藏东南色季拉山沟壑区土壤氮素空间分布特征

Spatial distribution of soil nitrogen in gully hillsides of Sejila Mountain,Southeastern Tibet

以西藏东南部色季拉山海拔3950—4350 m为研究区,采用30×50 m网格采样法,以地统计学半变异函数为工具,研究了色季拉山森林生态系统沟谷与坡面上土壤氮素空间变异特征及模型。结果表明:土壤全氮、硝态氮和铵态氮含量均表现为0—10 cm>10—20 cm,两个层次上空间变异性表现为全氮和铵态氮0—10 cm>10—20 cm,而硝态氮表现为10—20 cm>0—10cm;不同海拔高度土壤氮含量表现为随着海拔高度的升高而增加,但这种海拔梯度效应并未达显著水平(P>0.05);沟谷区土壤氮含量高于坡面,这可能与植被残体在沟谷区的堆积分解促进氮循环有关;土壤全氮、铵态氮和硝态氮均具有中等程度的空间依赖性,其中土壤全氮空间变异符合指数模型,块金值/基台值为50%;土壤铵态氮和硝态氮含量空间变异分布均符合高斯模型,块金值/基台值分别为70.91%和37.45%;该区域土壤全氮、铵态氮和硝态氮含量空间依赖性表现为:硝态氮>全氮>铵态氮,即土壤硝态氮更易受到空间结构因素的影响,而铵态氮含量空间变化则主要受随机因素的影响。

Nitrogen （N） is one of the most important nutrients for plant growth, yet it has high spatial variability because of the effects of topography, climate, and vegetation. Therefore, it is critical to demonstrate and model the distribution of N to enhance our understanding of N variability and related factors. We used Sejila Mountain （ elevation of approximately 3950-- 4350 m） in southeastern Tibet as a model area to examine the spatial pattern of N distribution. We applied a 30 × 50-m grid sampling method and the geostatistical semivariogram analysis to study the spatial variability and distribution of the soil total N （TN）, nitrate-N （NN）, and ammonium-N （AN） in both valleys and slopes of the Sejila Mountain. The TN, NN, and AN contents in the 0--10cm layer were higher than those in the 10--20cm layer： （3.40±1.19） g/kg and （2.32±0.50） g/ kg, respectively, for TN （P 〈 0.05）; （360.55±97.72） mg/kg and （273.15±64.97） mg/kg, respectively, for AN （P 〈 0.01） ; and （98.45±22.00） mg/kg and （83.72±33.52） mg/kg, respectively, for NN （not significantly different）. AN comprises a greater fraction of the mineral N than NN, and in the 0-10cm layer, the proportions of AN and NN were （78.16：t：3.97） % and （21.84±3.97）%, respectively. The spatial variability of TN and AN in the 0--10cm layer was higher than that in the 10--20cm layer, but the opposite was found for NN. The coefficients of variation in spatial distribution for TN, AN, and NN in 0--10cm and 10--20cm layers were 34.95% and 21.49% for TN, 27.10% and 23.78% for AN, and 22.35% and 40.04% for NN, respectively. The N content in 0--10cm and 10--20cm layers increased with increasingelevation, but the increase was not significant （P 〉 0.05）. The TN content showed a higher dependency on altitude in the lO--20cm layer than in the 0--10cm layer, whereas the opposite effect was found for NN and AN. The soil N contents in the valleys were higher than those on the slopes, which may have been related to high levels of accumulation and decomposition of vegetation residues in the gully areas. These results imply that the effects of microtopography should be considered when assessing the spatial heterogeneity of N. The distribution of soil TN, AN, and NN showed a moderate spatial correlation. The spatial variability of soil TN followed an exponential function model and the nugget：sill ratio was 50%. Gaussian models were the optimal models for AN and NN, and the nugget： sill ratios were 70.91% and 37.45% for AN and NN, respectively. The spatial autocorrelation of the soil TN, NN, and AN in the study area decreased from NN to TN and AN. The spatial variability of soil NN was affected more by spatial structural factors, whereas soil AN was affected by random factors.