Seth M. Dabney

Killing cover crops mechanically: Review of recent literature and assessment of new research results

Cover-crop residues left on the soil surface as a mulch in no-till crop production systems protect the soil from erosion, increase water infiltration and suppress weeds. Because many growers using cover crops want to reduce chemical inputs, non-chemical methods of killing or suppressing cover crops are needed. In the first part of this paper we review the current literature and discuss advantages and disadvantages of five mechanical methods for killing cover crops, i.e., mowing, rolling, roll-chopping, undercutting and partial rototilling. We also report on three new studies that broaden the current literature, including planting into freshly killed residue. In the first study, the use of planter attachments to remove surface residues from the planter row improved stands when cotton was no-till planted 2-7 days after mowing cover crops in Mississippi. In the second study, 100% of a rye/vetch cover crop in Missouri was killed by mowing, and greater than 90% was killed by roll-chopping. Cotton stands were reduced by the use of row cleaners that clogged when the cover crop was roll-chopped or mowed on the same day that the crop was planted. The third study evaluated three methods of mechanically killing summer cover crops in North Carolina. Undercutting provided greater than 95% kill for five of six broadleaf species, and two of five grass species. Mowing effectively killed all six broadleaf cover crops, but re-growth occurred with three of five grasses, with the exception of nearly mature German foxtail millet and mature Japanese millet. In general, rolling did not effectively kill broadleaf or grass cover crops, with the exception of nearly mature German foxtail millet, mature Japanese millet and mature buckwheat.

Landscape benching from tillage erosion between grass hedges

Grass hedges are narrow (1‐2 m wide) parallel strips of stiff, erect, grass planted near to or on the contour of fields but crossing swale areas at angles convenient for farming. They serve as guides for contour cultivation, retard and disperse surface runoff, cause deposition of eroded sediment, and reduce ephemeral gully development. After three years of tilled fallow between mixed-species hedges, the average grade of 18 m wide tilled strips between 1.5 m wide hedges was reduced from 0.068 to 0.052 as a result of surface lowering below hedges and on the shoulders of swale areas combined with increases in elevation above hedges. Annual surveys show progressive lowering of high spots and filling of low spots as contours lines more closely aligned with hedges. Survey data indicated annual erosion rates of nearly 250 t ha ˇ1 year ˇ1 . Both RUSLE and WEPP over-predicted erosion rates, partly because backwater and slope modification affects were not considered. A tillage translocation model predicted enough soil movement to account for 30‐60% of the observed changes. A combination of tillage translocation and water erosion/deposition provides the best explanation for the observed aggradation/degradation patterns. # 1999 Elsevier Science B.V.

TILLAGE AND RESIDUE EFFECTS ON RUNOFF AND EROSION DYNAMICS

The carry-over effects from one year to the next of surface residue and tillage management decisions on runoff and erosion are not clear. The dynamics of runoff and erosion processes during rainfall events are likely dependent on the tillage and residue management system. The objective of this study was to elucidate the effects of tillage practices and residue management, by removal of residue cover, on the properties that describe the dynamics of the runoff and erosion processes. Six-row, 12.2 m long . 5.5 m wide, plots under conventional tillage (CT) or no tillage (NT) corn (Zea mays L.) for nine years were used in this study. Plots had an average slope of 5.7% on a Grenada silt loam (Glossic Fragiudalf) soil. Rainfall simulations were conducted on a 10.7 m . 3.7 m area within each plot at a rate of 65 mm h -1 for 1 h under natural antecedent soil-water conditions (dry run), followed by a 0.5 h simulation 4 h later (wet run), and another 0.5 h application 30 min later (very wet run). The ten treatments consisted of an incomplete 2 . 2 . 3 factorial arrangement of two tillage histories (CTh and NTh), two tillage levels (tilled and not tilled), and three residue management levels (residue left, residue removed just prior to simulated rainfall, and residue removed one year prior to simulated rainfall). The missing treatments were the NTh-tilled and CTh-not tilled with residue left. The time of runoff initiation, maximum runoff rate, flow velocity, and maximum sediment concentration were used to describe differences in runoff and erosion dynamics. Residue removal resulted in significantly sooner runoff with the NT system. There was a significant carry-over effect of residue removal with runoff initiated 35% sooner the subsequent year of removal and sediment concentrations increasing by >100%. Maximum sediment concentrations were lower for the CTh land that was not tilled than for the tilled despite the untilled land experiencing sooner runoff and higher runoff rates. Tilling NTh land resulted in significantly lower sediment concentrations than tilling CTh land, suggesting that the soil quality of NT was not immediately lost when tilled, but these beneficial properties were fully lost within one year of residue removal.

History, residue, and tillage effects on erosion of loessial soil

Studies have shown that no-till (NT) management reduces soil erosion relative to chisel/disk-tillage (CT) and that this benefit may increase over time. There are fewer data, however, to separate the erosion-reduction contributions of surface residue mulch from those of improved soil properties under NT. The objective of this study was to quantify these separate contributions for a silt loam soil (Glossic Fragiudalf) used for corn (Zea mays L.) production in northern Mississippi for five to ten years with either CT or NT. The experiment had ten treatments. Two were normal CT and NT managements in which a crop was planted but had not emerged prior to simulated rainfall. The other eight treatments had surface crop residues removed and comprised a 2 × 2 × 2 factorial arrangement of two tillage histories (CTh or NTh), two levels of tillage immediately prior to rainfall simulation (disturbed or not disturbed), and two levels of residue removal (residue removed just prior to simulated rainfall or residue removed one year prior to simulated rainfall). Simulated rainfall was applied at a rate of 65 mm h-1 in a three-storm sequence on 10.7 × 3.7 m areas. NT exhibited greater runoff but much lower sediment losses than CT. Residue removal doubled erosion for both tillage histories. Surface disturbance decreased runoff from the first storm following tillage but increased total soil loss 26% to 47%. With residues removed, long-term NTh resulted in one-third the soil loss of CTh, and similar benefits were observed with or without surface disturbance. This residual benefit of NTh was lost within one year of fallow after residue removal. These results demonstrate that the erosion resistance of NT areas is due to both residue cover and improved soil quality factors. Although the erosion-resisting soil quality factors developed over several years of NT management may be lost within a single year of fallow management, these factors may protect the soil from excessive erosion if NT fields that must occasionally be tilled are quickly returned to NT management.

Apparent deposition velocity and compensation point of ammonia inferred from gradient measurements above and through alfalfa

Abstract Understanding the cycling of ammonia between croplands and the atmosphere is of importance to agriculturalists and atmospheric scientists. Flux densities of gaseous ammonia (NH3), particulate ammonium (NH4+), and total ammoniacal nitrogen (AN) were measured using an aerodynamic method above an alfalfa (Medicago sativa, L.) canopy between April and July 1981 at a rural location in central New York State. In air not influenced by local sources, NH3 and NH4+ averaged 1.5 and 3.0 ppb, respectively, at 1 m above the crop. Ambient NH4+ varied consistently with synoptic air masses, being lowest (2.3 ppb) for NW and highest (6.4 ppb) for SW flows. Concentrations and gradients of both species were higher during periods of hay harvest. Gradients of NH3 were much steeper than those of NH4+ within the alfalfa canopy, but NH4+ contributed appreciably (36% on average) to above-canopy AN gradients. Alfalfas NH3 compensation point was estimated by combining concentration and gradient data with transport resistances. Gaseous gradients indicated a compensation point of 2 ppb, lower than previously published estimates. Conversion of NH3 to NH4+ within the canopy air could have reduced NH3 gradients and caused a low estimate of the compensation point. Acidic aerosols, by keeping NH3 levels low, may compete with plants for NH3. Future studies of ammonia exchange should distinguish between NH3 and NH4+ if flux densities are to be related to ambient conditions. Total AN level is a poor predictor of soil-plant-atmosphere ammonia exchange since high AN was frequently associated with low NH3, and NH3 is more surface reactive than NH4+.

RUNOFF AND SOIL LOSS FROM COTTON PLOTSWITH AND WITHOUT STIFF-GRASS HEDGES

The performance of grass hedges and the effectiveness of no-till cropping systems in reducing soil loss was evaluated on standard erosion plots. No-till cotton with grass hedges, no-till cotton without grass hedges, conventional-till cotton with grass hedges, conventional-till cotton without grass hedges, and no-till cotton without grass hedges but with a winter wheat cover crop produced average annual soil losses of 2.2, 5.2, 12.3, 48.5, and 2.0 t/ha, respectively. The annual ratio of soil loss for no-till cotton plots with grass hedges to those without hedges averaged 0.43. The annual ratio of soil loss for conventional-till plots with grass hedges to without hedges was 0.25. Averaged over all plots (with and without grass hedges, but not including winter cover plots), no-till plots reduced soil loss from conventional-till plots by 88%. Notill plots without grass hedges had 57% less soil loss than conventional-till plots with grass hedges. Although grass hedges were effective in reducing soil loss on erosion plots with similar cropping practices as compared to plots without hedges, other studies of contoured grass hedges on field-sized areas are needed to determine their applicability on larger areas with greater concentrations of runoff.

Rehabilitation of an Incised Stream Using Plant Materials: the Dominance of Geomorphic Processes

The restoration of potentially species-rich stream ecosystems in physically unstable environments is challenging, and few attempts have been evaluated scientifically. Restoration approaches that involve living and dead native vegetation are attractive economically and from an ecological standpoint. A 2-km reach of an incised, sand-bed stream in northern Mississippi was treated with large wood structures and willow plantings to trigger responses that would result in increasing similarity with a lightly degraded reference stream. Experimental approaches for stream bank and gully stabilization were also examined. Although the project was initially successful in producing improved aquatic habitat, after 4 yr it had failed to effectively address issues related to flashy watershed hydrology and physical instability manifest by erosion and sedimentation. The success of ecosystem rehabilitation was thus governed by landscape-scale hydrological and geomorphological processes.

EROSION PROCESSES IN GULLIES MODIFIED BY ESTABLISHING GRASS HEDGES

Concentrated flow can cause gully formation on sloping lands and in riparian zones of floodplains adjacent to incising stream channels. Current practice for riparian gully control involves blocking the gully with an earthen embankment and installing a pipe outlet. Measures involving native vegetation would be more attractive for habitat recovery and economic reasons. To test the hypothesis that switchgrass (Panicum virgatum L.) hedges planted at 0.5 m vertical intervals within a gully would control erosion, we established a series of hedges in several concentrated flow channels. Two of the channels were previously eroded trapezoidal channels cut into compacted fill in an outdoor laboratory. The other channels were located at the margin of floodplain fields adjacent to an incised stream channel (Little Topashaw Creek) in Chickasaw County, Mississippi. While vegetation was dormant following two growing seasons, we created artificial runoff events in our test gullies using synthetic trapezoidal-shaped hydrographs with peak discharge rates of approximately 0.03, 0.07, and 0.16 m3 s-1, flow rates similar to those observed during natural runoff events in gullies at Topashaw. During these tests, we monitored flow depth, velocity, turbidity, and soil pore water pressures. Flow depths were generally <0.3 m, and flow velocities varied spatially and exceeded 2.0 m s-1 at the steepest points in some tests. Erosion rates remained modest for the conditions tested, as long as slopes were less than 3 horizontal to 1 vertical (33%) and step height between hedges was less than 0.5 m. Stability modeling of soil steps reinforced with switchgrass roots showed that cohesive forces were 3 times greater than shearing forces for 0.5 m step heights, and that therefore mass failure was unlikely even with the surcharge weight of a 0.2 m depth of ponded water. For step heights greater than 1 m, however, mass failure was observed and predicted to be the dominant erosion mechanism.

The application of the Revised Universal Soil Loss Equation, Version 2, to evaluate the impacts of alternative climate change scenarios on runoff and sediment yield

The Revised Universal Soil Loss Equation, Version 2 (RUSLE2), provides robust estimates of average annual sheet and rill erosion for one-dimensional hillslope representations. Extensive databases describing climate, soils, and management options have been developed and are widely used in the United States for conservation planning. Recent RUSLE2 enhancements allow estimation of erosion and runoff from a representative sequence of runoff events that are suitable for linkage with an ephemeral gully model. This paper reviews the sensitivity of RUSLE2 erosion estimates to possible climate change scenarios, demonstrates its ability to evaluate alternative management adaptations, and compares predictions with observations of runoff and sediment yield from a 6.6 ha (16 ac) research watershed located near Treynor, Iowa. When applied to a representative hillslope profile with conventional tillage corn (Zea mays L.), increasing monthly temperature by 0.8°C (1.5°F) and rainfall depth, rainfall erosivity density, and 10-year, 24-hour rainfall depth each by 10% cumulatively increased sheet and rill erosion by 47% and increased runoff by 33%, assuming there was no change in corn yield. If the climate changes decreased corn yield by 10%, the overall effect was to increase soil loss for conservation planning by 63%. These results demonstrate that modest and expected changes in climate will significantly increase the risk of soil erosion, and improved conservation management will be an important part of successful adaptation.

Conservation practices and gully erosion contributions in the Topashaw Canal watershed

Quantifying the effectiveness of conservation practices at the watershed scale throughout the nation has been identified as a critical need. Our objective was to determine the effectiveness of these conservation practices for reducing sediment yield. The Topashaw Canal watershed (TCW), an 11,000-ha (27,181-ac) area in northcentral Mississippi, exhibits flashy stream response to storms with mean sediment concentrations (117 mg L-1 [117 ppm]) almost double the median sediment concentration (60 mg L-1). The most prevalent conservation practice imposed by acreage, since 1985, is enrollment in the Conservation Reserve Program (e.g., planting of pine trees). Grade-stabilization structures (e.g., drop pipes) are the most common conservation practice used to control gully erosion within the TCW. These structures are estimated by the USDA Natural Resources Conservation Service to reduce annual sediment yield from 11.5 to 0.1 Mg ha-1 yr-1 (5.13 to 0.05 tn ac-1 yr-1), but measurements have not been made to determine the accuracy of these estimates. Nonetheless, an average of 58 drop pipes have been installed annually within the TCW using Environmental Quality Incentives Program funds, and an additional 5.4 large drop pipes have been installed each year using US Corps of Engineers funds. Annual gully erosion accounted for 54% of the total sediment yield of over 73,000 Mg (80,445 tn) from TCW. The shift in land use to Conservation Reserve Program, combined with channel incision, has resulted in streambank failure and gully erosion being the primary sources of sediment currently leaving the watershed.