The Interaction Between Soil Carbon And Nitrogen In An Oasis Drip-irrigated Cotton Field

[Objective] Soil organic C and N influence directly soil fertility and crop yields,therefore,they are the foundations for soil productivity and sustainable utilization.The objective of this study was to investigate the effect of water-N and C-N management practices on the distribution and accumulation of soil C and N in drip-irrigated cotton fields.The results of this study should provide information about how to increase soil C and maintain soil N so that agricultural soil can be sustainably utilized.[Method] A combined curvey and field experiment was included in this study.The field survey was conducted in Shihezi region of Xinjiang and 200 soil samples were collected using grid sampling method.The spatial distribution of soil organic matter(SOM)and total nitrogen(TN)were nalyzed by geostatistics method.The field plot experiment consisted of two parts:(i)water and N management and(ii)C and N management.The water and N management portion of the experiment included three irrigation amounts(360,480,600 mm,referred to as W360,W480,and W600,respectively)and three N application rates(0,300,450 kg N ha-1,referred to as N0,N300,N450,respectively).The C and N management part of the experiment included three C treatments [no amendment,amendment with cotton stalks(ST),and amendment with biochar(BC)] and three N application rates(0,300,450 kg N ha-1,referred to as N0,N300,and N450,respectively).[Result](1)The content of SOM and TN increased steadily in Shihezi region from 1960 s to 1980 s.Since1990s,SOM content remained stable,the TN content increased continuously,and the ratio of carbon to nitrogen(C/N)decreased.At present,the average SOM is 10.8 g kg-1,and the average TN is 0.68 g kg-1.The area of SOM content in deficient and very deficient accounted for 92%.The area of TN content in extremely deficient accounted for 28%,in deficient and very deficient accounted for 60%.The spatial distribution of SOM and TN in Shihezi region was similar.and showed the pattern of “south-high and north-low”.(2)The concent of total organic carbon(TOC),easily-oxidized organic carbon(WOC),water-soluble organic carbon(WOC)and TN and inorganic N in soil decreased with increasing irrigation amount.The degradation rate of organic matter was increased,and the C/N was not significantly changed with irrigation amount increasing.N fertilizer application decreased significantly TOC,EOC,and TN in soil.Soil WOC,microbial biomass carbon(MBC),and inorganic N contents were increased significantly with N application rate increased.N fertilizer application accelerated organic material decomposition and reduced soil C/N.(3)Irrigation and N fertilization increased significantly the activity of urease,?-glycosidase(BG),and N-acetyl-?-D-glucosamindase(NAG)in soil.In the N0 and N300 plots,soil enzyme activities increased with irrigation amount increased.In the N450 plots,the highest soil enzyme activity was found in the W480 treatment.Dry weight and N uptake of cotton increased with irrigation amount and N application rate increased.In the N0 plots,cotton yield was significant higher in W600 than in W480 and W360.In the N300 and N450 plots,W480 had the highest of cotton yield.(4)In the N0 plots,ST and BC had no significant effect on soil TN and even reduced soil TN.In the nitrogen application(N300 and N450)plots,soil TN in ST and BC were 39.2 and 18.6% greater than those in the corresponding CK treatments.Application of cotton straw and its biochar significantly increased TOC,EOC,and MBC of soil.Within different N treatments,all three variables decreased in the order CK >ST > BC.In the N0 plots,compared with CK treatment,ST and BC treatment significantly increased soil C/N.In the N300 and N450 plots,soil C/N was 15.7% lower in ST treatment than in CK.soil C/N in BC treatment was 23.3% higher than that in CK treatment.(5)Compared with the CK treatment,ST treatment increased the basal soil respiration by 8.3%,?-glycosidase(BG)activity by 43.1% and N-acetyl-?-D-glucosamindase(NAG)activity by 71.8%.The basal soil respiration was 14.6% in BC treatment than in CK treatment,and there were no significant differences for soil BG and NAG activities between BC and CK treatments.In the N0 plots,ST and BC treatments had no significant effect on cotton yield.In the N300 plots,cotton yield in ST and BC treatments were 17.9% and 24.8% higher than that in CK treatment,respectively.In the N450 plots,BC treatment increased significantly yield and N use efficiency of cotton.However,there was no significant difference for cotton yield between ST and CK treatments.[Conclusion] The contents of SOM and TN in soil are relatively low,and soil C/N show a decline trend in Shihezi region.However,excessive irrigation and N fertilization decrease the contents of SOM and TN in soil.Cotton straw combined with N fertilizer application can increase SOM and TN of soil,but reduce soil C/N.Biochar combined with N fertilizer application not only increase SOM and TN of soil,but also enhance soil C/N,and promote cotton yield and N use efficiency.Therefore,N fertilizer plus biochar is an optimal practice for soil management in drip-irrigated cotton field.