Saturday, 15 July 2006

Aeration in Grasslands, It Only Works Some-of-the-Times.

Dorcas H. Franklin, USDA ARS, J. Phil Campbell Sr., Natural Resource Conservation Center, 1420 Experiment Station Road, Watkinsville, GA 30677, Miguel L. Cabrera, Univ of Georgia, 3111 Plant Sciences Bldg, Athens, GA 30602-7272, and David Butler, Univ of Georgia, Crop and Soil Sciences, 3111 Miller Plant Sciences Bldg., Athens, GA 30602.

In an effort to efficiently utilize natural resources and leave them in best possible condition, it is necessary to build the soil and reduce unnecessary losses of nutrients beyond the site or point of intended use. Improving infiltration of rainfall or reducing losses to overland flow can help extend grass or forage productivity during droughty periods or it can retard flooding during periods of heavy rainfall. Application of broiler litter to grasslands may lead to elevated concentrations of phosphorus (P) in surface runoff, with its potential effect on eutrophication of surface waters. Thus, management practices are currently needed to reduce the amount of P loss in runoff from grasslands fertilized with broiler litter. Reductions in runoff P loss can be achieved by reducing the volume of runoff and/or reducing the concentration of P in runoff. A practice that may accomplish these results is aeration. Studies have been conducted to explore the viability of aeration to provide environmental and agronomic benefits. Most studies were related to increase productivity or yields and others were to determine its effectiveness to reduce nutrient losses. Both agronomic and environmental results varied. Agronomically, mechanical aeration did not increase yield when manures were applied, whereas in other cases forage yields were increased. Environmentally, even when soil textures were equivalent (silt loams), grasslands fertilized with liquid dairy manure were unaffected by aeration while in others aeration decreased nutrient losses in runoff when broiler litter was applied to grasslands. Of the studies primarily interested in environmental impacts, most were done using simulated rainfall but a few were done with natural rainfall. All were done at a small plot scale. All environmental studies are laden with variability. For example, in one study done on differing landscape position (plot scale) there was a two-fold difference in runoff volume between landscape positions on the same soil classification. Environmental factors related to soil can be quite variable across one soil classification unit and there are often multiple soil units at the field scale. Plot scale studies are useful in that they allow for more replications. It is important though to also look at management practices at scales that are more similar to real management units. In this presentation we will present the results from two small plot aeration studies and one field scale paired watershed study. Over all objective of all three studies was to determine the impact of aeration on runoff volume and P losses in runoff from a tall fescue-bermudagrass grasslands fertilized with inorganic fertilizer and different manures (diary slurry, layer manure and broiler litter). The small scale plots were done on two different soils using three different types of aeration implements. The field scale study was conducted in six 0.7–ha, tall fescue (Festuca arundinacea Schreb.)/bermudagrass (Cynodon dactylon L.) bermed fields or field-scale watersheds, located at the College of Agricultural and Environmental Sciences Central Research and Education Center, near Eatonton, Georgia. Bermed fields were paired based on their propensity to generate surface runoff. We used PROC REG in SAS 8.0 (SAS Institute Inc., Cary, NC) to regress loss of runoff volume, DRP and TKP from the aerated plots against loss of runoff volume, DRP, and TKP from the non-aerated plot before and after application of the aeration treatment. Then, PROC GLM in SAS 8.0 was used to determine if the slopes of the regression lines were different before and after applying aeration. Results varied depending on soils regardless of soil texture. Small plots study showed that core aeration decreased Total P and dissolved P from plots fertilized with broiler litter by 46 and 62%. Results from between paired watersheds varied while results of runoff and DRP loss were usually similar on each set of watersheds. In the fields with well drained soils, aeration reduced DRP losses by 35%. This overall reduction appeared to have been caused by a combination of reductions in runoff volume and DRP concentration.

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