COS 120-4 - Interactions between fire and the functional attributes of expanding woody plants modulate nitrogen fluxes in a semi-arid grassland

Abstract: Woody plant expansion (WPE) into grasslands is a well-documented phenomenon, but its effect on biogeochemical processes remains poorly understood. In particular, how the reintroduction of fire to manage WPE influences C and N fluxes remains an outstanding question. WPE increases the relative C and N biomass pools aboveground compared to grasslands, increasing vulnerability to volatile fire losses. However, the compensatory production of long-lasting pyrogenic material from the incomplete combustion of woody biomass into mineral soil after fire can buffer C and N losses. A thorough quantification of how expanding woody species differentially contributes to these fluxes from fire in grasslands is critical to understanding ecosystem C storage and future N availability. To investigate the effects of WPE and fire on C and N cycling, we used long-term monitoring plots in a region of the Northern Great Plains that has experienced extensive WPE from two conifer species (Pinus ponderosa and Juniperus scopulorum). We assess changes in aboveground biomass of both woody species and surface soil C, pyrogenic C, and N concentrations, in response to a low-severity fire and employ a space-for-time experimental design across a woody-cover gradient to assess interactive effects of fire and WPE on C and N fluxes. Here, we show that implicit functional and morphological differences between the two species influenced the distribution and vulnerability of ecosystem N in the study system. Junipers contained higher foliar to wood N ratios (15:1) compared to ponderosa (6:1) on a per mass basis (kg N), despite similarly distributed C storage between the two species. That low-severity fire caused >90% mortality of all mature juniper stems but only 36% for ponderosa resulted in juniper disproportionately driving total woody N losses, consuming 82% of the total 0.23 Mg N ha^-1 removed from woody combustion. Similarly, each species equally contributed to C combustion losses with a total of 11.7 Mg C ha^-1 being consumed despite less ponderosa mortality. Conversely, while the fire led to large estimated losses of woody C and N, surface soil concentrations increased by 33% and 36%, respectively, after fire. Additionally, we found that soil PyC increased by 1.2 Mg ha^-1. Together these results show that the interaction of WPE and fire leads to large ecosystem N fluxes that are modulated by woody plant traits related to fire resistance and canopy chemistry and that a soil PyC additions only buffered volatilized C flux by 10%.