黄土丘陵沟壑区典型人工林植被恢复过程对林地凋落物分解的影响机理研究

Litter decomposition is one of the key links of material cycling of ecosystem. During the vegetation restoration of artificial forests, the succession of understory plants and the long-term growth of single tree species will certainly cause alteration in the characteristics of litter and soil, and thus influence the decomposition of litter. However, there is no consensus about how does vegetation restoration affect litter decomposition, especially, the impacts of vegetation restoration of artificial forests on this process is still not clear. Based on the previous vegetation investigation, two types of typical artificial forest in the loess hilly-gully region, Robinia pseudoacacia and Hippophae rhamnoides forests are selected as objects. The influences of the alterations in the litter characteristics (the overall substrate quality, chemical diversity and litter microbial properties and enzymatic activities) and soil characteristics (soil physical and chemical properties and soil microbial properties and enzymatic activities) during vegetation restoration on litter decomposition are separately investigated by indoor forest litter decomposition experiments and field representative litter decomposition experiments, respectively. In addition, the forest litter in-situ field decomposition experiments are conducted to investigate the contributions of the aforesaid factors to the alterations in litter decomposition and their interrelations when they act together. The results of this project might be helpful to analyze the mechanisms of the influences of the vegetation restoration on the material cycling of artificial forests in the view of litter decomposition, and accordingly provide scientific basis for the reasonable management of the artificial forests.

凋落物分解是生态系统物质循环的关键环节。人工林植被恢复中，林下植被演替和单一林木长期生长势必改变凋落物自身和林地土壤性质，从而影响该过程。然而，目前关于植被恢复过程对凋落物分解影响的研究结果尚无统一结论，特别是人工林植被恢复过程对其影响机制尚不清楚。本项目在前期植被调查基础上，以黄土丘陵沟壑区刺槐和沙棘两种典型人工林为研究对象，分别通过林地凋落物室内分解试验和代表性凋落物野外分解试验，在排除彼此干扰的情况下研究人工林植被恢复过程中凋落物性质（混合凋落物整体基质质量、化学多样性、凋落物微生物和酶活性）和土壤性质（土壤理化环境、土壤微生物和酶活性）两方面因素单独变化对凋落物分解的影响机制。并通过林地凋落物野外原位分解试验研究两者共同变化时对凋落物分解影响的贡献及其交互作用。研究结果有望以凋落物分解为切入点分析人工林植被恢复过程对林地物质循环的影响机制，为选择合理的人工林管理措施提供科学依据。

植被恢复过程中凋落物自身和土壤环境的改变必然影响凋落物分解以及由其控制的林地物质循环，但其影响结果和具体机制尚不清楚。本项目通过林地凋落物室内分解、代表性凋落物野外分解以及林地凋落物原位分解实验，解析了两种人工林植被恢复过程中上述因素的变化趋势、其单独变化或共同作用时影响凋落物分解的途径。仅考虑凋落物自身性质时，其基质质量和化学多样性的提高通过改变定殖真菌群落或诱导协同分解效应提高凋落物木质纤维素酶活性，从而促进分解；仅考虑土壤环境时，土壤酶活性（菌群结构）、有效氮含量和温度的改善有利于凋落物分解，但未发现明确的路径关系；然而两种因素共同作用时，凋落物性质变化对其自身分解的影响途径不变，特定分解时期土壤水、热和（或）pH条件的改善则通过土壤微生物影响凋落物微生物群落结构特征，最终提高凋落物酶活性促进分解，反之则产生抑制作用。随植被恢复年限增加，刺槐林地凋落物基质质量提高而化学多样性降低，沙棘林地凋落物基质质量呈提高-降低-提高趋势而化学多样性提高，最终使其分解在相同土壤环境下分别呈加速和加速-减缓-加速趋势；两种林地土壤环境特性以及不同代表性凋落物分解速率随林龄增加的变化无明确统一规律；而林地凋落物在原位分解时，特定分解时期刺槐林地土壤水热条件随林龄增加的恶化以及沙棘林地土壤温度和pH条件的改善则分别使凋落物分解逐渐受到抑制和促进作用，最终使前者凋落物原位分解速率先降后增，而后者凋落物维持加快-减缓-加快变化趋势。本研究明确了人工林植被恢复过程中林地凋落物自身化学特性及特定分解时期土壤理化生物环境的改变均显著影响林地凋落物分解速率的变化趋势，且证明了上述因素的改变最终均是通过影响凋落物上定殖微生物的群落特性和酶（特别是木质素纤维素酶）活性控制其分解。研究结果有助于进一步明确人工林植被恢复过程对林地物质循环的影响机制，并据此为选择合理的人工林管理措施提供科学依据。