Characteristic is and evolution of soil nitrogen in dam land and terrace in Loess Hilly Region

صينى. 研究黄土丘陵沟壑区不同利用年限坝地和梯田土壤氮素的特征与演变，为正确评价土壤供氮能力及合理施用氮肥提供科学参考。采集不同利用年限坝地和梯田0~20、20~40和40~60 cm土样，用常规方法测定其有机质、全氮、碱解氮、矿质氮(NO3－-N和NH4+-N)含量，系统研究这些养分指标的特征、变异性及时空演化趋势。坝地和梯田表层氮素空间变异性较大；表层氮素中只有梯田碱解氮含量属“中”水平，在0~60 cm剖面内，有机质平均含量属“很低”水平，全氮和碱解氮含量属“低”水平；坝地和梯田硝铵比均大于1，表明NO3－-N是碱解氮的主体。各土壤形态氮素含量间及其与有机质含量间均达到极显著相关，它们的含量受土壤颗粒组成、坡向和坡位的影响。表层(0~20 cm)土壤有机质、全氮、碱解氮、NH4+-N和起始矿质氮含量与20~40和40~60 cm土层差异显著(P0.05)，而20~40与40~60 cm土层间差异不显著，在0~60 cm土层，其平均含量均是坝地高于梯田。坝地和梯田土壤有机质、全氮、碱解氮含量和硝铵比均随着利用年限的延长而先增加后降低；坝地土壤有机质和氮素含量开始衰退的年限(39~42年)比梯田(22~29年)滞后至少13年。黄土丘陵沟壑区坝地和梯田氮素整体缺乏；坝地氮素的现实供应优于梯田，且供应潜力更具持久性和稳定性。

إنجليزي. The characteristics and evolution of soil nitrogen under different use years in dam land and terrace in loess hilly region were studied to provide scientific instructions to evaluate exactly the supply potential of soil nitrogen and fertilize reasonably nitrogenous fertilizer. Soil samples of 0―20 cm, 20―40 cm and 40―60 cm under different use years in dam land and terrace were taken to test the contents of organic matter (OM), total nitrogen (TN), available nitrogen (AN) and mineral nitrogen (NO3－-N and NH4+-N) by using general methods. The characteristic, variability and temporal and spatial trend of these nutrient indicators were studied systematically by using statistical analytical method. The spatial variability of soil nitrogen in topsoil of dam land and terrace was large. Only average content of AN of terrace was at the “medium” level among average content of OM, TN, and AN in topsoil of dam land and terrace. Average content of OM belonged to “lower” level and average content of TN and AN was at the “low” level in 0―60 cm profile of dam land and terrace. NO3－-N/NH4+-N of dam land and terrace was more than 1,namely,NO3－-N was main body of AN. The interrelations of OM, TN, AN, NO3－-N and NH4+-N were very significant with each other, and their contents were affected to some extent by particle composition of soil, aspect and position of the slope. Significant differences (P0.05) between contents of OM, TN, AN, NH4+-N and initial mineral nitrogen of to psoil and that of 20―40 cm and 40―60 cm in dam land and terrace were found, but their content between 20―40 cm and 40―60 cm has no significant difference,and their average content of 0―60 cm in dam land were larger than that of terrace respectively. The contents of OM, TN, AN and NO3－-N/NH4+-N increased incipiently and decreased subsequently with the increase of dam land and terrace use years. The critical year (39―42 years) of the degradation for soil nitrogen in dam land hung behind 13 years at least than that of terrace (22―29 years). As a whole, soil nitrogen of dam land and terrace was poor in loess hilly region.Current supply ability of soil nitrogen in dam was higher and its potential capacity was more durative and steady than that of terrace.