Chronic Nitrogen Deposition Enhances Nitrogen Mineralization Potential of Semiarid Shrubland Soils.

Semiarid chaparral and coastal sage shrublands of southern California have been exposed to high levels of atmospheric N for decades, which has the capacity to increase both N and C storage and cycling in these N-limited systems. Thus we hypothesize that soil C and N mineralization will be higher in areas that have been exposed to high atmospheric N deposition. This hypothesis was tested in a 50-wk laboratoiy incubation experiment where the inorganic N (NH₄ + NO₃) and CO₂ production of chaparral and coastal sage soils were repeatedly measured. Soil was incubated in the dark at a constant temperature of 25°C and a soil moisture of 0.25 kg H₂O kg^-1 dry soil (65% water-filled pore space). Relative differences in N deposition exposure between the study sites were quantified by repeatedly rinsing and collecting the N accumulated on branch surfaces during 1 yr. Temporal trends in cumulative C and N mineralization were best described by single-pool first-order and zero-order models, respectively. Total N mineralization, but not C mineralization, increased linearly with relative N deposition, and NO₃ accounted for 95% of the total inorganic N accumulated during the 50-wk incubation. The soil δ¹⁵N natural abundance increased with relative N deposition (r = 0.85, P < 0.05) and the soil C/N ratio declined with relative N deposition (r = -0.74, P < 0.05), suggesting that N deposition exposure enhanced N mineralization in part because of increases in the soil organic matter quality (i.e., lower C/N ratio). Furthermore, soil C storage declined as a function of relative N deposition exposure, indicating that high atmospheric N inputs are not likely to stimulate soil C storage in these semiarid ecosystems. [ABSTRACT FROM AUTHOR]