Soil Carbon Sequestration And Crop Growth Affected By Straw Return Modes Under A Wheat-Maize Rotation System In The Guanzhong Plain,Northwest China

Increasing the amount of soil organic carbon（SOC）sequestration can effectivelyimprove soil fertility and alleviate global climate change.Although SOC storage in farmland accounts for only 10%of the global soil carbon pool,due to the strong impact of human agricultural production activities,it is of great significance to pay attention to SOC fixation in farmland.In addition,in arid and semi-arid areas,soil inorganic carbon（SIC）is an important form of soil carbon fixation,so there may be a close transformation relationship between SOC and SIC.At present,the research on the driving mechanism of SOC fixation and SOC-SIC transformation in farmland has become an important branch and hot issue of C cycle research.Although it has been known that the net SOC fixation in farmland depends on the break even between the newly formed SOC from organic materials and the original SOC mineralization loss,and it is also affected by the priming effect dominated by microorganisms and agronomic measures such as farmland nutrient management.However,the dominant process and influencing factors of SOC turnover and fixation in specific ecological regions and cropping systems still need to be further studied.The wheat maize rotation system is the most important cropping system in the Huang Huai Hai Plain and Fen Wei plain of northern China.In this system,the application of organic fertilizer has been negligible.The straw returned to the field has become the most important source of organic carbon input and a part of mineral nutrients.However,straw can be used for many other purposes besides returning to the field.Therefore,it is of great scientific and productive significance to seek a way of returning straw which can not only guarantee the improvement of soil fertility,but also meet other uses of straw.Firstly,this study focused on the effects and mechanisms of straw returning methods on SOC fixation,such as whether single crop straw returning（only wheat straw returning or only maize straw returning）can effectively maintain the SOC content without reducing or increase the SOC content under the premise of maintaining high and stable crop yield?Secondly,will the increase of SOC sequestration lead to the simultaneous increase of SIC retention?Therefore,this study was based on the long-term experiment of different straw returning modes which started in 2008.In this experiment,maize straw and wheat straw were respectively set up with three returning ways to form nine straw returning modes.By measuring soil organic carbon,enzyme activity,crop yield and other indicators,the research results were expected to reveal the response and mechanism of soil carbon sequestration,soil nutrient status,enzyme activity and crop yield to straw return modes,and provide important scientific basis for constructing the best returning model to achieve the goal of high carbon sequestration,stable crop yield and environment-friendly..The main research results are as follows:1.The study included five straw return modes（2008-2019）:（1）straw removal（WR-MR）;（2）return of both crushed maize and wheat straw（WC-MC）;（3）return of chopped wheat straw only（WC-MR）;（4）only high stubble wheat straw mulching（WS-MN）;（5）leaving high wheat stubble combined with returning crushed maize straw（WS-MC）.The distribution characteristics and relationship of soil organic and inorganic carbon among 0 to40 cm soil layer were compared.The results showed that the proportion of SOC storage was increased by 28.1%and 22.2%in WS-MC and WC-MC treatments,respectively,compared to WR-MR,whereas the proportion of SIC storage was increased by 20.4%and 17.3%.Compared with the initial value,the SOC sequestration among 5 treatments ranged from-0.84 Mg·ha^-1 to 6.55 Mg·ha^-1 and the SIC sequestration ranged from-0.26 Mg·ha^-1 to 8.61Mg·ha^-1.The sequestration efficiency of total residue organic carbon was 7.5%.The minimum carbon input to maintain the basic organic carbon balance was 4.65 Mg·ha^-1·a^-1.The content of labile carbon fractions in the WS-MC and WC-MC treatments significantly increased over the control in the 0-20 cm layer.The result of principal component analysis showed that the changes of soil carbon pool were mainly affected by amendments.Ca²⁺and Mg²⁺derived from irrigation water and plant residues can coprecipitate with CO₂ from SOC mineralization to form Ca CO₃,which may be the main mechanism for the increase of SIC in farmland without a large amount of exogenous Ca input.In conclusion,our results indicate that returning of crushed maize combined with high stubble wheat left in the field is sufficient to maintain soil carbon storage and may be the optimal straw-returning strategy.2.The study included four straw return modes（2008-2020）:（1）no return （WR-MR）;（2）leaving high wheat stubble combined with returning crushed maize straw（WS-MC）;（3）only high stubble wheat straw mulching（WS-MR）;（4）return of maize straw only（WR-MC）.The differences of SOC content,sequestration,carbon pool management index（CMI）and soil enzyme activity were compared.The results showed that the SOC and SCS was increased by 34.54%and 40.90%,respectively,compared to WR-MR,whereas in WS-MR and WR-MC,the SOC also increased by 17.70%and 20.37%,and SCS increased by 15.85%and 19.34%,respectively.Compared with the initial value,the SOC sequestration among 4 treatments ranged from-0.70 Mg·ha^-1 to 10.15 Mg·ha^-1.There was a significant positive correlation between soil organic carbon sequestration and exogenous carbon input,indicating that high amount of straw returning was more conducive to carbon sequestration.The minimum carbon input to maintain the basic organic carbon balance was 2.91Mg·ha^-1·a^-1.After straw return,the content of soil labile organic C（LOC）,dissolved organic C（DOC）,microbial biomass C（MBC）and nonlabile organic C（NLOC）extremely increased.Compared to WR-MR,the soil C pool lability（L）,soil C pool lability index（LI）,C pool index（CPI）and C management index（CMI）in each treatment increased in diverse degrees,and the WS-MC was the highest.The four soil enzyme activity was significantly increased in different straw return mode.Furthermore,the result of corrlation analysis showed that that crop yield had a significant correlation with C input,soil C fractions,C pool management index and enzyme activity.It is thus clear that straw return can improve the quantity and quality of SOC,especially the mode of high amount of straw returning.3.The study included six straw return modes（2008-2020）:（1）no return（WR-MR）;（2）return of both crushed maize and wheat straw（WC-MC）;（3）WC-MC with subsoiling to～40 cm depth after maize harvest（WC-MD）;（4）return of maize straw only（WR-MC）;（5）WR-MC with subsoiling to～40 cm depth after maize harvest;（6）return of wheat straw only（WC-MR）.The changes of SOC,soil total nitrogen（TN）,available nutrients and enzyme activities were compared.The results showed that after 12 years of straw return,the effects of different straw return modes on soil fertility were different,and the effect of double season straw return treatment was the highest.The SOC,LOC,available nitrogen,available phosphorus,available potassium,microbial biomass,nutrient cycle related enzyme activities and crop yield were apparently increased in the return of wheat and maize straw mode,compared to WR-MR,and the single straw return mode also has a certain improvement effect.The result of principal component analysis showed that the amount of straw input was the main factor to determine the soil nutrients and enzyme activities under different straw return modes.Under the condition of little difference in the improvement of soil fertility and crop yield between WC-MC and WC-MD,the WC-MD can break the plough bottom and promote the soil environment,which is helpful to the improvement of crop yield.Therefore,WC-MD mode is more suitable for agricultural development in this region.In conclusion,only wheat or maize straw returning mode can maintain soil carbon storage,but also improve soil fertility and microbial characteristics to a certain extent.High amount of straw returning is most conducive to soil carbon fixation,soil fertility and crop yield.Therefore,at least one season of crop straw should be returned to the field in the wheatmaize rotation area of Guanzhong Plain,and the remaining straw can be removed for other purposes.Soil carbon balance point and straw carbon mineralization are significantly correlated with C input.Increasing straw input will improve the balance point and straw-C mineralization,and continue to increase soil carbon sequestration potential,but it is too early to reach the carbon balance point.Compared with WC-MC,WS-MC and WC-MD modes are more suitable for sustainable development of agriculture.Among them,the management mode of wheat straw high stubble combined with chopped maize straw returning to field is easy to be popularized and applied in the North wheat-maize rotation area.It can improve the carbon sequestration,unstable organic carbon content,carbon bank management index,improve soil quality,reduce environmental pollution and reduce the operating cost of agricultural machinery,which is of great significance to promote the sustainable development of dry land agriculture in the north.