Open AccessAlteration of belowground carbon dynamics by nitrogen addition in southern California mixed conifer forests
Author(s) -
Nowinski Nicole S.,
Trumbore Susan E.,
Jimenez Gloria,
Fenn Mark E.
Publication year - 2009
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2008jg000801
Subject(s) - nitrogen , deposition (geology) , nitrogen cycle , soil water , organic matter , soil respiration , cycling , carbon cycle , ecosystem , lignin , soil carbon , environmental chemistry , soil organic matter , zoology , chemistry , ecology , environmental science , botany , biology , forestry , geography , paleontology , organic chemistry , sediment
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. Δ 14 C 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, Δ 14 C of respired CO 2 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.