| Soil Soil organic carbon and soil total nitrogen are core indicators of soil quality and fertility in farmland.Their quality and quantity will affect the productivity and stability of cultivated land,and their dynamics will affect soil quality and crop yield.In recent years,straw and plastic film mulching technologies have been widely used in dryland farming areas in northern China due to its good preservation of soil temperature and moisture,and its impact on soil carbon and nitrogen pools has also received considerable attention.Based on a long-term mulching experiment in dry highland of the Losses Plateau,this study investigated dynamic changes of soil organic carbon at different crop growth stages under straw and plastic film mulching conditions.The aim of the study was to explore seasonal responses of soil organic carbon and total nitrogen as well as their fractions to surface mulching and deepen the understanding of soil carbon and nitrogen cycle in dry farmland.The data were collected from the 8 years experiment with the treatments of straw mulching(SM),plastic film mulching(PM)and no mulching(CK)at pre-sowing(April 8th),seeding(May 17th),jointing(June 17th),belling-heading(July 17th),filling(August 17th)and harvest stage(September 17th)of spring maize.The seasonal dynamics of soil organic carbon and total nitrogen,microbial biomass carbon and nitrogen,potentially mineralizable carbon and nitrogen,particulate organic carbon and nitrogen,as well as two kinds of mineral nitrogen in 0~10 cm,10~20 cm and 20~40 cm soil layers were determined.The results of soil organic carbon fractions showed that:(1)the contents of soil organic carbon,microbial biomass carbon,potentially mineralizable carbon and particulate organic carbon showed an tendency of decreasing at seedling stage,increasing at jointing stage,decreasing again at belling-heading stage and recovering at filling and harvest stages.(2)Compared with CK,straw mulching effectively increased the contents of soil organic carbon and its fractions at most growth stages of spring maize,which improved soil fertility and enhanced carbon sequestration.Plastic film mulching did not shown significant effects at the early growth stages,but decreased these fractions at the later stages of spring maize.(3)The ratio of microbial biomass carbon to soil organic carbon did not change with growth stages among treatments.However,particulate organic carbon in 0~10 cm under straw mulching was significantly higher than that under CK and PM treatment,indicating that it hugely contributed to the dynamics of soil organic carbon in the topsoil layer under straw mulching treatment.The dynamics of soil organic carbon under plastic film mulching were mainly attributed to changes in particulate organic carbon and potential mineralized carbon.(4)The proportions of potential mineralized carbon and particulate organic carbon decreased at belling-filling stage with plastic film mulching or without any form of mulching,but did not change throughout the growing season with straw mulching.This indicated that straw mulching recharged soil potential mineralization carbon and particulate organic carbon during later growth stages.(5)There was a highly significant positive correlation between soil organic carbon and its fractions(P<0.01),and the responses of soil particulate organic carbon and microbial biomass carbon to the dynamics of soil carbon pool ware more rapid and sensitive.Microbial carbon was significantly positively correlated with potential mineralized nitrogen(P<0.01).The results of soil total nitrogen and its fractions showed that:(1)The contents of soil total nitrogen,microbial biomass nitrogen,potentially mineralizable nitrogen,particulate organic nitrogen and mineral nitrogen showed an tendency of decreasing at seedling stage,increasing at jointing stage,decreasing again at belling-heading stage and recovering at filling and harvest stages,except for nitrate nitrogen and ammonium nitrogen which increased at seedling stage.(2)Compared with CK,straw mulching effectively increased the contents of soil total nitrogen and nitrate nitrogen in 0-40 cm soil layer and soil ammonium nitrogen in 0-20 cm soil layer at most growth stages of spring maize,and increased soil microbial biomass nitrogen,potentially mineralizable nitrogen,and particulate organic nitrogen in 0-40 cm soil layer at nearly all growth stages.(3)Compared with CK,plastic film mulching improved soil nitrate nitrogen in 0-40 cm soil layer and ammonium nitrogen in 0-20 cm soil layer at most growth stages of maize,and decreased soil total nitrogen in 0-20 cm soil layer and particulate organic nitrogen in 0-40 cm soil layer at the later growth stages,and decreased microbial biomass nitrogen in 0-40 cm soil layer and potentially mineralizable nitrogen in 10-20 cm soil layer at most stages of maize.(4)Soil microbial biomass nitrogen,potentially mineralizable nitrogen and particulate organic nitrogen under mulching all had an important contribution to the dynamics of total nitrogen,except for the ratio of particulate organic nitrogen to total nitrogen in 20-40 cm layer under plastic film mulching.(5)There was a highly significant positive correlation between the contents of active organic nitrogen components and soil total nitrogen(P<0.01).The particulate organic nitrogen and microbial biomass nitrogen were more sensitive to the dynamics of soil nitrogen pool.There was a significant positive correlation between microbial biomass nitrogen and potential mineralizable nitrogen(P<0.01).Both nitrate nitrogen and ammonium nitrogen were significantly positively correlated with microbial biomass nitrogen and potentially mineralizable nitrogen(P<0.01).Nitrate nitrogen was significantly and positively correlated with ammonium nitrogen(P<0.01).In conclusion,straw mulching of spring maize in the Loess Plateau has a significant role in increasing the contents of soil carbon and nitrogen fractions,which is conducive to improving the soil fertility and carbon and nitrogen fixation levels in dry farmland and promoting high crop yields.While plastic film mulching will reduce the contents of soil carbon and nitrogen fractions at the later growth stages of maize,which will reduce soil fertility.At the same time,it will increase the soil nitrate nitrogen level and increase the nitrogen leaching risk of dry farmland. |
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