Potential And Mechanisms Of Dryland Leguminous Green Manure For Improving Soil Carbon/Nitrogen Stocks And Reducing Environmental Costs

The huge demand for food and the scarcity of cropland of China are the main reasons for the abuse of synthetic nitrogen?N?and the neglect of cropland protection under modern intensified cropping systems.This way of field management has caused environmental problems such as the decline of soil organic carbon stocks?SOCS?,the emission of more greenhouse gases?GHG?and the release of extra reactive nitrogen?Nr?,which are not beneficial for sustainable development of agriculture.How to restore soil fertility while substantially decrease the environmental costs of crop production without sacrificing the crop yield is a topic that needs urgent research.The Loess Plateau is an important area of China for wheat production,the proper use of the precipitation and heat resources during summer fallow after wheat harvest can be helpful to mitigate the issues regarding soil degradation and environment due to the application of intensified cropping system in this area.Leguminous green manure?LGM?has the ability to fix both carbon?C?and N at the same time biologically.It can replace part of the synthetic N and improve SOCS,thus LGM is the first choice to optimise the cropping system.Based on the long-term field experiment and the integration of measured data and multiple models,the target of this research was to explore the potential and mechanisms of LGM for improving soil carbon/nitrogen stocks and reducing environmental costs.The field experiment was arranged in a split-plot design,the main treatments was different field management practices during summer fallow,which included summer fallow–winter wheat?FW??control?,Huai bean–winter wheat?HW?,soybean–winter wheat?SW?and mung bean–winter wheat?MW?;the sub-treatments included four rates of synthetic N applied at wheat seeding,namely,0?N₀?,108(N₁₀₈),135(N₁₃₅)and 162(N₁₆₂)kg ha^-1.The main research results were listed below:?1?Simulating the dynamics and future sequestration potential of both SOCS and total nitrogen stocks?TNS?via the RothC model and the C:N ratio of the measured SOCS and TNS.Compared with the FW treatment,the LGM treatments increased the amount of C and N returned to the field by 67%-91%and 74%-125%,respectively;the corresponding SOCS and TNS were also increased by 15%-23%and 12%-22%,respectively,after 8 years?P<0.05?.Coupling the RothC model and the correlations between SOCS and TNS of the 3 LGM treatments generated acceptable simulations for the dynamics of SOCS and TNS.The projected SOCS for the LGM and FW treatments at the new equilibrium would be 37.67-47.29Mg ha^-11 and 13.97-17.58 Mg ha^-1,respectively while the corresponding TNS for LGM treatments would be 5.42-6.79 Mg ha^-1.The projection also indicated that under the LGM treatments,the SOCS and TNS at the new equilibrium would be 107%and 158%higher,respectively,than the baseline level prior to the initiation of the field experiment.Growing LGM during summer fallow period still has great potential to improve the SOCS and TNS,which is beneficial for improving soil fertility continuously.?2?Physical fractionation was used to separate different organic carbon?OC?fractions,among them,the intra-microaggregate fine particulate OC?iPOC?and OC associated with soil mineral particles?MOC?are considered as protected OC.Quantifying the effects of LGM on the content of protected OC makes it possible to determine whether this management practice can increase the stability of SOC.The results indicate that the HW treatment significantly increased the mean weight diameter at the 0-10 cm soil when compared with the FW.Treatments with LGM significantly increase the content of SOC by 0.93-1.18 g kg^-1 and 0.33-1.04 g kg^-1,respectively,at the 0-10 cm and 10-20 cm depths compared with the FW,69%-86%of which was attributed to the increase of protected OC.Moreover,only the increase of the protected OC was positively and linearly correlated with the increased SOC?P<0.05?,which further suggested that growing LGM during summer fallow mainly increased the SOC by promoting the formation of more protected OC.LGM can not only improve soil structure,but also increase the content and stability of the SOC.?3?The integration of life-cycle assessment?LCA?and the RothC model can be used to quantify the carbon footprint?CF?during wheat production after 8 years and when the SOCS of different treatments reach the new equilibrium.Choosing soybean as LGM increased the averaged wheat yield over different synthetic N rates by 8%compared with FW after 8 years?P<0.05?,and synthetic N rate was reduced by 33%without compromising the wheat yield for all the main treatments.Although LGM treatments induced higher GHG emissions from field inputs,the greater amount of C inputs and the corresponding increase of SOCS had reduced the CF by 25%-51%compared with the FW.When the SOCS of all treatments are at the new equilibrium,the CF for cropping systems with LGM would be 53%-62%lower than FW and 23%-37%lower compared with their current level.Including Huai bean and soybean as LGM in the cropping system to replace the local summer fallow practice is a valid method to maintain wheat yield and decrease the CF persistently.Coupling the RothC model and LCA is an alternative method to estimate the long-term GHG emissions of different cropping systems.?4?The nitrate holding capacity of the root-zone soil?0-100 cm?and the corresponding synthetic N limits of the 3 LGM treatments can be determined based on the standard of zero increase of soil nirate in the non-root zone soil?100-200 cm?between the LGM and FW treatments.The further integration of the N returned to the field via LGM and the total N threshold for high wheat yield,the optimal synthetic N rates could be calculated.The N accumulation for the LGM treatments ranged from 61-90 kg ha^-1,and that of Huai bean was46%higher than the average value of soybean and mung bean?P<0.05?.The linear+plateau model indicated that the threshold of total N for wheat to produce the highest yields was 141kg ha^-1.The estimated nitrate-holding capacity of the root-zone soil for the LGM treatments ranged from 104-117 kg ha^-1,and the corresponding synthetic N limits were 97 to 130 kg ha^-1.By synthesizing the abovementioned results from this section,the optimal synthetic N rates for the LGM treatments were 52-80 kg ha^-1.LGM has great potential to replace synthetic N,it can maintain high wheat yield while avoid nitrate accumulation in the non-root zone soil.?5?Using the farmers traditional practice(FW+N₁₆₂,BAU)as the control,new LGM-based management scenarios(HW+N₅₂,HNO and SW+N₇₉,SNO)were established based on the results from our previous studies.The future climate parameters from 36 general circulation models under both the representative concentration pathway?RCP?4.5 and RCP8.5 were obtained.In combination with the RothC model,LCA and empirical N models,the SOCS,CF and environmental costs for BAU,HNO and SNO were studied to clarify the impacts of LGM on environment.The SOCS of the HNO and SNO were projected to increase by 14-18 Mg ha^-1 compared with those of BAU until 2100.However,the potential of SOCS for differet treatments under RCP8.5 were lower than those under RCP4.5,mainly due to the increased temperature.The CF of HNO and SNO in different time slices was projected to be808-1388 kg CO₂ eq ha^-1 and 1280-1797 kg CO₂ eq ha^-1,respectively,which is significantly lower than that of BAU(2756-3456 kg CO₂ eq ha^-1).The total environmental costs of HNO and SNO were also projected to be 46%-70%lower than those of BAU?P<0.05?,which is mainly ascribed to the reduced environmental costs associated with the great potential for synthetic N replacement of LGM.Both HNO and SNO can decrease the environmental costs due to wheat production continuously for a long period of time in the future;besides SOC sequestration,more attention should be paied to the capacity of synthetic N replacement via LGM.In conclusion,growing LGM to replace summer fallow can ameliorate soil quality,it can also cap the environmental costs during wheat production by propelling soil C sequestration and replacing synthetic N,thus can be considered as the future-orinted sustainable field management.