『Abstract
Non-point nitrogen (N) loss from agriculture is an environmental
concern among scientists, decision-makers, and the public. This
study investigated NO3-N and total N losses
from a mixed land use watershed (39.5 ha) in the Appalachian Valley
and Ridge Physiographic Province as influenced by hydrology (flow
type, runoff volume, storm sizes, and precipitation amount) and
seasons (pre-growing, growing, and post-growing seasons) from
2002 to 2006. Stream discharge was monitored every 5-min and water
samples for NO3-N and total N analyses were
collected weekly for base flow and for every storm. The majority
of NO3-N (about 75%) and total N (about 65%)
were exported in base flow, which contributed about 64% of the
total flow in an average year and had greater flow-weighted mean
NO3-N concentration (5.6 mg L-1)
than storm flow (3.4 mg L-1). A substantial proportion
of total N was in the form of NO3-N in base
flow (58.1%) and small storms with <1-year return period (48.4%),
suggesting that base flow and small storm flow were probably dominated
by NO3-N-rich lateral subsurface flow. As
storm size (runoff volume, flow rate, and return period) increased,
the NO3-N concentration decreased following
a power relationship. In contrast, total N concentration increased
with increasing storm size, which was attributed to surface runoff
that flushed NH4-N and dissolved and particulate
organic N into the stream. The NO3-N and
total N losses from this watershed were greater during the pre-growing
(January-April) and post-growing (October-December) seasons, which
contributed >73% of the overall NO3-N and
total N losses. These two seasons also had greater flow-weighted
mean NO3-N (4.8 and 5.5 mg L-1,
respectively) and total N (9.8 and 10.1 mg L-1, respectively)
concentrations and greater discharge (40% and 32% of total discharge,
respectively) than during the growing season. Greater contribution
of NO3-N to total N loss in storm flow was
also observed during the pre-growing and post-growing seasons
than during the growing season, while contribution of NO3-N to total N loss in base flow was consistent
across seasons. Management practices to reduce N loss from this
watershed should target periods of base flow when NO3-N
and total N concentrations are greater (before and after the crop
growing season) and target areas along the stream where seeps
are present, perhaps including perennial plant species in localized
riparian buffers and introducing cover crops on the agricultural
land during the fallow season.
Keywords: Nitrate loss; Total N loss; Discharge; Storm; Watershed』
1. Introduction
2. Materials and methods
2.1. Watershed description
2.2. Data collection
2.3. Data analysis
3. Results
3.1. Trends in hydrology
3.2. NO3-N and total N trends in base flow
and storm flow
3.3. Differences across seasons
4. Discussion
4.1. hydrological features
4.2. N export mechanisms in base flow and storm flow
4.3. N export mechanisms in different seasons
4.4. management practices to reduce N loss
5. Summary
References