Martin J. Shipitalo

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.

Impact of Glyphosate-Tolerant Soybean and Glufosinate-Tolerant Corn Production on Herbicide Losses in Surface Runoff

Residual herbicides used in the production of soybean [Glycine max (L.) Merr] and corn (Zea mays L.) are often detected in surface runoff at concentrations exceeding their maximum contaminant levels (MCL) or health advisory levels (HAL). With the advent of transgenic, glyphosate-tolerant soybean and glufosinate-tolerant corn this concern might be reduced by replacing some of the residual herbicides with short half-life, strongly sorbed, contact herbicides. We applied both herbicide types to two chiseled and two no-till watersheds in a 2-yr corn-soybean rotation and at half rates to three disked watersheds in a 3-yr corn/soybean/wheat (Triticum aestivum L.)-red clover (Trifolium pratense L.) rotation and monitored herbicide losses in runoff water for four crop years. In soybean years, average glyphosate loss (0.07%) was approximately 1/7 that of metribuzin (0.48%) and about one-half that of alachlor (0.12%), residual herbicides it can replace. Maximum, annual, flow-weighted concentration of glyphosate (9.2 microg L(-1)) was well below its 700 microg L(-1) MCL and metribuzin (9.5 microg L(-1)) was well below its 200 microg L(-1) HAL, whereas alachlor (44.5 microg L(-1)) was well above its 2 microg L(-1) MCL. In corn years, average glufosinate loss (0.10%) was similar to losses of alachlor (0.07%) and linuron (0.15%), but about one-fourth that of atrazine (0.37%). Maximum, annual, flow-weighted concentration of glufosinate (no MCL) was 3.5 microg L(-1), whereas atrazine (31.5 microg L(-1)) and alachlor (9.8 microg L(-1)) substantially exceeded their MCLs of 3 and 2 microg L(-1), respectively. Regardless of tillage system, flow-weighted atrazine and alachlor concentrations exceeded their MCLs in at least one crop year. Replacing these herbicides with glyphosate and glufosinate can reduce the occurrence of dissolved herbicide concentrations in runoff exceeding drinking water standards.

Tillage effect on macroporosity and herbicide transport in percolate

Research suggests that pesticide transport to tile drains and shallow groundwater may be greater for no-till than tilled soil. Also, most pesticide transport through soil can be from macropore flow, but the effect of tillage on macropore transport is uncertain. Our objective was to investigate the effect of tillage on herbicide leaching through hydraulically active macropores. The number of percolate-producing macropores at 30 cm (nmacro) and the timing of initial percolate were measured from an experiment where atrazine, alachlor and rainfall were applied to moldboard plowed (MP) and no-till (NT) undisturbed soil blocks from two different silt loam soils. Alachlor and atrazine transport through the undisturbed soil blocks was simulated using the Root Zone Water Quality Model (RZWQM). The time of initial percolate breakthrough at 30 cm was significantly less for NT than for MP (p<0.001), but nmacro was not significantly different between MP and NT treatments. Additionally, nmacro was significantly different between the two silt loam soils (p<0.001). Multiple linear regression revealed that flow-weighted herbicide concentration in percolate decreased with increasing nmacro (cm � 2 ) and increasing time for initial percolate breakthrough (min) (R 2 =0.87 for alachlor and 0.85 for atrazine). Because a small fraction of nmacro produces the majority of percolate, we used half of measured nmacro for RZWQM input. Also, soil parameters were calibrated to accurately simulate the water flow component timing of percolate arrival and percolate amount through macropores. This parameterization strategy resulted in accurate predicted herbicide concentrations in percolate at 30 cm using RZWQM (within the range of observations). The modeling results suggest that differences in soil properties other than macroporosity such as a lower soil matrix saturated hydraulic conductivity and porosity in subsurface soil (8–30 cm) can cause percolate to occur sooner through macropores on NT than on

Water quality response times to pasture management changes in small and large watersheds

To interpret the effects of best management practices on water quality at a regional or large watershed scale, likely response times at various scales must be known. Therefore, four small (≤1 ha [≤2.5 ac]) watersheds, in rotational grazing studies at the North Appalachian Experimental Watershed near Coshocton, Ohio, were used to study management impacts on water quality and response times. Surface runoff was sampled on an event basis; groundwater discharge was sampled monthly from springs developed where a perching clay layer outcropped at the soil surface. In four large watersheds ranging from 18 to 123 ha (44 to 303 ac), base flow was over 50% of annual stream flow and approximately 20% of annual precipitation. Nitrate-N loads in base flow were 31% to 59% of total annual NO3-N load in stream flow. When the N fertilization rate in a “medium fertility” area that contains two small watersheds was increased from 56 to 168 kg ha-1 y-1 (50 to 150 lb ac-1 yr-1), NO3-N concentrations in groundwater discharge responded little in four years. Then NO3-N levels in groundwater discharge increased for 10 years. With discontinuation of N fertilization, NO3-N concentrations in groundwater discharge returned to pre-N increase levels after six years. In a “high fertility” grazing area with a similar perched water table, 224 kg N ha-1 (200 lb ac-1) was applied annually. Concentrations of NO3-N increased to >10 mg L-1 (ppm) after five years. Legumes were then interseeded into the grass forage, and mineral N fertilization was discontinued. Nitrate-N concentrations in groundwater discharge returned to their pre-fertilization levels after about five years. This multi-year response of groundwater discharge quality to management change in small watersheds indicates that the response time for measurable change in multi-square-mile watersheds will be equally long, if not longer, and trends will be muted.

MACROPORE COMPONENT ASSESSMENT OF THE ROOT ZONE WATER QUALITY MODEL (RZWQM) USING NO–TILL SOIL BLOCKS

In structured soils, macropores can contribute to rapid movement of water and solutes through the profile. To provide insight into these processes, model assessments should be performed under a variety of conditions. We evaluated the macropore component of the RZWQM using undisturbed soil blocks with natural macropores. To accomplish this, atrazine, alachlor, and bromide were surface–applied to nine 30 U 30 U 30 cm blocks of undisturbed, no–till silt loam soil at three water contents (dry, intermediate, and wet). One hour later, we subjected the blocks to a 0.5–h, 30–mm simulated rain. Percolate was collected and analyzed from 64 uniform size cells at the base of the blocks. After percolation ceased, the soil was sectioned and analyzed to determine chemical distribution. We tested the chemical sub–component of macropore flow using these blocks following hydrologic calibration, while a separate set of blocks was used to calibrate selected chemical parameters. Parameterization of the macropore component included measuring the effective macroporosity (50% of percolate producing macropores) and calibrating the effective soil radius (0.6 cm). The effective soil radius represents the soil surrounding the macropores that interacts with macropore flow. This parameterization strategy resulted in accurate simulations of the composite chemical concentrations in percolate (i.e., all simulated chemical concentrations were within a factor of 2.0 of the average observed value). However, observed herbicide concentration in percolate decreased with cumulative percolate volume, while simulated concentrations increased. Model modifications, such as incorporating a dynamic effective macroporosity (effective macroporosity increase with increasing rainfall) and chemical kinetics in macropores, may improve simulations.

Effect of tillage and rainfall on transport of manure-applied Cryptosporidium parvum oocysts through soil.

Most waterborne outbreaks of cryptosporidiosis have been attributed to agricultural sources due to the high prevalence of Cryptosporidium oocysts in animal wastes and manure spreading on farmlands. No-till, an effective conservation practice, often results in soil having higher water infiltration and percolation rates than conventional tillage. We treated six undisturbed no-till and six tilled soil blocks (30 by 30 by 30 cm) with 1 L liquid dairy manure containing 10(5) C. parvum oocysts per milliliter to test the effect of tillage and rainfall on oocyst transport. The blocks were subjected to rainfall treatments consisting of 5 mm or 30 mm in 30 min. Leachate was collected from the base of the blocks in 35-mL increments using a 64-cell grid lysimeter. Even before any rain was applied, approximately 300 mL of water from the liquid manure (30% of that applied) was transported through the no-till soil, but none through the tilled blocks. After rain was applied, a greater number and percentage of first leachate samples from the no-till soil blocks compared to the tilled blocks tested positive for Cryptosporidium oocysts. In contrast to leachate, greater numbers of oocysts were recovered from the tilled soil, itself, than from the no-till soil. Although tillage was the most important factor affecting oocyst transport, rainfall timing and intensity were also important. To minimize transport of Cryptosporidium in no-till fields, manure should be applied at least 48 h before heavy rainfall is anticipated or methods of disrupting the direct linkage of surface soil to drains, via macropores, need to be used.

Earthworm additions affect leachate production and nitrogen losses in typical midwestern agroecosystems.

Earthworms affect soil structure and the movement of agrochemicals. Yet, there have been few field-scale studies that quantify the effect of earthworms on dissolved nitrogen fluxes in agroecosystems. We investigated the influence of semi-annual earthworm additions on leachate production and quality in different row crop agroecosystems. Chisel-till corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation (CT) and ridge-till corn-soybean-wheat (Triticum aestivum L.) rotation (RT) plots were arranged in a complete randomized block design (n = 3) with earthworm treatments (addition and ambient) as subplots where zero-tension lysimeters were placed 45 cm below ground. We assessed earthworm populations semi-annually and collected leachate biweekly over a three-year period and determined leachate volume and concentrations of total inorganic nitrogen (TIN) and dissolved organic nitrogen (DON). Abundance of deep-burrowing earthworms was increased in addition treatments over ambient and for both agroecosystems. Leachate loss was similar among agroecosystems, but earthworm additions increased leachate production in the range of 4.5 to 45.2% above ambient in CT cropping. Although leachate TIN and DON concentrations were generally similar between agroecosystems or earthworm treatments, transport of TIN was significantly increased in addition treatments over ambient in CT cropping due to increased leachate volume. Losses of total nitrogen in leachate loadings were up to approximately 10% of agroecosystem N inputs. The coincidence of (i) soluble N production and availability and (ii) preferential leaching pathways formed by deep-burrowing earthworms thereby increased N losses from the CT agroecosystem at the 45-cm depth. Processing of N compounds and transport in soil water from RT cropping were more affected by management phase and largely independent of earthworm activity.

Impact of grassed waterways and compost filter socks on the quality of surface runoff from corn fields.

Surface runoff from cropland frequently has high concentrations of nutrients and herbicides, particularly in the first few events after application. Grassed waterways can control erosion while transmitting this runoff offsite, but are generally ineffective in removing dissolved agrochemicals. In this study, we routed runoff from one tilled (0.67 ha) and one no-till watershed (0.79 ha) planted to corn (Zea mays L.) into parallel, 30-m-long grassed waterways. Two 46-cm-diam. filter socks filled with composted bark and wood chips were placed 7.5 m apart in the upper half of one waterway and in the lower half of the other waterway to determine if they decreased concentrations of sediment and dissolved chemicals. Automated samplers were used to obtain samples above and below the treated segments of the waterways for two crop years. The filter socks had no significant effect (P <or= 0.05) on sediment concentrations for runoff from the no-till watershed, but contributed to an additional 49% reduction in average sediment concentration compared with unamended waterways used with the tilled watershed. The filter socks significantly increased the concentrations of Cl, NO(3)-N, PO(4)-P, SO(4), Ca, K, Na, and Mg in runoff from at least one watershed, probably due to soluble forms of these ions in the compost. The estimated additional amounts of these ions contributed by the socks each year ranged from 0.04 to 1.2 kg, thus were likely to be inconsequential. The filter socks contributed to a significant (P <or= 0.05) additional reduction in dissolved glyphosate [N-(phosphonomethyl)glycine] (5%) and alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] (18%) concentrations for the tilled watersheds, but this was insufficient to reduce alachlor concentrations to acceptable levels.