A significant portion of the NO3 from agricultural fields that contaminates surface waters in the Midwest Corn Belt is transported to streams or rivers by subsurface drainage systems or "tiles." Previous research has shown that N fertilizer management alone is not sufficient for reducing NO3 concentrations in subsurface drainage to acceptable levels; therefore, additional approaches need to be devised. We compared two cropping system modifications for NO3 concentration and load in subsurface drainage water for a no-till corn (Zea mays L.)-soybean (Glycine max [L.] Merr.) management system. In one treatment, eastern gamagrass (Tripsacum dactyloides L.) was grown in permanent 3.05-m-wide strips above the tiles. For the second treatment, a rye (Secale cereale L.) winter cover crop was seeded over the entire plot area each year near harvest and chemically killed before planting the following spring. Twelve 30.5x42.7-m subsurface-drained field plots were established in 1999 with an automated system for measuring tile flow and collecting flow-weighted samples. Both treatments and a control were initiated in 2000 and replicated four times. Full establishment of both treatments did not occur until fall 2001 because of dry conditions. Treatment comparisons were conducted from 2002 through 2005. The rye cover crop treatment significantly reduced subsurface drainage water flow-weighted NO3 concentrations and NO3 loads in all 4 yr. The rye cover crop treatment did not significantly reduce cumulative annual drainage. Averaged over 4 yr, the rye cover crop reduced flow-weighted NO3 concentrations by 59% and loads by 61%. The gamagrass strips did not significantly reduce cumulative drainage, the average annual flow-weighted NO3 concentrations, or cumulative NO3 loads averaged over the 4 yr. Rye winter cover crops grown after corn and soybean have the potential to reduce the NO3 concentrations and loads delivered to surface waters by subsurface drainage systems.
Seventy‐five percent of swine (Sus scrofa) production systems in North America use anaerobic or liquid‐slurry systems for waste holding or disposal. Accurate emissions data and emission factors are needed for engineering, planning, and regulatory agencies. These data are used for system design and evaluation of the effect of animal concentrations on the regional soil, surface and ground waters, and atmospheric environments. Noninvasive techniques were used to evaluate trace gases without disturbing the meteorology or lagoon system being measured. Micrometeorological and gas sensors were mounted on a submersible barge in the center of the lagoon for use with flux‐gradient methodology to determine trace gas fluxes, without disturbing atmospheric transport processes, over extended periods. Collateral measurements included lagoon nutrient, dissolved gas concentrations, and sludge gas mass flux. Ammonia emissions varied diurnally and seasonally and were highly correlated with windspeed and water temperature. Nutrient loading measurements showed that mobile ions, which were nonvolatile, were constant throughout four successive lagoons. Immobile ions concentrated primarily in the sludge layer of the first lagoon. Measurements of denitrification N2 losses suggest as much N2−N lost as from NH3‐N. Ammonia gas emissions are not as large a percentage of total nitrogen input to the lagoons as previously thought but unaccounted‐for nitrogen requires further research.
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
Microbial transformations of fertilizers and pesticides in the surface soil have a direct impact on the mass of the agrochemical that is susceptible to leaching losses. Thus, our greatest potential for controlling leaching losses of agrochemicals is through the management of these compounds in the surface soil. A variety of strategies have been employed to maximize the residence time of applied chemical in the surface soil, including: timing of application, formulation (e.g., slow-release fertilizers and encapsulated pesticides), and the use of compounds that modify microbial activity in soil (e.g., nitrification inhibitors). Although these strategies have met with some success, more precise quantification of the microbial transformations of agrochemicals is required to aid the development of improved management strategies. The high spatial variability exhibited by many microbial processes, in many cases, precludes precise quantification. A greater understanding of the factors contributing to the variability of microbial processes allows for improved estimation, as well as for the assessment of key driving variables controlling microbial processes in soil. This article reviews several aspects of spatial variability associated with microbial populations and processes. The discussion focuses on the scale at which variability is expressed, and the soil and environmental variables that serve to control variability at each scale. Implications for the development of new management strategies are also discussed, and finally, some statistical considerations for characterizing variability are presented.
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