Effects Of Forest Gap In Pinus Massoniana Plantations On Soil Organic Carbon And Aggregates

As the main tree species of plantations in the low, hilly land of the Sichuan basin, the total area of P. massoniana plantations are more than 2 million hm2. But they have widespread ecological problems such as soil degradation, pests and diseases and decreased above/below-ground diversity, which restrict the forest sustainable development. Artificial forest gap in commercial plantations is an efficient approach to improving forest ecosystem functions. The response of soil carbon pool and aggregates to forest gap disturbance is the main respect of forest ecosystem function and important factor of gap size selection, which determine the ecosystem productivity and sustainable development. The study took seven size gaps(Gl:100 m2, G2:225 m2, G3:400 m2, G4: 625 m2, G5:900 m2, G6:1225 m2, G7:1600 m2) as objects and pure 39-year-old plantation as a control, analyzed the soil organic carbon (SOC), microbial biomass carbon (MBC), water-soluble organic carbon (WSOC), readily oxidizable carbon (ROC), carbon management index (CMI) and weight, organic carbon content and labile carbon content distribution in aggregates in gap center and gap edge in two sampling season. The result shows that:(1) Forest gap size had significant effects on content and proportion of MBC, WSOC and ROC in wet and dry season, and this effect represent more significant in wet season. With the increasing size of forest gaps, the content and proportion of MBC, WSOC and ROC increased in gap centers, which represent different trend in gap edges due to the edge effect. Forest gap size had significant effects on weight proportion and associated organic carbon content of macro-aggregate and readily oxidizable carbon content of all aggregates only in wet season. Overall, there were higher activity of microorganism, higher labile carbon content and higher weight proportion of macro-aggregates, associated organic carbon and readily oxidizable carbon content in the soil of moderate sized forest gap (900～1225m2) in the P. massoniana plantations.(2) There were no difference of the content and proportion of MBC, WSOC and ROC between gap centers and edges. The MBC and ROC content and MBC/SOC were only significantly different in gap center and edge of G1. However, MBC content and MBC/SOC in gaps were higher than understorey, WSOC, ROC content and WSOC/SOC in gaps were lower than understorey. It was showed that, soil microbial activity had improved greatly, which benefit to the improvement of soil organic carbon pool, and the possibility of water soluble organic carbon may increase. Sampling position had significant effect on weight proportion and organic carbon content of macro-aggregates and readily oxidizable carbon content of all aggregates only in wet season. In wet season, there were significant differences of aggregates associated readily oxidizable carbon content between gap centers and edges. Specifically, the readily oxidizable carbon content of< 1mm fractions were significantly higher in the edge of forest gap than that in gap center, whereas the readily oxidizable carbon content of the>1 mm fractions were significantly higher in the gap center than edge. There were significantly lower>5 mm aggregate proportion but higher 1-0.5 and 0.5-0.25 mm proportion in G3 and G4 than understorey in wet season. The organic carbon contents of 0.5-0.25 and 2-1 mm aggregates in the center of all the gaps were significantly lower than CK in wet season.(3)Sampling season had significant effects on content and proportion of different soil labile organic carbon fractions. There were higher SOC、WSOC and MBC content in wet season than dry season, but the ROC content represent different tendency. This might correlate to the higher organic carbon accumulation in wet season. Meanwhile, sampling season also had significant effects on weight proportion of aggregates and associated organic carbon and readily oxidizable carbon content. The aggregates associated organic carbon and readily oxidizable carbon content were higher in wet season than dry season. And the effects of forest gap size and sampling position on aggregates were only significant in wet season.