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 bare 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 proportion of soil was quantified for each aggregate size class. Mineralassociated (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-/un 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 fan) 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. N GRASSLAND SOILS are generally highly structured and rich in SOM. Cultivation reduces SOM content (Jenny, 1941; Haas et al., 1957) and results in a deterioration of the aggregate structure (Chancy and Swift, 1984). The POM fraction, which is composed primarily of partially decomposed root fragments, accounts for the majority of the SOM initially lost as a result of cultivation of grassland soils (Cambardella and Elliott, 1992). Reductions in aggregate stability after cultivation are most pronounced in soil macroaggregates, while the stability of soil microaggregates remains unchanged (Tisdall and Oades, 1982; Oades, 1984). As macroaggregates disintegrate with tillage, the proportion of microaggregates increases, since microaggregates are not destroyed by cultivation (Tisdall and Oades, 1980; Elliott, 1986). Microaggregates have a lower organic matter concentration than macroaggregates (Dormaar, 1983) and this organic matter is less labile than that associated with macroaggregates (Elliott, 1986; Gupta and Germida, 1988). The disintegration of macroaggregates with cultivation into nutrient-poor microaggregates and the subsequent release of plant-available nutrients may be one explanation for the observed pattern of lower orC.A. Cambardella, USDA-ARS, National Soil Tilth Lab. , 2150 Pammel Drive, Ames, IA 50011; and E.T. Elliott, Natural Resource Ecology Lab. , Colorado State Univ., Fort Collins, CO 80523. Received 30 June 1992. "Corresponding author. Published in Soil Sci. Soc. Am. J. 57:1071-1076 (1993). ganic matter contents and reduced nutrient-supplying efficiencies in cultivated soils when compared with grassland soils (Elliott, 1986). Sustainable production has become a key issue in the management of cropping systems. Reduced tillage and no-till management have been initiated to mitigate some of the detrimental effects of intensive cultivation, such as reduction of soil organic matter and the concomitant increase in erosion and decrease in soil fertility (Fenster and Peterson, 1979). Little information exists in the literature on the effect of reduced tillage or no-till management on the nutrient contents of soil aggregates and their associated organic matter. This study examined the effects of bare fallow (moldboard plow), stubble mulch fallow (subtill), and notill management on aggregate size distributions and aggregate organic C and N contents compared with native grassland. MATERIALS AND METHODS