Impacts Of Forest Restoration On Soil Organic Carbon And Nitrogen Dynamics

Afforestation and restoration have been proposed as a major measure to sequester carbon (C) from the atmosphere and thus mitigate global climate change. Currently, human beings are experiencing increasingly severe test in the face of ecological environment problems, such as soil erosion, land use type conversion. The relationship between afforestation and environmental changes has constantly become the focus for scholars, because afforestation has not always positive effect on soil organic pool, also may brings negative effect or no significant net change in soil C pools. We investigated the effects of afforestation on soil organic C and nitrogen (N) dynamics in response to global environmental change, which could better provide theoretical guidance for government to implement large scale forestry engineering. In this study, we selected two study areas (the soil erosion area in South China with the Pinus Massoniana reforestation; and the Danjiangkou Reservoir area of central China following afforestation) to examine the impacts of reforestation/afforestation on the soil organic C and N dynamics by soil fractionation, C and N stable isotope and static chamber methods. The main conclusions are as following:1) The effects after reforestation of Pinus massoniana on soil organic carbon and nitrogen dynamics in eroded soil in south China:We found that18-30years after P. massoniana reforestation resulted in significant increases in the soil C and N stocks at our study sites. The soil C accumulation after reforestation was associated with biomass inputs and low SOM decomposition rates. But the soil C and N stock did not significantly increased after10years of P. massoniana reforestation, probably because of the fact that increased litter input was offset by fast C loss rate in the first few years. The change in the δ13C ratio indicated that the decay rates for old C in soils of the10years old site were faster compared to those at the18-30years old reforestation soils. The δ15N values of the soil organic matter (SOM) were more depleted with reforestation age, suggesting a decreased N loss with increasing reforestation age. Our results indicted soil C and N changes in this study area can be an indicator of the P. massoniana reforestation of different ages and a lesson for us to better understand how forest reforestation protects soil C and N elements from water and soil loss. Soil chemical fractionation methods combined with isotope analyses in our study were an attempt to better understand soil C and N dynamics in response to reforestation. This in turn will provide government and policy makers with useful science bases to maximize C sequestration and soil elements and water protection in eroded soil of south China2) Shifts in soil organic carbon and nitrogen fractions and dynamics for land use conversion in central China:The conversion of uncultivated areas into woodland and shrub land has been proposed as a primary means of sequestering carbon (C) from the atmosphere, thereby mitigating climate change. However, consequences of land use conversion on soil organic C and nitrogen (N) dynamics are not fully understood. In this study, we investigated soil organic C and N content, and513C and515N values of four soil aggregate sizes (>2000μm,2000-250μm,250-53μm and<53μm), and three density fractions [a free light fraction (LF), an intra-aggregate particulate organic matter fraction (iPOM) and a mineral-associated organic matter fraction (mSOM)] in the plant rhizosphere and open areas in the woodland, shrubland and adjacent cropland. Afforestation significantly increased macroaggregate (>2000μm) distribution, thereby enhancing the soil C and N content, primarily due to increases in soil organic C and N content of macro aggregates (>2000μm). The iPOM accounted for the largest (65%-87%) fraction of total soil organic matter (SOM) across land use types. Afforestation also increased soil C and N in LF, iPOM, and mSOM of macroaggregates (>2000μm). However, soil organic N content of aggregates(<2000μm) in woodland was lower than in shrubland and cropland. The C:N ratios of aggregates (2000-53μm) in woodland were significantly higher than in other land use types, whereas C:N ratios in LF, iPOM and mSOM generally decreased from woodland to shrubland to cropland across all aggregate sizes. The decay rates in LF of macroaggregates (>2000μm) was faster in woodland than in shrubland and cropland, whereas the fastest decay rates were observed in the iPOM of all aggregates in cropland. Our results suggest that physical distribution of C and N in soil strongly affect turnover and stability of SOM during land use conversion.3) Effects of land use conversions on greenhouse gas(CO2.CH4.N2O) emissions in central China:Afforestation (conversion from cropland to woodland/shrubland) increased CO2emission flux in spring and autumn, and reduced CH4emission flux in spring while increased CH4emission flux in other seasons. In contrast, afforestation increased N2O emission flux in autumn and winter. CO2emission flux reached the peak in summer, which was more sensitive to soil temperature, soil moisture and soil organic C content. CH4emission flux reached the peak in spring, which was strongly related to soil pH. N2O emission flux reached the peak in autumn and winter due to the lower soil NO3--N concentration in both seasons. The CO2emission flux in the forest soils mainly originated from the litter respiration in spring while from the root respiration in autumn. The CH4emission flux in the forest soils mainly originated from litter respiration in summer. Additionally, no significant differences between litter respiration and root respiration were found in greenhousegas (CO2, CH4, N2O) emissions in other restoration zones and seasons.