Evidence for nitrogen saturation in the San Bernardino Mountains in southern California

Abstract

Elevated N deposition has occurred in the Los Angeles Basin in southern California for at least the last 40 years. Elevated streamwater NO 3 − fluxes and high nitric oxide (NO) fluxes from soil, indicators of N saturation, have recently been reported for chaparral watersheds exposed to chronic N deposition in the San Gabriel Mountains north/northeast of Los Angeles. A number of nutritional and edaphic parameters across a deposition gradient in the San Bernardino Mountains (SBM) support the hypothesis that the mixed conifer forest in the western end of the range is also N saturated. Concentrations of NO 3 − in the soil solution or in soil extracts during the summer months were 14 to 44 times higher at Camp Paivika (CP), a western high N deposition site, than at Camp Osceola (CAO) or Barton Flats (BF), eastern low-pollution sites. Accumulation of NO 3 − in foliage of bracken fern ( Pteridium aquilinum var. pubescens Underw.) and overstory species was also much greater at CP than at CAO and a site near BF. Nitric oxide fluxes in mid-August from relatively dry soil at CP were ca. 20 times higher than for typical forests in North America. Nitrous oxide (N 2O) emissions, on the other hand, were low in the SBM sites. However, emissions of NO and N 2O were several-fold higher at CP than at BF, a relatively low-pollution site. High NO emissions from otherwise undisturbed and well-drained forest soils of the western US may prove useful as a diagnostic indicator of N saturation. Nitrogen mineralization was greater at CP and Dogwood (high-pollution sites) than at CAO and Heartbar (low-pollution sites). Additional indicators of N enrichment at CP compared with the low N deposition sites include: low C:N ratios in soil and foliage, high foliar N:P ratios, higher nitrification rates and high soil acidity. Lower pH and base saturation were observed in soil from two high-pollution sites compared with two low-pollution sites. In summary, high NO emissions and elevated NO 3 − concentrations in the soil solution and in foliage, and high foliar N:P ratios at CP, indicate N in excess of biotic demand, with potential above-normal loss of N from the ecosystem - and thus, a N-saturated condition. Model outputs from the nutrient cycling model (NuCM) agreed well with field data from the SBM on elevated soil solution NO 3 − concentrations, reduced soil base saturation, and lack of a growth response to increasing N deposition.