Microbial sulfur transformations in A-horizon soils of a Missouri Ozark forest managed for timber production by clear-cutting

Abstract

Sulfur transformations in A-horizon soils were determined for soils collected from sites managed for timber production by clear cutting between 2–3 or 8–10 y previously, and sites that were last harvested approximately 40 y ago (control sites) in Deer Run State Forest near Ellington, Missouri. Sulfur pools determined included total S, soluble sulfate, adsorbed sulfate and organic S. Sulfur transformations were quantified using trace additions of 35S-sulfate to aliquots of sieved soils, followed by 48 h aerobic incubations at the temperature of A-horizon soils determined on the date of sample collection. The 35S recovered from these soils was found in each of the S fractions quantified, with total recovery of radiolabel ranging from 85.6 to 94.6% for all samples. The amount of sulfate incorporated into soil organic matter was calculated using the percent of 35S-sulfate incorporated into the soil organic S fraction multiplied by the unlabeled adsorbed sulfate pool for that soil to account for isotopic dilution. Organic S production was significantly greater ( P < 0.05) in A-horizon soils from control sites than in either the 2–3 or 8–10 y post-harvest sites, and ranged from 0.7 to 27.3 nmol g −1 dw d −1. The short-term production of organic S in A-horizon soils was an aerobic microbial process, as addition of the cytochrome oxidase inhibitor sodium azide significantly reduced the incorporation of 35S-sulfate into the organic S fraction (1.85 vs 0.21% 35S-incorporated per 48 h incubation). A-horizon soils from clear-cut sites had approximately 40% less exchangeable K + than the control soils, and from 40 to 70% less exchangeable Mg +2 than the controls. Changes in S transformations observed for A-horizon soils in Deer Run State Forest clear-cut sites appear to be correlated with increased mobility of these cationic nutrients, and may affect the long-term nutrient status of the A-horizon soils of this ecosystem.