Summer maize grain yield and soil carbon emission response to subsoiling before winter wheat sowing in the North China Plain

ABSTRACT Food security and carbon emissions are major challenges for China and the international community. To investigate the effects of different subsoiling depths on summer maize grain yield and soil carbon emissions, in 2016 and 2018, three tillage treatments, that is, rotary tillage at the depth of 15 cm (R15), subsoiling tillage at the depth of 40 cm (S40), and subsoiling tillage at the depth of 35 cm (S35), were set before winter wheat sowing. The effects of different tillage treatments on summer maize grain yield and soil carbon emissions were analyzed. The results showed that compared with R15, subsoiling increased the microbial biomass carbon in the 20–40 cm soil layers, and improved the soil respiration rate and CO2-C cumulative emissions. The grain yield was increased by subsoiling, especially S35 (8.30% and 13.89% in 2018 and 2019, respectively). The carbon emission efficiency in S35 was significantly higher than that in S40. Altogether, we found that the best performance on grain yield of summer maize and carbon emission efficiency occurred in S35. S35 could be used to coordinate the relationship between summer maize grain yield and soil carbon emission. Therefore, this treatment can be applied and promoted in the NCP.


Introduction
The North China Plain (NCP) is the main winter wheat and summer maize production area in China , and the double cropping system of winter wheat and summer maize is the main planting system in this area. In the NCP, winter wheat is mainly sown after rotary tillage. Shortterm rotary tillage has a positive impact on soil moisture status and crop yield, on the contrary, rotary tillage only cultivates the soil at a shallow depth, and long-term of rotary tillage causes the bottom soil to be struck and squeezed by a rotary cultivator, forming solid plow bottom (Guan et al. 2014;Wang et al. 2019a), which affects crop roots and inhibits their growth and temporal and spatial distribution, and leads to a decrease in soil water and fertilizer retention, which adversely affect crop growth (Hammel 1994;Ghosh et al. 2005;Moreira et al. 2016). Researches have shown that subsoiling tillage can break the plow bottom formed by long-term rotary tillage, loosen the soil, reduce soil compaction, soil bulk density and strength, increase soil infiltration, improve soil permeability, promote root growth, and improve water and fertilizer utilization efficiency and crop yield (Kumar et al. 2013;Ahmad et al. 2018;Ren et al. 2018;Wang et al. 2021a). The soil water retention by subsoiling is mainly caused by the decrease of large water-conducting macropores and the increase of capillary pores (Tormena et al. 2017). Subsoiling can improve soil structure and organic materials by increasing aggregate and aggregate organic carbon content, thus increasing dry matter accumulation and grain yield (Li et al. 2015a;Shi et al. 2016;Rahmati et al. 2020;Wang et al. 2020a). In addition, subsoiling can alleviate the adverse effects of drought stress on crops and improve the potential of land production (Schneider et al. 2017). Subsoiling is of great significance for breaking the plow bottom, obtaining a larger tillage layer, and achieving high crop yield.
As NCP is one of the main grain-producing areas in China, its annual greenhouse gas emissions, mainly CO 2 emissions, increase rapidly. China's 14th Five-Year Plan clearly propose the goal of carbon neutrality and carbon peak, and the time period addressed in this plan is the key period in which China needs to achieve the goal of carbon neutrality (Huang 2021). CO 2 is one of the main greenhouse gases, and agricultural soil is the largest carbon pool in the terrestrial carbon cycle (Lal 2004). Farmland is an ecosystem which has strong carbon sequestration potential, and thus, it could effectively reduce atmospheric CO 2 concentrations (Deng et al. 2016;Smith 2016). Zhang et al. (2014) point out that soil respiration is the main mechanism by which soil releases CO 2 into the atmosphere. Soil tillage systems have an important effect on soil CO 2 emissions. The absence of tillage can reduce soil disturbance, improve the soil organic carbon pool, then effectively reduce carbon emissions (Pascal et al. 2011). Subsoiling can break the plow bottom, improve soil permeability, and promote organic matter decomposition and root growth, thus improving the soil respiration rate (Kumar et al. 2013;Zhao et al. 2014;Lou et al. 2021). Soil respiration mainly consists of root respiration and soil microbial respiration (Lu and Liao 2015;He and Xu 2021). Soil microbial biomass carbon (MBC) is a living component of soil organic matter, and it can reflect the number of soil microorganisms and has a significant influence on many microbial driving processes. Tillage practices affect the soil MBC content by changing the soil structure (Wang et al. 2007;Li et al. 2015b;Joergensen and Wichern 2018), thereby affecting the soil respiration rate.
Most studies on suitable depth of subsoiling in the NCP focused on the depth of 30-40 cm (Mu et al. 2016;Shao et al. 2016), and most explored the effects of different subsoil depths on crop growth and soil structure (Wang et al. 2019b(Wang et al. , 2020b(Wang et al. , 2021b. However, few studies have investigated the effects of different subsoil depths on soil carbon emissions. Furthermore, it has been pointed out that the soil respiration rate in the summer maize season is significantly higher than that in the winter wheat season (Tong et al. 2017). We speculated that subsoiling was able to increase soil carbon emissions and summer maize yield, but that it would have a more significant impact on grain yield, thus improving carbon emission efficiency. Therefore, to study the responses of different subsoiling depths to grain yield and soil carbon emission of summer maize, three tillage treatments were set before winter wheat sowing, namely rotary tillage at the depth of 15 cm (R15), subsoiling treatment at the depth of 40 cm (S40), and subsoiling treatment at the depth of 35 cm (S35). Soil respiration rate, grain yield, and carbon efficiency were measured. Our results will be useful for solving the problem of NCP food security, and they provide a theoretical basis for further studies on carbon emissions.

Experimental site
The experiment was carried out at the Agriculture Experimental Station of Shandong Agricultural University (36°10ʹ19" N, 117°9ʹ03" E). The experimental site is located in the temperate continental monsoon climatic zone. During the summer maize growth period, the average rainfall is 453.7 mm, which can satisfy the water requirement of summer maize during its growth season. In the growth season of summer maize in 2018 and 2019, rainfall was 447.0 and 354.2 mm, respectively. The daily rainfall, maximum temperature, and minimum temperature are shown in Figure 1. The contents of alkali-hydrolyzed nitrogen, available phosphorus, and available potassium in the 0-20 cm soil layers were 236.43 mg kg −1 , 86.73 mg kg −1 , and 85.36 mg kg −1 , respectively. Before the first subsoiling in 2016, the test field had been continuously rotary tillage for 15 years with a rotary cultivator (C250, Maschio Gaspardo China). The plow layer gradually thinned, and a solid plow layer appeared.

Experimental design
In 2018 and 2019, three tillage treatments were set up during the growth seasons of summer maize (variety Zhengdan 958): rotary tillage at the depth of 15 cm (R15), subsoiling tillage at the depth of 40 cm (S40), and subsoiling tillage at the depth of 35 cm (S35). The first subsoiling was carried out in 2016, and by 2019, the experiment had been carried out for four years. Every two years (in 2016 and 2018), we use a vibration-type subsoiling machine (ZS-180, Gongli Company Yuncheng County) to subsoil before winter wheat sowing to loosen the compacted deep soil layers, after which a rotary cultivator (C250, Maschio Gaspardo China) was used to prepare the ground. Before winter wheat sowing in 2017, we only use rotary tillage to prepare the land without subsoiling. Before summer maize sowing in 2018, a rotary tiller (C250, Maschio Gaspardo China) was used to prepare the land, with the tillage depth of 15 cm. Before winter wheat sowing in 2018, we use a vibration-type subsoiling machine (ZS-180, Gongli Company Yuncheng County) to subsoil, after which a rotary cultivator (C250, Maschio Gaspardo China) was used to prepare the ground. Before summer maize sowing in 2019, a rotary tiller (C250, Maschio Gaspardo China) was used to prepare the land, with the tillage depth of 15 cm. For R15 treatment, before sowing winter wheat and summer maize, a rotary tiller (C250, Maschio Gaspardo China) was used to prepare the land, with the tillage depth of 15 cm. The summer maize was sown using a spoon-wheeled precision planter (SHB-2, Hebei Nonghaha Agricultural Machinery Co., Ltd.). After winter wheat and summer maize harvested, the straw returning machine (180 F, Maschio Gaspardo China) was used to return all the straw to the field. The planting density of summer maize was 75,000 plants ha −1 . Each treatment had three replicates; thus, there was a total of nine plots, each with an area of 4 m × 18 m. Before summer maize sowing, urea (46.6% available N, 280 kg ha −1 ), diammonium phosphate (225 kg ha −1 ), and potassium chloride (169 kg ha −1 ) were applied, and at the jointing stage, urea (46.6% available N, 280 kg ha −1 ) was applied as topdressing before the onset of rainfall. We sowed summer maize on 9 June 2018 and 15 June 2019, and harvested on 7 October 2018 and 30 September 2019, respectively. Irrigation was not used during the growth period. Herbicides and insecticides were sprayed at the seedling stage and a corn rust control pesticide was sprayed at the V12 stage.

Soil microbial biomass carbon (MBC)
In 2019, during summer maize jointing, V12, tasseling, filling, and harvest stages, soil samples were taken from 0-40 cm soil layers with a depth interval of 10 cm; in each treatment' each layer, we took three soil samples. Soil MBC was determined by the substance-induced respiration method (Li et al. 2015c): briefly, 5 g of fresh soil were placed in a 280 mL wide-mouth flask, to which 5 mL of glucose solution with a concentration of 1.98 g L −1 and 0.025 g of talc were added; the flask was sealed with a rubber plug and incubated at 22°C for 2 h. The GXH-3052 L infrared gas analyzer (ADC Bioscientific Ltd., Hoddesdon, UK) was utilized to measure the amount of released CO 2 , and MBC content was determined based on the linear relationship between the amount of released CO 2 and MBC. Soil quality was measured as dry soil.

Soil respiration rate
The GXH-3052 L infrared gas analyzer (ADC Bioscientific Ltd., Hoddesdon, UK), which equipped with a PVC chamber (height 0.15 m, diameter 0.25 m), was utilized to measure the soil respiration rate. In each cell, a fixed point was selected for measurement, and the measurement time was 2 min. Before measurement, the measuring chamber was buried 0.03 m below the ground to prevent air leakage. During summer maize jointing, V12, tasseling, filling, and harvest stages, the measurements were carried out between 09:00 am and 10:00 am on sunny days. In 2018, the measurement days were 37, 48, 64, 73, and 120 days after summer maize sowing; in 2019, the measurement days were 23, 44, 55, 70, and 107 days after summer maize sowing.
The soil respiration rate was calculated as follows (Guo et al. 2019a): where F (g m −2 h −1 ) is the soil respiration rate, 60 is the coefficient for converting minutes into hour, H (m) is the effective height of the measuring chamber, M (g mol −1 ) is the relative molecular weight of CO 2 , P is the atmospheric pressure at the experimental site (101 kPa), T (°C) is the atmospheric temperature, and dc dt is the change rate of indoor CO 2 concentration with time.

CO 2 -C cumulative emission
The CO 2 -C cumulative emission was calculated as follows (Chai et al. 2014): where F (g m −2 h −1 ) is the soil respiration rate, t is the number of days after summer maize sowing, i is the number of measurements, and 24 is the coefficient for converting days into hours.

Grain yield and yield compositions
5 m double rows with uniform growth were selected in the center of each plot to record the ear number and harvest by hand, when summer maize was mature. Ten ears were randomly selected from each plot, and the number of rows per ear and number of kernels per row were recorded. We determined the grain weight and 1000-kernel weight in each plot after air drying and threshing.

Carbon emission efficiency
The carbon emission efficiency was calculated as follows (Guo et al. 2021): where CEE is the carbon emission efficiency, Y (g m −2 ) is the grain yield, and CE (g m −2 ) is the cumulative emission of CO 2 -C.

Statistical analysis
We used Microsoft Excel 2019 for data processing, and SPSS Statistics 18.0 for statistical analysis of MBC and carbon emission efficiency. We used the least significant difference (LSD) test to conduct comparisons (ɑ = 0.05), and Origin 2017 for plotting. Figure 2 shows the MBC values of 0-40 cm soil layers in the main growth stages of summer maize in 2019. The MBC was lower in deeper soil layers. In the 20-30 cm soil layer, the soil MBC in S35 was higher than R15 by 8.60%, 22.13%, 14.52%, and 20.80% at the V12, tasseling, filling, and harvest stages, respectively, and the soil MBC in S40 was 10.75% higher than R15 at the V12 stage. In the 30-40 cm soil layer, MBC in S35 was higher than R15 by 34.86% and 17.50% at the jointing and V12 stages, respectively. In 30-40 cm soil layer, the MBC increased with the tillage depth. Figure 3 shows the soil respiration rates in the main growth stages in the 2018 and 2019 summer maize growing periods. The soil respiration rate first increased and then decreased in these two growing seasons. In 2018, the soil respiration rate peaked during the filling stage. The soil respiration rate in S40 and S35 increased by 9.99% and 8.14% in comparison with that in R15, respectively. On the other hand, in 2019, the peak soil respiration rate was recorded at the V12 stage, and the soil respiration rates in S40 and S35 increased in comparison with those in R15 by 27.80% and 15.53%, respectively. In these two growing seasons, from the highest to the lowest, the soil respiration rate was S40> S35> R15. Figure 4 shows the soil CO 2 -C cumulative emissions in the 2018 and 2019 summer maize growing seasons. In 2018, the CO 2 -C cumulative emissions in S40 and S35 increased by 9.59% and 4.42% in comparison with those in R15, respectively. In 2019, the CO 2 -C cumulative emissions in S40 and S35 increased by 13.98% and 5.67% in comparison with those in R15, respectively. Table 1 shows the grain yield and yield compositions in 2018 and 2019 summer maize growing seasons. In 2018, compared with those in the R15 and S40 treatments, the kernel number per row in S35 significantly increased by 0.81% and 2.04%, respectively. In 2019, compared with that in the R15 treatment, the kernel numbers per row in S40 and S35 significantly increased by 11.96% and 6.44%, respectively. In these two growing seasons, compared with that in R15 treatment, the 1000-kernel weight in S40 and S35 were significantly increased. In 2018, the grain yield in S35 significantly increased by 8.30% in comparison with that in R15 treatment. In 2019, the grain yields in S40 and S35 significantly increased by 11.77% and 13.89% in comparison with that in R15 treatment, respectively. Figure 5 shows the carbon emission efficiency in 2018 and 2019 summer maize growing seasons. The order of carbon emission efficiency was S35> R15> S40 in these two growing seasons from the highest to the lowest. Carbon emission efficiency in S35 significantly increased in comparison with those in R15 and S40. In 2018, the carbon emission efficiency in S35 increased by 3.70% and 8.96% in comparison with those in R15 and S40, respectively, and in 2019, it increased by 7.85% and 9.96%, respectively.

Discussion
In the present study, subsoiling increased the soil respiration rate and CO 2 -C cumulative emissions in comparison with rotary tillage. It is known that soil respiration mainly includes root respiration as well as soil microbial respiration (Lu and Liao 2015;He and Xu 2021). Compare with rotary tillage, subsoiling can break the plow layer, reduce soil bulk density and strength, and increase the cumulative infiltration amount in the soil, improve soil permeability, so as to promote the growth of deep root system, promote the increase of root system, especially the deeper root dry weight, root system distribution and depth keep root higher physiological activity, and thus increased root respiration (Baumhardt et al. 2008;Zhang et al. 2015). At the same time, subsoiling can promote microbial respiration by reducing soil compaction, improving soil permeability, accelerating the migration and diffusion of gas in the soil, thus promoting the microbial decomposition of organic matter. (Zhao et al. 2014;He et al. 2019). Therefore, soil respiration rate and carbon emission of subsoiling treatments were higher than those of rotary tillage treatment. We observed greater MBC values in 20-40 cm soil profiles of the subsoiling treatments compared to R15, and greater still in S40 compared to S35. Soil respiration rate was positively correlated with MBC (Hassan et al. 2016), this is likely a large factor contributing to heightened CO 2 -C emissions in S35 and S40 treatments in comparison with rotary  tillage. Our results showed that soil respiration rate and cumulative CO 2 -C emission were significantly higher in S40 than in S35. This agrees with results from other studies that have found that tillage depth is significantly positively correlated with carbon emissions (Marakoglu and Carman 2015). S40 has a deeper tillage depth than S35, which has a larger effect on root growth and microbial. The respiration rate had a significant linear positive relationship with soil moisture at the interannual scale , and soil moisture was mainly affected by rainfall since irrigation was not used. In our experiment, the results clearly showed that the peak value of the soil respiration rate in 2018 occurred at the filling stage, while in 2019, it occurred at the V12 stage; the reason for this was because the rainfall in 2018 mainly concentrated on the late growth stage of summer maize, while in 2019, it was mainly concentrated on the early growth stage (Figure 1). The soil CO 2 -C cumulative emission during the summer maize growth season in 2019 was higher than that in 2018 because the rainfall during the summer maize growth season in 2019 was 26.20% higher than that in 2018. Moreover, the subsoiling tillage treatment was prior to the sowing of winter wheat in 2016 and 2018. Studies have shown that the disturbance effect of subsoiling on soil carbon emissions gradually decreases with time, lasting no more than two years (de Moraes et al. 2016;Nunes et al. 2018;Yan et al. 2020). Therefore, the subsoiling tillage had a more significant effect on improving the soil respiration rate and CO 2 -C cumulative emission of summer maize in 2019 than in 2018.
Many studies have shown that subsoiling can improve crop yield, which was consistent with the results of the present study. Subsoiling can increase soil organic carbon storage and improve soil fertility and crop yield stability effectively (Xu et al. 2019). Subsoiling contributed to a higher leaf area index, leaf area duration, net assimilation rate, and growth rate. Subsoiling also effectively delayed leaf senescence and increased dry matter accumulation, which laid the foundation for yield formation in the later stage (Sun et al. 2019). The subsoiling tillage enhanced the soil water holding capacity, provided good moisture conditions for plant growth (Wang et al. 2012;Yin et al. 2021). The soil conditions for root distribution and root growth could be improved by subsoiling, which could maintain higher photosynthetic capacity of post anthesis canopy and increase dry matter accumulation (Sun et al. 2017). The grain filling rate of maize was increased by subsoiling, and the 1000-grain weight and grain yield were increased (Wang et al. 2021c). The grain yield in S35 significantly increased in comparison with that in R15 during those two growing seasons. Kuang et al. (2021) combined field experiments with model simulation and showed that compared with S40, S35 reduced deep leakage and provided better soil moisture conditions for crop growth. Therefore, S35 performs better in summer maize yield. In the present study, although these two subsoiling treatments increased the soil carbon emissions, the S35 treatment had the highest carbon emission efficiency among all treatments. Altogether, subsoiling at 35 cm performed best on grain yield and carbon emission efficiency, and this treatment could be used for better control of the relationship between summer maize grain yield and farmland soil carbon emission and for achieving the highest grain yield and carbon emission efficiency. Crop roots contributed significantly to soil carbon emissions, indicating that the effects of different subsoil depths on summer maize roots could be studied in the future to reveal the mechanism by which different subsoil depths affect summer maize yield and soil carbon emission.

Conclusions
In our study, the results verified that subsoiling increased soil MBC in 20-40 cm soil layers, and MBC in S40 was higher than that in S35. Furthermore, subsoiling increased the soil respiration rate and CO 2 -C cumulative emission. Meanwhile, they increased with the increase in subsoiling depth. In those two growing seasons, grain yield in S35 significantly increased in comparison with that in R15, and carbon emission efficiency in S35 significantly increased in comparison with those in S40 and R15. Subsoiling tillage at the depth of 35 cm had the best effect on summer maize grain yield as well as carbon emission efficiency, and it could be used to coordinate the relationship between farmland soil carbon emission and summer maize grain yield. Therefore, it can be applied and promoted as an appropriate subsoiling depth in the NCP.

Disclosure statement
No potential conflict of interest was reported by the author(s).