In terrestrial ecosystems, anthropogenic activities have created a novel disturbance regime, which may alter the role of natural recovery mechanisms. One such mechanism is the emergence of trees capable of symbiotic nitrogen fixation (SNF), which replenishes N lost following disturbance. In the southern Appalachians, the dominant early successional tree species is black locust (Robinia pseudoacacia), a nitrogen-fixing legume. Our previous study on SNF by black locust indicated that some stands fixed a total of 300 kg ha-1 following disturbance. This high input of N led to the question: does historical SNF increase rates of reactive N cycling and abundance of N cycling organisms today? To address this question, we selected 43 long-term vegetation plots at the Coweeta Hydrologic Lab that ranged in disturbance history and topographic position. Soil moisture and potential nitrification and mineralization rates were determined for each plot. Potential denitrification, or the gaseous loss of N as N2O, was determined in the lab using an assay approach that supplied nutrients and substrate for the denitrifying microbial community. The abundances of microbial genes responsible for the rate limiting steps of nitrification (i.e., amoA) and denitrification (i.e., nirS, nirK, nosZ) were determined using qPCR.
Results/Conclusions
We found a complex relationship between SNF, potential denitrification rates, and microbial gene abundance that can only be explained in the context of hillslope hydrology. Soil moisture, an indicator of suitable conditions for the facultative anaerobes responsible for denitrification, was positively related to potential denitrification rate. Soil moisture also varied predictably with topography, increasing from ridges to toe-slopes. SNF was highest along the ridge and lowest at the toe-slope, while potential denitrification rates and abundances of amoA, nirS, nirK, and nosZ showed the opposite spatial pattern—lowest at the ridges and highest at the toe-slopes. Overall, increasing levels of forest disturbance led to higher rates of SNF, thereby supporting larger populations of nitrifying and denitrifiying organisms, which may result in greater gaseous and hydrologic losses of N. However, our results indicate a spatial decoupling of natural N inputs and losses along a hillslope gradient. These findings elucidate the long-term effects of human disturbance on the mobilization of reactive N in ecosystems.