林冠氮添加和林下植被去除对杉木林土壤有机碳组分的影响

为探讨氮添加和林下植被管理对杉木人工林土壤有机碳组分的影响，以福建沙县官庄国有林场杉木人工林为对象，设置对照（CK）、林冠氮添加（CN）、林下植被去除（UR）、林冠氮添加和林下植被去除（CNUR）4个处理的野外控制实验，研究林冠氮添加和林下植被去除对土壤总有机碳、惰性有机碳、易氧化有机碳、颗粒有机碳、微生物生物量碳和水溶性有机碳的影响。结果表明：5年CN添加处理显著降低易氧化有机碳（10—20 cm）和微生物生物量碳（20—40 cm）含量，增加表层土壤颗粒有机碳占总有机碳的比例。UR处理对土壤有机碳组分的作用不显著，而CNUR处理显著降低表层土壤水溶性有机碳含量及其比例。土壤各有机碳组分均与土壤含水量、可溶性有机氮、微生物生物量氮和铵态氮呈显著正相关。研究表明，土壤活性有机碳比惰性有机碳对林冠氮添加（5年）的响应更敏感，且表现为中下层土壤响应大于表层土壤，短期氮添加能促进土壤活性有机碳的分解，而林下植被去除在短时间内可能通过改变土壤含水量和可利用氮减缓有机碳的分解与转化，从而补偿由于氮添加引起的土壤活性有机碳下降，未来需要通过长期氮添加实验进一步研究土壤有机碳动态变化的响应机制。

Effects of canopy nitrogen addition and understory removal on soil organic carbon fractions in a Chinese fir plantation

Increased nitrogen (N) deposition due to anthropogenic activities has been recognized to have important effects on forest carbon dynamic and plant diversity. However, most N manipulative experiments from the understory have suggested that external N addition can influence soil organic carbon pool, very little is known about the effects of canopy N addition on soil organic carbon fractions. To investigate these effects, a field experiment was carried out with four different treatments including control (CK), canopy N addition (CN, 25 kg N hm^-2a^-1, understory removal (UR), and canopy N addition plus understory removal (CNUR, 25 kg N hm^-2a^-1) in a Chinese fir plantation at Shaxian State Forest Farm, Fujian Province. The experiment was started in June 2014 and was a random block design with four replicates. After 5-years treatment, soil samples were collected by a stainlessness core (3.5 cm diameter, 60 cm length) and separated into three depths of 0-10 cm, 10-20 cm, and 20-40 cm. Then, total organic carbon (TOC), recalcitrant organic carbon (ROC), readily oxidizable organic carbon (ROOC), particular organic carbon (POC), microbial biomass carbon (MBC), and water-soluble organic carbon (WSOC) were determined. The results showed that soil TOC and ROC contents in the CN treatment had no significant changes, but the contents of soil ROOC at the 10-20 cm layer and MBC at the 20-40 cm layer, and the proportion of WSOC to TOC in surface soil layer significantly decreased and increased, respectively, compared to CK treatment. Moreover, single UR treatment did not influence soil organic carbon components, while CNUR treatment significantly decreased soil WSOC contents and proportions to TOC. Pearson analysis showed that both soil labile and recalcitrant organic carbons were positively correlated with soil moisture, water-soluble organic nitrogen, microbial biomass nitrogen, and ammonium nitrogen. These results indicated that soil labile organic carbon (LOC) was more sensitive to the short-term CN (5 years) compared to ROC, and the responses of subsoil (10-40 cm) were larger than top soil (0-10 cm). CN could promote soil LOC decomposition and consequently have a negative effect on the accumulation of soil organic carbon. However, UR was likely to offset the decrease of soil LOC induced by CN in the short time since UR would change the soil water and available nitrogen to decline the decomposition and transformation of soil organic carbon. All these findings suggest that the effects of CN and UR on soil organic carbon fractions and dynamics are complex, which requires a much longer-term manipulative experiment to understand the responding mechanism of soil carbon dynamics in the future.