Effects Of Forest Gap Disturbance On Microbial Biomass And Bacterial Community Structure In The Process Of Foliar Litter Decomposition

Based on the idea of "Close-to-Nature Management of plantation", by artificial cutting forest gap, we explored effects of forest gap disturbance on microbial biomass and bacterial community structure in the process of Pinus massoniana and Toona ciliata foliar litter decomposition in the artificial forest gap. Our experiment began in November 2013. We chose 42-year mature P. massoniana plantation as the research object in low hilly land in the upper Yangtze river valley (in Gao county, Sichuan province), where we logged 7 kinds of square forest gap in different size (G1:100 m2, G2:225 m2, G3:400 m2, G4:625 m2, G5:900 m2, G6:1225 m2, G7:1600 m2), and understory in the pure forest as the control. The litterbag method and aperture method were adopted to analyze the effects of gap size, gap location (gap center and gap edge), species, decomposition time and soil fauna on microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and the bacterial community structure in the process of foliar litter decomposition. In addition, we also monitored ecological environment factors such as the temperature (AT), water content of foliar litter (WC), measured the content of total carbon (C), total nitrogen (N), total phosphorus (P) and dissolved organic carbon (DOC) of foliar litter, and analyzed the correlations between environmental changes caused by forest gap disturbance, litter chemical properties and microbial biomass, bacterial community diversity. The results are as follows:(1)Gap size and decomposition time had significant impacts on MBC and MBN of foliar litter, but gap location had no significant effect. The MBC and MBN of foliar leaf peaked in medium gap size (G3, G4) and small gap size (G1, G2) respectively. Overall, small and medium gap size (G1 to G4) were more helpful for the increase of the microbial biomass of foliar litter. Along with the change of decomposition time, the MBC and MBN of T. ciliate foliar litter reached the maximum in 90 d and 30 d respectively, and both of the MBC and MBN of P. massoniana foliar litter reached the highest in 180 d. Both the MBC and MBN of T. ciliate foliar litter was significantly higher than those of P. massoniana foliar litter. Besides AT and WC, the MBC and MBN in foliar litter were also closely associated with chemical properties of foliar litter (C, DOC, DOC/C, N, C/N, P).(2) Gap size, gap location, decomposition time, and their interaction determined the impacts of soil fauna under forest gap disturbance on MBC and MBN of P. massoniana foliar litter. In G1, soil fauna significantly increased the MBN of foliar litter after one year of decomposition. At gap edge, soil fauna also promoted the increment of MBN in foliar litter. From the point of decomposition time, after 30-d and 90-d decomposition, soil fauna significantly increased the MBN and MBC of foliar litter respectively. However, after 180-d decomposition, soil fauna significantly decreased the MBC. After 270-d and 360-d decomposition, soil fauna had no significant effect on the MBC and MBN of foliar litter.(3)Gap size and decomposition time showed significant effects on bacterial community structure and diversity in P. massoniana foliar litter. The formation of forest gap changed the bacterial community structure and composition in foliar litter, reduced the richness index, diversity index and evenness index of bacterial community, but increased the dominance index. The difference of bacteria community structure between 90-d and 270-d was very significant. The bacteria groups of 90-d (7 phylum and 10 class) were more than those of 270-d (5 phylum and 7 class). The richness, diversity and evenness index of bacterial community in 90-d were higher than those in 270-d, but the dominance index was lower than that in 270-d. In 90-d, the diversity of bacterial community exhibited very close correlation with gap size, WC, AT and C, N, DOC content of foliar litter. In 270-d, it was only related to AT and C. Overall, Proteobacteria was the dominant group in the process of P. massoniana foliar litter decomposition and was conducive to the cycle of carbon and nitrogen in the process of foliar litter decomposition.(4) Under forest gap, gap size determined the impacts of soil fauna on bacterial community structure in P. massoniana foliar litter, but there was no obvious regularity. Decomposition time significantly influenced the impacts of soil fauna on bacterial community structure. In 90-d, soil fauna had no significant effect on dominant groups in P. massoniana litter, but had effects on the proportion of different bacteria groups. The effects varied with gap size, but there was also no obvious regularity. In 270-d, soil fauna had significant effects on dominant bacterial groups of litter. There existed significant differences in dominant bacterial groups in different aperture.When we transformed P. massoniana plantation in the way of "Close-to-Nature Management" in test area and similar areas, we could log forest gap with an area of 100?625 m2. This way was helpful to increase the microbial biomass carbon and nitrogen in the process of foliar litter decomposition, accelerate the decomposition of foliar litter, and improve soil fertility of plantation forest. At the same time, the small and medium scale forest gap also could enhance the effects of soil fauna on microbial biomass in the earlv decomposition of P. massoniana foliar litter.