HR: 1340h
AN: B13B-0221    [Abstracts]
TI: Nitrogen Addition as a Result of Long-Term Root Removal Affects Soil Organic Matter Dynamics
AU: * Crow, S E
EM: crows@science.oregonstate.edu
AF: Oregon State University, Botany and Plant Pathology Deptartment, Cordley Hall 2082, Corvallis, OR 97333
AU: Lajtha, K
EM: lajthak@science.oregonstate.edu
AF: Oregon State University, Botany and Plant Pathology Deptartment, Cordley Hall 2082, Corvallis, OR 97333
AB: A long-term field litter manipulation site was established in a mature coniferous forest stand at the H.J. Andrews Experimental Forest, OR, USA in 1997 in order to address how detrital inputs influence soil organic matter formation and accumulation. Soils at this site are Andisols and are characterized by high carbon (C) and low nitrogen (N) contents, due largely to the legacy of woody debris and extremely low atmospheric N deposition. Detrital treatments include trenching to remove roots, doubling wood and needle litter, and removing aboveground litter. In order to determine whether five years of detrital manipulation had altered organic matter quantity and lability at this site, soil from the top 0-5 cm of the A horizon was density fractionated to separate the labile light fraction (LF) from the more recalcitrant mineral soil in the heavy fraction (HF). Both density fractions and whole soils were incubated for one year in chambers designed such that repeated measurements of soil respiration and leachate chemistry could be made. Trenching resulted in the removal of labile root inputs from root exudates and turnover of fine roots and active mycorrhizal communities as well as an increase of available N by removing plant uptake. Since 1999, soil solution chemistry from tension lysimeters has shown greater total N and dissolved organic nitrogen (DON) flux and less dissolved organic carbon (DOC) flux to stream flow in the trenched plots relative to the other detrital treatments. C/N ratio and C content of both light and heavy fractions from the trenched plots were greater than other detrital treatments. In the lab incubation, over the course of a year C mineralization from these soils was suppressed. Cumulative DOC losses and CO$_{2}$ efflux both were significantly less in soils from trenched plots than in other detrital treatments including controls. After day 150 of the incubation, leachates from the HF of plots with trenched treatments had a DOC/DON ratio significantly higher than other detrital treatments. DOC/DON ratio of both LF and bulk soils for the trenched plots did not vary among treatments. In trenched plots, under high N conditions with root-associated labile C inputs gone, there is preliminary evidence that the relative amounts and lability of soil C pools is changing. In particular, the relative concentration of recalcitrant compounds may be increasing and decomposition may be slowing down.
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting