Effect Of Straw Returning And N Fertilizer Reduction On Soil Organic Carbon And N Content And Crop Yield

Guanzhong Plain is one of the major crop production area in China. Winterwheat-summer maize rotation system is the primary cropping system in this region, with largeamouts of grain and straw production every year. Fully and proper utilization of the strawresources is of great importance to the economic development, environment protection andsoil quality improvement. Based on the local agriculture production situation, a four year fieldexperiment was conducted in A&F university station in2008to2012to study the effect ofnine different straw returning modes on soil C pool and crop yield, and therefore to screen theoptimum straw returning mode in this region. Meanwhile, the impact of reduced Nfertilization on soil NO₃^--N, soil C pool, crop yield, crop nutrients concentration and contentunder the condition of straw returning were investigated. The main results were as follows:1. The effect of N fertilizer application rate reduction on crop yields, soil carbon pool andNO₃^--N accumulation with straw returning to soil in winter wheat-summer maize rotationsystems of GuanZhong plain was studied to determine the optimal N rates for crop productionin this region. A four year field experiment was conducted during2008to2012. There arethree N treatments: were used in this experiment, including conventional N fertilization rate（187.5，150kg N/hm²in maize and wheat growing seasons, respectively）,15%N fertilizerreduction （159.4,127.5kg N/hm²in maize and wheat growing seasons, respectively）,30%Nfertilizer reduction （131.25,105kg N/hm²in maize and wheat growing seasons, respectively）.Straw and grain yield of maize and wheat, soil NO₃^--N content, soil organic carbon and labileorganic carbon were analyzed.15%N fertilizer reduction increased crop year-around seedyield by7.2%，and30%N fertilizer reduction did not decrease either grain or straw yieldwhen compared to conventional N fertilization treatment. N fertilizer reduction treatment didnot affect soil organic carbon and active organic carbon. The soil organic carbon wasincreased year by year with returning of straw to soil, and the soil active organic carbon wassignificantly higher than it was before returning. Compared with conventional N fertilizationtreatment, accumulation of NO₃^--N in soil profile was significantly decreased with N ratereduction and the N application rate could be further decreased based on the results ofapparent N balance. Appropriate reduction of N fertilization rate under the condition of straw returning could ensure no decline in crop yield and with gradually increasing of soil organiccarbon and decreasing of NO₃^--N accumulation. Therefore, a beneficial result for obtainingboth economic efficiency and environmental protection could be performed.2. To determine the optimum N fertilization amout in wheat-maize rotation system inGuanzhong Plain, the effect of reduced N fertilization on crop yield, crop nutrientsconcentration and content under straw returning condition were studied under field condition.The results showed that the crop yield of2008to2012growing seasons were not significantlyaffected by the reduced N fertilization, therefore, the N fertilization amount could be furtherreduced.15%N fertilizer reduction didn’t significantly affect the crop nutrientsconcentrations, except significantly decreased wheat grain P concentration.30%N fertilizerreduction significantly decreased straw N concentration.30%N fertilizer reductionsignificantly decreased crop grain and straw N content by12.9%and41.9%, and crop straw Kcontent by18.5%. In the long run,30%N fertilizer reduction may cause soil N and Kdeficiency and yield reduction. Overall,15%N fertilizer reduction can be the optimal Nfertilization amount in the studied region.3. The effect of different straw returning mode on soil C pool and crop yield in winterwheat-summer maize rotation system in Guanzhong Plain were investigated to screen theoptimal straw returning mode in the region.Results showed that straw returning generallyincreased crop grain and straw yield. WH （wheat straw high-stubble returning） treatmentsignificantly increased crop grain and straw yield compared to the WN （no wheat strawreturning） treatment. MC （maize straw chopping returning） treatment significantly increasedcrop grain and straw yield compared to the MN （no maize straw returning） treatment.Combined maize straw chopping with wheat straw high-stubble （WC-MC） or chopping（WH-MC） returning modes resulted in the largest increase of year round crop yield, whichwas24.2%and22.8%, repectively. The long-term straw returning could increased soil organiccarbon, active organic carbon and CMI （Carbon management index）, especially in theWH-MC, WH-MM and WC-MM treatments.Generally, in the Guanzhong Plian, N fertilization amount could be reduced more than30%. However, with respect to its environmental and economical effects, soil fertility, cropyield and nutrients concentration impacts,160kg/hm²N fertilizer application in the maizegrowing season and130kg/hm²in the wheat growing season is the optimal N fertilizationamount in the region. The WH-MC treatment not only increased soil C pool, but alsoimproved soil fertility and crop yield. therefore, it can be suggest that WH-MC is the optimal straw returning mode in the studied region and worth to be extended.