The Effect of Biochar Addition in Potato Fields on Microbial Communities in the Arid Region of Northern China

Abstract

Biochar is an effective soil amendment for improving soil function; however, the effects of biochar produced at different pyrolysis temperatures on soil microbial community structure and enzyme activities remain insufficiently studied. A field experiment was conducted from 2023 to 2024 in the arid and semi-arid region of Northern China to investigate the effects of biochar produced at different pyrolysis temperatures (T1: 300 °C; T2: 500 °C; T3: 700 °C) and application rates (C1: 10 t ha⁻¹; C2: 20 t ha⁻¹; C3: 30 t ha⁻¹) on soil chemical properties, microbial community structure, enzyme activity, and potato nutrient use efficiency. The results indicated that the C2T2 treatment was most effective in enhancing soil fungal and actinomycete populations, increasing total microbial biomass, significantly improving soil enzyme activities, and ultimately promoting crop yield. Structural equation modeling indicated that biochar regulates soil nutrient supply, drives microbial community succession toward functional specialization, prolongs microbial regeneration cycles, and ultimately enhances potato nutrient use efficiency. The results of this research provide scientific evidence to support the sustainable development of potato farming in the North China region.

1. Introduction

North China is an important grain production region, and potatoes, a key food crop in this area, are crucial in ensuring food security and improving nutrition [1]. However, in pursuit of high yields, farmers often over-irrigate and over-fertilize [2], leading to increased drought stress, nutrient leaching, loss of biodiversity, and soil degradation. This, in turn, exacerbates soil-borne diseases, threatening the sustainable development of potato cultivation [3]. Biochar, as a potential soil amendment, has been demonstrated to optimize microbial habitats and boost enzyme activity [4]. However, to fully realize the application potential of biochar in potato farming, additional studies are required to investigate its impact on soil chemical and biological properties, as well as the mechanisms that improve crop nutrient utilization efficiency.

Biochar significantly enhances soil carbon storage by improving its physical, chemical, and biological properties [5]. Additionally, it regulates soil pH and enhances nutrient use efficiency, thereby promoting plant productivity [6]. However, some studies have also suggested that biochar may have negative or negligible effects on soil nutrients and carbon storage potential. For example, its application can reduce soil mineral nitrogen availability in the short term [7] and may decrease crop productivity [8]. Several factors can affect the ability of biochar to enhance soil quality [9], with pyrolysis temperature being a key determinant of biochar’s physicochemical properties and a critical factor affecting the long-term effects of biochar on soil functions [10]. Biochar produced at higher temperatures generally exhibits greater specific surface area, porosity, pH, ash content, and carbon concentration, while also demonstrating lower toxicity to soil biota [11]. However, when the pyrolysis temperature exceeds a specific range, it may negatively affect crops’ ability to absorb soil nutrients [12]. The effects of biochar produced at different pyrolysis temperatures on microbial community composition and the associated metabolic changes in enzyme activity and crop nutrient use efficiency remain unclear.

The abundance and structure of soil microbial communities have been widely used as critical microbial indicators reflecting soil environmental changes and soil quality [13]. The physicochemical properties of biochar play a crucial role in determining its impact on microbial performance [14]. Biochar influences microbial community structure across different soils by modifying pH, organic carbon content, and the carbon-to-nitrogen ratio. Studies indicate that shifts in microbial composition driven by biochar–nitrogen interactions are closely linked to changes in soil pH and organic carbon levels [15]. Additionally, biochar can promote the growth of specific microorganisms, leading to an increased fungal-to-bacterial ratio and a higher proportion of Gram-negative to Gram-positive bacteria, thereby enhancing the activity of related organisms and improving the overall microbial community structure [16]. The impact of biochar on microbial communities is influenced not only by its properties but also by its application rate. Studies have demonstrated that microbial functions and community structure only change when the biochar application rate is sufficiently high, which is also a key factor affecting soil microbial diversity and composition [17]. A higher application rate of biochar provides microorganisms with more habitat space and pores, improving the soil environment, thereby significantly enhancing soil microbial activity [18]. However, excessive biochar application may also reduce the relative abundance of fungal and bacteria and decrease phospholipid fatty acid (PLFA) content [19]. Therefore, to fully explore biochar’s role in carbon storage and enhancing soil quality in agricultural ecosystems, additional research is required to examine how various biochar management approaches influence soil microbial community structure.

Soil enzymes are often seen as key microbial markers for assessing soil quality [20]. Applying biochar directly influences enzyme activity through the co-localization of enzymes and their interactions with the biochar surface [21]. Additionally, biochar improves soil physicochemical properties (such as specific surface area, water retention capacity, and unstable carbon compounds), enhancing microbial activity and biomass, which indirectly causes changes in enzyme activity [22]. Several studies have shown that pyrolysis temperature is a key factor determining the impact of biochar on soil enzyme activity [23]. Firstly, pyrolysis temperature affects the aromaticity, pH, mineral nutrient content, and active carbon pool of biochar, which in turn influences the abundance, composition, and activity of soil microorganisms [24], ultimately altering soil enzyme activity [25]. Secondly, biochar’s adsorption capacity is considered a significant factor leading to decreased enzyme activity in the soil [26], and pyrolysis temperature is a key factor affecting the ability of biochar particles to adsorb organic and inorganic molecules [27]. However, the threshold for the effect of biochar pyrolysis temperature on soil enzyme activity remains unclear. Research found that when biochar pyrolysis temperature was between 450 °C and 600 °C, extracellular enzyme activity related to carbon transformation first increased and then decreased [28]. The study also found that biochar produced at 350 °C in temperate sandy loam increased dehydrogenase activity, while biochar produced at 700 °C reduced dehydrogenase activity [29]. Current research indicates that the impact of biochar application on soil enzyme activity may exhibit a linear relationship with the application rate. However, it may also display a nonlinear or no relationship [23]. Therefore, how to combine pyrolysis temperature and the application rate of biochar to regulate soil enzyme activity, as well as the specific processes involved in this interaction, requires further investigation. Meanwhile, changes in microbial communities may indirectly or directly affect soil enzyme activity and crop nutrient uptake efficiency. However, the quantitative relationship between these processes has not yet been fully clarified.

This study aims to achieve the following: (1) to investigate the effects of biochar on the soil microbial community structure and related enzyme activity in potato fields; (2) to analyze the effects of biochar on soil biomass mortality rate and biomass turnover time; (3) to evaluate the impact of biochar on crop yield and nutrient use efficiency (PFP) and to explore the underlying regulatory mechanisms.

2. Materials and Methods

2.1. Experimental Site

Field trials were carried out from 2023 to 2024 at the Kebu’er Potato Science and Technology Backyard in Inner Mongolia (41°17′59.81″ N, 122°33′26.65″ E). The region has an average annual precipitation of 300 mm. The experimental area primarily comprises sandy loam soil, with a bulk density of 1.41 g cm⁻³ and organic matter content of 9.96 g kg⁻¹ in the 0~100 cm soil layer. The soil contents of total nitrogen, phosphorus, and potassium in the 0~40 cm plow layer are 1.42 g kg⁻¹, 12.22 mg kg⁻¹, and 98.91 mg kg⁻¹, respectively, with a soil pH of 7.95. Refer to Table S1 for details.

2.2. Experimental Design

2.2.1. Biochar

Biochar materials were prepared and provided by Zhengzhou Haosen Environmental Protection Technology Co., Ltd., Zhengzhou, China. Detailed information is presented in Table S2.

2.2.2. Experimental Arrangement

The planting dates for the potatoes were May 9, 2023, and May 10, 2024, with growing periods of 128 and 129 days, respectively. The variety used was Jinshu 16. The experiment included 10 treatments: a control without biochar application (CK) and nine biochar treatments formed by the combination of three pyrolysis temperatures (300 °C, 500 °C, and 700 °C) and three application rates (10, 20, and 30 t·ha⁻¹). A completely randomized design was adopted with three replicates per treatment. Each plot measured 4 m × 5 m, resulting in a total of 30 plots.

Biochar for each treatment was first evenly applied to the soil surface and then incorporated into the 0–20 cm soil layer using a rotary tiller. To prevent cross-contamination between treatments, a 1.5 m-wide bare soil strip was left between adjacent plots as a buffer zone (Figure S1). The irrigation water source was groundwater, and irrigation was applied every 7 to 15 days during the study period. A water meter (accuracy of 0.001 m³) was installed to monitor water usage. Detailed information is provided in Table S2. Fertilizers included urea (46% N, 300 kg·ha⁻¹) with a base ratio of 3:3:4, calcium superphosphate (46% P₂O₅, 180 kg·ha⁻¹), and potassium sulfate (45% K₂O, 300 kg·ha⁻¹) as basal fertilizers.

2.3. Measurements and Calculations

2.3.1. Soil Chemical Properties

Soil pH was measured using a pH meter (Eutech pH 510, USA) [30]; organic carbon was determined by the potassium dichromate titrimetric method with external heating [31]; total nitrogen was measured using the Kjeldahl digestion method [32]; and nitrate and ammonium nitrogen were extracted with KCl solution and determined using an AA3 continuous-flow analyzer (Bran+Luebbe, Germany) [33].

2.3.2. Soil PLFA Extraction and Measurement

Lipid extraction and analysis were conducted by adding chloroform–methanol–citrate buffer (1:2:0.8, v:v:v) to 4.0 g of freeze-dried soil. Lipid classes were separated using a silica gel column and sequentially eluted with chloroform, acetone, and methanol to obtain neutral lipids, glycolipids, and phospholipid fatty acids (PLFAs), respectively. The purified PLFAs were methylated to fatty acid methyl esters (FAMEs), followed by the addition of an internal standard (19:0 FAME). Fatty acid identification was performed using gas chromatography (Agilent 7890B) combined with the Sherlock Microbial Identification System (MIS) version 4.5, and δ¹³C values of individual PLFAs were determined using gas chromatography–combustion–isotope ratio mass spectrometry (GC-C-IRMS, MAT 253, Thermo Fisher Scientific, USA) [34].

This study used the following fatty acids as biomarkers to represent the biomass of different microbial communities: 16:0, 18:0, i15:0, a15:0, i16:0, i17:0, a17:0, 16:1ω7c, 18:1ω7c, cy17:0ω7c, and cy19:0ω7c to represent bacterial biomass [35]; 18:2ω6c and 18:1ω9c to represent fungal biomass [35,36]; 10Me 16:0, 10Me 17:0, and 10Me 18:0 to represent actinobacterial biomass [36]; i15:0, a15:0, i16:0, i17:0, and a17:0 to represent Gram-positive bacterial biomass [37]; and 18:1ω7c, cy17:0ω7c, and cy19:0ω7c to represent Gram-negative bacterial biomass [36]. Additionally, the total sum of microbial PLFAs was used to represent the total microbial biomass in the soil [38].

2.3.3. Soil Enzyme Activity Soil Enzyme Dynamics

Soil samples at different depths were collected during different growth stages of potatoes to determine the activities of soil urease, alkaline phosphatase, sucrase, and catalase. Urease and alkaline phosphatase activities were measured using a spectrophotometric method [39,40], sucrase activity was determined using a colorimetric method [32], and catalase activity was measured by a titration method [41].

2.3.4. Soil Microbial Activity

The soil microbial activity substrate experiment is used to determine the soil microbial mortality rate and regeneration cycle. First, 40 g of the soil sample is placed into a 1.5 L brown bottle and sealed with a rubber stopper for incubation. At the same time, 20 mL of 1 mmol L⁻¹ sodium hydroxide (NaOH) solution is added to a 50 mL beaker. The soil is incubated under 50% field capacity conditions. The water lost during incubation is replenished whenever NaOH is extracted for analysis, and NaOH solution is re-added [42]. During the incubation, the beakers are placed at 25 °C for continuous cultivation. The CO₂ emitted from the soil during incubation is measured using gas chromatography coupled with a thermal conductivity detector [43]. The soil microbial death rate (SDRB) is primarily related to substrate concentration (P) and the distribution coefficient (β) and can be calculated using a first-order kinetic equation [42].

$${\displaystyle \frac{\mathrm{d}\mathrm{B}}{\mathrm{d}\mathrm{t}}}=\mathsf{\beta }·\mathrm{P}-\mathrm{S}\mathrm{D}\mathrm{R}\mathrm{B}·\mathrm{M}\mathrm{B}\mathrm{C}$$

(1)

where ${\displaystyle \frac{\mathrm{d}\mathrm{B}}{\mathrm{d}\mathrm{t}}}$ represents the rate of change in the distribution coefficient B with respect to time t; dB refers to the change in the distribution coefficient B; dt denotes the change in time t; MBC represents soil microbial biomass carbon (mg kg⁻¹); P is the substrate concentration (mg kg⁻¹); β is the distribution coefficient for the conversion of the substrate to soil microbial biomass carbon; and SDRB is the soil microbial death rate (mg kg⁻¹ d⁻¹).

When soil microbial activity reaches a stable state, the equation can be rewritten as follows:

$$\mathsf{\beta }·\mathrm{P}=\mathrm{S}\mathrm{D}\mathrm{R}\mathrm{B}·\mathrm{M}\mathrm{B}\mathrm{C}$$

(2)

$$\mathrm{C}\mathrm{E}\mathrm{R}=\mathsf{\alpha }·\mathrm{P}$$

(3)

$$\mathrm{S}\mathrm{D}\mathrm{R}\mathrm{B}={\displaystyle \frac{\mathsf{\beta }}{\mathsf{\alpha }}}·{\displaystyle \frac{\mathrm{C}\mathrm{E}\mathrm{R}}{\mathrm{M}\mathrm{B}\mathrm{C}}}$$

(4)

$$\mathrm{T}\mathrm{T}={\displaystyle \frac{1}{\mathrm{S}\mathrm{D}\mathrm{R}\mathrm{B}}}$$

(5)

where CER is the CO₂ release rate, mg kg⁻¹ d⁻¹; α is the substrate conversion coefficient to CO₂; and TT is the turnover time, d.

Adenosine triphosphate (ATP).

$$\mathrm{M}\mathrm{B}\mathrm{C}=83.7·\mathrm{A}\mathrm{T}\mathrm{P}$$

(6)

where ATP is adenosine triphosphate, mg kg⁻¹. Since ATP is closely related to soil microbial biomass carbon content, this study estimated the soil ATP content using the equation proposed by Joergensen et al. [42].

Microbial biomass nitrogen and carbon were quantified using the chloroform fumigation–extraction technique [44].

2.3.5. Potato Yield and Nutrient Use Efficiency

Fresh potatoes from an area of 2 m × 1.8 m were harvested and weighed for measurement [45].

Nutrient use efficiency [46].

$$\mathrm{P}\mathrm{F}\mathrm{P}={\displaystyle \frac{\mathrm{Y}}{\mathrm{F}\mathrm{T}}}$$

(7)

where Y represents the yield (kg ha⁻¹); PFP is the fertilizer partial productivity (kg kg⁻¹); and FT is the total amount of N, P₂O₅, and K₂O applied (kg ha⁻¹).

2.4. Data Processing and Analysis

The experimental data were processed in Excel 2019, followed by statistical analysis in SPSS 26.0 using LSD and Duncan’s multiple range tests. Data visualizations were generated with OriginPro 2021, while the Mantel test and structural analysis were performed in R Studio 1.1.

3. Results and Analysis

3.1. Soil Microbial Community Structure

As the growing season progressed, the total soil microbial community (total PLFAs), fungi, bacteria, Gram-positive (G+) bacteria, and Gram-negative (G−) bacteria exhibited a downward-opening parabolic trend, whereas actinomycetes showed a linear increase. Biochar application significantly altered the activity of total soil microorganisms (total PLFAs), fungi, and actinomycetes but had no significant effect on the activity of soil bacteria, G⁺ bacteria, or G⁻ bacteria (Figure 1). Throughout the growing season, the activity of total soil microorganisms (total PLFAs) and actinomycetes was highest under the C2T2 treatment, significantly exceeding the other treatments by 7.29–18.35% and 7.02–16.21%, respectively (with no significant difference from C1T2 and C2T1 mean values across growth stages in 2023, p > 0.05). Similarly, fungal abundance was highest under the C2T2 treatment, significantly surpassing other treatments by 7.51–27.52% (with no significant difference from C1T2, mean values across growth stages in 2023, p > 0.05).

The temporal dynamics of soil microorganisms during the 2024 growing season were generally consistent with those observed in 2023. The total PLFAs and actinomycetes were highest under the C2T2 treatment, with no significant difference compared to C1T2 and C2T1. Compared to these treatments, C2T2 increased total microbial biomass and actinomycetes by 8.33–21.14% and 7.63–20.76%, respectively (p < 0.05, mean values across growth stages in 2024). Additionally, the fungal abundance in the C2T2 treatment was 8.62–32.87% higher than in other treatments, with no significant difference compared to C1T2 (p < 0.05, mean values across growth stages in 2024).

This study demonstrated that biochar application significantly altered soil F/B across different potato growth stages, whereas it had no significant effect on the G+/G− ratio (Figure 2 and Figure 3). Throughout the growing season, the impact of an increasing biochar pyrolysis temperature and application rate on F/B exhibited a unimodal trend, with the highest F/B value observed under the C2T2 treatment. There was no significant difference between C2T2 and C1T2 or C2T1; however, compared to other treatments, C2T2 significantly increased F/B by 10.42–22.93% (p < 0.05, mean values across growth stages in 2023 and 2024).

3.2. Microbial Death Rate and Regeneration Cycle

Microorganisms with longer regeneration cycles can gradually release nutrients, providing a stable nutrient supply, supporting continuous crop growth, and enhancing nutrient use efficiency. During the maturation period of potatoes, the C2T2 treatment showed the highest soil microbial carbon and nitrogen content, significantly higher by 6.46–31.93% and 6.21–30.51% compared to other treatments (

Table 1. Effects of biochar on soil microbial activity in potato farmland in 2023 and 2024.

Years	Treatments	MBC (mg kg−1)	MBN (mg kg−1)	CER (mg kg−1 d−1)	ATP (mmol kg−1)	SDRB (mg kg−1 d−1)	TT (d)
2023	CK	168.55f	17.23f	13.62d	2.01f	0.060a	16.72f
	C1T1	187.75cd	19.52bcd	12.49bc	2.24cd	0.049d	20.31c
	C2T1	203.51b	19.98bc	12.28ab	2.43b	0.045e	22.40b
	C3T1	177.81def	18.50de	13.05bcd	2.12def	0.054c	18.41de
	C1T2	198.80bc	20.53b	12.20ab	2.38bc	0.045e	22.02b
	C2T2	217.20a	22.18a	11.57a	2.59a	0.039f	25.37a
	C3T2	182.35de	19.04cde	12.70bc	2.18de	0.052cd	19.40cd
	C1T3	183.07de	18.50de	12.90bcd	2.19de	0.052bcd	19.18cde
	C2T3	189.30cd	20.26bc	12.68bc	2.26cd	0.050d	20.17c
	C3T3	174.83ef	17.82ef	13.17cd	2.09ef	0.056b	17.94e
2024	CK	165.82f	16.67f	11.71d	1.98f	0.052a	19.14h
	C1T1	193.92cd	19.21cd	10.62bc	2.32cd	0.041e	24.68d
	C2T1	210.82b	20.63ab	10.48abc	2.52b	0.037f	27.18c
	C3T1	175.14e	17.76ef	11.13cd	2.09ef	0.047bc	21.26gf
	C1T2	203.32bc	20.35bc	10.23ab	2.43bc	0.037f	26.86c
	C2T2	223.92a	22.06a	9.89a	2.68a	0.033g	30.60b
	C3T2	179.15e	18.46e	10.94bc	2.14e	0.045bc	22.13gf
	C1T3	182.50de	18.23de	10.97bcd	2.18de	0.044cd	22.48ef
	C2T3	192.46cd	19.92d	10.83bc	2.30cd	0.042de	24.01e
	C3T3	172.69ef	17.53ef	11.20cd	2.06ef	0.048b	20.84g

Note: MBC represents microbial biomass carbon; MBN represents microbial biomass nitrogen; CER represents CO₂ release rate; ATP represents adenosine triphosphate; SDRB represents soil microbial death rate; TT represents turnover time. The letters a–h are used to denote significant differences between different treatments. The same procedure was followed as described above.

, 2023 and 2024 average). The microbial biomass primarily influenced soil microorganisms’ death rate and corresponding regeneration cycle. Considering the energy release and regeneration cycle of microorganisms, the C2T2 treatment performed the best, with a CO₂ release rate of 11.57 mg kg⁻¹ d⁻¹, ATP content of 2.59 mmol kg⁻¹, death rate of 0.039 mg kg⁻¹ d⁻¹, and regeneration cycle of 25.37 days (2023 and 2024 average).

3.3. Soil Enzyme Activity

As there were no significant differences in soil enzyme activity between 2023 and 2024, the analysis was conducted using 2023 as a representative year. The activities of soil urease, alkaline phosphatase, sucrase, and catalase exhibited a trend of increasing first and then decreasing throughout the potato growing season (Figure 4). Overall, biochar application significantly enhanced soil enzyme activities. Across all growth stages, the highest activities of urease, catalase, alkaline phosphatase, and sucrase were observed under the C2T2 treatment, with values ranging from 349.1to 652.8, 42.6 to 67.6, 2.78 to 4.59, and 264.6 to 473.0, respectively. While no significant differences were detected between C2T2, C1T2, and C2T1, C2T2 significantly outperformed the other treatments, with increases of 6.58–17.35%, 10.64–19.32%, 9.91–26.20%, and 7.82–22.90%, respectively (mean values across growth stages in 2023).

3.4. Nutrient Use Efficiency of Potatoes and Its Relationship with Soil Chemical Properties, Microbial Community Structure, and Enzyme Activity

Both potato yield and nutrient use efficiency were highest under the C2T2 treatment, with values 8.44% to 27.15% higher than those of other treatments, and the differences were significant (p < 0.05) (Figure 5 and Figure 6, p < 0.05, mean values for 2023 and 2024). In addition, this study found that biochar application notably influenced the contents of soil nitrate nitrogen, organic carbon, and total nitrogen, while having little impact on ammonium nitrogen and pH (Table S4). Mantel test analysis (Figure 7) showed that microbial community structure was significantly positively correlated with soil organic carbon, total nitrogen, enzyme activity, CO₂ release rate (CER), soil microbial death rate (SDRB), and adenosine triphosphate (ATP) (p < 0.05). The regeneration cycle was strongly positively correlated with soil enzyme activity, microbial biomass carbon and nitrogen, and ATP (p < 0.001), while it was significantly negatively correlated with CER and SDRB (p < 0.001). Soil organic carbon and total nitrogen were significantly positively correlated with soil enzyme activity (p < 0.001) and significantly negatively correlated with CER (p < 0.05), while soil nitrate nitrogen showed the opposite trend.

Based on the structural equation model (SEM) analysis (Figure 8), soil nutrients had a direct positive effect on soil microbial community structure, enzyme activity, microbial regeneration cycle, and PFP, with path coefficients of 0.638 (p < 0.01), 0.630 (p < 0.01), 0.580 (p < 0.05), and 0.665 (p < 0.01), respectively. Soil microbial community structure directly affected the soil enzyme activity and microbial regeneration cycle (path coefficient 0.986, p < 0.001; path coefficient 0.999, p < 0.001). The soil enzyme activity and soil microbial regeneration cycle directly affected PFP (path coefficient 0.683, p < 0.01; path coefficient 0.735, p < 0.01). This indicates that soil nutrients not only directly improve nutrient use efficiency but also, through enhancing soil microbial community structure, indirectly increase soil enzyme activity and the microbial regeneration cycle, further enhancing nutrient use efficiency.

4. Discussion

4.1. The Effect of Biochar on Soil Microbial Abundance and Community Composition

Research has demonstrated that the application of biochar can increase the activity of total soil microorganisms [47]. However, most studies on the regulation of soil microbial communities by biochar have focused on humid and sub-humid regions, while research in potato farmlands of arid and semi-arid areas remains limited.

In this study, under the same biochar application rate, a pyrolysis temperature of 500 °C was most effective in enhancing the activity of total microorganisms, fungi, and actinomycetes in the soil. (Figure 1 and Figure 2). This pattern can be attributed to the fact that medium-temperature biochar, unlike low- and high-temperature biochar, contains a balanced level of organic nutrients and has a more optimal pore structure and surface area, both of which support increased microbial abundance and activity [18]. Additionally, an optimal biochar pyrolysis temperature can activate microbial activity, accelerate organic matter decomposition and transformation, promote organic carbon stability, and reduce its mineralization rate, thereby enhancing soil carbon sequestration and providing a more stable and sustained nutrient supply for potato growth [48].

Furthermore, at the same pyrolysis temperature, applying biochar at a rate of 20 t ha⁻¹ was found to be the most effective in enhancing soil microbial biomass and activity. This could be due to the increased habitat surface area and porosity, which lower soil bulk density and improve water and nutrient retention. However, excessive biochar application may disrupt the microenvironment required for microbial growth, leading to a decline in microbial diversity due to selective pressure on microbial populations [49]. In addition, high biochar application rates may introduce toxic compounds that negatively affect certain soil microorganisms, further reducing microbial diversity [50]. This impact may be more pronounced in arid and semi-arid environments with limited water availability, as microbial resilience is weaker, and potentially harmful biochar-derived compounds may persist in the soil for extended periods.

4.2. Effect of Biochar on Soil Enzyme Activity

Urease, alkaline phosphatase, and sucrase play crucial roles in the soil nitrogen, phosphorus, and carbon cycles, respectively, influencing soil nitrogen supply intensity, plant-available phosphorus content, and the bioavailability of soil organic matter [12]. Catalase activity reflects soil oxidation intensity and is closely associated with soil organic matter and microbial activity [15]. Adding biochar to the soil not only serves as an energy source for microbial activity but also offers abundant substrates for enzymatic reactions [51].

This study found that soil urease, alkaline phosphatase, sucrase, and catalase activities were highest when biochar produced at a medium pyrolysis temperature was applied (Figure 4). This could be because biochar pyrolyzed at 500 °C contains a higher proportion of oxygen-containing functional groups, a lower aromatic structure content, and a greater abundance of readily degradable compounds, which collectively enhance enzyme activity [52]. In contrast, high-temperature biochar (700 °C) exhibits greater aromaticity and hydrophobicity, increasing its stability in soil but reducing its bioavailability, thereby suppressing enzymatic activity [53]. In addition, biochar produced at higher pyrolysis temperatures typically has a smaller pore size (S2), which not only increases its specific surface area and enhances its adsorption capacity but may also reduce microbial abundance and enzyme activity in the soil [54,55], potentially limiting potato’s uptake of essential nutrients. However, some studies have reported that biochar pyrolysis temperature has no significant effect on soil enzyme activity [56], while others have even found negative effects [57]. These discrepancies could be due to differences in experimental conditions and the properties of biochar.

This study further showed that, at the same pyrolysis temperature, applying biochar at a rate of 20 t ha⁻¹ led to the highest soil enzyme activity (Figure 4). This could be because the optimal biochar application rate enhances soil water retention and improves the microbial environment, thereby supplying ample substrates for enzymatic reactions [51], thereby enhancing enzymatic efficiency, promoting nutrient uptake by potato roots, and improving drought tolerance and yield. However, excessive biochar application may increase its surface area and porosity, which could restrict substrate accessibility to enzyme active sites, ultimately reducing soil enzyme activity [53].

4.3. Impact and Mechanisms of Biochar on Potato Nutrient Use Efficiency

In arid and semi-arid environments, soil moisture scarcity and nutrient deficiency are the primary factors limiting potato growth. The application of biochar improves soil physicochemical properties and regulates microbial communities, thereby enhancing the soil’s ability to retain moisture and nutrients. This, in turn, increases nutrient availability, promotes crop growth, and boosts yield [58]. Our analysis through the Mantel test (Figure 7) and structural equation modeling (Figure 8) indicates that biochar directly improves soil microbial community structure by regulating soil nutrient status, which in turn indirectly affects soil enzyme activity and turnover cycles, ultimately enhancing soil nutrient use efficiency. This mechanism not only improves soil fertility but also contributes to the stability and sustainability of the soil ecosystem.

Based on the results of this study, applying 20 t ha⁻¹ biochar, in addition to local fertilization and irrigation practices, can achieve increased yield and efficiency. However, the application of biochar inevitably increases farmers’ investment costs. Therefore, to promote the large-scale application of biochar technology, further consideration should be given to the resource use efficiency and cost-effectiveness of biochar as a partial substitute for chemical fertilizers. By constructing a green potato production system that balances both ecological and economic benefits, this study provides scientific support for sustainable agricultural development in arid and semi-arid regions and offers practical evidence for reducing fertilizer dependence and minimizing environmental impact.

5. Conclusions

This study systematically investigated the impact of biochar on the microbial community environment in potato fields in North China through a two-year field trial conducted from 2023 to 2024. The results were as follows:

(1) In the potato planting system of arid and semi-arid regions, biochar application had no significant effect on soil bacterial, G+ bacterial, or G− bacterial activity or the G+/G− ratio. However, biochar application significantly affected the activity of total soil microorganisms, fungi, actinomycetes, and soil enzymes, with the highest sensitivity observed under a medium pyrolysis temperature (500 °C) and medium application rate (20 t·ha⁻¹).

(2) Structural equation modeling indicated that biochar improved nutrient supply, which drove the optimization of microbial community structure, subsequently enhancing soil enzyme activity, slowing down microbial turnover cycles, and ultimately increasing soil nutrient availability.

This study confirms that applying biochar produced at a pyrolysis temperature of 500 °C at a rate of 20 t ha⁻¹ is an appropriate strategy for synergistically enhancing the soil microenvironment, yield, and nutrient use efficiency in potato farmland in the dryland regions of North China. Future research should further assess the resource use efficiency and economic benefits of partial biochar substitution for chemical fertilizers, providing scientific support for the green and sustainable development of potato agriculture in arid regions.