Tuesday, November 6, 2007 - 10:30 AM
230-3
Manure Nutrient Management for Groundwater Protection in Irrigated Forage Systems.
Marsha Campbell Mathews, University of California, 3800 Cornucopia Way, Ste. A, Modesto, CA 95354 and Thomas Harter, University of California, Davis, Department of Land, Air, and Water Resources, 125 Veihmeyer Hall, Davis, CA 95616-8628.
Many dairies in California ’s Central Valley are located on light soils with shallow groundwater, and degradation of groundwater quality as a result of misapplication of dairy wastewater has been documented. Most of these dairies operate flush systems as the primary means of managing manure. The liquid manure is stored in retention ponds until it is land applied to forage crops through the existing surface irrigation systems, which may be inefficient but are prohibitively costly to replace. A system to inject lagoon water nutrients into the irrigation freshwater at predetermined application rates was developed. This synchronized rate nutrient application technique is used to apply targeted amounts of dairy lagoon nutrients to forage crops multiple times during the season, at rates and timings that closely anticipate crop nitrogen requirements. This paper reports on a ten year project to determine the impact of these improved nutrient management practices on groundwater quality under dairies. The early years of the study demonstrated that careful management of manure nutrients can dramatically improve shallow groundwater quality. However, in carrying this study forward for several more years, the groundwater nitrate concentrations increased again from the initial lows, but to levels not as high as those measured prior to implementation of the improved practices. This is attributed to a higher proportion of the total liquid manure nitrogen applied being in the organic form relative to the available nitrogen form. Crop yields were initially maintained when liquid manure was used as the exclusive nutrient source, however, in the latter years of the study, corn yields suffered despite excess total nitrogen being applied. Synchronizing the rates of applied nitrogen with anticipated crop uptake was shown to be an effective nutrient management practice even under these high leaching conditions. Management of organic form nitrogen, both by minimizing the amount of organic form nitrogen present in the applied wastewater and by better crop utilization of mineralizing organic nitrogen, is an essential component of a synchronized rate nutrient application technique if groundwater quality under dairy land application fields is to be protected.