (2015 and 2016). The remaining N was applied in subsequent growth periods via drip irrigation at 35 kg ha⁻¹ of N on each occasion (3 × jointing stage, 1 × tasseling stage, and 1 × grain-filling stage).

Table 2. Synthetic chemical fertilization during the maize growing periods.

	Phosphorus (kg ha−1)	Potassium (kg ha−1)	Nitrogen (kg ha−1)
	Prior to Seeding	Prior to Seeding	Prior to Seeding	Late-June	Mid-July	Late-July	Mid-August	Late-August	Total
CK	192	19	164	35	35	35	35	35	339
B15	192	19	164	35	35	35	35	35	339
B30	192	19	164	35	35	35	35	35	339
B45	192	19	164	35	35	35	35	35	339

describes the topdressing scheme.

2.3. Measurements

2.3.1. Soil Water Storage and Actual Evapotranspiration (ET_a)

The moisture content of the soil was determined by employing a thermo-gravimetric technique. Soil core samples were collected at intervals of 10 cm, ranging from 0 to 40 cm, between two plants in the fourth row using a portable power sampler every 15–20 days. Each sampling procedure was replicated three times.

Soil water storage was determined as follows [20]:

$$H=\sum _{i=1}^n\left(∆{\theta }_i·C_i·Z_i\right)$$

(1)

where H is soil water storage (mm), ∆θ_i is the soil water content for each layer i (kg kg⁻¹), C_i is the soil bulk density in soil layer i (g cm⁻³), and Z_i is the thickness for soil layer i (mm).

Relevant calculations were performed to estimate the actual evapotranspiration (ET_a) [21] of maize based on the soil water balance, as follows:

$${ET}_a=Prec+I+K+R-D+∆H$$

(2)

where Prec is rainfall during the entire crop growth period (mm), I is the irrigation quota for the crop growth period (mm), K is groundwater recharge (mm), R is runoff (mm) considered zero given that the land is flat, D is deep percolation (mm), also considered zero as data collected by a lysimeter (installed 80 cm beneath a furrow) detected no deep percolation during the growing season, and ∆H is the change in soil water storage in the 0–80 cm soil profile (mm).

2.3.2. Maize Yield and Water and Fertilizer Productivity

Four 3 m² plots were selected for each treatment to calculate the yield on 23 September 2015, and 22 September 2016, in order to determine the grain yield. The sampled plants underwent a drying process at a temperature of 105 °C for one hour to eliminate moisture content and then at a constant weight of 75 °C [22] as follows:

$$CWP=Y/ETa$$

(3)

where CWP is crop water productivity (kg m⁻³), Y is maize grain yield (kg ha⁻¹), and ET_a is water consumption during the entire maize growing period (mm).

Partial fertilizer productivity was calculated as follows [23]:

$$PFP=Y/F$$

(4)

where PFP is partial fertilizer productivity, Y is yield (kg ha⁻¹), and F is fertilizer (N, P, and K) input (kg ha⁻¹).

2.3.3. Gas Flux Calculation

The field measurements of CO₂ and CH₄ emissions were conducted from 2015 to 2016, employing the static chamber method for simultaneous in situ quantification [24]. Between 9:00 and 11:00, gas samples were obtained from the upper compartment using a polypropylene syringe with a nylon stopcock, with each collection occurring at 10 min intervals [25]. The gas sample was analyzed using the Agilent 6820 Gas Chromatograph (Agilent, Santa Clara, CA, USA) system within a time frame of 48 h. Each plot yielded three gas samples. CO₂ and CH₄ emissions were the focus of this study due to the region’s arid nature.

The concentration of gas samples (C_s) collected in the steel chambers was determined as follows:

$$C_S=A_S·C_0/A_0$$

(5)

where C₀ is the standard gas concentration, A_s is the peak area of the sample, and A₀ is the standard gas peak area.

The gas exchange flux (F) was determined as follows:

$$F={\displaystyle \frac{d_c}{d_t}}·{\displaystyle \frac{M}{V_0}}·{\displaystyle \frac{P_a}{P_0}}·{\displaystyle \frac{T_0}{T}}·H$$

(6)

where the measured gas flux (F) is determined by the variation ratio of measured gas in the top box (d_c/d_t), the molar mass of the measured gas (M), atmospheric pressure (Pa), absolute temperature (T), volume at standard condition (V₀), pressure and absolute temperature at standard condition (P₀ and T₀ respectively), and the height of the top box (H).

The estimation of total greenhouse gas emissions (G) throughout the entire duration of the growth season was conducted in the following manner:

$$G=(\overline{F}\times 24\times d)/100$$

(7)

$$\overline{F}={\sum }_{i=1}^n{\displaystyle \frac{F_i\times d_i}{d}}$$

(8)

where F is the average gas flux in the growing period, d is growth days, F_i is the measured gas flux for the ith sample (mg m⁻² h⁻¹), and d_i is the interval day of adjacent sampling events.

2.3.4. Soil Organic Carbon Content (SOC)

The determination of SOC content was conducted through the external heating method [26] using the K₂Cr₂O₇ volumetric technique. Soil cores were utilized to collect soil samples from two layers, namely 0–15 cm and 15–30 cm, during the maturation stage of maize.

2.4. Data Analysis

Daily greenhouse gas emissions, cumulative emissions, SOC content, soil nutrient concentrations, and diversity values were compared for each treatment using one-way ANOVA in SPSS version 20.0 (SPSS Inc., Chicago, IL, USA). Statistical distinctions between treatments were assessed utilizing Duncan’s test with a significance level set at p = 0.05. Origin 2021 was used for all figure making and principal component analysis (PCA); this software has a built-in Shapiro test that can detect whether the data conform to the normal distribution law.

3. Results

3.1. Soil Moisture and Water–Fertilizer Productivity

In both study years, soil water storage increased non-linearly with an increasing biochar application rate (

Table 3. Evapotranspiration in the 80 cm soil profile.

Year	Treatment	Irrigation Amount (mm)	Precipitation (mm)	Recharge of Groundwater (mm)	Soil Water Storage (mm)	ETa (mm)
2015	CK	369.6	73.4	74.4	84.1 c	601.5 c
	B15	369.6	73.4	74.4	85.9 b	603.3 b
	B30	369.6	73.4	74.4	87.6 a	605.0 a
	B45	369.6	73.4	74.4	85.9 b	603.3 b
2016	CK	278.1	119.1	34.6	61.0 c	492.8 c
	B15	278.1	119.1	34.6	73.7 a	505.5 a
	B30	278.1	119.1	34.6	69.2 a	501.0 b
	B45	278.1	119.1	34.6	70.1 b	501.9 b

Note: Different letters represent significant differences (p < 0.05) among biochar treatments in the same year.

). In 2015, the B30 plots had the most stored soil water (87.6 mm), increasing by 4.2% relative to the control. In 2016, the B15 plots had the most stored soil water (73.7 mm), increasing by 20.8% relative to the control. The ETa measured in the maize field was consistent with the changes in soil water storage, initially increasing and then decreasing. Biochar application significantly (p < 0.05) increased ETa by 2.1–4.2% and 14.9–20.8% in 2015 and 2016, respectively, relative to the control. Applying biochar also significantly improved CWP, with an increase of 13.8–19.5% observed in 2015 compared with the control group. In 2016, CWP notably improved in the B30 and B45 plots, showing increases of 8.7% and 1.3%, respectively, relative to the control group. However, notably, the B15 plots slightly decreased in CWP by 1.1%. Over the two-year study, the B30 plots increased soil water storage, water consumption, and CWP on average by 8.8%, 1.1%, and 14.1%, respectively, relative to the control. Furthermore, we discerned a quadratic function relationship between maize yield and the biochar application rate (Figure 2).

The N fertilizer PFP and amount of biochar added were quadratic functions, linearly increasing with the biochar application rate (Figure 3). The B30 and B0 plots had the highest and lowest PFP of N fertilizer, respectively. In 2015, a notable 14.1–20.4% increase in the PFP of the N fertilizer was observed in the B15, B30, and B45 plots compared with the control plots. In 2016, the nitrogen fertilizer PFP in the B15 and B45 plots did not significantly differ from the control plots, while the nitrogen fertilizer PFP in the B30 plot increased by 10.8%. Variations in P and K PFP were consistent with those of the N fertilizer.

Maize yield, CWP, fertilizer PFP and the amount of biochar added are quadratic functions that first increase and then decrease with an increasing biochar application rate (Figure 2 and Figure 3). In 2015, the yields of B15, B30, and B45 increased by 19.3%, 20.4%, and 14.2%, respectively, compared with the control. After calculating the increase in biochar yield per ton under different treatments, the results were 162.2, 86.1, and 39.8 t ha⁻¹, respectively. The R² values for maize yield and fertilizer PFP were >0.9 (maize yield: R² = 0.98; CWP: R² = 0.97; N, P, and K fertilizer PFP: R² = 0.98). According to the regression curve in 2015, the yield was the highest when the biochar addition reached 27.63 t ha⁻¹. CWP was the largest when the biochar addition reached 26.67 t ha⁻¹. Additionally, the productivity of N, P, and K fertilizers was the largest when the biochar addition reached 27.58, 27.64, and 27.63 t ha⁻¹, respectively. In 2016, the yields of B15, B30, and B45 increased by 1.5%, 10.8%, and 3.2% respectively compared with the control. After calculating the increase in biochar yield per ton under different treatments, the results were 11.6, 42.5, and 8.4 t ha⁻¹, respectively. The R² values for maize yield and fertilizer partial productivity were <0.6 (maize yield: R² = 0.57; CWP: R² = 0.33; PFP of N, P, and K fertilizer: R² = 0.57). According to the regression curve in 2016, the yield was the highest when the biochar addition reached 28.75 t ha⁻¹. CWP was the largest when the biochar addition reached 25.25 t ha⁻¹. Additionally, the productivity of N, P, and K fertilizers was the largest when the biochar addition reached 28.85, 28.87, and 28.77 t ha⁻¹, respectively. Considering crop yield, CWP, and fertilizer PFP, biochar application at 27 t ha⁻¹ is the optimal application rate for maize production in the HID.

3.2. Soil CO₂ and CH₄ Emissions

Biochar application affected soil carbon fluxes in maize fields (Figure 4). In 2015 (Figure 4a), biochar applications increased soil CO₂ emission fluxes during early maize growth but decreased them during the mid- and late-growth stages. Biochar amendment generally mitigated CO₂ emissions in 2015, with B15 having the greatest effect and B30 having the least effect. In 2015, total CO₂ emissions in the control, B15, B30, and B45 plots were 5.36, 4.04, 4.42, and 4.17 t ha⁻¹, respectively. The CO₂ emission fluxes during the maize growing period continually decreased in 2016 (Figure 4b). The CO₂ emission flux of the B15-amended soil on day 151 decreased by 43.7% compared with the control plots, whereas the B30 plots on day 164 decreased by 26.9% compared with the control plots. In 2016, the total CO₂ emissions of the control, B15, B30, and B45 plots were 7.88, 6.36, 5.84, and 4.68 t ha⁻¹, respectively. Soil CH₄ emission fluxes (Figure 4c) of CK, B15, B30, and B45 varied in the range of −38.88–17.77, −70.09–13.26, −52.52–0.00, and 43.89–74.12 mg m⁻² h⁻¹, respectively. The soil CH₄ total emissions were −0.19, −0.70, −0.55, and 0.04 kg ha⁻¹, respectively. Compared with CK, B15 and B30 reduced total CH₄ emissions by 268.4% and 189.5%, respectively, while B45 increased total CH₄ emissions by 121.1% compared with CK. Moderate biochar addition to soil contributes to soil CH₄ uptake during the growing season. The change pattern in CH₄ emission fluxes in 2016 (Figure 4d) was the same as in 2015, with temperature change being one of the main influences on CH₄ change. The soil CH₄ emission fluxes of CK, B15, B30, and B45 exhibited a range of variability from −25.36 to 4.79, −42.40 to 0.64, −30.64 to 12.58, and −36.39 to 17.82 mg m⁻² h⁻¹, respectively. Soil CH₄ total emissions were −0.40, −0.42, −0.66, and −0.19 kg ha⁻¹, respectively. Compared with CK, B15 and B30 reduced total CH₄ emissions by 0.5% and 65%, respectively, while B45 increased total CH₄ emissions by 52.5% compared with CK.

The regression relationships between the amount of biochar applied and total CO₂ emissions during the crop growth period are shown in Figure 5a. In 2015, the total CO₂ emissions increased first and then decreased with increasing biochar application (R² = 0.75), with a threshold application rate of 31.3 t ha⁻¹; however, in 2016, total CO₂ emissions linearly increased with increasing biochar application (R² = 0.97). The regression relationships between the amount of biochar applied and total CH₄ emissions during the crop growth period are shown Figure 5b. In 2015, the total CH₄ emission increased first and then decreased with increasing biochar applications (R² = 0.99), with a threshold application rate of 20.4 t ha⁻¹; however, in 2016, total CH₄ emissions linearly increased with increasing biochar application (R² = 0.60), with a threshold application rate of 22 t ha⁻¹.

3.3. Soil Organic Carbon

Biochar application initially increased the SOC content (Figure 6); however, over time, the SOC content decreased linearly with increases in the biochar application rate. Over 2015 and 2016, B15 increased the SOC content by 3.40 g kg⁻¹ compared with the control, with a 0.91 g kg⁻¹ increase in the first growing season. B30 and B45 increased the SOC content by 5.66 and 9.69 g kg⁻¹ compared to the control, with a 0.94 and 1.08 g kg⁻¹ reduce in the first growing season. In the dormant season. The SOC levels decreased in the biochar treatments, specifically B15, B30, and B45, with decreases of 2.5 g kg⁻¹, 22.2 g kg⁻¹, and 26.5 g kg⁻¹ respectively, whereas no significant changes were observed in the control plots.

3.4. Synergistic Impacts of Biochar on Greenhouse Gas Emissions, Soil Fertility, and Organic Matter Levels

Greenhouse gas emissions, soil fertility, and organic matter results under conditions wherein biochar was applied to maize were analyzed using principal component (PC) analysis (Figure 7) to obtain the optimal amount of biochar to be applied and the biochar replenishment time. The cumulative contribution of the three principal components accounted for 91.65%. PC1 mainly reflects soil fertility information. PC1 contributed 40.6% and exhibited a strong positive load of fertility indicators (N, P, K, SOC), along with a negative indicator for GHG emissions; By contrast, PC2 mainly reflects soil CO₂ emission information owing to the indicator with a larger load being CO₂. Similarly, the contribution of PC2 was 34.2%, with a positive N, K and CO₂ indicator, and a negative P, SOC and CH₄ indicator. Similarly, the contribution of PC2 was 34.2%, with positive N, K, and CO₂ indicators and negative P, SOC, and CH₄ indicators. In PC3. CH₄ had a larger load than other indicators, primarily reflecting soil CH₄ emission information. The PC3 contribution was 16.8%, with positive CH₄, CO₂, SOC, K, and N indicators and negative P indicators. The composite score in

Table 4. Principal component analysis comprehensive score.

Year	Treatment	PC1	PC2	PC3	The Composite Score
2015	CK	−1.87	−0.08	0.15	−0.76
	B15	−0.33	−0.92	−1.65	−0.73
	B30	−0.23	−1.03	−0.92	−0.60
	B45	−0.91	−2.02	1.47	−0.81
2016	CK	−1.35	2.39	0.65	0.38
	B15	0.28	0.97	−0.13	0.43
	B30	1.55	1.20	−0.41	0.97
	B45	2.87	−0.50	0.84	1.13

shows that in 2015, B30 > B15 > CK > B45, and all scores are negative, whereas the composite score shows that B45 > B30 > B15 > CK in 2016.

4. Discussion

4.1. Effects of Biochar on Crop Yield and Water Productivity

Applying biochar significantly increased CWP, consistent with the results of other studies [7,27]. Over the past two years, maize yields saw a significant rise, corresponding to increased crop water consumption following biochar application (Figure 2). Notably, while CWP increased with higher amounts of biochar, crop water consumption initially increased and then decreased [28]. Under irrigated conditions, biochar improved the soil environment, enhancing soil moisture and nitrogen fertilizer use efficiency, which boosted crop production [29]. The changes observed in maize water usage and output during 2015 suggest that the relationship between biochar application and maize yield is not linear. This implies that creating a more permeable soil structure may reduce soil evaporation [30]. Biochar also increases the active surface area of the soil, promoting water absorption by maize roots, which enhances transpiration, photosynthesis, and, consequently, yield [31]. Even without reapplication in the second year, the positive effects of biochar on yield persisted. A regression analysis of yield and CWP showed that the impact of biochar application follows a quadratic equation, with an optimal range between 27 and 28 t ha⁻¹ (Figure 2). At the same time, according to calculations, we know that adding 15 t ha⁻¹ of biochar can increase the yield per ton of biochar the most, reaching 162.2 t ha⁻¹ in the first year. The lower correlations between biochar application and crop yield, as well as CWP, observed in 2016 could be due to the absence of reapplication. However, the slow decomposition of biochar provided a residual effect from the initial application, enhancing crop yield and CWP in the second year.

4.2. Effects of Biochar on Fertilizer Productivity

Soil nutrients are essential for plant growth and development [32]. The stoichiometric ratios of N, P, and K are important indicators of soil nutrient cycling [33]. Biochar, with its high carbon concentrations and available nutrient contents (Table 1), significantly enhances SOC and available nutrients when applied to soil [15,34]. Under consistent irrigation and fertilization conditions, our study found that biochar application significantly increased the partial productivities of N, P, and K fertilizers. This improvement can be attributed to two key factors. Combined with relevant papers, we can draw similar conclusions. This is mainly due to the unique properties of biochar, which include larger pores, a greater surface area, a negative charge, and a complex pore structure [35]. These characteristics contribute to biochar’s strong cation exchange capacity and adsorption capacity [36], allowing it to effectively retain nutrients and enhance soil fertility, ultimately leading to higher crop yields and improved fertilizer efficiency. Biochar acts as a slow-release fertilizer, delaying nutrient release, thereby reducing N and P fertilizer losses and leaching while improving fertilizer retention [37]. Liu et al. [17] demonstrated that biochar-based organic fertilizers significantly reduce the fertilizer applied. Regression analysis verified the existence of quadratic associations between the partial productivities of N, P, and K fertilizers and the quantity of biochar applied. The optimal biochar application rate was identified at 27 t ha⁻¹ rather than the highest rate assessed (45 t ha⁻¹) (Figure 3).

4.3. Effects of Biochar on SOC and Greenhouse Gas Emissions

Research has shown that a significant proportion of the overall organic carbon content in soil is concentrated within the uppermost 20 cm layer, ranging from 74.23% to 82.20% [38]. Our study found that biochar application positively correlated with SOC in the 0–30 cm soil layer (Figure 6). This increased SOC concentration is crucial for enhanced crop yield for the following reasons: (1) The biochar used in this study introduced exogenous organic matter directly into the soil owing to its inherently high organic matter content (Table 1); (2) Biochar largely comprises highly concentrated aromatic ring structures, with the surface abundant with various functional groups [39] such as carboxyl, hydroxyl, and lactone. These structures and groups suppress external physical, chemical, and biological interference and enhance microbial degradation resistance [40]; and (3) biochar is inherently nutrient-rich, including Ca²⁺, Ma²⁺, and P [41]. Under the root pressure of crops, soil moisture acts as a carrier, transporting soluble nutrients for crop root adsorption [42,43]. Research shows that, owing to its porous structure and extensive surface area, biochar improves soil moisture conditions, promoting the proliferation and conservation of fungi [44]. This action provides additional carbon sources, enhancing soil biological activity and plant biomass. The increased biomass subsequently enhances the capacity of plants to fix CO₂ through photosynthesis, indirectly reducing the emissions of decomposed organic matter into the atmosphere, yielding a ‘carbon negative’ effect [45] (Figure 4 and Figure 5).

The high organic matter content in biochar readily fuels soil microorganisms, increasing soil microbial activity and respiration [46]. The low-temperature fracturing of biochar could also expose incompletely converted cellulose, hemicellulose, and other sugar substances readily metabolized by soil microorganisms [47], reducing the carbon sequestration potential of biochar and promoting soil CO₂ release [48]. Biochar application also reduces greenhouse gas emissions, which may be linked to changes in soil microbiology [49]. Notably, the soil organic carbon content decreased significantly in 2016 compared with 2015, likely because of the soil surface tillage carried out before sowing and after harvest. In regions characterized by aridity and semi-aridity, sandstorms are a common phenomenon in the spring season. These sandstorms often disperse lighter biochar particles through wind action after surface tillage.

4.4. The Regulation Mechanism of Biochar in Compensating Soil Carbon Loss

Our study reveals a complex interaction of factors influenced by biochar application. Under consistent irrigation conditions, biochar application led to quadratic increases in the maize yield, CWP, and PFP, as shown in Figure 2 and Figure 3. At the same time, it caused a linear decline in the SOC content, as illustrated in Figure 6 [16]. This interplay can be understood through a compensatory mechanism. In northwest China’s inland region, soil fertility is categorized based on organic carbon content. Soils with 10–20 g kg⁻¹ of carbon need pre-planting fertilization, while soils with 20–30 g kg⁻¹ are suitable for cultivation. Data show [50] that applying 15 t ha⁻¹ of biochar maintains the SOC content at the lower end of this scale. However, applying 30 and 45 t ha⁻¹ of biochar increased the SOC content to a more favorable range of 20–30 g kg⁻¹. According to our regression analysis, with 30 t ha⁻¹ of biochar, SOC is expected to drop below 20 g kg⁻¹ by the end of the second year (612 days), indicating that biochar should be reapplied before the third year’s planting. For 45 t ha⁻¹ of biochar, SOC is projected to fall below this threshold by the third year (1234 days), suggesting that it should be reapplied before the fourth year’s planting.

Given the PCA findings, in 2015, the B30 treatment exhibited the highest composite score among all treatments applied with biochar. Conversely, the B45 treatment demonstrated the lowest composite score during this period. The composite scores of the treatments in the second year (2016) decreased with the decreased amount of biochar applied. This shows that excessive biochar application can negatively affect soil fertility. Combining the compensatory effect and the results of the principal component analysis, we recommend a 30 t ha⁻¹ biochar application, supplemented in the third year after application to ensure soil fertility.

4.5. Prospects for Future Research

In this study, we only discussed the mechanism of biochar compensation for maize, a cash crop in an arid irrigation area. Subsequent studies can explore the biochar compensation mechanism with different crops and crop rotation modes in irrigated areas, to cope with different farming modes in irrigated areas. At the same time, considering the high cost of biochar, determining its residual effects can better reduce costs and increase production. The residual effect of biochar will also be limited by climate, soil type, crop type, field management system and other factors [16,51,52,53]; thus, it is necessary to conduct field trials for many years to more accurately determine the residual effect of biochar and promote it. Determining biochar residual efficiency can not only improve soil fertility in farmland, but also reduce greenhouse gas emissions, especially in arid irrigated areas around the world.

5. Conclusions

Under the same irrigation and fertilization conditions, biochar application improves soil water storage, water consumption, and crop water productivity in maize farmland. Biochar amendment also positively affects the partial factor productivity of fertilizer, with an initial increase and subsequent decrease in response to increasing biochar application rates. The addition of 15 t ha⁻¹ of biochar increased the yield per ton of carbon to 162.2 t ha⁻¹, which was 46.9% higher than B30 and 75.3% higher than B45. The quadratic equation derived from the calculations indicated that the fertilizer productivity peaked when 27 t ha⁻¹ of biochar was applied. With time, a gradual decline in the augmentation of the SOC content was observed, spanning −0.8% to 10.1%, which is attributable to the biochar incorporation. After calculation, the greenhouse gas emissions were reduced the most when the biochar was applied at a rate of 22 t ha⁻¹. Biochar application decreased the total soil CO₂ and CH₄ emissions during crop growth over two years by 17.6 to 40.6% and −1.21 to 268.4%, respectively. Our findings suggest that a biochar application rate of 22 t ha⁻¹ is suitable for enhancing maize cultivation under drip irrigation in arid climates. In the third year, an additional 20 t ha⁻¹ of biochar was incorporated to mitigate carbon loss and supplement SOC. However, extended and continuous observations are recommended to validate the long-term effects of biochar on SOC dynamics and greenhouse gas emissions.