Cows, humans and hydrology in the nitrogen dynamics of a grazed rural watershed

Abstract

Nitrogen inputs to the Tomales, California, watershed (a rural area of 56 000 ha) from the atmosphere are about equal to outputs via runoff and groundwater flow. This balance was initially interpreted to suggest that the system was neither releasing nor taking up nitrogen. A more detailed budgetary analysis suggests otherwise. In the present analysis, food imported for dairy cows and humans, waste management and milk export are incorporated into the nitrogen budget. Cattle contributions to the budget are influenced by nutrition as a function of age, lactation state and milk yield, as well as population density. The cow contribution distinguishes grazing (i.e. internal nutrient cycling) from the introduction of nitrogen in feed grown outside the watershed, and the budget incorporates nitrogen losses due to waste management and export of milk. Food imported for cattle is almost 10 times the import for humans, but cows and humans contribute approximately equal net nitrogen additions to the system. This inclusion of cows and humans in the nitrogen budget demonstrates that nitrogen inputs to the system exceed hydrological outputs by about 2 kg ha−1yr−1. Alternative sinks which may account for this ‘extra nitrogen’ include storage in biomass or soil organic matter, or loss from the system by the difference between nitrogen fixation and denitrification. These alternatives become testable hypotheses for future research.1998 Academic Press Limited