林窗尺度对侧柏人工林土壤微生物量和功能多样性的影响

微生物活性是影响土壤碳循环等地下生态系统过程的重要因素。以徐州市侧柏(Platycladus orientalis(L.)Franco)人工林为研究对象,以未受干扰的侧柏林为对照(CK),设置半径分别为4、8、12m的3种尺度近圆形林窗,从林窗边缘(D1)到距林缘4m(D2)及8m处林下(D3)水平梯度上分析土壤微生物量碳(MBC)、氮(MBN)和代谢功能多样性的变化。结果表明:1)与CK相比,林窗样地土壤MBC总体降低,MBN含量显著下降(P0.05),MBC/MBN显著上升(P0.05)。在3种尺度林窗中,MBC在大林窗偏小,MBN在小林窗偏小;MBC/MBN总体上随林窗尺度增大而减小。2)与CK相比,大中林窗降低了土壤微生物代谢活性(AWCD),小林窗变化不大。从D1到D3点,小林窗的AWCD先降后升,中林窗呈上升趋势,大林窗则相反。而且林窗降低了土壤微生物对各类碳源的利用,主要利用聚合物类和氨基酸类碳源,中林窗样点对碳水化合物和氨基酸类小分子碳源的利用最低。3)林窗总体提高了土壤微生物功能多样性,其中多样性(H')、丰富度(S)和均匀度(E)3个指数在各点之间均无显著差异,小林窗和CK的优势度指数(D_s)显著大于(P0.05)大林窗。侧柏林人工林窗对土壤微生物量和功能多样性的影响有着明显的尺度和位置梯度效应,林窗有望促进侧柏林土壤碳固持和大分子物质降解,提高其应对全球气候变化的能力,综合而言,中尺度林窗对侧柏林生态功能的发挥较为有利。

Effects of forest gap size on the biomass and functional diversity of soil microbes in Platycladus orientalis plantations

The metabolic activity of microbes profoundly influences underground ecological processes, similar to the soil carbon cycle. However, even though forest gap research has addressed underground processes, studies of microbial functional diversity remain scarce. In the present study, we mainly investigated changes in the microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial functional diversity (AWCD) of the soil along a horizontal gradient of large, medium, and small forest gaps (radii of 4, 8, and 12m, respectively) in a plantation of Platycladus orientalis (L.) Franco in Xuzhou (China) during the summer. The corresponding gap sizes, which were calculated as the ratio of gap radius to the height of trees at the edge of the gap, were 1.5, 1, and 0.5, respectively. Three forest gap gradients, namely, forest gap edge (D1), 4m outside the gap (D2), and 8m outside the gap (D3), were chosen. After forest gap formation, soil MBC decreased on the whole and MBN decreased significantly (P<0.05), where as the MBC/MBN ratio increased significantly (P<0.05). Among the three gap types, the MBC and MBN were lowest in the large and small gaps, respectively, with minimum levels of 2069.42 and 126.21mg/kg. Meanwhile, the MBC/MBN ratio varied from 10.50 to 19.96 and generally decreased with increasing gap size. In addition, forest gaps reduced soil AWCD, and the effect size varied with gap size, with the smallest effects observed in the small gaps. From D1 to D3, the AWCD of the small gaps initially declined and then increased, whereas it continuously increased in the medium gaps and continuously decreased in the large gaps. Furthermore, forest gaps also reduced the carbon consumption of soil microbes, and the use of carbohydrates and amino acids was the lowest in the medium gaps, whereas that of polymers was the highest in the medium and small gaps. In D1, the soil microbes consumed less carbon in the medium gaps than in the small gaps. The present study also revealed that forest gaps improved microbial functional diversity. Although there were no significant differences between the diversity (H''), richness (S), and evenness (E) indices of the three gradients, the dominance index (D_s) values of D3 and D1 were significantly higher in the small gap and control plot and were significantly greater than those of D1 and D2 in the large gap (P<0.05). The effect of forest gap on the diversity indices varied with gap scale and location. The main carbon sources used by the soil microbes were polymers and amino acids. In short, the effect size of forest gap on soil MBC, MBN, and microbial functional diversity in P. orientalis plantations vary remarkably with scale. Therefore, forest gaps can promote soil carbon sequestration and macromolecular substances degradation and improve the ability of forest ecosystems to adapt to global climate change. Furthermore, medium gaps were the most efficient for improving the ecological function of forest plantations.