Abstract: The variation characteristics of soil carbon and nitrogen in alfalfa grassland with different growth years and fallow were studied in the Loess Plateau. Results showed that the soil organic carbon, total nitrogen and available nitrogen contents of all tested alfalfa grassland and fallow decreased in 0~300 cm soil layers with the increase of soil depth. The soil organic carbon contents of tested alfalfa grassland were higher in 0~200 cm soil layers compared to others. Soil carbon contents were significantly improved in the alfalfa grasslands of 5 and 10 growth years (P<0.05). The soil total N contents of the 0~60 cm soil layers increased with the growth years of alfalfa grassland increasing. The alfalfa grassland of more than 3 growth years significantly increased the soil total nitrogen of 60~180 cm soil layers. The alfalfa grasslands of 5 and 10 growth years significantly increased the total nitrogen contents of 0~300 cm soil layers. The alfalfa grasslands of 3, 5 and 10 growth years significantly increased the soil available nitrogen contents of 0~300 cm soil layers. The alfalfa grasslands of more than 3 growth years significantly increased the nitrate nitrogen contents of above 60 cm soil layers, decreased the nitrate nitrogen contents of 140~240 cm soil layers (P<0.05), reduced the nitrate leaching to deep soil, and increased the ammonium nitrogen contents of below 60 cm soil layers. Therefore, a rotation should be conducted after 5 growth years to maintain the balance of soil carbon and nitrogen in alfalfa grassland.

Key words: Loess Plateau, Alfalfa, Number of growth years, Changes of carbon and nitrogen

摘要： 对黄土高原地区生长年限分别为1，3，5，10年苜蓿（Medicago sativa）草地和休闲地土壤碳氮的变化特征进行研究。结果表明：在0~300 cm土层，不同生长年限苜蓿草地和休闲地土壤有机碳、全氮与碱解氮含量均随土层深度增加呈下降趋势。各年限苜蓿草地均在0~200 cm土层具有较高的土壤有机碳含量，且主要集中在0~80 cm土层内，其有机碳含量显著高于以下土层及休闲地（P<0.05）；10 a和5 a苜蓿草地提高土壤含碳量最显著（P<0.05）。在0~60 cm土层，苜蓿草地土壤全氮含量随生长年限的延长而增加，3 a以上苜蓿草地对60~80 cm土层土壤全氮增加最为显著，5 a和10 a苜蓿草地显著增加了0~300 cm土壤全氮含量。5 a，10 a和3 a苜蓿草地均显著提高了0~300 cm土壤碱解氮含量，且在140 cm以上土层提高土壤氮有效性的作用均较强。3 a以上苜蓿草地显著提高了60 cm以上土层的硝态氮含量，显著降低了140~240 cm土层的硝态氮含量（P<0.05），减少了硝态氮向土壤深层的淋溶，并提高了60 cm 以下土层的铵态氮含量，生长初期这种作用不明显。因此，当苜蓿生长5 a以后，应进行轮作以维持土壤的碳氮平衡。

关键词: 黄土高原, 苜蓿, 生长年限, 碳氮变化