CARBON AND NITROGEN DISTRIBUTION IN AGGREGATES FROM CULTIVATED AND NATIVE GRASSLAND SOILS

Long-term cultivation of grassland soils reduces soil organic C and N content and has been associated with a deterioration in the aggregate structure of the soil. This study examined the effects of fallow (mold board plow), stubble mulch fallow (subtill), and no-till fallow manage ment on aggregate size distribution and aggregate organic C and N cont ents compared with a native (virgin) grassland soil. Aggregate size fr actions were separated by wet sieving and the portion of soil was quan tified for each aggregate size class. Mineral-associated ( silt and cl ay) organic matter was isolated by dispersing aggregates in sodium hex ametaphosphate and removing the sand and particulate organic matter (P OM) by passing the dispersed aggregates through a 53-mum sieve. The PO M fraction is composed primarily of partially decomposed root fragment s and has an average C/N ratio of about 16. A large proportion of the total soil dry weight (50-60%) was isolated in the small macroaggregat e (250-2000 mum) size class. The native grassland soil was more stable than the cultivated soils when slaked, and the no-till soil was more stable than the stubble mulch and bare fallow soil when slaked. Reduce d tillage management is effective at increasing the proportion of macr oaggregates and results in the accumulation of wheat (Triticum aestivu m L.) derived POM within the aggregate structure compared with bare fa llow soil. It has previously been shown that the POM fraction accounts for the majority of the soil organic matter (SOM) initially lost as a result of cultivation of grassland soils. The data reported in this s tudy relates the loss of structural stability from cultivation to loss es of organic C and N from the POM fraction.