黄土丘陵区不同种植年限沙棘人工林土壤可溶性氮组分时空变化特征

为探究黄土丘陵区不同种植年限沙棘人工林对土壤可溶性氮组分积累的季节和坡位动态变化的影响，以志丹县金丁镇不同时间种植沙棘人工林（20年生、15年生、5年生）为研究对象，以荒草地为对照，采集0~20 cm土壤样品，分析土壤可溶性氮组分含量和比例的季节和坡位动态变化。结果表明，沙棘人工林可显著增加土壤硝态氮、铵态氮和可溶性有机氮（soluble organic nitrogen，SON）含量，沙棘人工林种植年限越长，土壤可溶性氮养分增加越明显。沙棘人工林土壤硝态氮、铵态氮和SON分别比荒草地增加109.72%、112.27%和19.62%。土壤可溶性氮组分存在显著的季节性动态变化，土壤SON春冬季（84.46 mg·kg^-1）较高，夏秋季（37.89 mg·kg^-1）较低，硝态氮春夏季（7.37 mg·kg^-1）较高，冬季（6.75 mg·kg^-1）较低，铵态氮秋季（6.58 mg·kg^-1）较高，其他季节变化不明显。土壤SON季节变化规律与硝态氮变化规律相反。土壤可溶性氮组分在坡位间的变化规律为坡下最高，坡上较低，坡下的硝态氮、铵态氮和SON分别比坡上高28.7%、3.89%和20.3%。土壤可溶性氮组分以土壤SON为主，占土壤TSN平均为81.1%，其次是土壤硝态氮，平均为10.0%，土壤铵态氮所占比例最低，平均为8.9%。在陕北黄土丘陵区，营造沙棘林能有效提高土壤氮素，并且林龄越长，土壤可溶性氮素提升越明显。

Spatial Distribution Characteristics of Soil Soluble Nitrogen Component in Hippophae rhamnoides Plantations with Different Stand Ages in Loess Hilly Region

In order to investigate the spatial distribution characteristics of soil soluble nitrogen component in Hippophae rhamnoides plantations with different stand ages in loess hilly region,three 5-,15- and 20-year-old H.rhamnoides stands were selected in Jinding Town of Zhidan County.The artificial grassland was used as the control.Soil samples were collected in 0-20 cm soil layers,and seasonal dynamic variations of the proportion of soil soluble nitrogen component were analyzed.The results showed that the measures of returning farmland to forest significantly increased the contents of soil nitrate nitrogen,ammonium nitrogen and soluble organic nitrogen.There were significant differences in the above indexes among different soil layers and different aged stands and artificial grassland.The seasonal variation of soluble organic nitrogen content was opposite to that of nitrate nitrogen.The content of soluble organic nitrogen in spring and winter (84.46 mg·kg^-1) was higher than that in summer and autumn (37.89 mg·kg^-1).The nitrate nitrogen in spring and summer (7.37 mg·kg^-1) were higher than that in autumn and winter (6.75 mg·kg^-1).The ammonium nitrogen was the highest in autumn (6.58 mg·kg^-1),and the others were not significantly different.The soluble nitrogen content of lower slope was higher than that of upper and middle slopes.The soluble nitrogen was mainly soluble organic nitrogen,accounting for 81.1%,followed by nitrate nitrogen,accounted for 10.0%,and the proportion of ammonium nitrogen was the least,which was below 8.9%.In the all,the soluble nitrogen content increased differently in different aged stands,the content of soil soluble nitrogen in the 20-year-old stand was significantly higher than that in 15- and 5-year-old stands.H.rhamnoides plantation is a suitable afforestation model,which would help improve soil nitrogen.