Nitrogen Cycling Through Secondary Succession in a Northwestern Virginia Chronosequence

Abstract

Forest clearing for agricultural use followed by cropland or pasture abandonment is a leading cause of forest disturbance. While theoretical models broadly predict the biogeochemical and structural dynamics of secondary forest succession following disturbances, much remains unknown regarding how specific components of biogeochemical cycling vary through secondary succession. Here we investigate two post-agricultural disturbance chronosequences at Blandy Experimental Farm in Boyce, VA, each consisting of an early, mid, and late successional field (∼20, ∼35, and ∼100 years old, respectively). We collected data observing a wide range of ecosystem N pools, transformations, and fluxes, including soil, litter, and foliar N; net N mineralization and nitrification; soil N leaching potential; and soil and foliar ¹⁵N natural abundance. We found that total soil N increased throughout secondary succession; while litter N concentration decreased in late succession, total litter mass increased, so total litter N increased as well. Foliar N concentration increased from early to late succession, among and within species. While soil ammonium concentration decreased through succession, soil nitrate concentration increased. Net N mineralization and nitrification both increased throughout succession, and a greater proportion of mineralized N was nitrified later in succession. Isotopic analysis suggested high N-fixation in mid-succession, and these observations taken together indicated high N availability and a relatively open N cycle later in succession in this system. Comprehensive field observations such as these are essential for honing a mechanistic understanding of successional systems and making predictions about the biogeochemical cycling and ecosystem function of current and future successional forests.