The Effect of Soil Depth and Crop Cover on Enzymatic Activities in Pahokee Muck

Abstract

The activities of acid phosphatase, invertase, cellulase, xylanase, amylase, peroxidase, and polyphenol oxidase in a Pahokee muck soil (lithic medisaprist) of the Everglades agricultural area of south Florida were measured over a 1-year period. Study sites were cropped to sugarcane (Saccharum sp. L.), St. Augustinegrass [Stenatophrum secundatum (Walt) Ktze.], and paragrass [Brachiaria mutica (Forsk.) Stapf] or were maintained as fallow. Except for peroxidase, variations in enzymatic activities caused by differences in soil depth and/or crop were greater than those caused by the variability of the seasons. Hydrolytic enzyme activities generally decreased significantly with increasing soil depth. Compared to the enzymatic activities in the fallow field, invertase, xylanase, and acid phosphatase activities were significantly enhanced by the grasses and the latter two by sugarcane. In most cases, the crop effect was restricted to the upper portion of the soil profile. Peroxidase activity was high in the surface soil of the cropped fields when the crops were actively growing and ranged from 20- to 200- fold the activity of the fallow field. Peroxidase activity in the cropped field soils declined rapidly when the soil was incubated under laboratory conditions. Polyphenol oxidase activity did not vary with soil depth in the sugarcane field but declined with soil depth in the other fields. Soil from the fallow field exhibited oxidative activity toward a variety of hydroxy- and hydroxy-methoxy-substituted benzoic and cinnamic acids but not to either the di-methoxy- or the unsubstituted acids. Arguments for the adoption of a field-moist volume method of expressing enzymatic activities in soils are presented. On this basis, the values for the enzymatic activities measured in Pahokee muck were comparable to those reported for mineral soils. Collectively, the measurements of enzymatic activities and oxygen uptake studies indicated that microbial activity existed to at least 85 cm in the soil profile. The effect of laboratory incubation on CO2 evolution and enzymatic activities suggested that microbial activity was restricted by unfavorable environmental conditions deep in the profile.