Nitrogen (N) saturation is a condition signaling the extent to which N inputs have exceeded the capacity of ecosystems to assimilate N. Determining when ecosystems become N saturated has mostly relied on monitoring N losses—high N losses signal N saturation. However, this approach often fails in xeric environments where biological access to N is limited during the dry summer months, and where the onset of winter rain can promote rapid N gas evasion and leaching. While conceptual models have considered why elevated N losses can occur in xeric systems, we lack the fundamental understanding of the processes controlling N loss. Which biotic and abiotic processes interact to control N loss and our interpretation of N saturation?
Results/Conclusions
To answer this question, I combine several of our ongoing field and laboratory experiments evaluating ecosystem N losses via emission of nitric oxide (NO) and nitrous oxide (N2O). These experiments aim to disentangle i) how soil C and Fe availability control N emissions; ii) whether shifting precipitation patterns influence how microbes process soil N; and iii) how increasing rates of atmospheric N deposition affect the processes controlling N emissions. Results from these experiments suggest that soil N2O emissions increase with proximity to plants that act as “islands of fertility,” reaching levels as high as ~1,200 ng N g-1 h-1 as soil C (~0.1 – 2 %) and Fe (II) (60 – 160 mg kg-1) availability increase. Isotope labeling experiments suggest that the production of this N2O is fast, occurring within 15 min of wetting summer dry soils, and controlled by the rapid reduction of nitrate, not by the oxidation of ammonium. Xeric ecosystems appear to inherently lose N through both biotic and abiotic mechanisms, complicating assessments of N saturation.