Size, activity and catabolic diversity of the soil microbial biomass as affected by land use

Abstract

Although the effects of land use on soil organic matter are well known, the effects on the size, activity and diversity of the soil microbial community are less well documented. In this study the effects of agricultural land use (maize ( Zea mays), annual ryegrass ( Lolium multiflorum), permanent kikuyu grass pasture ( Pennisetum clandestinum), gum ( Eucalyptus grandis) and pine ( Pinus patula) forest) on microbial biomass C, microbial and exocellular enzyme activity and heterotrophic functional diversity (by analysis of catabolic response profiles to 36 substrates) was investigated on a site where the long-term history of land management was known. In comparison with native grassland, permanent kikuyu pasture resulted in an increase in organic C, microbial biomass, arginine ammonification rate, flourescein diacetate hydrolysis rate and arylsulphatase activity. By contrast, under maize under both conventional tillage (CT) and zero tillage (ZT) and annual ryegrass there was a reduction in organic matter and microbial biomass content and in measurements of microbial and enzyme activity. However, in these three arable soils, basal respiration, arginine ammonification and fluorescein diacetate (FDA) hydrolysis rates, when expressed per unit of microbial biomass, were higher than under the other land uses, suggesting the presence of a small but highly metabolically active microfloral community. Soils below pine forest tended to have a higher organic C content than those below gum forest, yet microbial biomass C, basal respiration, arginine ammonification rate and arylsulphatase activity were lower under pine forest. This was attributed to the inhibitory effects if phenolic compounds in pine needles. Principal components analysis of catabolic response profiles demonstrated that there were large differences in the catabolic capability of the soil microbial communities inhabiting the various land use types. Values for Shannon’s and Simpson’s diversity indices followed the order: pine<gum<kikuyu pasture=annual ryegrass=maize (ZT)=maize (CT)<native grassland indicating that greatest catabolic diversity existed under native grassland and least diversity existed under exotic forest. It was concluded that land use had substantial effects on the size, activity and diversity of the soil microbial community and that these changes could be broadly related to changes in soil organic matter content.