Impacts of aboveground litter and belowground roots on soil greenhouse gas emissions: Evidence from a DIRT experiment in a pine plantation

Plant detritus alteration may influence soil greenhouse gas emissions, but the direction, extent and mechanisms remain uncertain. Here we conducted a DIRT (Detritus Input and Removal Treatment) experiment in a Pinus sylvestris var. mongolica plantation in Saihanba (Hebei Province, China), manipulating aboveground litter (L-: litter removal, L: normal litter, L+: litter doubling) and belowground roots (R-: root removal, R: with root) resulting in six treatments. We measured CO2, N2O, and CH4 fluxes during 2019 and 2020 growing seasons, along with soil physicochemical, enzymatic and microbial variables. Our findings revealed that soil CO2 and N2O emissions increased with litter input, while CH4 uptake decreased. The absence of roots decreased soil CO2 emission and CH4 uptake but increased N2O emission. Aboveground litter enhanced the carbon cycling variables, such as extractable organic carbon (EOC), the ratio of soil extractable organic carbon to extractable total nitrogen (EOC:ETN), soil carbon mineralization rate (Cmin), and peroxidase activity (PER), resulting in increased CO2 emission. In contrast, the absence of roots increased the nitrogen cycling variables, such as soil extractable total nitrogen (ETN), nitrate nitrogen content (NO3−-N), the ratio of soil extractable organic carbon to extractable total nitrogen (EOC:ETN), soil net nitrogen mineralization rate (Nmin), and the abundance of ammonia-oxidizing bacteria (AOB), leading to increased N2O emission. Moreover, soil CH4 uptake was mainly influenced by soil microclimate: aboveground litter input reduced CH4 uptake by lowering soil temperature, while belowground roots benefited CH4 uptake via reducing soil moisture by water uptake. Taken together, our study suggests that soil CO2, N2O, and CH4 emissions (or uptake) are influenced by both aboveground and belowground plant inputs through various pathways. While aboveground litter input promotes the emissions of greenhouse gasses, the mitigating effect of belowground roots on N2O and CH4 partially offsets the increment of CO2 emission in this plantation.