凋落物多样性及组成对凋落物分解和土壤微生物群落的影响

凋落物分解是生态系统营养物质循环的核心过程，而土壤微生物群落在凋落物分解过程中扮演着极其重要且不可替代的角色。随着生物多样性的丧失日益严峻，探讨凋落物多样性及组成对凋落物分解和土壤微生物群落的影响，不仅有助于了解凋落物分解的内在机制，而且可为退化草原生态系统的恢复提供参考。以内蒙古呼伦贝尔草原退化恢复群落中的草本植物为研究对象，依据植物多度、盖度、频度和物种的重要值及其在群落中的恢复程度筛选出排序前4的羊草（Leymus chinensis）、茵陈蒿（Artemisia capillaris）、麻花头（Serratula centauroides）、二裂委陵菜（Potentilla bifurca）的凋落物为实验材料，通过设置3种凋落物多样性水平（1，2，4），包括11种凋落物组合（单物种凋落物共4种，两物种凋落物混合共6种，四物种凋落物混合共1种），利用磷脂脂肪酸（PLFA）方法来研究分解60 d后凋落物多样性及组成对凋落物分解和土壤微生物群落的影响。结果表明：（1）凋落物物种多样性仅对C残余率具有显著影响，表现在两物种混合凋落物C残余率显著低于单物种凋落物，而凋落物组成对所观测的4个凋落物分解参数（质量、C、N残余率以及C/N）均具有显著影响；（2）凋落物物种多样性对细菌（B）含量具有显著影响，而凋落物组成对真菌（F）含量具有显著影响，两者对F/B以及微生物总量均无显著影响；（3）冗余分析结果表明凋落物组成与凋落物分解相关指标（凋落物质量、C、N残余率及C/N）和土壤微生物（真菌、细菌含量）的相关关系高于凋落物多样性。（4）进一步建立结构方程模型（Structural Equation Model，SEM）发现，凋落物初始C含量对凋落物质量、C、N残余率及C/N有显著正的直接影响；凋落物木质素含量对凋落物质量、C、N残余率有显著正的直接影响；凋落物初始N含量对N残余率有显著正的直接影响，而对C残余率及C/N有显著负的直接影响；凋落物初始C/N对凋落物质量、N残余率有显著正的直接影响，而对C/N有显著负的直接影响。此外，凋落物初始C、N、木质素含量及C/N均对真菌含量具有显著正影响，并可通过真菌对凋落物质量分解产生显著负的间接影响。以上结果表明该退化恢复区域优势种凋落物分解以初始C、木质素为主导，主要通过土壤真菌影响凋落物的分解进程，这将减缓凋落物的分解速率进而减慢草原生态系统的进程。这些结果为凋落物多样性及组成对自身分解和土壤微生物群落的影响提供了实验依据，也为进一步分析凋落物分解内在机制以及草原生态系统的恢复提供了数据参考。

Effects of litter diversity and composition on litter decomposition characteristics and soil microbial community

Litter decomposition is the core process of the nutrient substance cycles in the ecosystem, and the soil microbial community plays an important and irreplaceable role in litter decomposition. Recently, the loss of diversity has become a serious issue because of climate changes and human activities, which would influence litter diversity and composition in the community. Therefore, in the present study, we examined how litter diversity and its composition affect the soil microbial community and litter decomposition characteristics. We focused on the typical steppe community of Hulun Buir in the Inner Mongolia Steppe, China, where the grassland has degraded in the past few decades. In a restoration community, we collected the litter of the first four most abundant species:Leymus chinensis, Artemisia capillaris, Serratula centauroides, and Potentilla bifurca. Litter diversity treatments were set up under controlled conditions:1-, 2-, and 4-species treatments. The 1-species litter treatment included 4 types of composition (each of the four species), 2-species litter treatment included 6 types of composition (litter of pairwise species among the four species), and 4-species litter treatment included 1 type of composition (all the four species). After 60 days of decomposition, the litter decomposition characteristics were estimated and corresponding soil microorganisms were analyzed using the phospholipid fatty acid (PLFA) method. The results showed as follows (1) Litter diversity only had a significant effect on the C remaining rate. The C remaining rate of mixed litter was significantly lower than that of single litter, whereas litter composition had a significant effect on the four decomposition parameters (mass, C, N remaining rate, and C/N). (2) Litter diversity had a significant effect on bacterial PLFAs content, whereas litter composition had a significant effect on fungal PLFAs content. Both factors had no significant effects on F/B or total PLFAs. (3) The redundancy analysis showed that litter composition had a greater impact on the decomposition parameters (mass, C, N remaining rate, and C/N) and soil microbe (bacteria and fungi) than litter diversity. (4) The structural equation model showed that the initial litter C content had significantly and directly positive effects on litter mass, C and N remaining rate, and C/N. The initial litter lignin content had significantly and directly positive effects on litter mass, C and N remaining rate. The initial litter N content had significantly and directly positive effects on the N remaining rate, whereas it had significantly and directly negative effects on litter C remaining rate and C/N. The initial litter C/N had significantly and directly positive effects on litter mass and N remaining rate, whereas it had significantly and directly negative effects on litter C/N. In addition, initial litter C, N, and lignin content and initial litter C/N all had significant effects on the fungal content, which had significantly and indirectly negative effects on the litter mass remaining rate. The results showed that the litter of dominant species in the restoration area was difficult to decompose, even though it was in the beginning stage. The decomposition process was mainly controlled by fungi, which would decrease the litter decomposition rate and thus slow down the grassland ecosystem process. These results will provide scientific evidence for the effects of litter diversity/composition on microbial community and theoretical proof for further analysis of the effects of species composition on the ecological function of restoration grassland.