The Detrital Input and Removal Treatment (DIRT) Network

Abstract

Ecological research networks functioning across climatic and edaphic gradients are critical for improving predictive understanding of biogeochemical cycles at local through global scales. Geographically broad networks can also test the generality of results seen at an individual site, and mixed results in different ecosystems can help with the discovery of unrecognized controls on ecosystem properties. One such international network, the Detrital Input and Removal Treatment (DIRT) Project, was established to assess how rates and sources of plant litter inputs influence accumulations or losses of organic matter in forest soils. DIRT employs chronic enhancements and exclusions of aboveground litter inputs and exclusion of root ingrowth to investigate how soil organic matter (SOM) dynamics are influenced by plant detrital inputs across ecosystem and soil types. Across the DIRT network described here, SOM pools decreased slightly in response to chronic exclusion of aboveground litter, but responded only slightly, or not at all, to chronic doubling of aboveground litter inputs. Explanations for the slow or even negative response of SOM to litter additions include increased decomposition resulting in greater soil respiration, priming (increased microbial utilization of old soil organic matter following inputs of new organic matter inputs), and increased dissolved organic carbon (DOC) output. Partitioning of belowground contributions to total soil respiration was predictable based on site-level soil C and N as estimates of site fertility. Contributions to soil respiration from root respiration were negatively related to soil fertility and contributions from decomposing aboveground litter in soil were positively related to site fertility. Finally, aboveground litter exclusion had an effect on C levels similar to that of root exclusion, thus we did not see evidence that root-derived C is more critical to soil C sequestration.