Indicators of nitrogen status in California forests

Abstract

Indicators of ecosystem nitrogen (N) status are needed for monitoring and for identifying ecosystems that are at risk of becoming N saturated. The N chemistry of a number of plant, soil and hydrologic components were analyzed to assess the N status of mixed conifer forests across an N deposition gradient in the San Bernardino Mountains east of Los Angeles, California. All of the measured parameters at Camp Paivika on the western high-deposition end of the gradient indicated that the forest is N saturated, while forest stands on the eastern end of the gradient are N-limited. On the basis of these and other studies, the following parameters are recommended as indicators for monitoring forest N status: foliar nitrogen:phosphorus (N:P), foliar nitrate (NO₃^-), foliar growth response to N fertilization, soil carbon:nitrogen (C:N) ratio, NO₃^- in soil extracts or soil solution, and streamwater NO₃^-. However, foliar NO₃^- levels are more temporally sensitive than the other indicators mentioned, and foliar NO₃^- levels vary widely among plant species. Nitrate levels measured in KCl extracts or in soil saturation extracts were as effective as soil solution NO₃^- measurements for comparing soil NO₃^- concentrations among sites across the N deposition gradient. Extractable NO₃^- may be the preferred indicator of soil NO₃^- because it can be measured when soils are wet or dry, whereas soil solution cannot easily be obtained from soils with low water content. Elevated N trace gas emissions from soil (nitric oxide [NO] is dominant under the arid conditions of California) seem to be diagnostic of excess soil N, but field measurement of N trace gases (particularly NO) is not routine for most laboratories. Streamwater NO₃^-- concentration is a highly useful integrative indicator of the net outcome of N cycling processes within a watershed. Temporal trends in streamwater NO₃^- are indicative of the N retentiveness of the watershed. Studying a combination of suitable indicators is recommended when evaluating forest N status.

Parent Publication

• Proceedings of the International Symposium on Air Pollution and Climate Change Effects on Forest Ecosystems

Citation

Fenn, Mark E.; Poth, Mark A. 1998. Indicators of nitrogen status in California forests. In: Bytnerowicz, Andrzej; Arbaugh, Michael J.; Schilling, Susan L., tech. coords. Proceedings of the international symposium on air pollution and climate change effects on forest ecosystems. Gen. Tech. Rep. PSW-GTR-166. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 123-130