Study On N₂O Emissions And Nitrogen Transformation In Leaching Solution In Cracking Paddy Soils

Different water regimes have important effects on N₂O emissions and nitrogen solute transport in paddy soils. In the process of gradual drying or the alternation of dying and wetting, clayey paddy soils will produce cracks. The appearance of cracks accompanying with soil water content reducing, soil temperature increasing and the disturbance of microorganism changed the physical and chemical characters in soils. According to these mentioned, the article tried to study some rules including N₂O emissions, related reductase activities and nitrogen transplant in leaching solution under the cracking conditions in rice field by the simulations of soil columns and rice field experiments which included the different water regimes, such as flooding, gradual soil drying, intermittent irrigation and alternation of wetting and drying, and different amount of sulfate ammonium and DCD inhibitor adding. In addition, the article made use of the DNDC model to simulate and compare the N₂O emissions rules in rice fields, in order to get the fittest model in Southern China paddy fields. The main results were summarized as follows: (l)The bigger N₂O emissions(Y) flux appeared in the thrice soil drying in paddy fields. Soil water content(xl), soil temperature(x2) and Eh(x3) had the effects on N₂O emissions by the equation: Y= -1498.95+2895.48X1+50.63X2 -96.99X1 *X2+0.006X2*X3. There are significant negative correlations between hydroxylamine reducatse activity and N₂O emissions flux in rice fields. When nitrogen and phosphor fertilizers added to fields, the key contributors occurred at the medium and upper stage of rice growth. Nitrogen and phosphors fertilizers promoted strongly the N₂O emissions since the first soil drying ( after the 34th transplant day). But lower nitrogen fertilizers adding resulted in worse stimulation of N₂O emissions than higher nitrogen, and there are no significant differences in N₂O emissions among N2(180kgN ha"1), Nl(90kgN ha'1) and N0(0 kgN ha"1) treatments. But when the soil drying was end, the supplemental urea nitrogen with higher level N content (N3, 270kgN ha"' and N4, 360kgN ha"1 ) could stimulate the N₂O emissions.(2) The produce of cracks had big differences on some reductase activities .The soil columns experiments showed Qingzini and Huangbiantian with high clay content produced cracks, but Xiaofentu with low clay content didn't appear cracks. The total length, maximal width and depthof cracks in same area had similar curves between Qingzini and Huangbiantian during the course of incubation. The produce of cracks had important effects on the activities of nitrate reducatse and hydroxylamine reducatse, except nitrite reductase. From produce to stabilization of cracks,the acitivities of nitrate reductase, nitrite reductase and hydroxylamine reductase changed similialy in cracked soils.The nitrate reductase activity decreased quickly, and hydroxylamine reductase activity fluctuated in soil drying of rice tillering in fields. In addition, we almost couldn't get the nitrite reductase activity in the process.(3) Cracks had significant effects on nitrate and nitrite concentration in leaching solution in the process of natural soil drying, but there are no effects on ammonium concentration. There are negative linearity correlations between cracks mean volume, surface area and depth and nitrate concentration in leaching solution. At the same time, the variation of nitrate concentration had effects on the nitrite concentration in soil leaching solution. The generation of cracks changed the soil redox and Eh value in soil leaching solution. There are negative correlations among nitrate, nitrite and total nitrogen in Xiaofentu without cracking leaching solution. But there are no significant correlations in cracked Qingzini and Huangbiantian leaching solution.(4) Under different water regimes conditions, The dissolved N₂O concentration was super-saturated in every experiment; the ammonium concentration in leaching solution was the lowest and highest occurred at the intermittent irrigation experiment...