The efficiency of long-term straw return to sequester organic carbon in Northeast China’s cropland

WANG Shi-chao, ZHAO Ya-wen, WANG Jin-zhou, ZHU Ping, CUl Xian, HAN Xiao-zeng, XU Minggang, LU Chang-ai

1 Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences/National Engineering Laboratory for Improving Quality of Arable Land, Beijing 100081, P.R.China

2 Jilin Academy of Agricultural Sciences, Changchun 130124, P.R.China

3 Heilongjiang Academy of Agricultural Sciences, Heihe 164300, P.R.China

4 Northeast Institute of Geography and Agro-ecology, Chinese Academy of Sciences, Harbin 150081, P.R.China

1. lntroduction

Soil contains the largest carbon (C) pool of the global terrestrial ecosystem. The total soil organic carbon (SOC)pool is approximately 1 500 Pg C, which is three times that of the atmospheric carbon pool (Songet al.2014). Soil organic matter (SOM) not only plays a vital role in global carbon cycling, but also contributes considerably to improvements in soil quality, crop production, and terrestrial ecosystem health(Wanget al.2008; Luet al.2009). However, increasing SOM has become a major global problem (Fanet al.2013;Keesstraet al.2016). SOC dynamics are strongly influenced by agricultural management practices, such as fertilization,crop residue return, and tillage (Luet al.2009; Luoet al.2010; Louet al.2011; Ouyanget al.2013; Douet al.2016).

It has recently been proposed that straw input can lead to SOC accumulation and improvements in soil structure(Luet al.2009; Liu Cet al.2014; Lu 2015; Wanget al.2015a). For example, Liu Cet al.(2014) found that straw return increased SOC by 12.8%, with a 42.0% increase in the active C fraction after 22 years. Previous research has also demonstrated that continuous straw return boosts SOC accumulation in early years, but that these effects decrease after a decade (Liu Cet al.2014; Wanget al.2015a).Specifically, Liu Cet al.(2014) reported that straw return enhanced SOC sequestration over the first three years, but that these effects were minimal in the experimental site after 15 years. However, opposing results have also been reported(Pittelkowet al.2015), which may be attributed to different environmental conditions, land management approaches,and soil types. Additional factors, such as straw-C input rate,number of cultivation years, initial SOC levels, and methods for straw disposal, are also critical in explaining why certain soil carbon pools react differently to straw return. Therefore,evaluating the changes in SOC under long-term cultivation would be beneficial for determining how land management practices influence soil and global carbon cycling.

The black soil of Northeast China is one of the main crop production areas in the country, and provides a third of the country’s grain commodity. The black soil area covers 1.1×108ha, including arable land (2.4×107ha in Heilongjiang Province, 1.1×107ha in Jilin Province, 0.6×107ha in Liaoning Province, and 0.6×107ha in Inner Mongolia) that is characterized by black soil (Mollisols) which is rich in organic matter (Liang A Zet al.2011; Zhanget al.2013). The large amount of agricultural straw that is produced in this region every year is not fully utilized. Instead of being returned to the soil, the straw is usually used as livestock feed or to provide energy for heating (Douet al.2016). Hence, soil degradation,a result of the loss of SOC and nutrients, is a considerable challenge for sustainable cultivation in this region (Liuet al.2010), as intensive cultivation has reduced SOC by 50%over about 60 years of tillage (Yuet al.2006), with a loss of 0.5% each year (Liuet al.2010). Rather than being used to provide energy, the organic material of straw could be used to promote the SOC stock. Many studies have investigated how straw return affects soil fertility (Yanet al.2007; Sun Bet al.2013). However, the approach of using straw return to reverse SOC reduction faces some problems. For example,the extent we depend on straw return to improve SOC is unknown. Also, the short- and long-term effects of using straw return to improve SOC in the black soil region in China have not been assessed.

Long-term experiments can provide reliable data for assessing and predicting the influence of straw return on SOC sequestration and soil fertility. There is currently minimal knowledge about the influence of different environmental conditions on the long-term effects of straw return (Dinget al.2014; Songet al.2014), and limited research has focused on black soils. In this study, we used long-term field experiments in Northeast China to study the influence of straw return on SOC. The objectives of this study were: 1) to analyze the SOC dynamics of longterm straw return; 2) to examine the relative contribution of straw return to SOC sequestration; and 3) to evaluate the sustainability of straw return for improving SOC and straw-C sequestration efficiency. We hypothesized that straw return could efficiently increase the SOC stock of the black soil region in Northeast China. However, we also expected clear differences among different regions in Northeast China.

2. Materials and methods

2.1. Site description

This study was conducted at three different sites in the Northeast China: Heihe (50°15′N, 127o27′E); Hailun(47°27′N, 126°55′E); and Gongzhuling (43°30′N, 124°48′E)(Fig. 1, Table 1). Three long-term straw return experiments with different fertilization treatments were initiated in 1979,1990, and 1990 for Heihe, Hailun, and Gongzhuling,respectively. Prior to the experiment, the field was under crop production for at least 60 years at Gongzhuling site, 110 years at Hailun site, or 150 years at Heihe site. The soil at all three sites originated from quaternary loess-like sediments.The study sites are characterized by a temperate sub-humid climate. The average annual precipitation is 528 mm, with most of the rainfall occurring from July to September. The frost-free period ranges from 105 days at the Heihe site to 140 days at the Gongzhuling site. Crop rotations include wheat (Triticum aestivumL.)/soybean (Glycine max(L.)Merrill.) at the Heihe site, wheat/maize (Zea maysL.)/soybean at the Hailun site, and a mono-cropping system with maize at the Gongzhuling site. Each year, spring wheat seeds are sown in strips in early April and harvested in mid-August. Soybean seeds are sown in holes in early May and harvested in late September. Maize seeds are sown in holes in early May and harvested in early October. The average annual temperature ranges from –0.5°C at Heihe to 4.5°C at Gongzhuling. Soil textures are clay loam, heavy loam, and loamy clay soil at Heihe, Hailun and Gongzhuling,respectively. The fine mineral contents were 33.4% (Heihe),34.5% (Hailun), and 31.1% (Gongzhuling). All sites had black soils, with initial SOC contents between 13.5–31.3 g kg–1, initial total nitrogen (TN) contents between 1.24–2.23 g kg–1, and soil pH values between 6.1–7.6 (Table 1).

Fig. 1 Locations of the study sites in Northeast China.

2.2. Experimental design and sampling

Three treatments were implemented at each site: (1) no fertilization (CK); (2) inorganic fertilization N, P and K (NPK);and (3) NPK plus straw return (NPKS). The plots were arranged in a randomized block design, and each treatment was replicated three times. The plot sizes were 212 m2at Heihe, 224 m2at Hailun, and 400 m2at Gongzhuling.The chemical fertilizer and straw input rates are in Table 2.The applied chemical fertilizers included urea, calcium superphosphate, and potassium sulfate, with the exception that no potassium fertilizer was used at the Heihe site. The application rates of inorganic fertilizer for each growing season ranged from 37.5 to 245.3 kg N ha–1, 16.4 to 62 kg P ha–1, and 0 to 68.5 kg K ha–1(Table 2).

The aboveground straw was removed from the CK and NPK treatments after each harvest (around October 10th).In the NPKS treatment, which included straw return, the aboveground straw was removed after each harvest and re-incorporated into the soil. At the Heihe and Hailun sites,wheat/maize/soybean organic material was returned by deep plowing, while at the Gongzhuling site maize straw was used for mulching material. The average annual rates of straw return were 3.0, 4.5, and 7.5 Mg ha–1(dry weight) at the Heihe, Hailun, and Gongzhuling sites, respectively. Straw was applied after cutting it into pieces of 5–15 cm length after harvesting. The amounts of C (straw-C and root-C) added to soil were 0.44–1.97 Mg ha–1at Heihe, 1.59–3.35 Mg ha–1at Hailun, and 4.4–5.9 Mg ha–1at Gongzhuling (Fig. 2).

2.3. Soil analysis

Surface soil samples (0–20 cm depth) were collected from each treatment plot after each harvest (around October 10th),after which they were air-dried and sieved through either a 1-mm mesh for pH and available P analyses or a 0.15-mm mesh for organic matter, total N, and total P analyses. Soil organic matter was determined by quantifying the oxidizable soil carbon in a heated dilution of K2Cr2O7subjected to a volumetric oxidation method (Nelson and Sommers 1996).Soil available P was measured using Olsen’s method (Van Reeuwijk 2002), while total nitrogen was measured using the micro-Kjeldahl digestion, distillation, and titration method(Bremner and Mulvaney 1982). All results were normalized to a dry mass basis. The soil bulk density (g cm–3) was determined by the core sampler method described by Piper(1966). We also collected soil samples from the 0–20 cm layer at the beginning of this study, in 1979, 1990, and 1990 for the Heihe, Hailun, and Gongzhuling experiments,respectively. These samples were analyzed to determine the basic physical and chemical properties.

2.4. Calculation of C sequestration rate and C sequestration efficiency

SOC stock (Mg ha–1) was calculated using the following equation:

?

Where, C is SOC content (g kg–1), BD is soil bulk density (g cm–3), anddis soil depth(m). In this study, we only calculated SOC stocks for the top layer of soil (0–20 cm).

The annual SOC changes (DSOC, Mg C ha–1yr–1) were calculated using the following equation:

全區(qū)櫻桃產(chǎn)業(yè)發(fā)展的專業(yè)化、專職化的人才團(tuán)隊(duì)十分匱乏，嚴(yán)重制約烏當(dāng)櫻桃產(chǎn)業(yè)的發(fā)展壯大。在全區(qū)各政府部門中，農(nóng)業(yè)局負(fù)責(zé)統(tǒng)籌全區(qū)櫻桃全產(chǎn)鏈發(fā)展，專業(yè)、專職農(nóng)業(yè)技術(shù)推廣人才團(tuán)隊(duì)成員約5名，在數(shù)量和專業(yè)結(jié)構(gòu)上都遠(yuǎn)遠(yuǎn)不能滿足發(fā)展好3.6萬余畝櫻桃種植基地及中下游產(chǎn)業(yè)的需求。烏當(dāng)櫻桃產(chǎn)業(yè)正面臨著很多技術(shù)瓶頸，特別是在病蟲害防治方面，如下壩鎮(zhèn)出現(xiàn)櫻桃根癌病、流膠病、病毒病及果實(shí)褐腐病等，造成櫻桃樹勢衰退和減產(chǎn)，嚴(yán)重時(shí)成片死亡，而現(xiàn)有人員的技術(shù)能力難以有效解決這一問題。

Where, SOCTand SOC0represent the SOC stock at the end and beginning of a year, respectively, andtis the duration of the experiment in years.

The relative changes of SOC stock are represented as SOCNPKS(or SOCNPK) minus SOCCK, and the relative contribution (RC) of straw return to the soil was calculated using the following equation:

The carbon sequestration rate (CSR, Mg C ha–1yr–1) was calculated using the following equation:

Where, DSOCTand DSOC0represent the respective annual change in the SOC stock of a particular treatment (CK, NPK, or NPKS). DSOCTdescribes the DSOC aftertyears of experiments whereas DSOC0represents the DSOC at the beginning of a year.

The carbon sequestration efficiency (CSE) for each paired-trial was calculated as follows:

Where, SOCNPKis the SOC stock of the NPK treatment, SOCNPKSis the SOC stock of the NPKS treatment, and TSC is the total straw C input (Mg C ha–1) over the duration of the experiment.

2.5. Statistical analyses

Analysis of variance (ANOVA) and multiple comparisons were performed in SAS (ver.8.1, SAS Institute Inc., Cary, NC, USA) to identify between treatment differences in SOC sequestration, carbon sequestration rate, and/or straw-C sequestration efficiency of the topsoil. If the ANOVA results were significant, further comparisons using leastsquares differences were conducted in SAS. A correlation analysis was carried out in SPSS (ver. 17.0, IBM, Armonk, NY, USA) to examine how different treatments affected various factors, e.g., soil C/N ratio, initial SOC (ISOC), available phosphorus (AP),pH, the cumulative straw-C input (SC), SC/ISOC ratio, the SC/SMBC (soil microbial biomass carbon) ratio, and cultivation year, and to construct the soil organic carbon sequestration property matrix.

3. Results

3.1. Changes in the SOC stock under straw return

The SOC stock of CK treatment plots at the three sites decreased throughout the entire long-term experiment period (Fig. 3). In this study, the SOC stocks of NPKS treatment plots varied between 31.7 and 58.3 Mg ha–1. When compared to the CK treatment plots, the SOC stock of NPKS treatment plots increased, on average, by 11.7% (Heihe site), 3.0% (Hailun site), and 7.0% (Gongzhuling site), respectively. The SOC stocks of NPKS plots were 4.0 and 5.7% higher than those of the NPK plots at the Hailun and Heihe sites, respectively. However, the straw return, when compared with the NPK treatment, did not significantly affect the SOC stock at the Gongzhuling site. These results demonstrate that long-term NPKS treatment maintained, and sometimes increased SOC, while long-term NPK treatment maintained (or slightly decreased) SOC compared with the original SOC stock. On the other hand, a lack of fertilization (CK treatment) tended to decrease SOC stocks.

The DSOC values for the fertilization treatments were≥0, while the CK treatment showed negative DSOC values(Fig. 4). The DSOC values ranged from –1.65 to 6.60 Mg C ha–1, with an average of –0.05 Mg C ha–1. The DSOC values for the three treatments at the Heihe and Hailun sites were ranked as NPKS＞NPK＞CK, while an equilibrium state between the three treatments was observed at Gongzhuling.At the Heihe site, the DSOC values for the NPKS and NPK treatments were 102.0 and 41.8% higher, respectively, than the DSOC value for the CK treatment. Both of the fertilization treatments significantly increased DSOC compared to the CK treatment (P＜0.05). In addition, both fertilization treatments at the Hailun site, when compared to the CK treatment,significantly increased DSOC (P＜0.05). The initial DSOC value for the NPK treatment was higher than that for the NPKS treatment. However, the NPK treatment showed a five-fold decrease in DSOC over 5 years at the Gongzhuling site. In contrast, the DSOC value for the NPKS treatment remained stable over the course of the experiment.

Table 2 Fertilizer inputs for each treatment

Fig. 2 Changes in straw-C inputs and reclamation years at three experimental sites in Northeast China.

3.2. Soil carbon sequestration rate in response to straw return

As expected, straw return primarily enhanced CSR at the Heihe site, relative to the Gongzhuling and Hailun sites(Fig. 5). The CSR value decreased over time at the Heihe site. In addition, straw return had a significant effect on CSR at the Heihe site (F=6.28,P=0.02); CSR ranged from 0.24 to 1.50 Mg C ha–1yr–1, compared with 0.15–0.50 Mg C ha–1yr–1for NPK. The Hailun site also showed a decreasing trend in CSR over time. However, at this site, both fertilization regimes yielded similar CSR values ((0.32±0.23) Mg C ha–1yr–1) (Fig. 5-C). At the Gongzhuling site, the NPK treatment resulted in a significantly higher CSR value ((0.84±1.08)Mg C ha–1yr–1,F=7.8,P＜0.05) than the NPKS treatment((0.11±0.24) Mg C ha–1yr–1) (Fig. 5-D). Regression analysis results showed that the CSR for the NPKS treatments equated to cultivation times of 17, 11, and 8 years at the Heihe, Hailun and Gongzhuling sites, respectively. This result indicates that the CSR of the black soils in northeastern China is close to a stable equilibrium state.

Fig. 3 Soil organic carbon (SOC) stocks for different fertilization treatments, as well as initial condition, at the three sites over the whole experimental period. CK, no fertilizer application;NPK, inorganic fertilization N, P and K; NPKS, NPK plus straw return. The results are presented as mean values with standard deviations (Heihe, n=11; Hailun, n=16; Gongzhuling,n=19). Mean values with different letters represent statistically significant differences (P＜0.05).

3.3. Straw-C sequestration efficiency

The CSE calculated for the Heihe site (46.8%) was significantly higher than the calculated values for the Hailun(13.3%) and Gongzhuling (–1.6%) sites (P＜0.01) (Fig. 6-A).The CSE ranged from –22.1 to 97.1% over 1–33 years. On average, (15.3±27.8)% of the straw-C input was converted to SOC. In addition, the CSE at the Heihe and Hailun sites decreased exponentially as cultivation increased, appearing to attain equilibrium at the end of the experiments (Fig. 6-B).The CSE at the Gongzhuling site, on the other hand,consistently remained in a steady state. The regression analyses for the Hailun and Heihe sites show that after a continual decline for 13 and 17 years respectively, the CSE remained stable at 1.0–6.0% and 15.6–38.8%, respectively.At the Gongzhuling site, CSE remained at approximately 0% over 18 years.

Fig. 4 Annual soil organic carbon (DSOC) changes for different fertilization treatments. CK, no fertilizer application; NPK,inorganic fertilization N, P and K; NPKS, NPK plus straw return;HH, Heihe site; HL, Hailun site; GZL, Gongzhuling site.

4. Discussion

4.1. lmpact of straw return on the SOC stock

Fig. 5 Soil carbon sequestration rate (CSR) under different fertilization treatments. A, all three experiment sites. Box-and-whisker diagrams show the median, 5th, 25th, 75th and 95th percentiles for CSR. The solid lines represent median values and the short dashed lines represent mean values. B, Heihe site. C, Hailun site. D, Gongzhuling site.

Fig. 6 Straw-C sequestration efficiency (CSE) in the treatment with straw. A, the range of CSE at different sites. Box-and-whisker diagrams show the median, 5th, 25th, 75th and 95th percentiles for CSE. The solid lines represent median values and the short dashed lines represent mean values. B, variation with cultivation year of CSE at three sites. All, three experiment sites; HH, Heihe site; HL, Hailun site; GZL, Gongzhuling site.

Our experiments showed that the application of fertilizer(NPK and NPKS treatments), when compared to the CK treatment, led to higher SOC stocks in absolute terms(Fig. 7) Straw return at the high SOC density site (Heihe)was an exception, as SOC rapidly increased during the earlier stages of the experiment only to decline in the later stages. Although the SOC stocks at the Hailun site only changed slightly and gradually increased at the Gongzhuling site (Fig. 8), straw-C input has been shown to be more effective than NPK and CK treatments in driving microbial activity (Majumder and Kuzyakov 2010; Bastidaet al.2013). This difference may stem from the shorter history of reclamation, lower straw-C input, and higher SOC density at the Heihe site relative to the other two sites(Figs. 2 and 7) (Lehtinenet al.2014; Lu 2015; Poeplauet al.2015). At Heihe, the SOC stock could continue to decrease for 50 years under natural conditions (Yuet al.2006). Furthermore, the SOC increase could probably be related to an increase of the C input (Duiker and Lal 1999;Panet al.2003; Wiesmeieret al.2015); in Hailun and Heihe,stagnating C inputs over time led to a constant SOC level(Hailun) or even a decline of SOC (Heihe). This is in line with our results that stagnating C inputs in agricultural soils led to SOC decreases over longer periods. On the other hand, temperature increases and low precipitation caused by climate change will result in intensified decomposition of native SOC (Wiesmeieret al.2016). The effects of straw return on improving SOC at the Heihe and Hailun sites were not evident, probably due to the increasing temperature over the experimental period (Fig. 9).

It is worth emphasizing that straw return increased SOC stocks in this region by either 6% (when compared to fertilized plots) or 12% (when compared to unfertilized plots)(Fig. 7-B). Interestingly, straw return increased SOC stocks by 5.7% in Northeast China, a result that was lower than previously reported values for upland soils in China (9.9%),Europe (7%), and North America (10.8%) (Powlsonet al.2011; Lehtinenet al.2014; Wanget al.2015a). Overall,straw return had a relatively weak effect on the SOC stocks in Northeast China.

Fig. 7 Average relative changes in soil organic carbon (SOC) stocks for two different fertilization management practices over the experimental period. A, all three experiment sites. B, Heihe. C, Hailun. D, Gongzhuling. NPK, inorganic fertilization N, P and K;NPKS, NPK plus straw return; ?NPKS, NPKS minus NPK. Different letters indicate statistically significant differences at P＜0.05.Box-and-whisker diagrams show the median, 5th, 25th, 75th and 95th percentiles for relative change in SOC stocks. The solid lines represent median values and the short dashed lines represent mean values.

Fig. 8 Soil organic carbon (SOC) stock dynamics under straw return in the black soils at the three long-term experimental sites.A, Heihe. B, Hailun. C, Gongzhuling.

4.2. Factors regulating straw-C sequestration efficiency

Our findings show that the average for all sites straw-C sequestration efficiency was 15.3% over the experimental duration of 33 years (Fig. 6). This CSE value is in agreement with a previously reported average CSE (15.9%) value for the upland area of northern China (Zhanget al.2010), but lower than the CSE (30%) value reported for upland India by Srinivasaraoet al.(2012). In contrast, the CSE value for wheat and maize crops has been reported to be only 7.6%in the USA (Konget al.2005). The differences in straw-C sequestration efficiency are mainly related to soil fertility,climate, and cultivation (Luoet al.2010; Ouyanget al.2013).The CSE value for Northeast China was correlated with soil C/N, available P, pH, reclamation year, SC/SOC, SC/SMBC, mean annual temperature, and annual precipitation(Tables 3 and 4).

Microbial communities differ based on climatic conditions,with the climate in Northeast China potentially affecting the structure of the microbial community, which can influence the rate of straw decomposition (Sun Bet al.2013). Straw addition clearly improved the carbon content and CSE in the northern region (Heihe), and this result was probably due to this site having a shorter cultivation history (Fig. 2),higher SOC content, and lower inputs of C relative to the other studied sites.

Our data indicated that the CSE value was significantly higher at the Heihe site (46.8%) than at the Hailun (13.3%)or Gongzhuling (?1.6%) sites (P＜0.01). Soil carbon sequestration may be restricted by certain aspects of initial soil quality associated with low inputs of C, such as initial SOC content, pH, or the total carbon and nitrogen ratios(Wanget al.2015b). Despite corroborating Wangetal.(2015b) that reported a significant correlation between changes in SOC and the amount of C input, a comparison of data from different experimental sites suggests that the continuous maize system in Gongzhuling is less efficient at sequestering C from added C inputs than systems that include wheat and/or soybean. Gongzhuling had the lowest CSE value of all three sites, a finding that is likely a consequence of a long history of reclamation (Fig. 2), and the high input of C (straw mulch at Gongzhuling site) coupled with low SOC density. However, straw return can accelerate the turnover rate of native soil organic carbon and introduce large amounts of labile C, such as microbial biomass carbon(Jenkinsonet al.1985; Kuzyakovet al.2000; Blagodatskaya and Kuzyakov 2008) and water-soluble organic carbon(WSOC) (Guenetet al.2012; Zhanget al.2012; Kirkbyet al.2014; Anet al.2015). Previous research in Gongzhuling has shown that NPKS treatment, when compared to CK treatment, increased WSOC content by 40.1% (Zhenget al.2006; Liang Yet al.2011). Furthermore, Pang and Huang(2006) showed that straw mulch can significantly improve SOC content at the soil surface, but can also impede straw decomposition and the accumulation of SOC.

Fig. 9 Mean annual temperature and mean annual precipitation at three experimental sites in our studied regions.

4.3. How to improve long-term straw-C sequestration with straw return in black soil

Our results showed that the equilibrium value of CSR equated to cultivation times of 17, 11, and 8 years for the NPKS treatment at the Heihe, Hailun, and Gongzhuling sites, respectively. These relatively short time could cause apparent new equilibrium states, which could be related to the C saturation status of the investigated soils (Wiesmeieret al.2014). Significantly positive linear correlations between SOC stocks for the NPKM treatment at the three sites and experiment duration (Fig. 10) indicated that the black soil was not saturated in C sequestration. Moreover,a significant increase in SOC stock at Hailun was also observed with higher organic manure input after 10 years(Dinget al.2014). The degree of increase in SOC was lower in the NPKS treatment compared to the NPKM treatment in our study area and did not reach the saturation level; this result was explained by different net carbon production rates(accumulation of straw derived SOC and decomposition of old C) with straw return. After the SOC equilibrium, CSE cannot improve further. Interestingly, when the relationships among CSE and soil properties, climatic factors, amount of straw returned, and the cropping system are investigated(Tables 3 and 4), CSE has the strongest correlation with SMBC and soil availability. However, the most effective way to translate straw return into soil C sequestration still remains unclear.

Previous studies have found that the major limiting factors of increasing SOC under straw return conditions include tillage, fertilization practices, and cropping system(Liang A Zet al.2011; Mathieuet al.2015). For example,the application of organic manure may stimulate the decomposition of added straw-C due to increased microbial biomass (Kuzyakow 2010). In addition, a laboratory experiment (25°C incubation over 95 days) by Aitaet al.(2012), in which both manure and straw were added to thesystem, found that the mineralization of straw-C accelerated by 24% over 95 days compared to a treatment in which only straw was added. This resulted in higher SOC stock and positive effects for soil fertility. Similarly, Zhanget al.(2017)observed an increase in SOC variables after coupling the long-term application of nitrogen fertilizer with straw return.These findings suggest that the optimized fertilization with straw return leads to higher SOC content.

Table 3 Correlation coefficients between straw-C sequestration efficiency (CSE) and soil factors in Northeast China1)

Table 4 Correlation coefficients between straw-C sequestration efficiency (CSE), cultivation years and climatic factors1)

Fig. 10 Soil organic carbon (SOC) stock dynamics under the N, P, K fertilizer in combination with manure (NPKM) treatment in the black soils at the three long-term experimental sites.

It is also important to consider cultivation modes that may affect straw-C sequestration. The combination of no-tillage and straw mulching resulted in a loss of straw-C,as the straw was left on the soil surface (Luet al.2009).According to a meta-analysis, a no-tillage approach had a 9% advantage over chopped straw in straw return situations,which increased to 12% with plough tillage, and to 16%with rotary tillage (Lu 2015). These increases occurred because straw buried in deep soil layers is favorable for straw-C sequestration (Caiet al.2015). Reasonable crop rotation (e.g., rotated soybean) can also improve the decomposition and transformation of straw-C returned to soil, as this could provide the nitrogen necessary for microorganism activity (Liu Set al.2014).

Overall, reasonable agronomic management practices,such as ploughing and rotary tillage, crop rotation, and optimized fertilization, have a significant effect on straw-C sequestration, but their long-term effects should be further investigated in future studies.

5. Conclusion

The results from our study indicate that the SOC stocks of black soil in the southern region of Northeast China were approximately at a stable equilibrium state, whereas the SOC stocks continued to decrease in the northern region. Furthermore, the rate of change of SOC stocks was directly related to the initial SOC density. Straw return and fertilization enhanced soil carbon sequestration in the northern and high latitude regions that were characterized by high SOC density, but the input of straw maintained the SOC stocks at a stable state in the southern region.However, a longer period of straw return would result in a very low or even negative CSE after which SOC would not increase further. Additional variables, such as reasonable agronomic management practices, i.e., tillage, rotation, and optimized fertilization, should be considered if the objective is a long-term improvement in the straw-C sequestration in black soil.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (973 Program, 2013CB127404)and the Collaborative Innovation Action of Scientific and Technological Innovation Project of the Chinese Academy of Agricultural Sciences. We are grateful to the staff that worked on the project, and contributed to the field management of the long-term experiments.

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