BIOTIC MANIPULATION EFFECTS ON SOIL CARBOHYDRATES AND MICROBIAL BIOMASS IN A CULTIVATED SOIL

Plant roots, fungi and soil fauna are important soil ecological consti tuents, which have substantial influences on soil C dynamics and nutri ent cycling. Four biotic treatments (root exclusion, earthworm additio n, fungicide and insecticide applications) were designed to investigat e the influences of plant roots, fungi, microarthropods and earthworms on soil C dynamics and nutrient cycling in the field. Soils under the se four treatments and their untreated controls in a 40 year old mixed meadow in the Georgia piedmont were analyzed for total organic C, soi l N, microbial biomass C and N, and carbohydrates for 2 years. Root ex clusion significantly reduced the concentrations of soil carbohydrates (P < 0.05) as determined by gas chromatography, with only slight diff erences in total C between the treatments and the controls. Microbial biomass C and N were significantly lower under root exclusion and fung icide treatment than under the control. Rod exclusion and inhibition o f soil fungi significantly reduced soil N content, suggesting that bot h roots and fungi contribute to the retention of soil organic N or con tribute as a source of N in cultivated soils. Microbial biomass determ ined in this study was strongly related to total carbohydrates (P << 0 .01). Mannose, a sugar mainly of microbial origin, significantly decre ased in the surface 5 cm soils under fungicide treatment, while little effects of the arthropod repellant naphthalene were found in any of t he measurements. Significant reductions in xylose and glucose but not in mannose after earthworm additions suggested that earthworms acceler ated the turnover of plant materials in soils. The results confirmed t hat the short-term effects of soil biota on SOM dynamics are more prof ound on soil carbohydrates and microbial biomass than on total C. High mannose-to-xylose ratio indicates that the microbial-derived carbohyd rates are relatively dominant in the total carbohydrate pool. When com bined with information on microbial biomass, soil carbohydrate ratios may be a useful indicator of changes in SOM status as a function of bi otic and management regimes in cultivated soils.