Effects Of Temperature Change And Nitrogen Addition On Soil Organic Carbon Mineralization Under A Dryland Maize Cropping System With Different Mulching Measures

Soil organic carbon?SOC?mineralization is one of the keys regulating the carbon cycling process,especially in the agroecosystems.Climate warming and increased atmospheric nitrogen deposition both have important impacts on the soil carbon cycle process.Similarly,agricultural practice managements can directly change the soil water and heat conditions,which in turn affect the organic carbon mineralization.The application of surface mulching measures?straw mulching or film mulching?have been widely used in the dryland area in northwest China recently.However,the response of soil organic carbon mineralization to temperature increase and nitrogen deposition under different mulching measures is still not clear.In this study,soil samples were collected from a 10-yr field experiment with straw mulching,plastic film mulching and no mulching,and then incubated at 5??15??25??35?with additions of 0?25?50 kg N·hm^-2for 105 d,respectively.Soil organic carbon mineralization rate,soil organic C and N fractions,as well as soil properties and extracellular enzyme activity were measured systematically.The main results are as follows:1)The dynamic of the soil organic carbon mineralization rate during the incubation can be divided into three periods.The early unstable stage was found during the days after incubation?DAI?1-10.The middle slow-down stage was found in DAI 10-30,the SOC mineralization rate decreased slowly at this stage.The final stable stage was found DAI30-105.The mineralization rate of organic carbon did not change much and gradually stabilized at this stage.2)The accumulated SOC mineralization during 105 d incubation was ranged from1.782 to 5.860 mg C·g^-1dry soil.Mulching,temperature,and nitrogen deposition all had significant effects on SOC mineralization.Compared with the control without mulching,the accumulated mineralization increased by 33.86%with straw mulching but not change with film mulching averagely.The increase in temperature significantly promoted SOC mineralization,and the accumulated SOC mineralization at 15?,25?and 35?were1.29 times,1.45 times and 2.40 times of that at 5?,respectively.Similarly,compared with zero nitrogen addition,low nitrogen addition increased SOC mineralization by 8.01%,and high nitrogen addition increased by 17.17%.Significant interactions were found among mulching,nitrogen addition and incubation temperature.Nitrogen addition significantly promoted SOC mineralization in the un-mulched soil and film-mulched soil,but had no effect in the straw-mulched soils.The SOC mineralization temperature sensitivity was higher with straw mulching than with no mulching or film mulching,and increased with increasing of nitrogen addition.3)The nitrogen addition promoted the SOC content after incubation in the film-mulched and un-mulched soils but had no effect in the soil with straw mulching.With the increase of temperature,the soil NO₃^--N content increased while the NH⁺₄-N content decreased significantly.However,as the nitrogen addition increases,soil NO₃^--N content after incubation increased and the NH⁺₄-N content decreased significantly.The temperature increases and nitrogen addition significantly promoted the Soil DOC content after incubation increased with the increase of temperature and nitrogen addition for the soils with film mulching and no mulching,however,only increased with increasing of temperature for that with straw mulching.The increase of temperature significantly reduced the content of soil microbial biomass carbon and nitrogen after incubation.Nitrogen addition promoted the soil microbial biomass carbon and nitrogen contents for the soils with film mulching and no mulching,but had no effect for that with straw mulching.4)Increased temperature significantly increased soil?-1,4-glucosidase?BG?,?-1,4-xylosidase??X?,?-Dcellobiohydrolase?CBH?,and N-acetyl-?-glucosaminidase?NAG?.Nitrogen addition significantly increased the activities of four carbon and nitrogen metabolism enzymes as BG,?X,CBH,and NAG for the soils with film mulching and no mulching,but had no effect for that with straw mulching.Significant positive correlations were found among different enzyme activities.Moreover,soil enzyme activities were negatively correlated to MBC and MBN,and positively to DOC.Soil BG and CBH also correlated with mineral nitrogen.The accumulative SOC mineralization was negatively correlated to soil p H,MBC and MBN but positively to DOC and hydrolases.DOC and BG can indicate that the sudden soil soluble organic matter caused from the soils with different mulching measures.To sum up,this study indicates that the decline in soil carbon sequestration capacity in dryland cropping systems may become an important carbon source in the context of global warming and nitrogen deposition,which can be regulated by management practices as surface mulching.