Twenty Years of Litter and Root Manipulations: Insights into Multi-Decadal SOM Dynamics

Abstract

An understanding of the controls on carbon (C) stored in soil organic matter is of critical importance to models of biospheric C sequestration. Although most models of ecosystem C balance assume a strong relationship between plant litter inputs and soil C accumulation, there is little experimental evidence in support of this relationship. The Detritus Input and Removal Treatment (DIRT) experiment at Harvard Forest was designed to assess how rates and sources of plant litter inputs control the accumulation and dynamics of organic matter and nutrients in soils over decadal time scales. After 20 years of litter manipulation, doubling litter inputs did not increase bulk soil C content, light or heavy fraction pools of C, or measures of labile C including laboratory-based C respiration rates, all suggestive of priming by added litter early in the experiment. The activities of two key enzymes were increased by 30% with litter additions, suggesting that soil enzyme activities are sensitive indicators of changes in residue management or detrital inputs. Sustained doubling of litter inputs did not increase soil N concentrations in either 0-10cm or 10-20cm mineral soil layers, suggesting that these soils are at or close to their N retention capacity under current climatic conditions at the site. Exclusion of either aboveground litter or roots, or both litter and roots, resulted in significant but slight (5-10%) decreases in total soil C, light fraction C, and soil respiration, but there were no differences among removal treatments. Soil C pools were remarkably resistant to destabilization over two decades, even in plots without roots and thus root-derived aggregation. Similarly, there appeared to be slow recovery of soil C and N pools after removal of O and A soil horizons but with normal litter inputs. Clearly soil C pools are not tightly coupled over decadal time-frames to changes in productivity or management that affect litter inputs.