『Abstract
Nitrogen loss laws on the field scale or the watershed scale
are often explored independently. Few studies have addressed the
fate characteristics of nitrogen in runoff from a “source” farmland
to a small watershed outlet during different natural rainfall
events. The dynamic processes of surface runoff and the concentrations
of three nitrogen forms at outlets of two representative “source”
farmlands (paddy field and upland), as well as the total outlets
of a small agricultural watershed on the south of Huaihe river
in China, were simultaneously monitored for three typical natural
rainfall events during one crop season. The results showed that
the fate of nitrogen in runoff from the small agricultural watershed
varied remarkably with time and space. On the scale of “source”
field, the peak flows, surface runoff fluxes, concentration peaks
and transfer fluxes of TN, NO3--N
and NH4+-N from the farmlands
for the three typical rainfall events were significantly different,
and were all in the magnitude order of heavy rainstorm event >
rainstorm event > moderate rain event. For the same rainfall event,
due to the influence of the antecedent soil moisture condition,
the runoff flux from the paddy field was less than from the upland
during the rainstorm, while this relation was reversed during
the heavy rainstorm and moderate rain events. Based on the observed
rainfall events, the variation extent of the concentrations of
three nitrogen forms with time in the runoff water from the upland
was all greater than from the paddy field. The nitrogen in runoff
water from the upland was transferred dominantly in particulate
form in the heavy rainstorm and rainstorm events and in dissolved
form in the moderate rainfall event. In contrast, rainfall characteristics
had a minimal effect on the main form of nitrogen loss from the
paddy field, while dissolved nitrogen was dominant in every rainfall
event. In addition, a significant nonlinear relationship between
the loss loads of the three nitrogen forms and runoff volumes
for each rainfall event was identified (P<0.01). On the scale
of the small watershed, the runoff volumes, average concentrations
and loss loads of each nitrogen form also varied consistently
with rainfall volume. Dissolved nitrogen was dominant in runoff
water at the small watershed outlet in very rainfall event. The
surface runoff volumes from the whole small watershed were larger
than those from the farmlands accounting for about 80% of the
small watershed area; however, the average concentrations and
loss loads of each nitrogen form in the runoff of the former were
all lower than those of the latter. The main form of nitrogen
loss in the runoff of the former was more stable, which indicated
that a large number of small ponds and drainage ditches in the
small watershed had a strong reduction and retention effect on
the “source” farmland nitrogen. Therefore, the retention or establishment
of regional landscape structure of “field-ditch-pond” had practical
significance on the control of agricultural non-point source nitrogen
pollution.
Keywords: Agricultural watershed; Upland; Paddy field; Fate of
nitrogen; Rainfall runoff』
Introduction
Study area
Methods
Hydrological monitoring
Monitoring of rainfall
Monitoring of overland flow from the upland and paddy field
Monitoring of stream flow at the outlet of study area
Water sampling
Sampling of overland flow from the upland and paddy field
Sampling of stream flow at the outlet of study area
Chemical analysis
Estimating methods
Statistical analysis
Results and discussion
Characteristics of rainfall runoff at the farmland outlets
Typical characteristics of rainfall events
Processes and characteristics of runoff from the farmlands
Fate characteristics of nitrogen at the farmlands outlets
Processes and characteristics of nitrogen losses from the farmlands
The relationship of nitrogen loss load and runoff volumes on
the farmlands
Response of nitrogen loss in runoff of the small watershed to
the farmlands
Response of runoff processes
Response of the variation of nitrogen concentration
Response of the main form of nitrogen loss
Comparison of nitrogen loss loads in runoff water
Conclusions
Acknowledgments
References