Illinois Fertilizer
Conference Proceedings
January 27-29, 1992
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R.M. Vanden Heuvel, R.L. Mulvaney, D.P. McKenna, and S.C. Schock1
Agricultural chemicals are a concern for the rural population since 90 percent of the rural population relies upon ground water for domestic use. With the threat of increasing nitrate-N (NO3--N) levels in these supplies, the need for assessing the contribution from various sources, such as fertilizer N, soil organic matter, and animal wastes, becomes essential before remedial N management decisions can be made to reduce environmental contamination. Identification of these sources and their relative influences is often very difficult. Traditionally, field history of past N management practices has been the best indicator of contributing N sources. A nontraditional method, involving the use of natural nitrogen isotopes, has also been used but is controversial.
A two-year, interdisciplinary project was begun in March of 1990 to study and evaluate the validity of natural abundance 15N techniques (d15N values) to identify and quantify sources of NO3--N contaminating ground water. This is a cooperative effort between the University of Illinois Agronomy Department, Illinois State Geological Survey (ISGS) and Illinois State Water Survey (ISWS). The Illinois study is a companion to a similar study currently being conducted on field lysimeters in North Dakota.
The desire to assess the contribution of fertilizer N to surface and ground water NO3-levels on a larger scale, particularly entire watersheds, has prompted research efforts relying on differences in natural abundance 15N levels of fertilizer and NO3- mineralized during soil incubations (Kohl et al., 1971; Kohl et al., 1973). These techniques are based on the hypothesis that there is a consistent and measurable difference in the natural 15N concentration (d15N) of NO3- derived from commercial fertilizers, manure, and soil organic matter. These differences are then used to identify and quantify contributing sources of NO3-. This approach has been criticized by a number of workers (Hauck et al., 1972; Edwards, 1973; Bremner and Tabatabai, 1973; Edwards, 1975; Meints et al., 1975; Broadbent et al., 1980) for several reasons, but most notably for the difficulty in establishing a single background 615N value for NO3-. In general, the work performed by these authors has shown extensive spatial variability of d15N values for soil derived NO3- for small incubated samples taken from surface soils or soil cores. Despite the body of evidence that has accumulated against the approach, a number of investigations have apparently used the d15N technique successfully to identify various sources of NO3- in ground water and estimate their relative contributions (Kreitler and Jones, 1975; Kreider et al., 1978; Gormly and Spalding, 1979). In addition, more recent work has been performed that indicates the variability of d15N values in soil is not a prohibitive factor in ,.their use for studying N cycling in soils (Karamanos et al., 1981; Selles et al., 1986).
To resolve this apparent conflict, we are conducting four different studies
to evaluate the methodology. Only the Illinois well water study will be discussed
here.
The State of Illinois has funded a study to determine the occurrence of NO3- and pesticides in rural, private water supply wells. As part of the that study, the State Geological Survey and the State Water Survey has selected for sampling, after an extensive screening process, 240 private wells in five: areas. Forty-eight wells from each of five township-size areas were selected for the year-long; sampling period. Subsamples from these water samples will be analyzed for their variation in d15N values. The study areas are located in Mason, Kankakee, Livingston, Piatt, and Effingham Counties and represent a diversity of hydrogeological conditions. Descriptions of the five areas are as follows:
Mason County Heavily irrigated, the top of the sand and gravel aquifer in the study area is within 5 feet of the surface. Most soils are Mollisols with about 85% of the area being planted to corn and soybeans. Most of the private wells are shallow.
Kankakee County The depth to the dolomite aquifer varies from 5 to 20 feet. Soils are mostly Mollisols and are cropped to corn and soybeans. Less than 10% of the area is irrigated. Well depths in the area are generally 40 to 125 feet deep.
Livingston County Depth to the sand and gravel aquifer is about 34 feet below the surface. Soils are mostly Mollisols under corn and soybeans with very little irrigation. Well depths are from 35 to 40 feet.
Piatt County The uppermost sand and gravel aquifer is found between 70 to 90 feet from the surface. Mollisols predominate here and are under corn and soybean production. Well depths range from 60 to 195 feet. No irrigation occurs in the study area.
Effingham County Soils are exclusively Alfisols mostly under non-irrigated corn and soybean production with about 30% of the land under forest. The depth to the aquifer is greater than 50 feet. Well depth varies from 20 to 70 feet.
Wells are being sampled biweekly for a one-year period. Characterization of the five study areas, including land use, agricultural practices, agricultural chemical usage, soils, hydrogeology, and hydrology have been conducted by the ISGS and ISWS.
Information is available about such items as well proximity to barnyards or septic systems, well type, depth and construction, and past manure and N management practices. These and other pertinent information that can be related to N isotope data have been collected by the ISGS and ISWS.
Calculation of d15N Values
The 15N content of the NO3- in the water samples is expressed as:
d15N = [%15N (sample) - %15N
(standard) / %15N (standard)] x 1000
The range of d15N values in water is generally about 0-22. According to the literature on this methodology, as the value decreases, the level of NO3- originating from commercial fertilizer increases.
Data for NO3- concentrations vs d15N (delta values) for well samples are given for four counties in Figures 1, 2, 3, and 4. Piatt county is not included because so few samples were obtained with significant NO3- concentrations. The data for Effingham and Livingston counties show little relationship between d15N and NO3- values (correlation coefficients of 0.076 and 0.18, respectively). However, Kankakee and Mason counties show rather strong relationships (correlation coefficients of 0.31 and 0.51, respectively). One explanation for the decline of d15N values with increasing NO3- concentrations is that greater leaching of fertilizer N is occurring. Fertilizer N naturally has a lower d15N value than soil N. However, a second possible explanation is that denitrification could be occurring at some of the well sites. Denitrification in ground water will decrease the NO3- concentration while also increasing the d15Nvalue. Since Mason county has primarily sandy soils, with a low denitrification potential, leaching of fertilizer N is likely the_ probable cause of NO3- contamination. This is not surprising, however, considering soil characteristics and the extensive irrigation in the county. The results from Kankakee county are not easily attributable to either leaching or denitrification because of the medium-textured soils there.
Pooled data for all counties (Figure 5) illustrate that use of d15N techniques cannot be used for broad (multi-county) investigations.
We have not yet related d15N data to
the various site data. That information should help clarify some of our data
trends. More will be said about those relationships during our presentation.
Results from the Illinois study indicate that d15N
techniques may help identify sources of NO3- contamination
in sandy irrigated soils. However, field history and management practices give
just as strong an indication as to possible sources. The use of d15N
techniques cannot be considered a reliable means to access sources of NO3-
contamination in broad environmental monitoring programs. Other work we have
conducted indicates that extensive variability of natural soil d15N
values can occur even in sandy soils. Such variability prevents clear identification
and quantification of the various possible sources of NO3-.
Figure 1. Delta 15N values and nitrate-N concentrations for Effingham County, Illinois
Figure 2. Delta 15N values and nitrate-N concentrations for Livingston County, Illinois
Figure 3. Delta 15N values and nitrate-N concentrations for Kankakee County, Illinois
Figure 4. Delta 15N values and nitrate-N concentrations for Mason County, Illinois
Figure 5. Delta 15N values and nitrate-N concentrations for all counties
1Richard M. Vanden Heuvel is Assistant Professor and Richard L. Mulvaney is Associate Professor, Department of Agronomy, University of Illinois, Urbana, IL. Dennis McKenna and Susan Schock are Professional Scientists, Illinois State Geological Survey and Illinois State Water Survey, respectively, Champaign, IL.
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