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Organic Matter Stimulation of Elemental Sulfur Oxidation in a Calcareous Soil

Department of Agronomy and Horticulture, New Mexico State Univ., Las Cruces, NM 88003

*Corresponding author.

ABSTRACT

Oxidation of S^o to H₂SO₄ in some calcareous soils is a slow process that often does not increase nutrient availability. Our objective was to determine the effect of organic matter on S^o oxidation and its subsequent effect on nutrient availability. Belen clay (clayey over loamy, montmorillonitic [calcareous], thermic Typic Torrifluvent) was amended with 2.5 or 5 g S^o kg^–1 in laboratory and field experiments, respectively. Organic amendments were 6 g C kg^–1 as glucose, starch, cellulose, or sawdust in the laboratory and 15 g kg^–1 as composted horse manure, fresh cow manure, or bermudagrass [Cynodon dactylon (L.) Pers. var. dactylon] clippings in the field. The response variables included soil pH, SO₄, electrical conductivity (EC), available P and micronutrients, as well as the most-probable-number (MPN) of neutralphilic, chemoautotrophic S^o oxidizers in the laboratory experiment. Averaged across organic matter types and sampling dates, organic matter stimulation of S^o oxidation decreased pH values 0.24 and 0.16 units, increased soil SO₄ 246 and 1455 mg kg^–1, and increased soil EC 0.42 and 0.48 dS m^–1 in the laboratory and field, respectively. Organic matter amendment decreased the number of chemoautotrophic S^o oxidizers compared with soils amended with S^o alone. Organic materials capable of supporting microbial growth for longer periods resulted in greater stimulation of S^o oxidation. Enhanced S^o oxidation consistently increased only Mn availability in the field. Changes in soil EC, pH, and SO₄ were highly correlated. In calcareous soils with low organic matter, addition of organic matter with S^o stimulates S^o oxidation.

This research was supported by the New Mexico Agric. Exp. Stn.

Received for publication July 17, 1992.