Conservation tillage increases carbon sequestration of winter wheat-summer maize farmland on Loess Plateau in China

Xingli Lu; Xingneng Lu; Yuncheng Liao

doi:10.1371/journal.pone.0199846

Abstract

The idea of mitigating anthropogenic CO₂ emissions by increasing soil organic carbon (SOC) is notable. However, the estimation of the net ecosystem carbon balance after conversion from conventional tillage to conservational tillage has been poorly quantified for the Loess Plateau in China. A 2-year field experiment was conducted to estimate the agroecosystem carbon balance of a winter wheat–summer maize rotation system using a full carbon cycle analysis. The results showed that a positive net ecosystem carbon balance value in the cases of rotary tillage with straw incorporation, chisel plow tillage with straw incorporation, and no tillage with straw mulching treatments. Note that a negative value was detected for the conventional moldboard plowing tillage without crop straw treatment. The conversion from conventional tillage to conservational tillage substantially enhanced the carbon sink potential from 0.84 t C ha⁻¹ yr⁻¹ to 2.69 t C ha⁻¹ yr⁻¹ in both years. Our findings suggest that the expansion of conservational tillage could enhance the potential carbon sink of the rain-fed land in China.

Citation: Lu X, Lu X, Liao Y (2018) Conservation tillage increases carbon sequestration of winter wheat-summer maize farmland on Loess Plateau in China. PLoS ONE 13(9): e0199846. https://doi.org/10.1371/journal.pone.0199846

Editor: Dafeng Hui, Tennessee State University, UNITED STATES

Received: October 22, 2017; Accepted: June 14, 2018; Published: September 5, 2018

Copyright: © 2018 Lu et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the paper and its Supporting Information files.

Funding: This study was supported in part by the National Natural Science Foundation of China (Grant Nos. 3157162). In addition, Yinchuan Provincial Sub-branch, The People’s Bank of China, gave support in the form of salaries for author Xingneng Lu but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section.

Competing interests: Xingneng Lu received support in the form of a salary from Yinchuan Provincial Sub-branch, The People’s Bank of China. This does not alter our adherence to all the PLOS ONE policies on sharing data and materials.

Introduction

Agriculture accounts for approximately 10.0%–12.0% of the total global anthropogenic emissions of greenhouse gases (GHGs) [1]. The direct emission of CO₂ included soil respiration or indirect emission of CO₂ induced by the production of agriculture inputs (fertilizers and pesticides), fuel combustion, and application of machinery on the farm that is increasing year on year [2]. The winter wheat–summer maize rotation system under a rain-fed condition is one of the major grain productions in North China [3]. Therefore, it is important to study carbon balance in rain-fed fields to select appropriate tillage methods to develop low-carbon agriculture and promote the development of sustainable agriculture.

Previous studies have been conducted to evaluate the carbon source or sink by using several methods such as net carbon flux [4–5], net ecosystem productivity [6], and carbon sustainability [7]. Moreover, previous studies on the carbon balance were primarily focused on forest, grassland, and wetland ecosystems [8–11]. The carbon balance of an agricultural ecosystem is primarily observed in a rice paddy field [12–14]. Conservation tillage treatments (e.g., reduced tillage, no-tillage, and straw returning) are often suggested to improve the potential negative effects of crop residue removal, which may refer to the reduction of soil organic carbon (SOC), the increase in soil compaction, disruption of soil aggregates, and deterioration of soil health [15–17]. Conservation tillage operations have often been reported to enhance soil organic carbon sequestration whereas simultaneously mitigate the carbon (C) emissions associated with agricultural inputs such as fertilizers and on-farm fuels [5]. However, there is considerably uncertainty in the estimation of the carbon sink/source of an agricultural system. For example, Snyder et al. [18] has showed that the agricultural fields not only are a carbon sink but also a carbon source because of the application of tillage and fertilizer treatments. Tillage and fertilizer methods always support food, energy, and air for the development of soil organisms, thus increasing the decomposition rate of residues and soil respiration and ultimately resulting in that the stable soil organic carbon is awkward [19]. Li et al. [20] measured the carbon balance of winter wheat (−1.98 t hm⁻²) and summer maize (−1.38 t hm⁻²) in the North China Plain, suggesting that without considering the harvest grain carbon part, this ecosystem is a carbon source; however, this ecosystem is a carbon sink when considering the harvest grain carbon part. However, Zhao et al. [21] found that the carbon uptake of a farmland in China’s coastal areas is significantly higher than that of C emissions. In addition, Chen et al. [22] showed that rotary tillage with straw incorporation and no tillage with straw mulching display a C sink, while moldboard plow tillage with or without straw shows a carbon source in paddy soil. These differences may be attributed to the difference in the level of regional economic development, production layout, and agricultural management practices such as tillage, fertilizer use, and herbicide use.

Soil respiration plays a key role in determining the carbon balance [23]. The lack of available and comprehensive carbon balance data revealed an urgent need to increase the research on the effect of conservation tillage on the net ecosystem carbon balance for the Loess Plateau in China. Thus, the goals of our study are as follows: (1) to estimate the effects of different tillage treatments on soil respiration and its components and (2) to evaluate the effects of different tillage on the net ecosystem carbon balance in the winter wheat–summer maize rotation system.

Materials and methods

Study site

The study was conducted at the Dry-land Experimental Station of Northwest A&F University, Yangling Town, Shaanxi province, in the northwestern part of China (34°21´N and 108°10´E). The soil is classified as silt loam (19% sand, 77% silt, and 4% clay) based on the USDA Texture Classification System. The surface soil (0–20 cm) bulk density before the start of the experiment (in 2009 year) was 1.30 g cm^-3. The study area belong to a semiarid climate, and the annual average temperature, the mean annual precipitation, and the annual potential evaporation are 13°C, 622 mm, and 993 mm, respectively. The weather conditions including mean daily air temperature and daily precipitation are presented in Fig 1.

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Fig 1.

The average of daily air temperature (a) and daily precipitation (b) during the experimental period (from Oct 2013 to Oct 2015).

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Experimental design

The experiment had a randomized block design with three replications. Four tillage systems including conventional moldboard plowing tillage without crop straw (CT), rotary tillage with straw incorporation (RTS), chisel plow tillage with straw incorporation (STS), and no tillage with straw mulching (NTS) were included. Thus, 12 plots were designed; the plot size was 48 m² (3.2 m × 15 m). Moreover, three root-free plots with the same size (these were placed at approximately 10-m intervals adjacent to the whole-soil plots) were designed. The root-free plots were established to evaluate microbial respiration (R_h). The root-free plots were kept free of vegetation by cutting the plants manually throughout the sampling period.

Four tillage treatments, namely, STS, NTS, RTS, and CT, were arranged in three repetitions. The tillage treatments were the same every year since 2009. After harvest, crop straw was removed by hand from the field for the CT treatment before the tillage application. While for the STS, NTS, and RTS treatments, the crop straw was left in the field. The soil tillage was operated twice each year; one was operated on June 17 before planting the summer maize, and the other was operated on October 17 before planting the winter wheat. In the CT plot, the soil was plowed up to a depth of 20–25 cm, and a rotavator was then used to plow the soil up to a depth of 15 cm (Fig 2). In the RTS plot, a rotavator (15 cm) driven by a 95-horsepower tractor (Dong fanghong-LX954, Luoyang, China) was used. In the STS plot, chisel plow machinery was used (30–35 cm). The NTS plots were not disturbed by using the tillage machine either before or after the establishment of the experiment except during sowing with a planter.

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Fig 2.

Season variations of soil respiration (a, 2013–14; b, 2014–15), microbial respiration (c, 2013–14; d, 2014–15) during the cycle of wheat-maize rotation in 2013–2015 in Yangling, China. (NTS: no tillage with straw mulching; RTS: rotary tillage with straw incorporation; STS: chisel plow tillage with straw incorporation; CT: conventional moldboard plowing tillage without crop straw).

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Crop management

Crop cultivation and management were applied in the experiment from October 2013 to October 2015 (Table 1). Winter wheat cultivar Shanmai-139 was sown by using a wheat drill at the rate of 208–210 kg ha⁻¹ on October 18, 2013/2014. Summer maize (CV Shandan-609) was planted using a maize drill at the rate of 30 kg ha⁻¹ on June 17, 2014/2015. Every year after tillage in June before planting summer maize, all the plots received applications of P₂O₅ (172 kg P₂O₅ ha⁻¹) and N (68 kg N ha⁻¹) as the diammonium phosphate fertilizer broadcast. At the seven-leaf stage of summer maize, 172 kg N ha⁻¹ was applied as the urea fertilizer by broadcasting according to local recommendations. Similarly, every year after tillage in October, all the plots received applications of P₂O₅ (172 kg P₂O₅ ha⁻¹) and N (68 kg N ha⁻¹) as the diammonium phosphate fertilizer broadcast. At the same time, 160 kg N ha^–1 in the form of urea was applied. Weeds were killed using herbicides. The distance between the rows of wheat was 16 cm. The row spacing was 70 cm, and the plant spacing was 20 cm in the summer maize field. No irrigation was applied at any other time during the entire crop growing season.

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Table 1. Summary of experimental design with four tillage treatments on Loess Plateau in China.

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Crop yields and root biomass measurements

At the winter wheat maturity stage, the grain yield of the winter wheat was determined by harvesting three 1-m² sampling areas per treatment by hand. At the summer maize maturity stage, 20 plants from the middle rows per subplot were randomly selected and harvested by hand to determine the yield, and the straw was cut for the crop straw biomass determination. The remaining crop was harvested mechanically. In addition, wheat and corn roots were collected at the ripening stage. To get soil cores, a soil auger (diameter: 8 cm) was applied at three different locations, i.e., at the plant spots, intra-plant spots, and intra-row spots. Each core was taken from a depth of 0 to 100 cm in the soil profile and was incremented by 10 cm, i.e., 0–10, 10–20, 20–30, 30–40, 40–50, 50–60, 60–70, 70–80, 80–90, and 90–100 cm. The soil cores were soaked in a plastic container overnight, and then the root was carefully washed by swirling water through it. The soil material and old dead root debris were separated from the live roots manually. The aboveground and root dry weights were determined after drying the root samples in an oven at 105°C for 30 min and then at 75°C until constant dry weight. After weighing the dry weight, we crushed the aboveground and underground of crops and then, sieved the dry samples by using a 0.25-mm sieve. Then, we placed the samples in clean plastic bags to measure the carbon content. The carbon content of the plant samples was measured using potassium dichromate (K₂Cr₂O₄) and sulfuric acid (H₂SO₄) oxidation and ferrous sulfate (FeSO₄) [24].

Soil respiration measurements and estimation of root respiration

After planting the crop including winter wheat and summer maize, the PVC chambers (height: 20 cm; inner diameter: 11 cm) were placed and pressed by hand into the soil to a depth of 5 cm for the measurements of R_s and R_h by the closed chamber method using an infrared gas analyzer (GXH-3010EI, Beijing Huayuan Gas Chemical Industry Co., Ltd., Beijing, China). The samples were placed in field twice each year; one was placed on June 18 after plating the summer maize, and the other was placed on October 20 after planting the winter wheat. The date of measurement of the R_s and R_h values were shown in S1 and S2 Tables. In each plot, one chamber was located to measure R_s in the entire soil. As a result, three chambers were placed in each treatment to measure the R_s value in the entire soil for the three replications. Another three chambers were placed in the no-root zone for each root-free plot. The samples were kept free of vegetation by cutting the plants manually throughout the sampling period.

All R_s and R_h measurements were performed between 9:00 AM and 11:00 AM. local time to avoid the highest CO₂ emission at noon. The increase in the concentration of CO₂ within the chamber was measured after three minutes [25]. Root respiration (R_a) was estimated by subtracting the microbial respiration in the non-root zone from the soil respiration (R_s) in the whole soil.

The R_s was calculated using Eq (1): (1) where F is the R_s value (mg CO₂ m⁻² h⁻¹); K is the reduction coefficient, which is equal to 1.80 at 25°C and 1 Pa; H is the height inserted in soil; and is the time rate of the change in CO₂ concentration in the air within the chamber (mg CO₂ m⁻³ h⁻¹). The total R_s and its components were calculated as follows: (2)

Where FCO₂ is the total emission of CO₂-C (t ha^-1), F_i is the first CO₂ emission value (mg CO₂-C m^-2 h^-1) at time t_i (h), and F_i+1 is the following value at time t_i+1 (h); n is the total number of CO₂ emission values.

Measurements of carbon balance

System dynamics.

The net ecosystem carbon balance from the winter wheat-summer maize production system to the atmosphere was calculated using a full carbon cycle analysis [8–9]. Both carbon fixation within wheat-maize production system and emissions from agricultural practices were considered.

Net primary production (NPP).

The NPP (t C ha⁻¹) of crops was calculated as follows [26]: (3)

Grain, straw, and root biomass NPP were converted by applying the dry biomass weight at harvest. Litter was calculated to account for 5% of the aboveground and root dry biomass [27], while rhizodeposits accounted for 18% [28] and 12% [29] of the aboveground and root dry biomass of wheat and maize.

Net ecosystem productivity (NEP).

The net ecosystem productivity (NEP) was assessed according to [30] as follows: (4) where R_h is the microbial respiration measured using the root exclusion technique.

Calculation of net ecosystem carbon balance (NCF).

The net ecosystem carbon balance without considering the carbon emission from the farm inputs (NECB) was calculated according to Smith et al. [26]. (5) where NEP is the net ecosystem productivity and harvest means the grain harvest for the STS, NTS, and RTS treatments, while for the CT treatment, harvest means grain + straw.

The net ecosystem carbon balance considering the carbon emission from farm inputs (NCF) was then calculated as follows: (6) where C_AP is the carbon emission from the agricultural input and the data are taken from Lu and Liao [31].

Carbon productivity.

Carbon productivity (CP) can be calculated by using the following equation [32]: (7) where Y_c is the grain carbon content (kg C ha⁻¹) and C_AP is the carbon emission from the agricultural input (kg C ha⁻¹).

Data analysis

Data are shown as the mean values ± standard error. The two-way analysis of variance (ANOVA) with the SAS version 8 software package (SAS Institute, Cary, NC, USA) was used for analyzing the effects of the cropping years and the tillage treatments on the total soil respiration and its components, crop yield, carbon productivity, NPP, NECB, and NCF. When significant, the difference between treatments was determined at the 5% level by applying the least significant difference (LSD) test.

Results

Crop production

The grain yields of wheat and maize strongly showed the treatment differences (Table 2). The grain yields ranged from 6.14 to 6.87 t ha⁻¹ for wheat and 7.96 to 9.51 t ha⁻¹ for maize in 2013–14. No difference in the wheat yield was recorded among the different tillage treatments in 2013–14, while the STS significantly (p < 0.05) increased the maize yield by 19.5% as compared with CT. In 2014–15, the STS treatment significantly (p < 0.05) increased the wheat yield by 15.4% with respect to CT. No difference in the wheat yield among the NTS, RTS, and CT treatments was recorded. Similar to the 2013–14 cropping year, the STS significantly (p < 0.05) increased the maize yield by 20.6% as compared to CT.

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Table 2. Total soil respiration (R_s), microbial respiration (R_h), and wheat and maize grain yields, straw yields and aboveground biomass during the wheat- and maize- growing seasons during the 2013–2015 rotation.

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Seasonal variations in R_s, R_h, and R_a

Fig 2 shows the seasonal variation in R_s and R_h in the 2013–2015 cropping seasons. The seasonal patterns of R_s for both seasons showed a similar trend, with the peak appearing in August (at the heading stage of the maize crop) in both the cropping years. After winter wheat was sown, R_s was high because of the disturbance of the tillage application. Then, it decreased rapidly and the lowest values during the entire cropping year was recorded at the wintering stage (January 12–19, 2014/2015) because of the low temperature in the winter season. R_s then exhibited a dramatic increase when the air temperature recovered and fluctuated till the wheat harvest.

During the maize crop season, R_s increased rapidly from July to August and the highest value was recorded at the heading stage (August 10, 2014/2015). The peak value of R_s was 395.0 mg CO₂ m⁻² h⁻¹ for NTS, 557.8 mg CO₂ m⁻² h⁻¹ for RTS, 698.7 mg CO₂ m⁻² h⁻¹ for STS, and 696.2 mg CO₂ m⁻² h⁻¹ for CT in the 2013–14 cropping year. While in the 2014–15 cropping year, the peak value of R_s was recorded to be 427.8 mg CO₂ m⁻² h⁻¹ for NTS, 591.0 mg CO₂ m⁻² h⁻¹ for RTS, 602.0 mg CO₂ m⁻² h⁻¹ for STS, and 652.0 mg CO₂ m⁻² h⁻¹for CT. R_s then reduced and varied as waves till the maize harvest (Fig 2).

R_h showed a similar trend to that of R_s when the crop was small, but a contrasting trend to R_s was observed when the crop became big. The R_a and R_a/R_s values showed a similar trend, and the season pattern of R_a and R_a/R_s was a unimodal curve, corresponding to the growing seasons of wheat and maize (Fig 3). The seasonal pattern of R_a/R_s resembled that of Ra and corresponded clearly to the development of crop (Fig 3). R_a/R_s for wheat exhibited a peak at approximately the flowering stage (April 13–27, 2014/2015). For the maize crop, R_a/R_s exhibited a peak value at approximately the heading stage (August 10–14, 2014/2015). At this time, R_s increased and R_h decreased, which resulted in an increase in R_a/R_s.

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Fig 3.

Season variations of root respiration (a, 2013–14; b, 2014–15) and its ratio (c, 2013–14; d, 2014–15) during the cycle of wheat-maize rotation in 2013–2015 in Yangling, China. (NTS: no tillage with straw mulching; RTS: rotary tillage with straw incorporation; STS: chisel plow tillage with straw incorporation; CT: conventional moldboard plowing tillage without crop straw).

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Cumulative R_s, and R_h

The cumulative R_s emissions from the wheat–maize rotation ranged from 4.07 t C ha⁻¹ for NTS to 7.05 t C ha⁻¹ for STS (Table 2) in the 2013–14 cropping season. While in the 2014–15 cropping season, the STS treatment significantly (p < 0.05) increased the cumulative R_s emissions by 12.0% and 72.7% as compared to CT and NTS, respectively. When compared with CT, the NTS significantly (p < 0.05) reduced the total R_s emissions by 54.1%, no difference in the cumulative R_s emissions was found between the RTS and STS treatments (Tables 2 and 3).

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Table 3. ANOVA of total soil respiration (R_s), microbial respiration (R_h), wheat and maize grain yields, straw yields and aboveground biomass during the wheat- and maize- growing seasons during the 2013–2015 rotation.

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Similarly, the lowest cumulative R_h emissions were recorded in the NTS treatment. The NTS significantly (p < 0.05) reduced the cumulative R_h emissions from 28.4% to 36.4% as compared to the other three treatments in the 2013–14 cropping season. While in the 2014–15 cropping season, this reduction percentage varied from 28.1% to 38.1% (Table 2).

Net ecosystem carbon balance under conservation tillage treatments

The calculations of the NCF from the estimates of the potential carbon inputs from the aboveground biomass, root biomass, negative cumulative carbon loss via R_h, and agricultural input emissions resulted in differences among the different tillage treatments. The NPP values ranged from 10.4 to 11.7 t C ha⁻¹ yr⁻¹ in 2013–14 and from 10.5 to 12.0 t C ha⁻¹ yr⁻¹ in 2014–15 (Table 4). In the 2013–14 and 2014–15 cropping years, the carbon loss through R_h in the winter wheat–summer maize ecosystem accounted for NPP 25.1%–39.1% and 24.7%–40.4%, and the harvest part accounted for NPP 28.8%–90.5% and 45.6%–90.4%.

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Table 4. The agro-ecosystem C balance (NCF) and its main components for the annual cycle of wheat-maize rotation in 2013–2015 (t C ha^-1 yr^-1).

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The overall NCFs were significantly (p < 0.05) affected by the tillage practices (Tables 4 and 5). The lowest NCF value was found under the conventional moldboard plow tillage treatment in the rain-fed winter wheat–summer maize field on the Loess Plateau in China at −3.61 t C ha⁻¹ yr⁻¹ in 2013–14, and −3.53 t C ha⁻¹ yr⁻¹ in 2014–15, whereas the highest NCF value was found under NTS at 2.53 t C ha⁻¹ yr⁻¹ in 2013–14, and 2.69 t C ha⁻¹ yr⁻¹ in 2014–15. This may be attributed to the lower carbon loss via R_h and higher carbon returning in the NTS treatment. There was no significant difference in the NCF values between the RTS and the STS treatments. Over the cycle of wheat–maize rotation, all of the treatments, except for the CT, led to carbon gains of 1.05 to 2.53 t C ha⁻¹ yr⁻¹ in 2013–14, and 0.84 to 2.69 t C ha⁻¹ yr⁻¹ in 2014–15. The CT treatment had a negative NCF value, mainly attributed to the higher harvest part as compared to the other straw returning treatments. The carbon in an agroecosystem mainly depended on the addition of organic matter; in the present study, positive NCF under three conservation tillage treatments was mainly attributed to the amount of crop residues in the soil. These results pointed out the importance of using crop straw in no tillage and reduced tillage for increasing the carbon input in the wheat–maize field on the Loess Plateau in China.

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Table 5. ANOVA of net primary productivity (NPP), harvest, net ecosystem carbon balance and net carbon flux during the wheat- and maize- growing seasons during the 2013–2015 rotation.

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Effect of conservation tillage on carbon productivity

Tillage significantly (p < 0.05) affected the carbon productivity of the winter wheat (Fig 4). When compared with CT, STS and NTS significantly increased the carbon productivity of winter wheat by 28.3% and 36.0% in 2013–14, respectively. This increase percentage was 32.4% and 36.0% in the 2014–15 cropping year. Similarly, STS and NTS significantly improved the carbon productivity of summer corn from 31.1% to 36.8% in both the years. When compared with the CT treatment, STS, NTS, and RTS significantly increased the annual carbon productivity by 32.4%, 33.3%, and 17.6% in 2013–14, while in 2014–15, STS, NTS, and RTS significantly increased the annual carbon productivity by 34.6%, 33.7%, and 17.5% as compared to the CT treatment.

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Fig 4.

Effect of different tillage treatments on carbon productivity of winter wheat (a), summer corn (b), and annual (c) in both cropping years. (The different lowercase letters above the error bars represent significant difference between different tillage treatments within a two-year period at 5% levels according to the LSD test. CT: conventional moldboard plowing tillage without crop straw; RTS: rotary tillage with straw incorporation; STS: chisel plow tillage with straw incorporation; NTS: no tillage with straw mulching).

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Discussion

Effect of conservation tillage on crop yields

In the present study, STS significantly increased the wheat yield by 15.4% as compared to the CT treatment in the 2014–15 cropping year. For the maize crop, when compared with CT, the STS treatment significantly improved the maize yield by 19.5% in 2013–14 and 20.6% in 2014–15, respectively. Similarly, Wang et al. [33] reported that chisel plow tillage is beneficial for reducing the soil bulk density, improving soil structure, increasing soil water availability, and aeration. Xu et al. [34] also reported that when compared to conventional tillage, chisel plow could improve the yield of crops by loosening the soil and promoting the root growth of the crop. In the present study, no difference in the grain yield was recorded between the NTS and the CT treatments. However, Liu et al. [35] reported that no tillage is beneficial for improving the crop yield and water use efficiency in Weibei Highland, China. These inconsistent results may be attributed to the difference in the cropping years, crop, climate conditions, and the duration of conservation tillage. Our results suggest that long-term studies are required to identify the initial and long-term yield constraints of conservation tillage.

Effect of conservation tillage on R_s and its components

The two-year data showed that NTS reduced R_s by 13.0% as compared to the other tillage treatments, which was mainly attributed to the lack of disturbance in NTS after the harvest of summer maize. In addition, when compared with CT, the STS and RTS treatments increased the total annual R_s by 12.0% and 6.8%, respectively. This was mainly attributed to the organic carbon input and the tillage disturbance. Li et al. [36] also reported that R_s increases when the organic carbon is input into the soil. Our results suggested that the organic carbon input in the STS, RTSm and NTS treatments promoted the activity of microbial and ultimately affected the decomposition of the organic matter and the release of soil CO₂, and influenced the carbon balance. Although STS and RTS increased the R_h value, the less carbon input (i.e., diesel fuel and the carbon loss by residue removal) compensated for the higher carbon emissions; finally, STS and RTS showed the carbon sink. In addition, the highest R_h value was recorded in RTS, which was mainly attributed to the fact that the tillage depth was only 15 cm, which made it easier for the microbes to touch the residue.

NCF in winter wheat-summer corn ecosystem under different tillage treatments

This is a very important method to mitigate the atmospheric CO₂ emission by increasing the soil carbon pool [18]. This study focused on the effect of different tillage treatments on the carbon balance of the winter wheat–summer maize ecosystem. The carbon storage or loss mainly relied on the balance of carbon fixed into the soil through the addition of residues and carbon loss through the respiration in a dryland agricultural ecosystem. Our results showed that the carbon loss through R_h and agricultural inputs accounted for NPP 24.7%–40.4% in the different tillage treatments in the 2013–14 and 2014–15 cropping years, respectively. In addition, conservation tillage practices such as the NTS, RTS, and STS treatments were beneficial for carbon sequestration irrespective of the carbon input from the agricultural inputs, which agreed with the other reported results [37–39]. Moreover, the contribution of the agricultural input to the total carbon emission varied from 5.1% to 17.8% in the different tillage treatments, which indicated that the carbon emission from the agricultural input should also be included when evaluating the carbon sink.

Moreover, the positive value of NCF was mainly attributed to the large amount input of the crop straws in the three conservation tillage treatments. In the experiment, the carbon input under the NTS, RTS, and STS treatments varied from 5.90–6.34 t C ha^-1 yr^-1 in both years. Han et al. [40] also showed that SOC sequestration increases with the annual input rate of straw C input rate. The winter wheat–summer maize ecosystem showed a carbon sink for all the treatments without considering the carbon harvest part. However, when the carbon harvest part was derived from NPP − R_h, this ecosystem was a carbon source for the CT treatment. Our results were similar to those of Li et al. [41] who showed that the winter wheat–summer maize rotation ecosystem was a carbon sink without considering the harvest carbon part; however, the carbon sink changed into a source when the harvest carbon part was included. Moreover, when considering the carbon emission from the agricultural input, the carbon balance decreased for all the treatments. Thus, our results suggested that the purpose of the carbon sink could be realized by increasing the carbon inputs of the crop residues and roots.

Carbon productivity

When combining the winter wheat and the summer maize, the highest annual carbon productivity was observed in the STS and NTS treatments, and the lowest value in the CT treatment. In the NTS treatment, the higher carbon productivity was mainly attributed to the lower carbon emission from the farm input because of the similar grain yield to that in the CT treatment. While for the STS treatment, the higher carbon productivity was mainly attributed to the higher crop productivity. Similarly, van den Putte et al. [42] reported that deep conservation tillage performs better than superficial conservation tillage. This indicated that plants could benefit from the increased pore space and aeration at deeper depths. In addition, studies have already shown that the root growth conditions for cereals are less favorable in the case of the NT treatment than in the case of the CT treatment [43–44]. However, Boomsma et al. [45] showed that as compared to the moldboard plow, conservation tillage reduced the crop yields. This was mainly attributed to the relatively poor seedbed conditions, delayed seedling emergence, and crop development in the case of conservation tillage with respect to those in the case of the moldboard plow tillage treatment. Chisel plow tillage may therefore be expected to be more favorable than superficial tillage for crops. Our results also suggested that the conversion of treatment from conventional tillage to conservation tillage could increase the annual carbon productivity.

Conclusions

Our results showed that heterotrophic (microbial) respiration was lower in the NTS treatment than in other three tillage treatments. In the case of the CT treatment in the winter wheat–summer maize field on China’s Loess Plateau, carbon added as the aboveground biomass and root biomass was not sufficient to compensate for the loss of carbon from organic matter decomposition, rendering the rain-fed winter wheat–summer maize field as the net sources of atmospheric CO₂. The conversion from conventional tillage to conservational tillage substantially enhanced the carbon sink potential from 0.84 t C ha⁻¹ yr⁻¹ in RTS to 2.69 t C ha⁻¹ yr⁻¹ in the NTS treatment. Thus, the expansion of conservational tillage could enhance the potential carbon sink of rain-fed land in China’s Loess Plateau. Our results also showed the importance of the returning of crop straw to the field in order to change the winter wheat–summer maize ecosystem from carbon source to sink.

Supporting information

S1 Table. Soil respiration under different tillage treatments.

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S2 Table. Soil microbial respiration under different tillage treatments.

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S3 Table. Root Respiration under different tillage treatments.

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S4 Table. The ratio of root respiration to total soil respiration under different tillage treatments.

https://doi.org/10.1371/journal.pone.0199846.s004

(DOCX)

S5 Table. The total respiration (R_s), microbial respiration (R_h), wheat and maize grain yields, straw yields and aboveground biomass during the wheat- and maize- growing seasons during the 2013–2015 rotation.

https://doi.org/10.1371/journal.pone.0199846.s005

(DOCX)

S6 Table. The agro-ecosystem C balance (NCF) and its main components for the annual cycle of wheat-maize rotation in 2013–2015.

https://doi.org/10.1371/journal.pone.0199846.s006

(DOCX)

S7 Table. The carbon productivity under different treatments.

https://doi.org/10.1371/journal.pone.0199846.s007

(DOCX)

Acknowledgments

The authors appreciate the great and helpful comments of the editors and the three anonymous reviewers.

References

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34. Xu D, Schmid R, Mermoud A. Effects of tillage practices on the variation of soil moisture and the yield of summer maize. Transactions of the Chinese Society of Agricultural Engineering. 1999; 15(3): 101–106 (in Chinese with English abstract).
- View Article
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- View Article
- Google Scholar
45. Boomsma CR, Santini JB, West TD, Brewer JC, Mcintyre LM, Vyn TJ. Maize grain yield responses to plant height variability resulting from crop rotation and tillage system in a long-term experiment. Soil Till Res. 2010; 106: 227–240.
- View Article
- Google Scholar

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View Article
Google Scholar

[2] View Article

[3] Google Scholar

[ref2] 2. Metz B, Davidson OR, Bosch PR, Dave R, Meyer LA. Climate change 2007-mitigation of climate change: contribution of working group III to the fourth assessment report of the intergovernmental panel on climate change. United Kingdom and New York: Cambridge University Press; 2007.

[ref3] 3. Peng X, Wu X, Wu F, Wang X, Tong X. Life cycle assessment of winter wheat-summer maize rotation system in Guanzhong region of Shaanxi province. Journal of Agro-Environment Science. 2015; 34: 809–816 (In Chinese with English abstract).
View Article
Google Scholar

[6] View Article

[7] Google Scholar

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View Article
PubMed/NCBI
Google Scholar

[9] View Article

[10] PubMed/NCBI

[11] Google Scholar

[ref5] 5. West T, Marland G. Net carbon flux from agricultural ecosystems: methodology for full carbon cycle analyses. Environ Pollut. 2002; 116: 439–444. pmid:11822723
View Article
PubMed/NCBI
Google Scholar

[13] View Article

[14] PubMed/NCBI

[15] Google Scholar

[ref6] 6. Wang Y, Xu H, Wu X, Zhu Y, Gu B, Niu X, et al. Quantification of net carbon flux from plastic greenhouse vegetable cultivation: a full carbon cycle analysis. Environ Pollut. 2011; 159: 1427–1434. pmid:21277056
View Article
PubMed/NCBI
Google Scholar

[17] View Article

[18] PubMed/NCBI

[19] Google Scholar

[ref7] 7. Zhang MY, Wang FJ, Chen F, Malemela MP, Zhang HL. Comparison of three tillage systems in the wheat-maize system on carbon sequestration in the North China Plain. J Clean Prod. 2013; 54: 101–107.
View Article
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[21] View Article

[22] Google Scholar

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View Article
Google Scholar

[24] View Article

[25] Google Scholar

[ref9] 9. Tian Y. Advance in research on carbon cycling in wetland soils. Journal of Yangtze University (Nat Sci Edit). 2005; 2: 1–4 (In Chinese with English abstract).
View Article
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[27] View Article

[28] Google Scholar

[ref10] 10. Wei H, Ma X, Liu A, Feng L, Huang Y. Review on carbon cycle of forest ecosystem. Chinese Journal of Eco-Agriculture, 2007, 15: 43–46 (In Chinese with English abstract).
View Article
Google Scholar

[30] View Article

[31] Google Scholar

[ref11] 11. Zha T, Zhang Z, Zhu J, Cui L, Zhang J, Chen J, et al. Carbon storage and carbon cycle in forest ecosystem. Science of Soil and Water Conservation. 2008; 6: 112–119 (In Chinese with English abstract).
View Article
Google Scholar

[33] View Article

[34] Google Scholar

[ref12] 12. Li L, Wu F, Zhang H, Chen F. Organic carbon and carbon pool management index in soil under conversation tillage in two- crop paddy field area. Journal of Agro-Environment Science. 2008; 27: 0248–0253 (In Chinese with English abstract).
View Article
Google Scholar

[36] View Article

[37] Google Scholar

[ref13] 13. Luo Y, Wang Z, Gao M, Wei C. Effects of different tillage systems on soil labile organic matter and carbon management index of purple paddy soil. Journal of Soil and Water Conservation. 2007; 21: 55–58 (In Chinese with English abstract).
View Article
Google Scholar

[39] View Article

[40] Google Scholar

[ref14] 14. Rui W, Zhou B, Zhang W. A brief assessment of carbon sequestration effects of conservational farming systems in paddy soils of Yangtze delta plain. Resources and Environment in the Yangtze Basin. 2006; 15: 207–212 (In Chinese with English abstract).
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Google Scholar

[42] View Article

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[ref15] 15. Blanco-Canqui H, Lal R. Soil and crop response to harvesting corn residues for biofuel production. Geoderma. 2007; 141:355–362.
View Article
Google Scholar

[45] View Article

[46] Google Scholar

[ref16] 16. Graham RL, Nelson R, Sheehan J, Perlack RD, Wright LL. Current and potential US corn stover supplies. Agron J. 2007; 99: 1–11.
View Article
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[48] View Article

[49] Google Scholar

[ref17] 17. Wienhold BJ, Varvel GE, Johnson JMF, Wilhelm WW. Carbon source quality and placement effects on soil organic carbon status. Bioenerg Res. 2013; 6: 786–796.
View Article
Google Scholar

[51] View Article

[52] Google Scholar

[ref18] 18. Snyder CS, Bruulsema TW, Jensen TL, Fixen PE. Review of greenhouse gas emissions from crop production systems and fertilizer management effects. Agr Ecosyst Environ. 2009; 133: 247–266.
View Article
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[54] View Article

[55] Google Scholar

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View Article
Google Scholar

[57] View Article

[58] Google Scholar

[ref20] 20. Li X, Toma Y, Yeluripati J, Iwasaki S, Bellingrath-Kimura SD, Jones EO, et al. Estimating agro-ecosystem carbon balance of northern Japan, and comparing the change in carbon stock by soil inventory and net biome productivity. Sci Total Environ. 2016; 554: 293–302. pmid:26956176
View Article
PubMed/NCBI
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[60] View Article

[61] PubMed/NCBI

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[ref21] 21. Zhao R, Liu Y, Ding M, Jiao S. Research on carbon source and sink of farmland ecosystem in Henan province. Journal of Henan Agricultural Sciences. 2010; 40–44 (In Chinese with English abstract).
View Article
Google Scholar

[64] View Article

[65] Google Scholar

[ref22] 22. Chen ZD, Dikgwatlhe SB, Xue JF, Zhang HL, Chen F, Xiao XP. Tillage impacts on net carbon flux in paddy soil of the Southern China. J Clean Prod. 2015; 103: 70–76.
View Article
Google Scholar

[67] View Article

[68] Google Scholar

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View Article
Google Scholar

[70] View Article

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[ref25] 25. Gao CD, Sun XY, Gao J, Luan YN, Hao HD, Li ZJ, et al. A method and apparatus of measurement of carbon dioxide flux from soil surface in situ. Journal of Beijing for University. 2008; 30: 102–105 (In Chinese with English abstract).
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Google Scholar

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View Article
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Google Scholar

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[90] Google Scholar

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View Article
Google Scholar

[92] View Article

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View Article
Google Scholar

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[96] Google Scholar

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View Article
Google Scholar

[98] View Article

[99] Google Scholar

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View Article
Google Scholar

[101] View Article

[102] Google Scholar

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View Article
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View Article
Google Scholar

[108] View Article

[109] Google Scholar

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View Article
Google Scholar

[111] View Article

[112] Google Scholar

[ref38] 38. Roldan A, Salinas-Garcia J, Alguacil M, Caravaca F. Changes in soil enzyme activity, fertility, aggregation and C sequestration mediated by conservation tillage practices and water regime in a maize field. Appl Soil Ecol. 2005; 30: 11–20.
View Article
Google Scholar

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View Article
Google Scholar

[117] View Article

[118] Google Scholar

[ref40] 40. Han X, Xu C, Dungait JAJ, Bol R, Wang XJ, Wu WL, et al. Straw incorporation increases crop yield and soil organic carbon sequestration but varies under different natural conditions and farming practices in China: a system analysis. Biogeosciences. 2018; 15: 1933–1946.
View Article
Google Scholar

[120] View Article

[121] Google Scholar

[ref41] 41. Li J, Yu Q, Sun X, Tong X, Ren C, Wang J, et al. Carbon exchange and its regulating mechanism in farmland ecosystem of North China plain. Science in China Series D: Earth Sciences. 2006; 36: 210–223 (In Chinese with English abstract).
View Article
Google Scholar

[123] View Article

[124] Google Scholar

[ref42] 42. Van den Putte A, Govers G, Diels J, Gillijins K, Demuzere M. Assessing the effect of soil tillage on crop growth: a meta-regression analysis on European crop yields under conservation agriculture. Eur J Agron. 2010; 33: 231–241.
View Article
Google Scholar

[126] View Article

[127] Google Scholar

[ref43] 43. Pietola LM. Root growth dynamics of spring cereals with discontinuation of mouldboard ploughing. Soil Till Res. 2005; 80: 103–114.
View Article
Google Scholar

[129] View Article

[130] Google Scholar

[ref44] 44. Qin R, Stamp P, Richner W. Impact of tillage on root systems of winter wheat. Agron J. 2004; 96: 1523–1530.
View Article
Google Scholar

[132] View Article

[133] Google Scholar

[ref45] 45. Boomsma CR, Santini JB, West TD, Brewer JC, Mcintyre LM, Vyn TJ. Maize grain yield responses to plant height variability resulting from crop rotation and tillage system in a long-term experiment. Soil Till Res. 2010; 106: 227–240.
View Article
Google Scholar

[135] View Article

[136] Google Scholar

Figures

Abstract

Introduction

Materials and methods

Study site

Experimental design

Crop management

Crop yields and root biomass measurements

Soil respiration measurements and estimation of root respiration

Measurements of carbon balance

System dynamics.

Net primary production (NPP).

Net ecosystem productivity (NEP).

Calculation of net ecosystem carbon balance (NCF).

Carbon productivity.

Data analysis

Results

Crop production

Seasonal variations in Rs, Rh, and Ra

Cumulative Rs, and Rh

Net ecosystem carbon balance under conservation tillage treatments

Effect of conservation tillage on carbon productivity

Discussion

Effect of conservation tillage on crop yields

Effect of conservation tillage on Rs and its components

NCF in winter wheat-summer corn ecosystem under different tillage treatments

Carbon productivity

Conclusions

Supporting information

S1 Table. Soil respiration under different tillage treatments.

S2 Table. Soil microbial respiration under different tillage treatments.

S3 Table. Root Respiration under different tillage treatments.

S4 Table. The ratio of root respiration to total soil respiration under different tillage treatments.

S5 Table. The total respiration (Rs), microbial respiration (Rh), wheat and maize grain yields, straw yields and aboveground biomass during the wheat- and maize- growing seasons during the 2013–2015 rotation.

S6 Table. The agro-ecosystem C balance (NCF) and its main components for the annual cycle of wheat-maize rotation in 2013–2015.

S7 Table. The carbon productivity under different treatments.

Acknowledgments

References

Seasonal variations in R_s, R_h, and R_a

Cumulative R_s, and R_h

Effect of conservation tillage on R_s and its components

S5 Table. The total respiration (R_s), microbial respiration (R_h), wheat and maize grain yields, straw yields and aboveground biomass during the wheat- and maize- growing seasons during the 2013–2015 rotation.