Biotic manipulation effects on soil carbohydrates and microbial biomass in a cultivated soil

Plant roots, fungi and soil fauna are important soil ecological constituents, which have substantial influences on soil C dynamics and nutrient cycling. Four biotic treatments (root exclusion, earthworm addition, fungicide and insecticide applications) were designed to investigate the influences of plant roots, fungi, microarthropods and earthworms on soil C dynamics and nutrient cycling in the field. Soils under these four treatments and their untreated controls in a 40 year old mixed meadow in the Georgia piedmont were analyzed for total organic C, soil N, microbial biomass C and N, and carbohydrates for 2 years. Root exclusion significantly reduced the concentrations of soil carbohydrates (P < 0.05) as determined by gas chromatography, with only slight differences in total C between the treatments and the controls. Microbial biomass C and N were significantly lower under root exclusion and fungicide treatment than under the control. Root exclusion and inhibition of soil fungi significantly reduced soil N content, suggesting that both roots and fungi contribute to the retention of soil organic N or contribute as a source of N in cultivated soils. Microbial biomass determined in this study was strongly related to total carbohydrates (P << 0.01). Mannose, a sugar mainly of microbial origin, significantly decreased in the surface 5 cm soils under fungicide treatment, while little effects of the arthropod repellant naphthalene were found in any of the measurements. Significant reductions in xylose and glucose but not in mannose after earthworm additions suggested that earthworms accelerated the turnover of plant materials in soils. The results confirmed that the short-term effects of soil biota on SOM dynamics are more profound on soil carbohydrates and microbial biomass than on total C. High mannose-to-xylose ratio indicates that the microbial-derived carbohydrates are relatively dominant in the total carbohydrate pool. When combined with information on microbial biomass, soil carbohydrate ratios may be a useful indicator of changes in SOM status as a function of biotic and management regimes in cultivated soils.