1. Introduction

Climate change is a serious environmental problem due to the increasing population and expansion of agricultural fields. N₂O and CO₂, the leading greenhouse gases (GHGs), are emitted constantly into the atmosphere. By reducing emissions and increasing GHG sequestration, the production of biochar, and its application to the soil, will positively affect soil fertility and increase crop production ^[1]. Biochar promotes soil carbon sequestration ^[2], reduces the emissions of ammonia and CO2 ^[3], lowers soil compactness, optimizes compost ^[4], increases nutrients for plants, enhances water retention, and increases soil pH ^[5]. Strategies such as subsurface drip irrigation (SDI) and deficit irrigation aim to improve water-use efficiency (WUE), while ensuring adequate irrigation, by reducing water usage. These strategies attempt to decrease water loss and preserve water resources available for agricultural production ^[6][7]. Alternate drying and wetting techniques have been demonstrated to decrease water usage by up to 33% under rice cultivation ^[8]. In addition to water recycling and the adoption of localized irrigation systems such as sprinklers, drip and SDI, efficiencies greater than 90% have been achieved, and are other strategies that can enhance WUE ^[6]. SDI has been used in recent years. It results in almost zero soil evaporation, high WUE, minimal deep percolation, optimum fertilizer supply, and increased crop productivity compared with other irrigation methods, including surface drip irrigation ^[9][10]. Understanding vertical and horizontal water movement is essential for crop production concerning conserving water, reducing N₂O and CO₂ emissions, and increasing yield ^[11]. Arif et al. ^[12] reported improved water retention in soils due to increased porosity after biochar amendment. Research has shown that adding biochar to soil increases its ability to hold water and reduces the need for irrigation ^[12][13]. The high organic C content in biochar could increase CO₂ flux from the soil ^[14]. Reducing CO₂ emission from soils by biochar requires high soil organic matter OM content to increase dissolved organic carbon sorption ^[15][16]. With increasing pyrolysis temperatures, biochar’s carbon and ash contents increase. In contrast, low-temperature biochar produces greater carbon and nutrient recovery, which is usually lost at higher temperatures ^[17]. The greatest biochar-induced carbon dioxide emissions were mainly found in low-temperature biochar samples, and decreasing emissions with increasing pyrolysis temperatures ^[18]. The production of vegetables is a crucial agricultural aspect in China ^[19]. Vegetables have recently become the primary crops grown in China due to their high economic value for farmers and health benefits. Chinese cabbage, also known as pakchoi cabbage (Brassica rapa L. ssp. Chinensis), is an important vegetable cultivated in south and northeast Asia and accounts for 30% to 40% of China’s crop cultivation. It can be grown throughout the year and has lower production costs than other vegetable crops due to its simple seed production and short crop duration ^[20]. The influence of biochar addition on soil CO₂ and N₂O emissions has been thoroughly studied. However, to our knowledge, the effect of C content in wheat straw biochar under different SDI depths on soil CO₂ and N₂O fluxes, and plant growth performance and yield, have not been investigated. 

Based on the above, our study aimed to (a) assess the response of soil N₂O and CO₂ emissions to different C-content biochar, (b) determine the optimal depth of subsurface drip irrigation for high-efficiency gas mitigation while maintaining a high plant yield, and (c) investigate how different biochar C contents affect the growth and yield of pakchoi cabbage.

2. The Effect of Different Biochar C contents and Irrigation Depths on Soil Moisture Dynamics

Irrigation depths significantly affected the soil moisture distribution. The soil moisture peaked after irrigation and then gradually decreased until the following irrigation period, with the SDI5 (emitter buried at 5 cm) treatment having the highest soil moisture content. Soil moisture content declined as the lateral distance from the emitter increased ^[21]. The soil moisture distribution changed quickly after watering as water moved rapidly through the soil and then slowed down ^[21]. In the current study, soil moisture decreased as the horizontal distance increased from the emitter point. This is consistent with Kuang et al. ^[22], who concluded that soil moisture in areas away from the drip point would likely be even lower, limiting nitrification and denitrification processes. Similarly, Wei et al. ^[23] reported soil moisture decreased gradually in a horizontal direction with increasing radial distance to the emitter.in the vertical direction, soil moisture decreased as depth increased. The findings are consistent with Weldon et al. ^[24], who stated that soil moisture and the soil matrix’s depletion eventually decreased microbial activity, and thus, emissions. Biochar has been proposed as a potential substance for soil amendment to enhance soil moisture retention, soil structure, soil C storage, and greenhouse gas emissions (GHGs) ^[25]. Adding biochar to the soil increases its water-holding capacity and improves aeration ^[26][27]. In the current study, compared to the control (CK), L (low) and H (high) carbon content biochar increased soil moisture by (15.42, 29.54%), (7.60, 19.95%), and (6.41, 18.07%) for SDI5, SDI10, and SDI15, respectively, indicating that C content has a significant effect on soil moisture retention and soil moisture increases with C content increase. This is due to the increase in carbonization temperature during biochar production, which results in the C particles being crushed, the average pore size and soil bulk density decreasing, and micropores being formed. When micropores are small, they help retain moisture for a long time. These findings are in line with results of several studies. For example, Zhao et al. ^[28] reported that adding biochar impacts soil moisture content, probably due to hydrophilic domains, high porosity, and the high surface area of biochar. The pyrolysis temperature of biochar affected soil water retention ^[29].

3. N₂O Emissions Response to Irrigation Depths and Different C Content Biochar

In recent decades, GHG emissions from agricultural fields have received wide attention because of the fast development of intensive agriculture and a large amount of chemical fertilizer input. Biochar is a porous medium with a high surface area, and can absorb a significant amount of nitrous oxide when applied to soils ^[25][30]. Several recent studies indicate that biochar additions may reduce soil nitrous oxide (N₂O) emissions. For instance, ^[31] investigated the effect of biochar application on greenhouse gas emissions and soil C sequestration in corn cultivated under subsurface drip irrigation (SDI). It was found that N₂O flux could be reduced compared to the control treatment because of an increase in soil pH by adding biochar, which stimulates N₂O emissions, direct N₂O adsorption promoted by the high specific surface area of biochar, and changes in microbial community structure. In our study, biochar-amended soils emitted less N₂O than the control, and N₂O emission occurred immediately following irrigation, then decreased gradually. This can be explained by the fact that the rapid alternation between wetting and drying after irrigation stimulates microorganism activity in the soil and nitrogen (N) transformation, resulting in greater N₂O emissions ^[32]. Therefore, microbial activity eventually decreased due to the decline in soil moisture and the consumption of soil N, which reduced the emissions. In the current study, we noticed N₂O flux under SDI with high C content biochar (SDI5 + H) was significantly less than in other treatments. This phenomenon could be explained by emitters being buried at 5 cm with soil layers below 5 cm being wet, and the soil layers above gradually drying. The wetted part of the soil would be greater than the dry part, and oxygen diffusion decreased, which promotes the best conditions for biochar to dissolve into the soil and create good conditions for plant roots to uptake water and nutrients; thus, the emission was reduced. Another reason could be that the high biochar C-content decreased soil bulk density and enhanced its capacity for absorbing moisture. In general, in our study, gas emissions were low because winter temperatures were lower than those of the other seasons; various microorganisms’ activities in the soil were decreased, which decreased the soil respiration rate.

4. CO₂ Emission Response to Irrigation Depths and Different Biochar C Content

The increase in C content obtained by adding biochar to the soil stimulates the humification and C storage processes and improves soil density and water retention ^[5][33] Biochar amendment effectively reduced CO₂ release and increased organic C content compared to control soil ^[34]. In the study, CO₂ emissions peaked within three days of irrigation and then gradually decreased. The CO₂ flux was higher for the control than for both different C-content biochar treatments, indicating that biochar amendment suppressed CO₂ emissions. Adding biochar to the soil significantly decreased soil mineralization, which was the primary source of CO₂ emissions ^[35][36].

Compared to CK, the L and H treatments significantly decreased CO₂ emissions; among L and H treatments, CO₂ emissions under L were higher than under H. One possible explanation for these results is that the biochar used in L was produced at low temperatures. Thus, it could have contained many unstable materials such as cellulose, hemicellulose, and other complex carbohydrates. These materials are also partially used by microorganisms, subsequently increasing soil carbon dioxide emissions. Our findings were consistent with Lahijani et al. ^[37], who reported that biochar made at high pyrolysis temperatures has a more substantial carbon dioxide capture effect. This is because as carbonization temperature increases, the carbon particles are crushed, the average pore size decreases, and micropores form.

5. Influence of Irrigation Depths and Different Biochar C Content on Plant Growth and Yield

Biochar addition to soil would increases plant access to readily available micronutrients ^[38]. Improving the soil’s chemical, physical, and biological properties enhances plant growth and productivity by increasing the quantity and availability of nutrient components, minimizing nutrient leaching, and decreasing gaseous component losses ^[39][40]. In the current study, the overall means of plant height (PH), leaf area index (LAI), leaf number (LN), and maximum root length (MRL) under SDI5 were significantly (p ≤ 0.05) greater compared to SDI10 and SDI15. This indicates better conservation of soil moisture at this depth. In other words, the soil layers below 5 cm are wet. From 5 cm and above, layers are gradually dried, which means that the wetted part of the soil is greater than the dry part. Oxygen diffusion decreases, which promotes the best conditions for the biochar to dissolve into the soil and create the best conditions for roots to uptake water and nutrients. Regardless of the irrigation depths, the results showed that increasing the C content in biochar significantly (p ≤ 0.05) increased all plant growth indicators.

Regardless of the C content of biochar, SDI5 showed better results for aboveground biomass (AGB), root dry biomass (RDB), aboveground weight (AGW), and root fresh weight (RFW) relative to other irrigation depths. This could be due to better root distribution in this layer, which would allow roots to easily obtain water and nutrients because the soil is wet from 5 cm and below. In other words, almost the entire root zone is wet, and the root can easily obtain water compared to those at other depths. Roots, here, develop vertically rather than horizontally to obtain water and nutrients. The vertical roots fix and support the plant rather than provide nutrients. On the other hand, compared to the CK treatment, the AGB, RDB, AGW, and RFW significantly (p ≤ 0.05) enhanced with the increase of C content in biochar. We postulate that this is because high C-content biochar was produced at a higher pyrolysis temperature than low C-content biochar and CK. Our results are supported by Biederman et al. ^[41], who stated that biochar made at higher temperatures was more efficient at promoting aboveground production, and a combination of suitable depth (SDI5) and high C-content biochar gave a better performance, as in our study. The addition of biochar to agricultural production and GHG mitigation has been intensively studied, but the effects of wheat straw biochar properties, especially the carbon content of biochar under different subsurface drip irrigation depths on soil GHG emissions and crop production, have been rarely studied. To meet population growth demand, it is necessary to produce more food, and the balance between production and protection of the environment is important. Therefore, additional research is required to completely determine how biochar works to reduce the effects of climate change while enhancing crop productivity.