Microbial carbon and nitrogen cycle process response to calcium additions in a northern hardwood forest.
Groffman, Peter1, 1 Institute of Ecosystem Studies, Millbrook, NY, USA
ABSTRACT- Depletion of available soil Ca has emerged as a potential major factor affecting the structure and function of northern hardwood forest ecosystems in eastern North America. At the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, an 11.8 ha watershed was treated with wollastonite (a Ca-rich silicate mineral) to replace Ca leached from the ecosystem by acidic deposition over the past several decades. The application (1.2 metric tons per ha) was made in fall 1999 and soil, stream and vegetation parameters have been monitored in the treated watershed and in reference areas at HBEF since that time. We expected that rates of microbial C and N cycling would increase in response to the treatment, especially respiration and nitrification rates. By 2002, soil pH was elevated by nearly a full unit in the Oie and by nearly 0.5 units in the Oa soil horizons. However, these changes in pH were not accompanied by changes in soil C and N cycling processes. There appeared to be a transient increase in nitrification in 2001, but this was not sustained into 2002 or 2003 and there was no effect on soil nitrate levels. There has also been no marked increase in nitrate levels in soil solutions and streams draining the treated watershed. These results suggest that microbial C and N cycling are not responsive to Ca additions and pH changes and that microbes may be adapted to the acidic soil conditions at HBEF. It is also possible that other factors (e.g., phosphorus) may limit microbial biomass and activity in these ecosystems and/or that Ca additions cause complex changes in soil C chemistry (e.g. binding of labile compounds) that limit microbial response to changes in pH.
Key words: carbon, microbial, nitrogen, calcium
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