Long-term impacts of land-use change on dynamics of tropical soil carbon and nitrogen pools

Land-use changes, especially the conversion of native forest vegetation to cropland and plantations in tropical region, can alter soil C and N pools and N availability for plant uptake. Deforestation, followed by shifting cultivation and establishment of rubber tree plantation, is a common land-use change in Xishuangbanna, southwest China. However the influence of this kind of land-use change on soil C and N dynamics in this region remains poorly understood. This study was conducted to assess the effects of land-use change on soil C and N pools. Soil samples were collected on five adjacent plots, which belong to three land-use types including secondary forest-an acuminate banana (Musa itinerans) secondary forest and a male bamboo (Dendrocalamus membranaceae) secondary forest, shifting cultivation, and rubber tree (Hevea brasiliensis (H. B. K.) Muell. Arg.) plantation (one plot is 3-year-old, and another is 7-year-old). We measured soil bulk density (BD), pH value, moisture content and concentrations of soil organic carbon (SOC), total soil nitrogen (TSN), and inorganic N (NO3- -N and NH4+ -N) at 0-3, 3-20, 20-40 and 40-60 cm depths, and calculated C and N pools in 0-20, 20-40, 40-60, and 0-60 cm soil layers. Compared with the adjacent secondary forests, shifting cultivation and establishment of rubber tree plantations resulted in significant decline in concentrations and stocks of SOC and TSN in 0-20 and 0-60 cm soil layers, and increase in pH and bulk density at 0-3, 3-20, and 20-40 cm depths. Soil moisture content decreased only in 0-20 cm surface soils in shifting cultivation and plantations. The dynamics of mineral N was much more complex, which had different trends among depths and ecosystems. Compared with the secondary forests, SOC stocks in 0-20 cm surface soils in shifting cultivation and rubber tree plantations (3-year-old plantation and 7-year-old plantation) decreased by 34.0%, 33%, and 23%; and TSN stocks decreased by 32.2%, 20.4%, and 20.4%, respectively, whereas the decreases of SOC and TSN stocks in 0-60 cm soil layers were much less. The results indicated that C and N losses were mainly occurred in 0-20 cm surface soil, followed by 20-40 cm layer.