Effect of summer fallowing, perennial crop cover and conservation reserve practices on soil nitrate distribution

Abstract

Dryland agriculture is an economically feasible method of producing crops in some parts of the semi-arid regions of the mid-west and western United States. Summer fallowing, which is commonly practiced in these regions, serves to replenish soil moisture and plant-available nitrate-nitrogen (N0 3--N), while also stabilizing production and more uniformly distributing the work load. In areas where dryland agriculture is concentrated, summer fallowing may lead to N0 3--N contamination of ground-water due to lack of plant uptake of excess water and N. Under certain conditions of precipitation, soil percolation, slope, and cropping intensity, summer fallowing has been shown to cause elevated N03--N concentrations in shallow groundwater wells. These elevated N0 3--N levels have been detected in several areas of the U.S. Two areas where high N0 3--N concentrations have been repeatedly found in groundwater samples are in northeastern and central Montana. Judith Basin and Fergus Counties of Montana were selected for soil sampling to assess any differences in soil NO3--N concentrations under three different land use systems: 1) crop fallow rotation, 2) acreage enrolled in the USDA Agricultural Stabilization and Conservation Service (ASCS) conservation reserve program (CRP), 3) and rangeland. Soil samples were collected to a depth of three m (10 ft) at four different sites in each county. Soil samples from all sampling depths at each of the eight sites were analyzed for gravel percent (>2 mm diameter) , N0 3--N load, and N0 3--N concentration. Samples from 0 m to 0.6 m were also analyzed for total-N concentration. Trends in soil N0 3-N give evidence that summer fallowing may be the cause of N0 3--N in shallow groundwater in some areas Of Montana where dryland cropping is practiced. Overall, average soil N0 3--N concentration throughout the sampled soil profile was 4.2, 2.0, and 1.3 mgkg-1 for the crop fallow, CRP, and rangeland land use practices, respectively. Average N0 3--N concentration in Fergus County ranged from 2.5 mgkg-1 to 20.4 mgkg-1 under crop fallow while it ranged from only 0.9 to 6.2 mgkg-1 and from 0.9 to 4.2 mgkg-1 for the CRP and rangeland uses, respectively. Average N0 3--N concentration in Judith Basin County ranged from 1.4 mgkg-1 to 6.9 mgkg-1, 0.6 mgkg-1 to 2.0 mgkg-1, and from 0.8 mgkg-1 to 1.4 mgkg-1 under crop fallow, CRP, and rangeland land use systems, respectively. Assuming that extensive use of crop fallow causes increased N0 3--N concentrations under some conditions, alternative land use management practices may be effective in reducing N0 3--N levels in such areas.