Nitrogen retention in the soil of old-growth forests

The widespread use of fossil fuels and combustion engines, along with several particular agricultural practices, have transformed a lot of nitrogen from unavailable forms (atmospheric dinitrogen) into biologically usable (or “reactive”) forms. This process is called nitrogen fixation. Worldwide, human activities have tripled the rate of nitrogen fixation, compared to background rates prior to the Industrial Revolution. This accelerated conversion of nitrogen (N) into reactive forms has damaged air and water quality, created human health risks, and upended ecological communities composed of species adapted to low-nitrogen conditions.

Fortunately, soil organic matter has the capacity to capture and retain large masses of reactive N, potentially mitigating the problems of excess N. Soil organic matter is the decaying, non-living material that came from organisms (fallen leaves, dead microbial cells, molecules exuded by plant roots, etc). This organic matter is abundant in soil, but there is little information on exactly how it retains N, and on which forms of N it best retains. We recently published a paper in which we compared the retention of three forms of nitrogen (NH4, NO2, and NO3) by both biological and non-biological (abiotic) mechanisms. We added a stable isotope of nitrogen (15N) in each N form to both live and sterilized soil. We conducted this research using soils from old-growth forests, which were once thought not to retain N because their plants had stopped growing. But our renewed focus on the soils of these ancient places shows that they may indeed retain large quantities of N. This research was conducted out of Jason Kaye’s lab at Penn State.