冬小麦/夏玉米轮作中NO3-N在土壤剖面的累积及移动

通过田间试验研究了冬小麦 /夏玉米轮作中NO- 3 N在土壤剖面的累积及移动 ,结果表明 ,尿素施入旱地土壤后 ,硝化作用一般在 7d之内完成 ,NH 4 N只在施肥后的短期内保持较高浓度 ,其它时期NH 4 N含量基本在 1～ 3mgkg- 1 范围内 ,土壤剖面不同层次NH 4 N一般也低于 4mgkg- 1 ,NH 4 N的含量不能反映土壤有效氮的水平。土壤剖面中的NO- 3 N随施氮量的增加而显著升高。在低施氮量条件下 (N <12 0kghm- 2 ) ,NO- 3 N主要在 0～ 40cm土层内移动 ,但当施氮量高于N 2 40kghm- 2 时 ,冬小麦季即有相当数量的氮移出 0～ 10 0cm土体。NO- 3 N在土体中的移动存在着很大的年际变化 ,在干旱年份 ,即使夏玉米季 ,NO- 3 N向深层移动的可能性也很小。试验年份中 ,除 1999年夏玉米季发生了较严重的气体损失以外 (该季节特别干旱 ) ,其余季节损失的肥料氮主要以NO- 3 N的形式在深层土壤剖面中累积 ,这在两个试验点的结果相当一致。

ACCUMULATION AND MOVEMENT OF NO3-N IN SOIL PROFILE IN WINTER WHEAT-SUMM ER MAIZE ROTATION SYSTEM

Nonequilibrium transport of pesticides is frequently reported to greatly affect their transport and mobility in soil.Atrazine(2-chloro-4-ethylamino-6-isopropylamino-striazine) is a kind of herbicide that is widely used for selective control of grassy weeds in corn production but poses potential danger to the quality of ground water in China.Consequently,there is interest in predicting its mobility in agricultural soils. The objective of the present study was to apply the one-dimensional convection-dispersion equation based on the local equilibrium assumption model(LEA model)and nonequilibrium assumption model(NEA model,including the two-site model and two-region model) for describing nonequilibrium sorption characteristics of atrazine during miscible displacement in wate-rsaturated homogeneous soil.Soil column experiments were conducted in duplicate with a sandy loamy soil under steady-state flow at a specific pore water velocity.A nonlinear least-squares optimization approach(CXTFIT 2.0) was used to fit breakthrough curves(BTCs) for bromide and atrazine in Exp.1.The symmetrical BTC for tracer bromide was best described by the LEA model,indicating that there were no significant effects of immobile water on transport of bromide(i.e.,no significant physical nonequilibrium in this system).The asymmetrical shape and tailing of atrazine BTC showed that atrazine was mainly influenced by chemical nonequilibrium during transport and the best fit to observed atrazine BTC was obtained with the two-site model.Model verification procedures were based on bestfit parameters optimized from the first displacement experiment(Exp.1), and then these fitted parameters were used to simulate the transport of atrazine in the duplicate experiment (Exp.2)by the chemical nonequilibrium model.The results showed that there was a good agreement between measured and simulated concentration variations for atrazine leaching in this soil column.In addition,dynamics of the flux concentration and the cumulative leaching quantity of atrazine at different depths of the soil column in Exp.2 were simulated.This study suggests that the two-site model based on the nonequilibrium theory can be used as a useful approach to better quantifying environmental fate of atrazine in a given scenario.