Study on the Nitrogen Mineralization Characters of Paddy Soil in Cold Area

【Objective】Compare to southern paddy fields, total applied nitrogen amount is lower and nitrogen use efficiency is higher in northeast of China. Soil N supply ability is closely related to nitrogen application and nitrogen efficiency. It is very important to compared nitrogen supply ability between southern and northern paddy soil. This would be helpful to reveal the relationship between soil nitrogen supply and high N efficiency in northern paddy.【Method】The tested soil was composed of gleyed paddy soil with high fertility and hydromorphic paddy soil with medium fertility from Jiangsu province, as well as albic paddy soil with high and medium fertility from Heilongjiang province. The samples were incubated at 25℃,30℃ and 40℃ for 28 days, respectively. Soil ammonium nitrogen was detected before and after incubation. At the same time, soil organic matter, total nitrogen and organic N forms were determined before incubation. Effective cumulated temperature model and One-pool model were fitted to the observed mineral-N vs incubation days using non-linear regression procedure. 【Result】 Ratios of the hydrolysamino acid N and amino acid N to total N were higher in southern paddy soil than the northern soil with corresponding fertility, however, the C/N in northern soil was higher. There was no significant difference of cumulative mineralization N on 28 days after incubation between southern and northern paddy soil under 25℃. When the incubation temperature was increased from 25℃ to 40℃, the cumulative mineralization N of southern soil with high fertility or medium fertility was higher than northern soil with corresponding fertility due to high organic nitrogen forms of soil or ratio of organic nitrogen forms to total nitrogen in southern soil. One-pool model between cumulative soil mineralization N and incubation days showed that soil N mineralization potential(N0) of northern soil increased by 35.9%-36.3% compared to southern soil with corresponding fertility under 25℃. On the contrary, N0 of northern soil decreased by 6.1%-32.7% and 20.9%-36.7% than southern soil with corresponding fertility under 30 ℃ and 40℃, respectively. Low soil microbial activity under high temperature may be the reason of lower N0 in northern soil. Effective cumulated temperature model between cumulative soil mineralization N and incubation days showed that the n value of same soil decreased with the increase of temperature. Southern soil had higher K value and Northern soil had higher n value. Therefore, earlier mineralization rate of southern soil was clearly higher than northern soil, but later N mineralization rate was lower. 【Conclusion】 The content of mineralized nitrogen and N0 depends on soil microbial activity, ratio of soil carbon to nitrogen, content of soil organic nitrogen and the proportion of organic form nitrogen to total nitrogen. Nitrogen mineralization potential of northern paddy soil was higher than southern soil under 25 ℃. Compared to southern paddy soil, the rate of nitrogen mineralization in northern soil was slower and faster in the early and late stage, respectively. This nitrogen mineralization character for northern paddy soil matched to rice N uptake, which was one of the reasons for high nitrogen use efficiency in cold area.