COS 188-4 - How and why soil greenhouse gas fluxes change during forest succession?

Abstract: Global estimates of the climate mitigation benefits of forest regeneration are critical for achieving international climate agreements. Currently these continental- and global-scale estimates account for carbon sequestration of forest biomass accumulation but not soil fluxes of the greenhouse gasses nitrous oxide (N2O) and methane (CH4). Fluxes of soil N2O and CH4 can change during forest regeneration, but no large-scale, observation-based study exists to date that quantifies the magnitude and drivers of soil N2O and CH4 fluxes from ecosystem regeneration. To address this policy-relevant knowledge gap, we conducted a meta-analysis of existing studies that measure soil CO2, N2O and CH4 fluxes in unmanaged non-wetland ecosystems. Within our dataset, 72% of the observations were measurements through time; 8% were chronosequences, and the remaining 20% employed both approaches combined. To analyze the data we used generalized linear mixed models, with each study treated as a random effect. Ecosystem age as well as climate and environmental drivers were included as fixed effects.
Studies measuring N2O and CH4 fluxes showed a wide range of values, spanning both negative and positive annual fluxes. Most studies reported small negative CH4 fluxes regardless of ecosystem age, indicating that most ecosystems act as CH4 sinks throughout succession. In contrast, N2O fluxes remained mostly positive throughout succession (i.e., mostly N2O sources), although a decreasing trend with ecosystem age was observed (p< 0.05). We found an increasing trend in soil CO2 emissions throughout regeneration, consistent with our expectations of higher below-ground biomass causing higher levels of autotrophic respiration in the soil, though also possibly in part due to higher microbial respiration later in succession. Our results indicate that seasonality of precipitation and temperature rather than annual means better explain variation in greenhouse gas fluxes through forest regeneration. In particular, higher seasonality of precipitation was associated with higher CO2 fluxes, lower CH4 fluxes and higher N2O fluxes (p< 0.05). Meanwhile, higher temperature seasonality was associated with lower CO2 fluxes and higher N2O fluxes (p< 0.05). Our study provides the first globally comprehensive observation-based synthesis of soil fluxes of CO2, N2O and CH4 measurements – a contribution necessary for addressing many fundamental questions about the magnitude and drivers of the climate mitigation potential from large-scale ecosystem regeneration.