黄土丘陵区刺槐人工林林龄增加土壤微环境变化对凋落物分解的影响

凋落物所处的土壤微环境是影响其分解的关键因素之一,然而在黄土高原广泛栽植的刺槐人工林中,土壤微环境随林龄增加如何改变、其对凋落物分解过程的影响趋势尚不清楚。为明确上述问题,以油松凋落物(典型的难分解凋落物)和白三叶凋落物(易分解)为对象,分别在林龄为10、20、33 a和43 a的刺槐林地土壤表面进行为期592 d的模拟分解试验,检测凋落物分解特征以及地表土壤理化生物学性质随林龄增加的变化趋势,并分析凋落物分解速率与土壤微环境指标间的关系。结果表明:(1)随林龄增加,油松凋落物的分解速率呈先小幅降低后提高的趋势,白三叶凋落物的分解速率持续提高(P<0.05);(2)总体而言,随林龄增加林地表层土壤温度呈先降后增趋势,土壤湿度、有效磷含量和pH持续降低,而速效氮含量持续提高(P<0.05);(3)林龄增加显著改变了林地土壤微生物群落结构,特别是在各分解时间点时均导致真菌属的明显演替现象。土壤中9种凋落物分解酶的总酶活性和木质纤维素酶活性均在分解第108天时随林龄增加呈先降后增趋势,而在分解第389天和第592天时持续提高(P<0.05)。(4)油松凋落物分解速率仅与土壤总酶活性、真菌群落结构和铵态氮含量呈显著正相关,白三叶凋落物分解速率则与总酶和木质纤维素酶活性、细菌和真菌群落结构、温度和铵态氮含量显著正相关,而与土壤湿度和pH显著负相关(P<0.05)。综上所述,刺槐林龄增加引起的土壤理、化和生物微环境变化总体倾向于加速凋落物的分解过程。

Effects of the alterations in soil micro-environment with increasing stand age of Robinia pseudocacia plantation on the litter decomposition in the Loess Hilly Region

The microenvironment of litter affects its decomposition to a great extent. However, how the microenvironment alters in Robinia pseudocacia plantations with increasing stand age, which are widely distributed in the Loess Plateau, and how these alterations affect litter decomposition in plantations remain unclear. To answer these issues, the typical recalcitrant and labile litters of Pinus tabuliformis (Pt) and Trifolium repens (Tr) were chosen for this study. The tested litters were fixed on the floor of R. pseudocacia plantations with stand ages of 10, 20, 33 and 43 years for 592-day decomposition. The tendency of alterations in litter decomposition and nutrients release, and soil physical (temperature and moisture), chemical (content of available N and P, and pH) and biological properties (community composition and structure of fungi and bacteria, and enzymatic activities) at given decomposition stages with the increasing stand age were detected. The relations between litter decomposition rates and soil micro-environmental properties were analyzed. The results indicated that: (1) Pt litter exhibited decrease-increase decomposition rates with the increasing stand age, its C release rate at all stages of decomposition exhibited decrease-increase with the increasing stand age as well, while its N and P release rates only exhibited the same pattern at the late decomposition stage. Tr litter exhibited the increasing decomposition rates with increasing stand age (P<0.05), while its C, N and P release rate at all stages of decomposition the same pattern as well. (2) Generally, the surface temperature of soil in plantations exhibited a decrease-increased tendency. The soil moisture, available P content and pH decreased with the increasing stand age, while the available N content exhibited an opposite trend (P<0.05). (3) The increase in stand age significantly altered the soil fungal and bacterial community structure, especially, it caused significant successions of fungal genus. Comprehensively considering the activity of nine litter-decomposition associated enzymes, both the total enzymatic activity and lignocellulases activity exhibited a decrease-increase trend in the 108^th day of litter decomposition, while they significantly increased with increasing stand age in the 389^th and 592^nd days (P<0.05). (4) The decomposition rate of Pt litter was positively related with soil total enzymatic activity, fungal community structure and ammonium N content, while that of Tr was positively related with soil total enzymatic activity and lignocellulases activity, fungal and bacterial communities structure, temperature and ammonium N content, and negatively related with soil moisture and pH (P<0.05). In conclusion, the alterations in the physical, chemical and biological microenvironment in R. pseudocacia plantation caused by increasing stand age are generally in favor of the litter decomposition processes.