Sensitivity of soil CO2 efflux to environmental conditions regulates how the largest terrestrial source of CO2 is responding to global climate and land use changes. Yet, little is known about the sensitivity of in situ soil CO2 efflux to temperature and moisture in the tropics, and how it is affected by land use change. This represents a large gap in our understanding of global C cycling, since tropical regions have the highest rates of net primary productivity, soil CO2 efflux, deforestation and land use- land cover change. Our study aims to provide valuable information about the controls on soil CO2 efflux in a tropical forest of Xishuangbanna, southwestern China. We compared relationships among aboveground litterfall and soil temperature, moisture and CO2 production in a mature secondary forest and rubber plantation; the dominant land use change in the region. Litter production, soil temperature and moisture were measured using litter traps, temperature and time domain reflectometry (TDR) probes, respectively. Soil CO2 efflux was measured using static field chambers and an infrared gas analyzer. All measurements were made in 2m x 2m treatment plots within four experimental blocks in each site. Treatments included aboveground litter exchange and litter exclusion to assess the influence of aboveground litter input on soil CO2 efflux.
Results/Conclusions
Our data indicate that replacement of tropical evergreen rainforests with rubber plantations changes patterns of aboveground C input via litterfall. The forest site exhibited a rather steady litter input rate, with peak production occurring during the wet season. Litterfall in rubber plantations occurs as a pulse during the cold, dry winter months. Despite large differences in litterfall patterns, soil respiration was similar in both sites: increasing soil CO2 efflux was observed during the warm, wet summer months and decreased during the cool, drier months. Site differences in soil CO2 efflux were only detected during the cool dry season. Treatment differences were not detected, likely due to the high spatial variability of soil CO2 efflux in both sites, and relatively low contribution of heterotrophic litter respiration to overall soil respiration. Exponential regression models of soil CO2 efflux and temperature were different between sites, but not strong at either site (R2<0.20). Soil moisture only marginally improved the model for the rubber plantation, but the importance of moisture as a predictor of soil CO2 efflux is likely due to seasonal dormancy of the rubber trees rather than physiological limitations on soil microbes.
