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.

The amount of organic matter present in soil and the rate of soil organic matter (SOM) turnover are influenced by agricultural management practices. Because SOM is composed of a series of fractions, management practices will also influence the distribution of organic C and N among SOM pools. Our study examined SOM fractions that are occluded within the aggregate structure. Aggregates were disrupted by sonication and the disrupted soil suspensions were passed through a series of sieves to isolate size fractions. Densiometric separations were carried out on the size fractions, creating size‐density fractions. Fine‐silt‐size particles having a density of 2.07 to 2.22 g/cm3 isolated from inside macroaggregates contained the highest percentage of total soil C and N for all cultivation treatments and, because of its properties, will be referred to as the enriched labile fraction (ELF). As cultivation intensity was reduced, the amount of N in the ELF increased from 110 mg N/kg in the bare fallow treatment to 405 mg N/kg in the no‐till treatment. About 5% of the N in the ELF was mineralized during a 28‐d laboratory incubation, averaged across treatments. The proportion of N mineralized from the ELF (4.7%) was significantly higher than from intact macroaggregates (2.1%), which suggests this fraction may be protected from decomposition within the aggregate structure. We postulate that the ELF is a byproduct of microbial activity and that it contributes to binding microaggregates into macroaggregates in cultivated grassland soils.

It is hypothesized that particulate organic matter (POM) contributes to aggregate stability. However, little is known about the dynamics of the POM fraction or its role in aggregate formation. A simulated no‐till study was conducted to examine changes in free and aggregate‐associated POM during the decomposition of in situ 14C‐labeled roots during a 1‐yr incubation in a loess‐derived silt loam. Two water pretreatments (capillary‐wetted and slaked) were applied to soil samples collected during the incubation, and the samples were then wet sieved to obtain five aggregate size fractions. Densiometric separations were used to isolate free and released POM (frPOM) and intraaggregate POM (iPOM). Small macroaggregates (250–2000 μm) were enriched in iPOM‐14C on Day 0 which suggested that many of these aggregates formed around cores of new, root‐derived POM during the growth and senescence of the oat plants. Slaking resulted in the disruption of many of the small macroaggregates (250–2000 μm) and a large increase in frPOM‐14C on Day 0. The amount of 14C released into the frPOM pool with slaking declined with time. In contrast, there was a significant linear increase in the amount of new, root‐derived iPOM‐14C in large microaggregates (53–250 μm) that were released when unstable macroaggregates (>250 μm) slaked. These data support the hypothesis that new microaggregates are formed within existing macroaggregates and provide strong evidence that, in no‐till, aggregate formation and stabilization processes are directly related to the decomposition of root‐residue and the dynamics of POM C in the soil.

Nonpoint source contamination of surface and groundwater resources with nitrate-N (NO3-N) has been linked to agriculture across the midwestern USA. A 4-yr study was conducted to assess the extent of NO3-N leaching in a central Iowa field. Water flow rate was monitored continuously and data were stored on an internal datalogger. Water samples for chemical analysis were collected weekly provided there was sufficient flow. Twelve soil cores were collected in spring, early summer, midsummer , and after harvest for each of the 4 yr. Nitrate-N concentrations in shallow groundwater exhibited temporal trends and were higher under Clarion soil than under Okoboji or Canisteo soil. Denitrification rates were two times higher in Okoboji surface soil than in Clarion surface soil and the highest denitrificafion potential among subsurface sediments was observed for deep unoxidized loess. Soil profile NO~-N concentrations decreased with depth and were the same below 30 cm for fertilized corn (Zea mays L.) and soybean (Glycine max L. Merr.). Nitrate-N concentrations in subsurface drainage water exceeded 10 mg L-1 for 12 mo and were between 6 and 9 mg L-1 for 32 mo during the 4-yr study. The temporal pattern of NO3-N concentrations in subsurface drainage water was not related to the timing of fertilizer N application or the amount of fertilizer N applied. Total NO~-N losses to subsurface drains were greatest in 1993 (51.3 kg ha-1) and least in 1994 (4.9 kg ha-l). Most of the subsurface drainage water NO~-N was lost when crop plants were not present (November-May), except in 1993. Our results indicate that NO~-N losses to subsurface drainage water occur primarily as a result of asynchronous production and uptake of NO~-N in the soil and the presence of large quantifies of potentially mineralizable N in the soil organic matter. N ONPOINT SOURCE contamination of surface-and groundwater with NO3-N has been linked to agricultural production in the midwestern USA. This is especially true for surface waters in the upper Midwest due to extensive subsurface draining of the highly productive but poorly drained soils found in this region (Gast et al., 1978). However, the extent to which agriculture contributes to water-quality deterioration is not fully known. In some geographic regions, surface-water NO3-N concentrations in excess of the 10 mg L-1 drinking water standard frequently have been reported (Hallberg, 1986). Keeney and DeLuca (1993) found that NO3-N concentrations in Des Moines river water in central Iowa were above 10 mg L-1 for an average of 14 d per year, generally in the spring. Subsurface drainage water NO3-N concentrations exhibit yearly and seasonal variability (Kladivko et al., 1991). Nitrogen flux to subsurface drains appears to be primarilly a function of precipitation amounts and distribution, and is only slightly affected by crop N up

No‐till practices have the potential to increase soil organic C, but little is known about the relative contribution of surface residue and roots to soil organic C accumulation. In a simulated no‐till experiment, we studied the fate of 14C‐labeled surface residue and in situ roots during a 1‐yr incubation. Soil samples collected during the incubation were chemically dispersed and separated into five particle size and density fractions. The organic C, 14C, and total N content of each fraction was determined. Alkali traps were used to measure 14C losses due to respiration. After 360 d, 66% of the 14C contained in the surface residue on Day 0 had been respired as 14CO2, 11% remained in residue on the soil surface, and 16% was in the soil. In comparison, 56% of the root‐derived 14C in the soil was evolved as 14CO2 and 42% remained in the soil. The large (500–2000 μm) and small (53–500 μm) particulate organic matter (POM) fractions together contained 11 to 16% of the initial root‐derived 14C in the soil. In contrast, POM contained only 1 to 3% of the inital surface residue–derived 14C. These data show clear differences in the partitioning of surface residue– and root‐derived C during decomposition and imply that the beneficial effects of no‐till on soil organic C accrual are primarily due to the increased retention of root‐derived C in the soil.