Effects Of Different Management Practices On Soil Carbon And Nitrogen And Related Microbial Processes In Rain-fed Farmlands

Soil organic carbon（SOC）and nitrogen（N）in cropland play vital roles in assessing soil quality and soil suitability,and soil microorganisms are driving force for transformation of soil organic matter（SOM）,regulating the substrates and energy of ecosystems.Sensitive to micro-environment,soil microorganisms are susceptible to different management practices,which change transformation of soil C and N in turn.Plastic film mulching has been taken as an affective measure to increase soil water,temperature and grain yields,but how to maintain soil fertility and balance nutrients such as C,N and phosphorus（P）in soil with film mulchingis an urgent problem.Organic manure and biochar（originated from waste organic material）can effectively improve soil microenvironment and promote soil fertility,providing a new pathway to sustainability of cropland productivity.Based on a long-term cropland planted with spring maize in rain-fed region on the Loess Plateau,field experiments and incubation experiments were conducted.Considering the soil-microorganism-plant system,the objectives of this study were to assess dynamics of soil C,N and P,and soil microorganisms in soils with film mulching,organic manure and biochar,thereby evaluated effects of different management practices on transformation of soil nutrients and crop use in cropland.Our study provided certain scientific basics to optimize rain-fed farmland nutrient management,to increase C sequestration capacity of farmland,and to effectively regulate nutrient use efficiency and soil sustainability in the given region.The main results and conclusions were showed as follows:（1）Application of organic materials markedly affected the dynamics of soil C and N,and mineralization of soil C and N were more affected by maize straw.In the short-term experiment,maize straw alone significantly increased accumulative CO₂ emissions;biochar alone had no significant effect,but the accumulative CO₂ emissionsin soil with maize straw and biochar increased with biochar addition rates compared to the control.Application of organic materials markedly increased SOC and total N（TN）,decreased mineral N concentrations.Further incubation experimentbased on ¹³C and ¹⁵N showed that maize straw progressively incorporated into soil C and N during decomposition and maize straw-C preferred to contribute to CO₂ emissions while maize straw-N preferred to maintain in the soil.The priming effect of maize straw on soil C was positive at early stage but switched to negative later during incubation.Maize straw increased the turnover rate of soil C and promoted renewal of SOM;but decreased the potential loss of soil N in gases and increased soil N in the end.（2）Different agricultural practiceschanged soil C,N and their labile fractions,organic manure and biochar increased SOC and TN,but the labile fractions had obviously seasonal and annual variations.Compared to the control（CK）,film mulching（PF）had no significant effects on contents and stocks of SOC and TN,organic manure（FM）significantly increased them.However,SOC and TN in treatments CK and PF negatively increased and PF was more negative since the beginning of the experiment;while FM treatment positively increased SOC and TN,improving soil C and N in the long term.Fractions of soil C and N were affected by film mulching and fertilization during maize growing seasons in 2015 and 2016.PF increased water soluble organic C（WSOC）at PT and V6 stages in 2015 but decreased it at PT stage in2016;PF markedly increased water soluble organic N（WSON）at V6 and R5 stages in 2015and 2016,but organic manure significantly increased WSOC and WSON at different stages in both years;PF had no effect on soil easily oxidized C（EOC）,FM increased it at R6 stage（0-10 cm layer）and R5 stage（10-20 cm layer）in 2015.Similar to organic manure,biochar significantly increased contents and stocks of SOC and TN,especially 30 t ha^-1 biochar（BC30）.The SOC in soil with biochar positively increased since the beginning of the experiment thus prevented the loss of SOM.Biochar increased concentrations of WSOC,WSON and EOC,changing along with maize growth stages in 2015 and 2016.Therefore,organic manure and biochar application could increase levels of SOM and improve availability of soil C and N.（3）Organic manure under film mulching markedly increased soil microbial biomass C（MBC）,N（MBN）and P（MBP）at different growth stages in both experimental years;increased accumulations of soil total amino sugar,caused the soil microbial community turned to bacterial community,and improved soil nutrients availability.However,the effects of biochar on soil microbial biomass and functional diversity showed obvious difference.Organic manure application under film mulching significantly increased MBC,MBN and MBP in 2015 and 2016,while effects of film mulching on them varied with growth stages.Compared with soil with biochar application（BC0）,BC30 significantly decreased MBC and MBN in 2014,but increased MBP in 2014 and microbial biomasses in 2015.Thus,organic manure and biochar could generally increase soil microbial biomass.After 7 years of film mulching and organic manure application,PF significantly decreased accumulations of total amino sugar by 19.2%in 0-10 cm layer;FM significantly increased accumulations of total amino sugar by 26.3%and 38.1%in 0-10 cm and 10-20 cm layers,respectively.The order of individual amino sugar was GluN>GalN>MurA in different treatments.FM significantly decreased GluN:MurA relative to PF and CK,suggesting a change of microbial community to bacteria community in soil.The responses of soil microbial functional diversity to biochar at jointing stage during maize growing season showed that biochar significantly decreased soil AWCD and C utilization,the decrease effect in soil applied with 10 t ha^-1 biochar（BC10）was more significant than BC30;BC10 significantly decreased richness index S in 0-10 cm and10-20 cm layers but significantly increased evenness index E in 10-20 cm layer,BC30significantly decreased richness index S in 10-20 cm layer.Therefore,the different effects of biochar on soil microbial functional diversity were related to the selectivity of microorganisms that preferred to use specific substrates.（4）Ecological stoichiometry characteristics of soil,microbial and enzyme had certain consistency and relative specificity under different practices in rain-fed farmland.PF significantly decreased SOC:TN in 0-10 cm layer,FM significantly increased SOC:TP in10-20 cm layer and TN:TP in 0-10 cm and 10-20 cm layers;but FM markedly decreased MBC:MBP in 0-10 cm and 10-20 cm layers,both PF and FM significantly decreased MBN:MBP in 10-20 cm layer.FM markedly increasedβG:NAG in 0-10 cm layer andβG:AP in 10-20 cm layer relative to CK.Effects of biochar on these ecological stoichiometry characteristics were related with biochar application rates.Compared to BC0,BC10 and BC30 significantly increased SOC:TN in both layers and SOC:TP in 0-10 cm layer;BC30significantly increased MBC:MBN by 9.8%in 0-10 cm layer and by 36.9%in 10-20 cm layer,BC10 significantly decreased MBC:MBP in both layers,and BC10 and BC30 markedly decreased MBN:MBP by 26.8%and 37.4%in 10-20 cm layer,respectively.BC10significantly increasedβG:NAG in 0-10 cm layer,and BC30 significantly increasedβG:NAG andβG:AP in both layers.Correlation analysis showed that MBC:MBN andβG:NAG were both positively related with SOC:TN and SOC:TP,while MBN:MBP was negatively related with SOC:TN,SOC:TP and NAG:AP.Thus,organic manure and biochar aggregated the needs of C and N in microorganisms and stimulated the corressponding enzyme activities,made soil microorganisms capture more C and N and increased soil C and N in the end.（5）The results of¹⁵N signatures showed that PF and FM significantly increased grain yields and N accumulation,but decreased absorption of fertilization N in the first crop season.N absorbed by crop was transferred to grains in prior,and the ratios of absorption of residual N in grains was larger than other organs in the second crop season,the residual effect of fertilization N could not be ignored.After a growing season,the proportions of residual fertilization N were 26.7%-62.7%,FM significantly increased residual N in soil and supplemented the loss of soil N.The residual fertilization N was mainly in the upper soil,which was effective to increase N use efficiency.The proportions of residual N were7.5%-20.2%after two growing seasons,FM significantly increased the residual N and decreased the losses but PF decreased residual N compared to CK;both PF and FM significantly increased N use efficiency but the increase effect was more obvious in treatment FM.Therefore,organic manure application could effectively regulate soil nutrients and change their availability,increase soil N use efficiency,maintain soil N and decrease losses of N under film mulching conditions,which were beneficial to efficient use of nutrients and soil sustainability in rain-fed farmland.