W. M. Edwards

Conservation tillage and macropore factors that affect water movement and the fate of chemicals.

A thorough understanding of how conservation tillage influences water quality is predicated on knowledge of how tillage affects water movement. This paper summarizes the effects of conservation tillage on water movement and quality mainly based on long-term experiments on Luvisols at the North Appalachian Experimental Watershed near Coshocton, OH, USA. Conservation tillage can have a much larger effect on how water moves through the soil than it does on the total amount percolating to groundwater. Soil macroporosity and the proportion of rainfall moving through preferential flow paths often increase with the adoption of conservation tillage and can contribute to a reduction in surface runoff. In some medium- and fine-textured soils most of the water that moves to the subsoil during the growing season (May-October) is probably transmitted by macropores. If a heavy, intense storm occurs shortly after surface application of an agricultural chemical to soils with well-developed macroporosity, the water transmitted to the subsoil by the macropores may contain significant amounts of applied chemical, up to a few per cent, regardless of the affinity of the chemical for the soil. This amount can be reduced by an order of magnitude or more with the passage of time or if light rainstorms precede the first major leaching event. Because of movement into the soil matrix and sorption, solutes normally strongly adsorbed by the soil should only be subject to leaching in macropores in the first few storms after application. Even under extreme conditions, it is unlikely that the amount of additional adsorbed solute transported to groundwater will exceed a few per cent of the application when conservation tillage is used instead of conventional tillage. In the case of non-adsorbed solutes, such as nitrate, movement into the soil matrix will not preclude further leaching. Therefore, when recharge occurs during the dormant season thorough flushing of the soil, whether macropores are present or not, can move the remaining solutes to groundwater. Thus, the net effect of tillage treatment on leaching of non-adsorbed solutes should be minimal.

Role of lumbricus terrestris (L.) burrows on quality of infiltrating water

Abstract Long-term watershed studies at the North Appalachian Experimental Watershed, Coshocton, Ohio have shown that when corn ( Zea mays L.) is planted in soil covered by the residue of the previous crop (i.e. no-tillage management), surface runoff from summer storms is greatly reduced. In addition, the residue cover provides a favorable environment for various soil invertebrates, especially earthworms. During high-intensity rainstorms, some of the water that infiltrates in no-till corn fields moves rapidly downward in burrows made by the earthworms Lumbricus terrestris L. Samplers were developed for collecting infiltrating rain water flowing in L. terrestris burrows at a depth of 45cm below the soil surface. With annual surface applications of 175 kg N ha −1 as NH 4 NO 3 , concentrations of NO 3 -N in water flowing in individual burrows during growing season storms ranged up to 152 mg l −1 . Concentrations of NO 3 -N tended to be lowest after prolonged wet soil conditions and highest after intermittent warm, dry periods. Distilled water poured directly into the surface openings of L. terrestris burrows and immediately collected as it drained into samplers, contained up to 40 mg of NO 3 -N I −1 , a value greater than that measured in many of the samples resulting from natural rain storms. Water and herbicide mixtures poured through L. terrestris burrows showed that the linings of the burrow, or drilosphere, may contribute nitrogen to the infiltrating water while greatly reducing the concentrations of atrazine and alachlor.

Runoff and erosion control with conservation tillage and reduced-input practices on cropped watersheds

The North Appalachian Experimental Watershed near Coshocton, OH was established in 1935 to develop, evaluate, and refine conservation practices that reduce runoff and erosion under the hilly, humid conditions of the northeastern United States. Small (0.5 to 1 ha), single-practice, gaged watersheds comprised of sandstone- and shale-derived residual soils are used to evaluate the interaction of management, climate, and soils. In a 28-year, nine-watershed study, 92% of the erosion occurred during the corn (Zea mays L.) years of a 4-year corn/wheat (Triticum aestivum L.)/meadow/meadow rotation. These watersheds were moldboard plowed prior to planting corn and cultivation was used for weed control. By tilling and planting on the contour and increasing fertility levels, soil loss was reduced more than 3-fold, but still averaged 4.7 Mg ha−1 during corn years. Thus, annual production of row crops on a sustainable basis was not without risk. A 6-year, six-watershed study indicated that by using reduced tillage (no-till, chisel, or paraplow) and herbicides, corn and soybean [Glycine max (L.) Merr.] can be grown in rotation with an average soil loss of 0.5 Mg ha−1 yr−1, well below the stipulated soil loss tolerance of 7.8 Mg ha−1 yr−1, if a winter cover crop of rye (Secale cereale L.) followed soybean. Under these conditions, however, concentrations of surface-applied herbicides and nitrate in runoff frequently exceeded drinking water standards, particularly in the first few runoff events after application, and may be a concern. A reduced-input management practice for corn and soybean production with light disking and cultivation for weed control and manure and a legume (red clover, Trifolium pratense L.) to supply some of the nitrogen was implemented to determine if a balance between losses of soil and purchased chemical inputs could be obtained. In a 6-year comparison, soil losses were similar to those under conservation tillage, but the risk of yield loss increased due to inability to cultivate in a timely manner due to weather conditions. Regardless of tillage practice, infrequent, severe storms during years when row crops were grown caused most of the soil loss from the watersheds. Erosion prediction models must account for the contribution of such events and management practices must limit erosion caused by these storms if long-term sustainability is to be maintained.

Impacts of antecedant moisture and soil surface mulch coverage on water and chemical transport through a no-till soil

Abstract Flow in macropores of no-tillage soils is often implicated as a principal mechanism responsible for accelerated movement of agrochemcials into groundwater. The objective of this study was to assess the impact of a surface mulch coverage and antecedent water content on water and chemical transport characteristics in a Typic Hapludult soil. SrBr2·6H2O and atrazine were surface-applied to four undisturbed 0.3 m × 0.3 m × 0.3 m surface soil blocks. Three simulated 30 mm rains were applied to the block surfaces, and leachate was collected from 64 cells at the bottom of each block. Leachate volume, chemical amounts, and conducting macropore areas were determined for each cell and block. A parameter, m, found by fitting sorted cumulative outflow curves to an exponential function, was used to desctibe the degree of flow preference in a block. The dominant factor producing transport differences betweent the four blocks was pre-rain moisture content, which correlated negatively with degree of flow preference and positively with total leachate volume in each block. In a drier soil only the more rapid flow pathways, marked by high cell leachate volumes, contributed to the flow, while the slower pathways having greater interaction with the bulk soil were mostly truncated. This resulted in a higher degree of flow preference, smaller total leachate volumes and smaller block-averaged concentrations of Br, Sr and atrazine in soil with lower pre-rain moisture content. The peak of chemical transport was observed after the first simulated rain regardless of pre-rain moisture and surface mulch coverage. Following the second and third rains the chemical transport was reduced twofold for the less reactive Br, three-fold for the more reactive atrazine and ten-fold for Sr, apparently due to the by-pass of chemicals by subsequent leaching events. Much had little effect on water movement, but slightly enhanced the Sr and atrazine transport through the block, most likely by prolonging the chemical contact with infiltrating water at the soil surface.