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Title: Alteration of belowground carbon dynamics by nitrogen addition in southern California mixed conifer forests

Author: Nowinski, N.S.; Trumbore, S.E.; Jimenez, G.; Fenn, M.E.;

Date: 2009

Source: J. Geophys. Res. 114(G02005): 15p.

Publication Series: Scientific Journal (JRNL)

Description: Nitrogen deposition rates in southern California are the highest in North America and have had substantial effects on ecosystem functioning. We document changes in the belowground C cycle near ponderosa pine trees experiencing experimental nitrogen (N) addition (50 and 150 kg N ha−1 a−1 as slow release urea since 1997) at two end‐member sites along a pollution gradient in the San Bernardino Mountains, California. Despite considerable differences in N deposition between the two sites, we observed parallel changes in microbial substrate use and soil enzyme activity with N addition. Δ14C measurements indicate that the mean age of C respired by the Oa horizon declined 10–15 years with N addition at both sites. N addition caused an increase in cellulolytic enzyme activity at the polluted site and a decrease in ligninolytic enzyme activity at the unpolluted site. Given the likely differences in lignin and cellulose ages, this could explain the difference in the age of microbial respiration with N addition. Measurements of fractionated soil organic matter did not show the same magnitude of changes in response to N addition as were observed for respired C. This lesser response was likely because the soils are mostly composed of C having turnover times of decades to centuries, and 9 years of N amendment were not enough to affect this material. Consequently, Δ14C of respired CO2 provided a more sensitive indicator of the effects of N addition than other methods. Results suggest that enhanced N deposition alone may not result in increased soil C storage in xeric ecosystems.

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Nowinski, N.S.; Trumbore, S.E.; Jimenez, G.; Fenn, M.E. 2009. Alteration of belowground carbon dynamics by nitrogen addition in southern California mixed conifer forests. J. Geophys. Res. 114(G02005): 15p. doi:10.1029/2008JG000801


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