Land use change effects on ecosystem carbon budget in the Sichuan Basin of Southwest China: Conversion of cropland to forest ecosystem.

In the humid subtropics, conversion of cropland to forest has been recognized to influence carbon cycling (e.g., soil CO2 emissions) and the associated ecosystem carbon balance. A three-year field study was conducted in situ to quantitatively evaluate effects of land use change on carbon budget in a cropland (under winter wheat-summer maize rotation) comparison with the adjacent forest ecosystem. During the three-year experimental period, on average, soil heterotrophic respirations were 35.19mgC·m-2·h-1 for the cropland and 40.02mgC·m-2·h-1 for the adjacent forest ecosystem. The quantified net primary production (NPP) were 8724.78kgC·ha-1·year-1 for the cropland (3218.14kgC·ha-1 for winter wheat season and 5506.64kgC·ha-1 for summer maize season) and 6478.99kgC·ha-1·year-1 for the adjacent forest ecosystem. Thus, the average positive net ecosystem production (NEP) of 5139.33kgC·ha-1·year-1 and 2790.43kgC·ha-1·year-1 were gained in the cropland and the adjacent forest ecosystem, respectively. Nonetheless, if take into consideration of crop grain harvest (i.e., removal), the mean NEP was only 976.69kgC·ha-1·year-1 for cropland which were over three-fold lower than for the adjacent forest ecosystem. The practice of conversion of cropland (maize-wheat rotation system) to forest consequently resulted in an average annual net carbon sequestration of 1813.74kgC·ha-1·year-1 in the study. Therefore, our findings highlight that practices of conversion of subtropical cropland to forest commonly conducted in the last decades act as sinks of atmospheric CO2 in southwest China.