Assessing the effects of livestock manure application on water environments and modeling water and nitrogen badgets in a watershed

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It is urgent ot prevent water pollution by livestock wastes and to establish a proper method to process livestock manure which is produced 95 million ton annually. We have assessed ground- and surface- water contamination of nitrate nitrogen (NO_3) due to dairy cattle manure applied to the surface of a grass filed. In the last three years, 50 to 60% of annual precipitation infiltrated into soil. Thus, it is a big concern that manure and fertilizer (NO_3) applied to the soil surface may be easily transorted into the groundwater surface. It was found, however, with measuremet of water and nitrate contents in soil using time domain reflectometry that most of NO_3 applied was consumed in the root zone between the surface and about 80cm deep. Chemical analysis of soil water, extracted from soil samples from the soil coring method, revealed that NO_3 was consumed in the root zone probably by denitrification, uptake by a plant, and adsorption on Andisol. As content of NO_3 in plants increased after the manure application and deceased with the growth of the plants, it was suggestd that NO3 was up-taken and consumed by plants. Observation of groundwater level in the experimental field revealed recharge of groundwater in the field. Although the current concentration of NO_3 is below the regulated level, concentration of No_3 in groundwater tended to increase vary gradually. Thus, assuming the continuation of the current trends of changes in groundwater quality and the management practice of the field, NO_3 concentration in ground water might reach the regulated level in approximate 40 years. Further investigation is needed to assess long-term effects of livestock manure applied to the grass field on groundwater quality.