亚高山针叶林土壤有机层有效氮动态及其对外源C、N增加的响应

Dynamics on soil available nitrogen in organic layer and its responses to carbon and nitrogen supply in two subalpine coniferous forests of western Sichuan

为了解川西亚高山针叶林土壤有机层有效N的动态及其对外源C、N增加的响应,2005年4～10月采用人为施加C、N的控制实验方法,研究了川西亚高山紫果云杉(Picea purpureaMasters)林和岷江冷杉(Abies faxonianaRehder&E.H.Wilson)林下土壤有机层(OL)的半分解层(FL)和完全分解层(HL)及矿质土壤层(MS)有效氮(NH4+-N+NO3--N)动态及其对外源C、N的响应。云杉和冷杉林FL和HL的有效氮含量均显著高于MS。云杉林FL、HL和MS的NH4+-N分别为总有效氮含量的77.4%、72.4%和76.6%,而冷杉林分别为65.2%、57.6%和67.2%。有效氮和NH4+-N含量均以10月份最高,而NO3--N含量以8月份最高。外源C输入使云杉林土壤FL、HL和MS有效氮含量提高了14.6%、21.2%和28.0%,使冷杉林提高了16.7%、25.3%和5.2%。外源N输入使云杉林土壤FL、HL和MS有效氮含量提高了53.9%、11.6%和13.2%,冷杉林提高了14.2%、23.8%和50.5%。OL对外源C、N输入的响应比MS更敏感,且云杉林大于冷杉林。处理时间越长,外源C、N输入对OL和MS有效氮含量的影响越大。

As a characteristic component in the subalpine forest ecosystem, soil organic layer （OL） plays an important role in soil and water conserving, and soil fertility maintaining. As yet, little is known about the response of nutrient availability in the OL to future climate change. In order to obtain a clear knowledge of biochemical process in the OL as well as their responses to exogenous carbon and nitrogen supplies in the subalpine forest, therefore, an experiment with adding 10% carbon and 10% nitrogen to the forest floor was carried out in spruce （Picea purpurea Masters） and fir （Abies faxoniana Rehder ＆ E. H. Wilson） forests, two representative subalpine forests in Wanglang National Reserve from April to October, J 2005, based on one hypothesis of ＂the ratio of carbon to nitrogen in the OL will increase or decrease 10% in the context of climate in the future＂ and the past results of carbon and nitrogen stocks in OL and mineral soil （MS） of the corresponding forests. OL was divided into a fresh litter layer （ LL）, semi-decomposed litter layer （FL） and decomposed litter layer （HL）, and available N contents in FL and HL of OL and MS with carbon and nitrogen supplies were measured simultaneously to understand the monthly dynamics of nitrogen availability and their responses to future climate change. Available nitrogen contents in FL and HL were （ 163.4±78.4） mg·kg^-1 and （ 125.5±50.7 ） mg·kg^- 1 in spruce forest,（ 242.2±107.2） mg·kg^ -1 and （ 192.3±70.2） mg·kg^- 1 in fir forest, respectively, while those in MS were （96.2± 47.6） mg·kg^-1 and （112.3±53.4） mg·kg^-1 in the corresponding forest, implying that the OL had significant higher available nitrogen content in comparison with the MS. In spruce forest, ammonium nitrogen content accounted for 77.4%, 72.4% and 76.6% of the total available nitrogen in FL, HL and MS, respectively, while in fir forest, the percentage was 65.2%, 57.6% and 67.2%, respectively. Regardless of the forests and soil layers, the maximum contents of both available nitrogen and ammonium nitrogen were found in October, while the maximum of nitrate content was found in August. Exogenous carbon and nitrogen inputs significantly increased available nitrogen content in the OL and MS in both spruce and fir forests. Carbon inputs made FL, HL and MS increase 14.6% , 21.2% and 28.0% in spruce forest, 16.7% , 25.3% and 5.2% in fir forest. Nitrogen inputs made FL, HL and MS increase 53.9%, 11.6% and 13.2% in spruce forest, 14. 2% , 23.8% and 50.5% in fir forest. Judged by available nitrogen content, the response of OL to exogenous carbon and nitrogen inputs was more sensitive than MS, and the responses in spruce forest were more sensitive than those in fir forest. In addition, available nitrogen contents in OL and MS increased more rapid from May to Oct, 2005 with supplied carbon and nitrogen.