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FOREST, RANGE & WILDLAND SOILS

Chronic Nitrogen Deposition Enhances Nitrogen Mineralization Potential of Semiarid Shrubland Soils

Dep. of Biological Sciences, California State Univ., San Marcos, CA 92096

^* Corresponding author (georgev{at}csusm.edu).

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 laboratory 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 ¹⁵N 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.

Abbreviations: ¹⁵N, ratio of ¹⁵N/¹⁴N of a sample relative to a standard • MRR, Motte Rimrock Reserve • SDEF, San Dimas Experimental Forest • SLW, specific leaf weight • SMER, Santa Margarita Ecological Reserve • SOFS, Sky Oaks Field Station