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 (moldboard plow), stubble mulch fallow (subtill), and no-till fallow management on aggregate size distribution and aggregate organic C and N contents compared with a native (virgin) grassland soil. Aggregate size fractions were separated by wet sieving and the portion of soil was quantified for each aggregate size class. Mineral-associated ( silt and clay) organic matter was isolated by dispersing aggregates in sodium hexametaphosphate and removing the sand and particulate organic matter (POM) by passing the dispersed aggregates through a 53-mum sieve. The POM fraction is composed primarily of partially decomposed root fragments 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 macroaggregate (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. Reduced tillage management is effective at increasing the proportion of macroaggregates and results in the accumulation of wheat (Triticum aestivum L.) derived POM within the aggregate structure compared with bare fallow 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 study relates the loss of structural stability from cultivation to losses of organic C and N from the POM fraction.