Digging deeper: Looking for missing nitrogen in the mineral soil

Printer-friendly version

Five decades of biogeochemical budgets at the Hubbard Brook Experimental Forest have revealed a shift from having an unexplained source of nitrogen (N) to the ecosystem in the 1960s and 70s to an unexplained sink in recent years. Recent low export of N in streamwater is contrary to nutrient retention theory and model predictions that suggest the forest should leach N since biomass is no longer accumulating and atmospheric deposition is elevated. The largest pool of N is organic matter in the mineral soil and detecting change with time is particularly challenging due to its size and heterogeneity. We are investigating mineral soil N dynamics as a function of forest age in order to explain both the former missing source of N and the current missing sink, with the hypothesis that young forests mine N from the soil in order to meet growth demands and in older forests which have reached peak biomass, N reaccumulates in soil until reaching a saturation level. Across a chronosequence of sites we are constructing N budgets and studying mineral soil N dynamics, focusing on measuring N mining process measures such as  root growth and enzyme activity, and N accumulation measures such as mineral soil organic matter adsorption capacities and isotopic tracer studies.