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    Author(s): D. F. Cusack
    Date: 2013
    Source: Soil Biology and Biochemistry 57:192-203.
    Publication Series: Scientific Journal (JRNL)
    Station: International Institute of Tropical Forestry
    PDF: Download Publication  (1.16 MB)


    Urban areas in tropical regions are expanding rapidly, with significant potential to affect local ecosystem dynamics. In particular, nitrogen (N) availability may increase in urban-proximate forests because of atmospheric N deposition. Unlike temperate forests, many tropical forests on highly weathered soils have high background N availability, so plant growth is unlikely to respond to increased N inputs. However, microbial activity and decomposition of carbon-rich plant tissue can respond positively to added N in these forests, as has been observed in a growing number of fertilization studies. The relevance of these controlled studies to landscape-scale dynamics in urban-proximate moist tropical forests requires further investigation. I used ten forest stands in three watersheds along an urban-remote gradient in Puerto Rico to test the hypotheses that urban activity has a positive effect on soil N availability, and that decomposition enzyme activities vary with soil N. I found that mineral N, total dissolved N (TDN), and ammonium:nitrate (NH4+:NO3-) ratios varied by nearly one order of magnitude across the urban-remote gradient, and variability among urban sites was high. On average, urban forests had higher soil NO3 -, lower NH4 +, and lower C:N values than remote forests, suggesting high nitrification rates and/or external inputs of NO3- to the urban forests, and enrichment in N relative to C. Total mineral N and total dissolved N were positively correlated with the activities of enzymes that acquire carbon (C) and phosphorus (P) from organic matter. Across this gradient soil N levels were stronger predictors of enzyme activities than soil C or pH, which drive enzyme activities globally. The ratio of NH4 +:NO3 - was the strongest predictor of oxidative enzyme activities. Compared to global averages, ratios of C:N:P enzyme activities across these tropical forests indicated lower relative N-acquisition and higher relative P-acquisition, with N-acquisition lowest in the urbanwatershed, and P-acquisition highest in the upper-elevation remote watershed. These results suggest a strong urban effect on forest soil N levels, and show a link between changes in N availability and microbial processing of soil organic matter.

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    • This article was written and prepared by U.S. Government employees on official time, and is therefore in the public domain.


    Cusack, D. F. 2013. Soil nitrogen levels are linked to decomposition enzyme activities along an urban-remote tropical forest gradient. Soil Biology and Biochemistry 57:192-203.


    Rural, Suburban, Nutrient acquisition, Ammonium, Nitrate, Puerto Rico, Microbial

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