OOS 24-4 - Hot moments in drylands: Biocrusts differentially respond to resource pulses on the Colorado Plateau

Background/Question/Methods
Biological soil crusts (biocrusts) are common features of drylands worldwide, composed of mosses, lichens, and cyanobacteria that live at the soil surface. Biocrusts are critical to dryland nutrient cycling as they can fix N₂, emit nitrogen (N) gases, and exchange CO₂ with the atmosphere. To predict changes in dryland nutrient cycling, the ‘pulse-dynamics’ framework postulates that wetting events create pulses of biological activity followed by an inactive period with the potential to create reserve of other resources. However, recent studies suggest that reserves may be less important to overall ecosystem function, limiting our ability to predict how global changes may alter nutrient cycling as a function of biocrust successional state. More specifically, biocrusts often deplete resources made available following a precipitation event, such as immobilizing available N leaving no reserve. Thus, nutrient losses from biocrusts may be better tracked than reserves and may help improve understanding of patterns and controls of resource pulses on ecosystem function. Here, we aim to test the sensitivity of biocrust successional states (bare soils, early and late successional crusts) to resource pulses as well as quantify N losses via gaseous pathways in response to increased rates of atmospheric N deposition.

Results/Conclusions
We experimentally wetted soils simulating a 5 mm precipitation event with three levels of N enrichment: 0, 8, and 50 kg N ha^-1 yr^-1. We found that all treatments increased CO₂ pulses after wetting and that the magnitude of the pulses increased proportionally with the added N. NO_x fluxes also increased proportionally with N, with early successional crusts producing the highest emissions around 15 µg N-NO m^-2 s^-1 at its peak. NO_x emissions from late successional crusts showed a bimodal increase – following N addition, whereas early successional crusts showed a single peak of NO_x fluxes. In contrast to CO₂ and NO_x, N₂O fluxes were low overall. Soil inorganic N remained elevated 24-hours post N addition indicating that some reserve remains, with more reserve as we move from late successional crusts to bare soil. These data suggest that biocrust successional states respond differentially to resource pulses, with early successional biocrusts favoring N losses via gaseous pathways relative to bare soils and late successional biocrusts. Climate change is known to shift successional states, thus, to better predict effects of increasing anthropogenic N on ecosystem processes, tracking losses using a successional framework is needed to understand the future of dryland N cycling.