Rr. Freifelder et al., COWS, HUMANS AND HYDROLOGY IN THE NITROGEN DYNAMICS OF A GRAZED RURALWATERSHED, Journal of environmental management, 52(2), 1998, pp. 99-111
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 sugges
t that the system was neither releasing nor taking up nitrogen. A more
detailed budgetary analysis suggests otherwise. In the present analys
is, food imported for dairy cows and humans, waste management and milk
export are incorporated into the nitrogen budget. Cattle contribution
s to the budget are influenced by nutrition as a function of age, lact
ation state and milk yield, as well as population density. The cow con
tribution 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 ex
port of milk. Food imported for cattle is almost la times the import f
or humans, but cows and humans contribute approximately equal net nitr
ogen additions to the system. This inclusion of cows and humans in the
nitrogen budget demonstrates that nitrogen inputs to the system excee
d hydrological outputs by about 2 kg ha(-1) yr(-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 bet
ween nitrogen fixation and denitrification. These alternatives become
testable hypotheses for future research. (C) 1998 Academic Press Limit
ed.