Soil Nitrogen Supply Character Of Protected Field Under Different Irrigation Methods

Soil N accumulation, nitrate leaching and ammonia volatilization were serious in protected field because of planting the same vegetable for many years, applying fertilizer and irrigation management measures unreasonably, which not only caused a lot of N fertilizer waste, but also had a series of soil degradation and environmental protection problems. According to the current actual situation, different irrigations were used in the protected field. The effect of different irrigation methods on N content, the mineralization of organic N, tomato yield and N absorption of plant were studied; N supplying capacity and N durative effect of protected field soil were discussed by chemical extraction and biological incubation methods. The experimentation began in 1998, and irrigation control upper and lower limits were seted to soil water suction for 6.3 KPa and 40 KPa. The water quantity of furrow irrigation was half of subsurface irrigation and drip irrigation's every time; tomato was cultivated in annual spring, fertilization amount was same in different plots, annual test scheme and test methods were exactly same. Soil samples were collected in the fall of 2004, the tomato fruit and plants were harvested in 2007 for the study. The results were showed as follows:1. The content and distribution of soil organic matter and total NAt 0～50 cm layers, the content soil organic matter and total N were decreased with layers' depth. At 0～20 cm layers, subsurface irrigation was advantage to accumulate soil total N, nitrate N and ammonium N, and furrow irrigation was secondly, drip irrigation was the smallest.At all layer levels, the percentage of soil organic N to total N was more than 95%, while soil inorganic N was generally less than 5% under three irrigation treatments. But the proportion of soil organic N to total N was lower at 0～10 cm , 10～20 cm of subsurface irrigation and 0～10 cm of furrow irrigation, the content were respectively 63.29%, 77.87 % and 77.21%.At 0～80 cm layers, acidic hydrolysable N was the main components of the organic N, the order of the content of Acidic hydrolysable N forms and the proportion to total N were unknown N>ammonia N>amino acid N>amino sugar N. Compared with three irrigation methods, except for a few layers, the proportion of amino acid N, amino sugar N and ammonia N to total N in various layers of drip irrigation and subsurface irrigation were higher than furrow irrigation; but the proportion of unknown N and Non-acidic hydrolysable N to total N of furrow irrigation were higher than drip irrigation and subsurface irrigation.2. The characteristics of soil organic N mineralizationTwo-pool model was fitted to describe the process of organic N mineralization, and it was optimal. N mineralization parameters showed that drip irrigation treatment not only could increase N mineralization potential but also could promote active organic N amount. For the three irrigation methods, furrow irrigation was advantage to accumulate organic N, drip irrigation could improve the quality of soil organic N and increase the capacity of organic N mineralization at 0～20 cm layers. organic N mineralization of subsurface irrigation was higher than furrow irrigation' and lower than drip irrigation'.Temperature and moisture were important factors to organic N mineralization. There had significant positive correlation between N mineralization rate and temperature or moisture, and had a significant interaction. At 0～20 cm layers, temperature of 35℃and the water content of 83% to 100% field capacity were appropriate temperature and moisture levels for protected field soil.3. The relationship between organic N forms and N mineralization potential, soil N supplying capacityAt 0～50 cm layers, there had closely relationship between organic N forms and N mineralization potential. Amino acid N had important contribution to mineralized N. The source of N mineralization potential was different under different irrigation treatments. For subsurface irrigation, N mineralization potential and amino acid N and amino sugar N had a close relationship; for drip irrigation, N mineralization potential was correlative to unknown N, while furrow irrigation, N mineralization potential was closely correlative to amino acid N and unknown N.4. A comprehensive evaluation for soil N supplying capacityThe content of initial mineral N was high at various layers under different irrigation methods, so this part of inorganic nitrogen must be considered in evaluation of soil nitrogen supplying capacity. The sum of initial inorganic nitrogen in the soil and nitrogen mineralization potential was considered as index of soil nitrogen supplying capacity. At 0～40 cm layers, soil N supplying capacity was stronger in protected field. At 0～20 cm layers, subsurface irrigation treatment had the largest soil N supplying capacity, furrow irrigation was Secondly, drip irrigation was the smallest under Long-term different irrigation treatments. At 20～30 cm layers, the order of soil N supplying capacity was furrow irrigation>drip irrigation>subsurface irrigation.Tomato production, biomass and plant N absorption was measured in 2007. The result showed that dry matter weight under drip irrigation was more than subsurface irrigation's and furrow irrigation's; Tomato yield and N absorption of fruit were increased under drip irrigation treament. The process of soil N supply had lasting stability under drip irrigation than furrow irrigation and subsurface irrigation.The difference of content, distribution, composition of Soil N and characteristics of organic N mineralization were due to the difference of distribution of soil water, the temperature, the pore of soil under different irrigation methods. Different water, gas, heat conditions affected not only the decomposition of soil organic matter but also the movement of salinity. So that inorganic N and some of low molecular organic N were continuously re-allocated. At 0～20 cm layers, furrow irrigation had a total moist, full drying process; subsurface irrigation was relatively dry and stable process, and drip irrigation was always in part of moist, relatively dry state, which were the direct causes for the difference of soil fertility characteristics.In short, according to irrigation technology, drip irrigation was best for protect field soil in saving water, improving the utilization of N and preventing soil degradation. We should be pay attention to preventing salt accumulation, and reducing the amount of applying nitrogen fertilizer under long-term subsurface irrigation. Irrigation water was wasted, the utilization of N fertilizer and tomato production were decreased under furrow irrigation treatment.N mineralization characteristics of protected field soil under long-term irrigation were discussed firstly. The results enriched the theoretical study of protect field soil N, and had great theoretical and practical significance for optimizing water and fertilizer management and preventing soil degradation.