Changes in soil CO2 and N2O emissions in response to urea and biochar-based urea in a subtropical Moso bamboo forest

Abstract

The application of biochar-based fertilizers is an effective strategy for enhancing plant growth and soil organic carbon stocks; however, the impacts of such practices on soil CO2 and N2O emissions and associated mechanisms in subtropical forests are poorly understood. A two-year field trial was conducted to determine effects of urea and biochar-based urea on variations in soil CO2 and N2O emissions, as well as various soil environmental factors in a subtropical Moso bamboo forest. Five treatments were established: a control (without fertilization), urea with low and high application rates (100 and 300 kg N ha−1, LU and HU), and biochar-based urea with low and high application rates (100 and 300 kg N ha−1, LBU and HBU). The soil CO2 emissions were increased by LU, HU, and HBU treatments during the first year compared to controls (P < 0.05). However, this stimulatory response was observed only under the HU treatment during the second year. The soil N2O emissions increased under the LU and HU treatments during the first year but were decreased under HBU (P < 0.05); during the second year, only HU had a stimulatory effect. Importantly, the soil CO2 emissions and their Q10 values under the biochar-based urea treatment were lower than those with urea (P < 0.05). The application of urea increased contents of NH4+-N, NO3−-N, water soluble organic C/N (WSOC/WSON), as well as activities of invertase, β-glucosidase, urease, and protease (P < 0.05). In comparison, application of biochar-based urea enhanced contents of WSOC, NH4+-N, and NO3−-N but decreased WSON and activities of β-glucosidase, urease, and protease (P < 0.05). Regardless of treatment, the CO2 emissions were correlated with WSOC concentration and invertase/β-glucosidase activities; the N2O emissions were associated with (P < 0.05) NH4+-N, NO3−-N, WSON and urease/protease activities. Structural equation models revealed application of urea/biochar-based urea influenced the soil CO2/N2O emissions primarily through altering the pool size of labile C/N fractions and the activities of enzymes regarding C/N cycling. In conclusion, compared with urea, application of biochar-based urea provided more advantages for mitigating soil CO2/N2O emissions in subtropical forest ecosystems.