Study On The Dynamic Variation Of Nitrogen Of Rice Paddy Soil Under Conservation Tillage

Through the field test on rape-paddy fields and wheat-paddy fields in foothills in Central Hilly Area of Sichuan (take Xinhe village in Jianyang as an example)which are applied under the different tillages and same fertilization condition, the dynamic changing feature of NH₄⁺-N,NO₃^--N in the leakage water and surface water are analyzed with the self-designed lysimeter.The treatments were:A for tillage and rice seeding,B for tillage with straw cover and rice seeding,C for no tillage with straw cover and rice seeding ,D for no tillage with straw cover and transplant seeding,and the research indicated:(1) Statistics and analysis showed that after autumn rice crop rotation in 2007,the bulk density about deal B in wheat-paddy field was lower the other deals, and which reach extremely significant level with another deals (P<0.05) .The deal B was treatment that tillage and cavered with straw. Straw and surface soil mixed, and the density of straw was less than the soli particles's,which could reduce the the bulk density. After did conservation tillage, the bulk density of rape-paddy field usually droped, it size could be expressed as D> C>A>B.In the wheat-rice rotations field, the differences in the 5% level between B deal and the other three deals were significant. The bulk density of B deal was reduced, and the total porosity was increased; No-Tillage increased the bulk density, and also decreases the total porosity of C and D deals. Compared to the two year ago, each deal of the total porosity of rape- rice field had increased, which was consistent with the bulk density changes of each deal. It definitely showed B>A>C>D, but the differences were not significant between the each deal.The respective steady infiltration rates of the four deals of the wheat-rice rotations field were 1.20mm/min(A deal), 0.20 mm/min(B deal), 0.33 mm/min(C deal), 1.00 mm/min(D deal). The initial infiltration rate of C was lower than the B, but the final steady infiltration rate was higher. The steady infiltration rates of the four deals of the rape-rice rotations field showed C>D>A>B. The statistics analysis showed that the differences of the saturated hydraulic conductivity K between A deal and B, C deal in the wheat-rice rotations field were significant.The differences of the saturated hydraulic conductivity between C deal and B deal in the rape-rice rotations field are significant (P<0.05). The result showed that No-Tillage-cover can definitely improves the soil saturated hydraulic conductivity and increases soil water infiltration. With the help of double-ring infiltrometer, The results of soil permeability showed that,at the beginning of the infiltration,the first infiltration rate of soil was remarkably quick.Then as time goes by,it had reduced gradually until reaching the final infiltration rate.(2) There were more NH₄⁺-N in the surface water and the biggest variation of concentration value was the treatment of D in the wheat-paddy field, which was between 0.90 mg/L and 28.47 mg/L. Meanwhile the treatment of D, which was26.51 mg/L, was also the highest in rape-paddy field after two-day' s fertilization. All the other three treatments had higher concentration of NH₄⁺-N in the water after two-day' s fertilization except traditional plowing treatment of A. If there is a heave rain and can form runoff during this period, it will cause nitrogenous fertilizer loss.The concentration value of NO₃^-- N in the surface water of rape-paddy field reached the highest point of 16.55 mg/L on the 10th day.The value of nitrogen concentration in surface water of no-tillage field is always higher than that of plowing treatment field. So to the filed of no-tillage with straw cover, the risk of loss nitrogen is bigger when there is rainfall.(3) Under the submerging condition, NH₄⁺ -N could not be lost with the water leakage, which was adsorbed quikly by soil. The leakage speed of NH₄⁺- N was depended on the saturation time of the soil absorbing the NH₄⁺-N. The faster the time was, the faster the leakage speed was.10 days after transplanting seedlings, the concentration values of NO₃^-- N in several leakage water of wheat-paddy field change between 1.05 mg/L and 12.92 mg/L. It shows A>B>D>C in the first three days and mostly D>OB>A hereafter. On the other hand, the concentration values of NO₃^--N in rape-paddy field shows D>OB>A on the 4th day after transplanting. C and D are treated as no-tillage with straw cover, which can make the soil loose and lacunal. Therefore it improves the water transmitting ability, accelerates soil water infiltration and the NO₃^---N can move and infiltrate more easily. Even though the concentration of NH₄⁺-N and NO₃^--N in leakage water increased after fertilizer is added, the concentration of mineral N in every leakage water were almost the same since the crops had entered into flourishing period and enough nutrient was absorbed.(4) After the rice harvest, the content of NO₃^--N was significantly higher than the content of NH₄⁺-N in the 0-60cm rice soil layer. The contents of NO₃^--N and NH₄⁺-N in the deals (0-20cm ,20-40cm,40-60cm) gotten from the soil of wheat-rice rotations field are analyzed using the method of analysis of variance. The result showed that the content of NH₄⁺-N was increasing with the deeper of the soil layer. The differences of contents of NH₄⁺-N between A and the other deals in the soil (0-20 cm, 20-40 cm, 40-60cm) were extremely significant. The differences of contents of NO₃^--N between regular tillage A and the other three deals in the soil 40-60cm were extremely significant. The differences of contents of NO₃^- -N and NH₄⁺-N between A and the other three deals in the each soil layer of canola-rice rotations field were all extremely significant.