Soil nitrogen dynamics within profiles of a managed moist temperate forest chronosequence consistent with long‐term harvesting‐induced losses

Abstract

AbstractThis study investigates whether clear‐cut forest harvesting leads to alterations in the decadal‐scale biogeochemical nitrogen (N) cycles of moist temperate forest ecosystems. Using a harvested temperate red spruce (Picea rubens Sarg.) forest chronosequence in Nova Scotia, Canada, representing <1 to >80 year old postharvest conditions, alongside a reference old‐growth (125+ year old) site with no documented history of disturbance, we examine harvesting‐related changes in soil N pools and fluxes. Specifically, we quantify soil N storage with depth and age across the forest chronosequence, examine changes in physical fractions and δ15N of soil N through depth and time, and quantify gross soil N transformation rates through depth and time using a 15N isotope dilution technique. Our findings point to a large loss of total N in the soil pool, particularly within the deep soil (>20 cm) and organomineral fractions. A pulse of available mineralized N (as ammonium) was observed following harvesting (mean residence time (MRT) > 6 days), but its MRT dropped to <1 day 80 years following harvesting, in contrast to the MRT of 2–3 days observed in the reference old‐growth forest site. These mineralization patterns coupled with inferred leaching losses to groundwater are consistent with storage estimates that suggest soil N may not reaccrue for almost a century following this disturbance.