Effects of land-use conversion from Masson pine forests to tea plantations on net ecosystem carbon and greenhouse gas budgets

A five-year in situ field experiment was conducted to explore the effects of land-use conversion from Masson pine forests to tea plantations on the net ecosystem carbon balance (NECB) and net greenhouse gas (GHG) budget. Compared with forests, tea plantations decreased soil methane (CH4) uptake by 23.3–73.1% and increased heterotrophic respiration (Rh) and nitrous oxide (N2O) emissions by 1.4–60.0% and 263.2–2796.7%, respectively, over the five-year study period. The forests acted as net carbon sinks (0.04 Mg C ha−1 yr−1 on average). Converting the forests to tea fields in the first and second years led to net carbon losses of 27.94 and 2.28 Mg C ha−1 yr−1, respectively. These losses could primarily be attributed to the sharp decrease in net primary productivity (NPP) by removal of the forests and increase in Rh outweighing organic fertilizer application in the first year, and Rh exceeding NPP in the second year. Tea plantations served as net carbon sinks (0.27–0.69 Mg C ha−1 yr−1) in the third to fifth years following land-use conversion, which was mainly due to organic fertilizer application and NPP outweighing Rh. The forests showed net GHG uptake (−0.07 Mg CO2-eq. ha−1 yr−1 on average). However, tea plantations functioned as net GHG emissions for the first and second years (104.3 and 8.9 Mg CO2-eq. ha−1 yr−1, respectively) following their conversion from the forests primarily due to net C losses and N2O emissions. Subsequently, tea fields transformed into net GHG sinks (−0.29 to −0.06 Mg CO2-eq. ha−1 yr−1) from the third to fifth years following land-use conversion, primarily due to the NECB overweighing N2O emissions. On a 5-year average, converting Masson pine forests to tea plantations resulted in net C losses and net GHG emissions of 5.76 Mg C ha−1 yr−1 and 22.53 Mg CO2-eq. ha−1 yr−1, respectively. In conclusion, net C losses and net GHG emissions in the first two years following the conversion of Masson pine forests to tea plantations should be considered when assessing the environmental impacts on land-use conversion.