Interactive effects of disturbance and nitrogen av

Authors: Corinne E Block Jennifer D Knoepp Jennifer M Fraterrigo

Publish Date: 2012/03/24

Volume: 112, Issue: 1-3, Pages: 329-342

PDF Link

Abstract

Understanding the main and interactive effects of chronically altered resource availability and disturbance on phosphorus P availability is increasingly important in light of the rapid pace at which human activities are altering these processes and potentially introducing P limitation We measured P pools and fluxes in eighteen mixed forest stands at three elevations low mid high subjected to increasing atmospheric N deposition where hemlock Tsuga canadensis was absent or declining due to infestation by the exotic hemlock woolly adelgid Adelges tsugae While total soil P was similar across the study area phosphorus fractionation revealed distinct differences in the distribution of soil P fractions as elevation and N availability increased Soils from high elevation plots where N availability was greatest had 139 larger organic P pools and 55 smaller residual and refractory P pools than soils from low elevation plots with less N availability suggesting that increased N availability has driven the depletion of recalcitrant P pools by stimulating biotic demand and sequestration These differences in P distribution among fractions influenced how tree mortality affected P dynamics At high elevations plots containing declining hemlocks had significantly greater foliar P concentrations and fluxes of P from the forest floor than reference plots at similar elevations whereas at low and midelevations there were no consistent differences between plots Across all elevation classes hardwood foliar NP ratios were lower in plots with declining hemlocks Collectively these results suggest that increased N availability enhances bioavailable P which is sequestered in vegetation until disturbances liberate itThis project was supported by the USDA National Institute of Food and Agriculture Hatch project 875353 by a grant from the National Science Foundation to the Coweeta Long Term Ecological Research LTER program DEB0823293 and by the US Forest Service Southern Research Station Coweeta Hydrologic Laboratory We acknowledge the support of many individuals at Coweeta Hydrologic Lab especially K Elliott P Clinton C Brown and J Love for logistical support and data collection and processing and thank M David M Weand and two anonymous reviewers for helpful comments on earlier versions of this manuscript

Keywords: