Elevated litter inputs indicated by variation in soil 13CO2 effluxes in response to long-term manipulations of a temperate forest soil
While studies have characterized effects of plant detrital quality and quantity on short-term decomposition rates, the long-term responses of temperate forest soil organic carbon pools (SOC) to changes in detrital inputs are still poorly understood. Recent long-term studies show that increasing litterfall does not consistently increase SOC; there is a nonlinear relationship between litter loading and SOC accumulation. ‘Priming’ of SOC decomposition has been hypothesized as a process that offsets SOC increases with additional plant inputs to soils: the proposed mechanism is that the additional litter increases dissolved organic carbon (DOC), stimulates microbial activity, and decomposes more old SOC. Enzyme activities associated with decomposition and have also been shown to increase with litter additions, and thus our study sought a 13CO2 respiration signal in response to litter manipulations. Given positive relationships between decomposition stage (‘age’ of material) and the 13C content in SOC, we assessed the effects of a decade of litter manipulations on the d13CO2 signature of soil CO2 efflux. We measured CO2 evolved from soils of replicated plots (n=3, 5 x 5 m each) in 2014. Treatments included control, double leaf litter, no leaf litter, no roots, no input, wood additions, and, given nitrogen effects on decomposition and thus SOC pools, N addition and double litter + N addition ,plots were also included.
The double litter and wood treatments released CO2 that was significantly depleted in 13C compared to controls by ~1.5 units (p=0.0332 and p=0.0451 respectively). The no input treatment released CO2 with a similar isotopic signature to control while all other treatments tended toward depletion. Therefore, the addition treatments reflect more recent C inputs but the litter removal treatments do not appear to be releasing more ‘older’ carbon. As a result, To further elucidate these mechanisms, future experiments should characterize the age of different components of SOC and which components temporal response and sensitivities of those components to soil carbon inputs.