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SOIL NITROGEN TRANSFORMATIONS AND ROLE OF LIGHT FRACTION ORGANIC MATTER IN FOREST SOILS
Citation:
Compton, J E. AND R. D. Boone. SOIL NITROGEN TRANSFORMATIONS AND ROLE OF LIGHT FRACTION ORGANIC MATTER IN FOREST SOILS. SOIL BIOLOGY AND BIOCHEMISTRY. Elsevier Science Ltd, New York, NY, 34:933-943, (2002).
Description:
Depletion of soil organic matter through cultivation may alter substrate availability for microbes, altering the dynamic balance between nitrogen (N) immobilization and mineralization. Soil light fraction (LF) organic matter is an active pool that decreases upon cultivation, and recovers slowly after abandonment. Here we examined two hypotheses: 1) microbial nitrate immobilization is lower in formerly cultivated soils, and 2) the LF organic matter is the major sink for added N. We measured gross N transformations in June and August 1995 at Harvard Forest, MA, USA, in soils with different land-use history and present vegetation. In August, we determined the incorporation of 15N-ammonium and 15N-nitrate into the LF (< 1.75 g cm-3) and heavy fraction after 18 hours.
Gross production ranged from 1-8 mg ammonium-N kg-1 soil d-1 and <0.1-1 mg nitrate-N kg-1 soil d-1. While gross nitrification was more rapid under hardwoods than conifers, gross ammonium production was affected only by sampling date. Gross immobilization of both N forms was more rapid in June than during the drier month of August, and was linearly related to gravimetric soil moisture. In contrast, gross production rates were not affected by soil moisture. Contrary to our first hypothesis, lower gross immobilization cannot explain the higher net nitrification rates in the formerly cultivated soils; these rates must be explained by higher net accumulation of nitrate in the longer-term (6-week) incubations.
The LF was a much greater short-term sink for N than the heavy fraction, retaining an average of 51% of the added ammonium and 16% of the added nitrate after 18 hours. The heavy fraction retained acted as a small (<5%) but rapid sink for added N. Only 11% of the ammonium and 6% of the nitrate were held in the extractable form after 18 hours, suggesting that the remainder of the 15N added was discarded in the metatungstate solution. The LF retained more 15N per unit organic matter than the heavy fraction, indicating that this less decomposed and less physically protected organic matter serves as a more important site of N incorporation. The LF organic matter was the most important measured short-term sink for inorganic N, and may be responsible for the rapid immobilization of N in soils. Management practices that deplete the light fraction pool are expected to decrease short-term soil N incorporation.