Organic Matter Stimulation of Elemental Sulfur Oxidation in a Calcareous Soil

Abstract

Abstract Oxidation of S o to H 2 SO 4 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, SO 4 , 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 SO 4 246 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 SO 4 were highly correlated. In calcareous soils with low organic matter, addition of organic matter with S o stimulates S o oxidation.