Soil nitrogen (N) supply limits plant health and productivity in many terrestrial ecosystems. Plant-available N is highly mobile in soils and thus prone to leaching or gaseous loss. Moreover, the specific origins in soil of this essential nutrient remain unclear and measuring its availability to plants is challenging. We commonly assume that detrital polymers are the primary source of N for plants and microbes, but here we examine another, more overlooked source of N: mineral-associated organic matter (MAOM). Although MAOM is an important long-term storehouse for N, an emerging body of evidence points to the N-supplying capacity of MAOM and the plant-mediated processes driving its turnover. Specifically, root-deposited exudates may enhance the microbial and non-microbial destabilization of MAOM, which we simulated and tested in a laboratory incubation.
MAOM was physically isolated using a particle size-based technique and diluted with sterile sand. We applied three solution treatments: 13C-labelled glucose, to stimulate microbial activity and potentially the production of extracellular enzymes capable of liberating N; 13C-labelled oxalic acid, which has been demonstrated to dissolve metal-organic bonds and possibly destabilize mineral-bound and N-rich organic matter; and water, to serve as a control. Over the course of the 12-day incubation we measured CO2 respiration rates, 13C-CO2, potential enzyme activities and gross N ammonification rates.
Results/Conclusions
Both C additions stimulated CO2 respiration and MAOM degradation. Compared to the control, glucose and oxalic acid-treated soils respired 139% and 89% more C, respectively. In both cases we also observed a net positive priming effect indicating that microbes were releasing C from MAOM at a faster rate than in the control. The magnitude of this priming effect was significant: increases of 200-400% in glucose treatments and up to 280% in oxalic acid treatments. Likewise, C treatments stimulated C/N-acquiring and oxidative enzyme activities, although the response was specific to the substrate. Glucose additions specifically enhanced the production of an exo-cellulase and a chitinase, while oxalic acid suppressed most N-acquiring enzyme activities and enhanced peroxidase and phenol oxidase activities. Finally, gross ammonium production was positively associated with the priming of MAOM-C. Our results indicate that common root exudates, like glucose and oxalic acid, can significantly increase the turnover and potential release of C and N from MAOM.