Forest soil CO2 efflux models improved by incorporating topographic controls on carbon content and sorption capacity of soils.

Improved models are needed to predict the fate of carbon in forest soils under changing environmental conditions. Within a temperate sugar maple forest, soil CO2 efflux averaged 3.58 µmol m-2 s-1 but ranged from 0.02 to 25.35 µmol m-2 s-1. Soil CO2 efflux models based on temperature and moisture explained approximately the same amount of variance on gentle and steep hillslopes (r2 = 0.506, p < 0.05 and r2 = 0.470, p < 0.05 respectively). When soil carbon content and sorption capacity were added to the models, the amount of explanation increased slightly on a gentle hillslope (r2 = 0.567, p < 0.05) and substantially on a steep hillslope (r2 = 0.803, p < 0.05). Within the organic-rich surface of the mineral soil, carbon content was positively related and sorption capacity was negatively related to soil CO2 efflux rates. There were general patterns of smaller carbon pools and lower sorption capacity in the upland positions than in the lowland and wetland positions, likely a result of hydrological transport of particulate and dissolved substances downslope, leading to higher soil CO2 efflux in the upland positions. However, the magnitude of the soil CO2 efflux was mitigated by the higher sorption capacity of the organic-rich surface layer of the mineral soils, which was negatively correlated to soil CO2 efflux. More accurate estimates of forest soil CO2 efflux must take into account topographic influences on the carbon pool, the environmental factors that affect rates of carbon transformation, as well as the physicochemical factors that determine the fraction of the carbon pool that can be transformed.