Effects Of Drying-Rewetting On Soil Nitrogen Transformation And Soil Biological Characteristics

More frequently drying-rewetting is likely to be expected for soils under the comprehensive effects of climate change, drought, deluge and human activities. Drying-rewetting has strong effects on nitrogen transformation. The paper was taken semi-disturbed soils which sampled from a farmland in Songjiang, Shanghai as research object. Soils were incubated in laboratory under4different moisture regimes (drying-rewetting\constant wet\constant dry\constant fooled) to study the effects of drying-rewetting on soil nitrogen transformation and biological characteristics. The soil moisture depletion regulation under multiple drying-rewetting and the correlations between soil moisture content (g/g) and nitrogen in DW cycles were analysised. This paper also revealed the effects of drying-rewetting on soil nitrogen mineralization and nitrification. Besides, the effects of dry-wet cycles on soil microbial biomass and enzyme activities which closely related with nitrogen transformation were carried out. The main conclusions in this paper were as follows:(1) The daily moisture contents (v/v) of DW soils were monitored to calculate the soil moisture depletion coefficients. The results indicated that soil moisture depletion coefficients were decreased during DW cycles. Persistent drought altered the structure of soil porosity and speed up the depletion of soil moisture. However, such influence weakened with the DW cycles going. The damage caused by drought was irreversible for soils.(2) Correlation analyses (Pearson) between soil nitrogen and moisture of DW treatment soil were conducted. Results showed that moisture contents (g/g) had a significant negative correlation with soil NO3--N and soil total mineral N content. And it had a significant positive correlation with soil NH4+-N. But the significant correlation between TDN and soil moisture was not be found.(3) Experiments were conduct in the artificial climate incubator for71d (3DW cycles) under4regimes of soil moisture. The results indicated that drying-rewetting contributed to the accumulation of NO3--N, which showed a "pulse" increasing after rewetting. The dynamics of NH4+-N among4regimes did not have significant differences. Drying-rewetting reduced the loss of soil TDN, increasing soil mineralization and nitrification which showed a "pulse" increasing. The drying and rewetting soil had the highest nitrification intensity when the soil moisture content (g/g) ranging at15.82%～17.06%.(4) The total bacterial count, total fungal count and MBN before and after rewetting were contrasted. Results showed that the total bacterial count decreased after persistent drought and rapidly increased after rewetting. The total fungal count did not have a significant fluctuations, indicating that fungus had strong drought resistance. Multiple drying-rewetting changed the community structure among bacterium and fungi. MBN increased after rewetting and showed a pulse increasing with the DW cycles going.(5) Denitrifying enzyme activities had a significant positive correlation with soil moisture content (g/g). But urease, nitrite reductase and hydroxylamine reductase activities had no significant correlations with it. Rewetting stimulated the enzyme activities increasing, but the effect worn off with the DW cycles going. Denitrifying enzyme activities showed a "pulse" characteristic. When the soil moisture content ranged between31.51%and15.82%, nitrite reductase activities decreased. The urease activities in different moisture regimes ranked as CW> DW>CF>CD; The denitrifying enzyme activities ranked as CF>CD>CW>DW; The nitrite reductase activities ranked as CF>CD>DW>CW; The hydroxylamine reductase activities ranked as CW>DW>CD>CF.