Response Of Soil Carbon And Nitrogen Pools To Mulching And Organic Materials Amendment In Rain-fed Spring Maize Farmland

The Loess Plateau is an important dry and rainfed agricultural area in China.Low temperature and drought in the early spring crop growth period（April and May each year）are the main factors limiting the yield and water use efficiency（WUE）of spring maize in this region.Soil surface mulching is an effective method to improve crop yield and WUE in this region.However,improved hydrothermal conditions also increase the risk of soil organic carbon（SOC）mineralization,which may lead to depletion of SOC.Currently,the effects of long-term（>10 years）mulching on the characteristics of changes in soil carbon（C）and nitrogen（N）and their fractions,the stability of soil aggregate and the distribution of C and N in different particle size fractions of soil aggregate are unclear,while film mulching with organic material inputs is an effective method to improve soil fertility.To investigate the effects of different C and N management practices on the characteristics of changes in soil C and N and their fractions and on the distribution of C and N in the aggregate,this study was conducted on cultivation system of spring maize in dry farming on the loess Plateau,based on the 12-year-long long-term mulching measures and organic fertilizer application field positioning experiments（CK:no mulching;GM:gravel mulching;FM:film mulching,FCM:film mulching chemical fertilizer with organic manure）and the 7-year positioning straw returning experiments（CU:film mulching mixed with one application of fertilizer;CS:film mulching mixed with one application of fertilizer with straw returning）,we systematically analyzed the effects of different C and N management measures.The effects of different C and N management practices on yield,above-ground dry matter and N accumulation,water use efficiency（WUE）,soil C and N and their fractions,as well as the distribution of C and N in aggregates were systematically analyzed to provide a theoretical basis for the screening of green and sustainable agricultural development technology models for long-term maintenance of high yield and improvement of soil fertility in arid and semi-arid regions.The main results of this study are as follows.（1）Long-term（10-12 years）GM and FM treatments reduced SOC content.Compared to the control,FM resulted in a significant decrease of SOC in the 0-20 cm and 20-40 cm soil layers by an average of 8.19%and 5.05%,respectively,and long-term gravel mulching resulted in a significant decrease of SOC in the topsoil（0-20 cm）by an average of 3.43%.In terms of C fractions,the decrease of C content in the topsoil under FM treatment came from water-soluble organic C（WSC）,potassium permanganate organic C（POX-C）and particulate organic C（POC）,which were significantly decreased by 13.04 mg kg^-1,0.08 g kg^-1and 0.92 g kg^-1,respectively.From the perspective of oxidizable organic C fractions（OOCFs）,the decrease of C content in the topsoil under FM treatment came from the very labile C(C_VL)and the less labile C(C_LL)in the topsoil by 0.60 and 0.66 g kg^-1,respectively.Compared to the control in the topsoil,long-term mulching significantly decreased the mean values of C pool management index（CMI）.（2）Long-term GM and FM treatments reduced soil total N（TN）content.Compared with the control,FM treatment significantly reduced the mean values of TN in the 0-20 cm and 20-40 cm soil layers by 6.73%and 5.81%,respectively.Long-term mulching resulted in a significant decrease in the mean value of TN in the topsoil.In terms of N fractions,FM treatment significantly reduced the mean values of water-soluble N（WSN）and particulate fraction N（PFN）in the topsoil by 0.75 mg kg^-1and 0.08 g kg^-1,respectively,and GM treatment significantly reduced the mean value of PFN by 0.05 g kg^-1.However,long-term mulching had no significant effect on hot-water extractable N（HWN）and mineral-associated fraction N（MAFN）.Compared to the GM treatment,the FM treatment had lower content of TN and N fractions.（3）Long-term different soil surface mulching had significant effects both particle size distribution（LM:>2 mm;SM:2-0.25 mm;MI:0.25-0.053 mm;SC:<0.053 mm）and structural stability of soil aggregates in the topsoil.Compared with the CK,the content of>0.25 mm the mechanically-stable and water-stable aggregates,weight mean diameter（WMD）,geometric mean diameter（GMD）of the FM treatment in the topsoil were significantly increased by 2.94%and 5.00%,8.95%and 11.71%,respectively,but there was no significant effect on the destruction rate（PAD）and structure coefficient（K）.The effect of GM on the distribution and stability of aggregates was no longer significant.Under long-term mulching treatments,the C and N contents of soil aggregates showed a tendency to increase with the increase of aggregates particle size,while the C and N ratio of aggregates had no significant difference.The long-term mulching significantly reduced the unit SOC and TN contents in the SM,MI and SC fractions of aggregates,with the largest decreases of 0.63 and 0.08 g kg^-1in the MI fraction of aggregates,respectively.The relative importance analysis showed that the particle size distribution of aggregates contributed more to the SOC content in the bulk soil and the N distribution in the aggregates contributed more to the TN content in the bulk soil.The highest contribution to the SOC in the bulk soil was made by the SM fraction,the highest contribution to TN in the bulk soil was made by the MI fraction.（4）Compared with FM treatment,FCM treatment significantly increased the mean value of SOC and TN contents in 0-20 cm and 20-40 cm soil layers,and the mean value of SOC and TN contents in the topsoil were significantly increased by 48.12%and 40.21%,respectively.Compared with FM treatment,FCM treatment significantly increased the labile C and N fractions,and C_VL,labile C（C_L）,C_L,in the topsoil.Compared with FM treatment,the proportion of>0.25 mm aggregate was significantly increased by 16.25%under FCM treatment,and the average SOC and TN contents of each aggregate fraction and the average unit SOC and TN contents of LM and SM fractions were significantly increased.The total effect of aggregate size distribution and C distribution in the aggregates contributed more to SOC in the bulk soil,and N distribution in the aggregates contributed more to TN in the bulk soil.SM and MI fractions contributed the most to SOC and TN,respectively.Compared with CU treatment,CS treatment significantly increased the mean value of SOC and TN contents in 0-20 cm and 20-40 cm soil layers,and the mean value of SOC and TN contents in the topsoil were significantly increased by 15.18%and 14.29%,respectively.Compared with CU treatment,the proportion of>0.25mm aggregate under CS treatment was significantly increased by 10.80%.The SOC and TN contents in each aggregate fraction and the unit SOC and TN contents of LM and SM aggregate fractions were significantly increased.C distribution in the aggregates contributed more to SOC in the bulk soil,and aggregate particle size distribution contributed more to TN in the bulk soil.LM and SC fractions contributed the most to SOC and TN,respectively.（5）Both FM and FCM treatments could increase soil yield and WUE,but FM treatment significantly reduced SOC and TN stocks in the topsoil.FCM treatment significantly increased SOC and TN stocks by 8.9 and 1.92 Mg ha^-1,respectively.The amount of organic manure（fresh cow dung with 7.25%C content）applied to achieve SOC balance under mulching was about 10 Mg ha^-1yr^-1on the Loess Plateau.However,long-term exogenous soil organic matter input resulted in a stable state of SOC（no longer increasing significantly）,and significantly decreased the saturation deficit of SOC and TN in the topsoil.The results of SOC and TN saturation deficit showed,compared with the control,FCM treatment significantly increased soil current SOC and TN pool,significantly reduced SOC and TN saturation deficit in the topsoil,but had little effect on the C and N saturation deficit in the subsoil（20-40 cm and 40-60 cm）.（6）Compared with CU treatment,CS treatment had no significant effect on yield,WUE and nitrogen use efficiency.The CS treatment significantly increased SOC and TN stocks in the topsoil by 1.55 and 0.20 Mg ha^-1,respectively.The current saturated c and N pool in the topsoil was significantly increased,and the saturation deficit of C and N in the topsoil was significantly reduced.In conclusion,in terms of long-term maintenance of high yield and improved fertility,FCM is the best practice to achieve long-term sustainable development of spring maize farmland in arid and semi-arid regions.