The Nitrification Characteristics Of Paddy Soils After Winter Plantation Of Milk Vetch(Astragalus Sinicus L.)

Nitrification is the central part of global nitrogen-cycling,and is the main process in the usage and loss of nitrogen in agricultural ecosystem.The researches about nitrification have significant importance in agriculture sciences.Application of green manure has been proved to be one of the most important methods to achieve stable and high yields of rice in Southern China.There were little studies about the effects of winter plantation of milk vetch on nitrification in paddy soil.Our study was based on the experiments which had established for many years in the typical rice planting regions in Southern China,and combined with the incubation experiment.The influences of milk vetch on soil types in different areas,sampling periods,fertilizer levels and water status were studied.The main results were as follows:The combined experiments located in Anhui,Fujian,Hunan,Henan,Hubei and Jiangxi province were studied.Milk vetch combined with chemical fertilizer improved soil properties and fertility in all the six sites.Application of milk vetch increased the soil total nitrogen,organic matter,available phosphorus and nitrate nitrogen to some extent in all experimental sites.In yellow clayey soil of Jiangxi province,ammonium nitrogen content was significantly higher than that in other sites,while the green manure treatment decreased the content of ammonium nitrogen in yellow clayey soil.The nitrification activity of alkaline purple alluvial soil was the highest one among the six sites,the average value was up to 0.187 ?g/(g·h).The responses of nitrification activity on milk vetch and chemical fertilizer were different in the six sites with different soil types.Application of milk vetch reduced nitrification activity in Hunan and Henan sites,and increased the nitrification activity in Jiangxi.Green manure combined with chemical fertilizer increased nitrification activity in Hunan,while reduced the nitrification activity in Henan,and no significant difference was found at other sites.The nitrification ability of purple alluvial soil and yellow clayey soil were different.The responses of nitrification on the treatments were similar at the two sampling stages,i.e.after the harvest of late rice and the full-blossom stage of green manure.Nitrification activity and nitrification potential for yellow clayey soil was obviously lower than that in purple alluvial soil.In purple alluvial soil in Hunan,the nitrification activity ranged from 0.269 ?g/(g·h)to 0.325 ?g/(g·h)in all the treatments,and there was no significant difference among the treatments.The potential nitrification rate in purple alluvial soil reached to 10.25% at the fifth week of incubation,the milk vetch combined chemical fertilizer treatment inhibited nitrification ability to some extent.In yellow clayey soil in Jiangxi,the nitrification activity ranged from 0.010 ?g/(g·h)to 0.021 ?g/(g·h)in all the treatments,the potential nitrification rate in yellow clayey soil increased after the third week of incubation and reached to 5.41% at the fifth week.Compared with control,winter green manuring improved the soil nitrification activity and potential nitrification rate,and had almost the same effects as the chemical fertilizer treatment.The strongest promotion effect was found in the milk vetch combined chemical fertilizer treatment for the nitrification ability in the yellow clayey soil.AOA were predominant in both purple alluvial soil and yellow clayey soil,the abundance of AOA-amoA in purple alluvial soil was significantly higher than that in yellow clayey soil.The application of milk vetch had no significant effect on the abundance of AOA-amoA and AOB-amoA in purple alluvial soil,but promoted the abundance of both AOA-amoA and AOB-amo A in yellow clayey soil significantly,same as the effects of milk vetch on nitrification activity and potential nitrification rate.Milk vetch combined with different proportion of chemical fertilizer had different effect on soil fertility and nitrification activity in purple alluvial soil in Hunan and yellow clayey soil in Jiangxi.Milk vetch combined with different proportion of chemical fertilizer decreased soil pH in purple alluvial soil,and milk vetch combined 100% chemical fertilizer treatment was the most significant one.The application of chemical fertilizer also reduced the pH in yellow clayey soil.Milk vetch combined different proportion of chemical fertilizer increased soil total nitrogen and organic matter to some extent in the two kinds of soils,particularly the treatment with milk vetch combined 40% chemical fertilizer.Milk vetch combined 60% chemical fertilizer had the highest total nitrogen and organic matter content in yellow clayey soil.Green manure increased the content of ammonium nitrogen in two type of soils,increased by 22.4% and 16.0%,respectively.The nitrification activity of purple alluvial soil was significantly higher than that in yellow clayey soil.Milk vetch combined different proportion of chemical fertilizer significantly increased the nitrification activity in two type of soils,and milk vetch combined 60% chemical fertilizer in purple alluvial soil and milk vetch combined 80% chemical fertilizer in yellow clayey soil were the most significant ones.The trends of pH were the same during the dry-wet rotation incubation whether in purple alluvial soil or in yellow clayey soil,and all revealed a trend of increased at the flooding period and decreased at the drying period.Dry-wet rotation reduced the Eh at flooding period in yellow clayey,the tendency of soil Eh was opposite with pH.The ammonium nitrogen content of dry-wet rotation treatment was increased gradually during drying period in the two type of soils.The content of nitrate nitrogen increased during drying period of dry-wet rotation in purple alluvial soil,especially during the second drying period.Nitrate nitrogen content had decreased during the drying period in yellow clayey soil and increased after rewetting.Nitrification activity had different tendency of different treatment in dry-wet rotation incubation.Nitrification activity increased gradually during the second drying period in purple alluvial soil,and milk vetch combined chemical fertilizer treatment increased nitrification activity in the later period of the incubation.The yellow clayey soil had lower nitrification activity and didn't change a lot during the incubation.