Plant nitrogen demand, not soil carbon availability, decouples net mineralization and nitrification following forest canopy disturbances

Abstract

Nitrification is a key biogeochemical process, with higher rates indicative of higher soil nitrogen availability and potential nitrogen losses from soils to waterways and the atmosphere. Heterotrophic microbes and plants compete with nitrifiers for mineralized nitrogen, thereby influencing the fraction of ammonium converted by nitrifiers to nitrate. Higher soil carbon availability fuels heterotrophic microbial ammonium demand, which can weaken the positive relationship between net nitrogen mineralization and nitrification by limiting ammonium supply to nitrifiers. Whether soil carbon availability remains a central control on the coupling of these processes under altered plant nitrogen demand remains relatively unexplored even as disturbances that reduce plant biomass increase globally. Using partially disturbed forests that vary in aboveground biomass and soil carbon availability, we test the generalizability of microbially available carbon as a control on the coupling of net nitrogen mineralization and nitrification. We analyze differences between harvested and unharvested forest stands, changes over time since harvest, and the effects of retained overstory trees. Higher levels of disturbance consistently strengthened the positive relationship between net nitrogen mineralization and nitrification. Yet reduced plant biomass, rather than microbially available carbon, primarily mediated the coupling of these processes. Our findings suggest that plant-mediated nitrogen demand can be a stronger control on the decoupling of nitrogen mineralization and nitrification than heterotrophic soil microbes following partial canopy disturbances. These results have important implications for understanding coupled carbon and nitrogen cycling processes in forests globally, highlighting a need to consider how shifting disturbance regimes could influence controls on nitrification.