Henry G. Spratt, Jr. 1(Henry-Spratt@utc.edu)
Department of Biological and Environmental Sciences, University of Tennessee,
Chattanooga, Tennessee, U.S.A.
Retention of nutrient cations, such as K+, Ca2+ and Mg2+, by forest soils, is essential to long-term sustainability of forest ecosystems. Cation exchange sites in forest surface soils include mineral (e.g., clays) and organic sites. The retention of anions in soils also minimises cation leaching (Johnson, 1992). Conversion of SO42- to organic sulphur (OS) by microorganisms may be important to cation retention, particularly since mineralisation of OS has been associated with ecosystem wide cation loss (Watwood et al., 1993). This study focuses on the conversion of SO42- to OS by microorganisms, and the role of this OS in retention of nutrient cations in A-horizon soils of forests in Missouri, USA.
A-horizon soils were collected from 9 plots in Deer Run State Forest (DRSF), southeastern Missouri on two sample dates in fall 1992, and from 27 permanent plots that are part of the Missouri Ozark Forest Ecosystem Project (MOFEP) near DRSF, seasonally from August 1993 until May 1996. The DRSF plots had three different experimental treatments based on timber harvest; control (approx. 40 y post clear-cut), 2-3 y post clear-cut, and 8-10 y post clear-cut. The MOFEP plots were approx. 40 y post clear-cut. At the laboratory, analyses performed on these sieved soils included: total carbon (TC), total sulphur (TS), water soluble SO42- (WS), adsorbed SO42- (AS), exchangeable K+, Ca2+ and Mg2+, rates of microbial incorporation of SO42- into OS, and estimations of microbial activity, using a lignocellulose mineralisation assay. Organic sulphur was calculated by difference [OS=TS-(WS+AS)].
Total sulphur in A-horizon soils from DRSF and MOFEP was composed primarily of OS, which accounted for 95.2 % to 97.2 % of the TS for all plots (Table 1). The production of OS by soil microorganisms ranged from 3.4 to 16.8 nmol g-1 d-1 for all plots, exhibited seasonal trends, and was correlated with lignocellulose mineralisation. In general, seasonal trends were observed for exchangeable K+, Ca2+ and Mg2+, in A-horizon soils from MOFEP plots, and the trends correlated with TC and OS. For the DRSF plots that were clear-cut, very large losses (p<0.01) of exchangeable K+ and Mg2+, OS, and reductions of OS production rates were observed in comparisons with control plot data. Although TC was also lost from clear-cut plots, the strongest correlation between loss of exchangeable K+ and Mg2+ was with loss of OS from those soils. These data suggest that the production of OS in forest soils is critical to the retention of nutrient cations, as observed by Watwood et al. (1993). It is possible that reduced input of organic matter from litter in the DRSF clear-cut sites has resulted in diminished microbial activity in these soils, leading to reduced OS production. Clear-cutting has been found to lead to reduced soil microbial biomass after labile carbon pools have been exhausted, approximately 2 y post harvest (Pietikäinen & Fritze, 1995). Whether SO42- retention in the soil, or some unique cation exchange quality of microbial OS is more important to nutrient cation retention in the forest soils tested here is currently under study.
Johnson, D. W. In Atmospheric Deposition and Forest Nutrient Cycling (eds Johnson, D. W. & Lindberg, S.E.) 275-340 (Springer-Verlag, New York, 1992).
Pietikäinen, J. & Fritze, H. (1995) Soil Biology & Biochemistry 27, 101-109 (1995).
Watwood, M. E., Sommer, A. S. & Fizgerald, J. W. Soil Sci. (Trends in Agril. Sci.). 1,103-111 (1993).
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