Pulses of Biogenic Nitrogen Cycling Lead to Atmospheric-Based Nutrient Spiraling in Southern California

Nitrogen deposition into arid ecosystems is increasingly shaping biogeochemical dynamics worldwide. We propose a framework to investigate the role of pulsed soil N emission as a mechanism that relays the influences of atmospheric anthropogenic N deposition to areas that otherwise would be minimally affected. We use nutrient spiraling theory, developed in lotic ecosystems, to quantify how regeneration of N by soils influences N deposition downwind of an urban plume. Our hierarchical framework of landscape functioning thereby connects ecosystem processes occurring from microbial (<1cm and hours) to regional (>100km and inter-annual) scales through reciprocal interactions among soils and microbes, pollution sources, and the atmosphere. We use southern California, USA as a case study for evaluation where we test the terrestrial nitrogen spiraling framework using a combination of field experiments, isotopic measurements, theoretical models, and atmospheric transport and chemistry model outputs. Initial results from field wetting experiments, isotope measurements and contrasting modeling approaches all support a spiraling framework and the increasing importance of soil regeneration of nitrogen to deposition farther from the urban source. Soil microbiome communities associated with nitrogen cycling vary both spatially across the deposition gradient and temporally in response to wetting events. From these results we derive terrestrial spiraling metrics that can identify consequences of both soil and anthropogenic inputs to regional nitrogen cycling. New landscape frameworks for evaluating the role of transport and transformations on N cycling can help understand and predict spatial variation in ecosystems connected across multiple scales.