SOIL CARBOHYDRATES IN AGGRADING AND DEGRADING AGROECOSYSTEMS - INFLUENCES OF FUNGI AND AGGREGATES

Differences in soil organic carbon, nitrogen and carbohydrates as a result of different tillage practices under continuous cropping were studied in a 12 year old sorghum/rye rotation experiment in the Georgia piedmont. Soil organic C and N concentrations in an aggrading agroecosystem (no-tillage, NT) were significantly higher than in a degrading agroecosystem (conventional tillage, CT) at all dates. Soil carbohydrates followed a pattern similar to total organic C. Carbohydrate concentrations in macroaggregates (> 250 mum) from NT surface soils were significantly higher than in microaggregates (<250 mum), although > 2000 mum aggregates contained the lowest total organic C concentration. A significantly higher mannose to xylose ratio in microaggregates than in macroaggregates suggested that organic matter (OM) in microaggregates was strongly metabolized by soil microorganism, but OM in macroaggregates was much less processed. The inhibition of fungi by the fungicide captan significantly reduced the concentrations of soil C and acid-hydrolyzable carbohydrates in NT soils but not in CT soils, indicating that fungi play a more significant role in organic C retention in the NT system than in the CT system.