Illinois
Fertilizer Conference Proceedings
January 28-30, 1991
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R. M. Vanden Heuvel, R. L. Mulvaney, D. P. McKenna, and S. C.
Schock1
Contamination of ground-and surface water from agricultural activities has become a nationwide concern because of the public's dependence on these sources of water for drinking supplies. With the threat of increasing No 3- 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.
Unfortunately, the only procedure that has been suggested for identifying sources of NO3- in water is controversial. A two-year, interdisciplinary project was begun in March of 1990 to study and evaluate the validity of natural abundance 15N techniques (δ15N values) to identify and quantify sources of nitrate nitrogen (NO3 -N) contaminating groundwater. 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 study 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 groundwater 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 (δ15N) 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 δ15N value for NO3-.
In general, the work performed by these authors has shown extensive spatial variability of δ15N 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 δ15N technique successfully to identify various sources of NO3- in groundwater and estimate their relative; contributions (Kreitlei and Jones, 1975; Kreitler et al.; 1978; Gormly ando Spalding, 19J9). In addition, more recent work has been performed that indicates the variability of δ15N values in soil is not a prohibitive factor in their use for studying N cycling in soils (Karimanos et al., 1981; Selles et al., 1986).
To resolve this apparent conflict, a direct comparison of conventional 15N-labeled and δ15N techniques is being performed with field lysimeters as originally suggested by Edwards (1973). To date, no comparison of this type has been reported, presumably because of the lack of suitable lysimeter installations and mass spectrometric facilities. Information from this study will allow a more meaningful interpretation of the drinking well samples from Illinois.
Field Lysimeters
The field lysimiters to be used in the first part of this study, located in southeastern North Dakota, will provide a unique opportunity to evaluate the use of δ15N values in tracing NO3-; leaching and studying N transformations. The comparison between the two techniques, conducted on adjacent lysimeters, will allow the limitations of the non-labeled approach to be evaluated in terms of the data obtained with the more sensitive 15N-enriched approach. The lysimeters are ideally suited to these studies because they allow for intensive soil solution sampling on a year-round basis (Montgomery et al., 1987). Moreover, soil variability, a significant source of error in δ15N studies, should be minimal since the lysimeters were filled with a reconstructed soil. This is significant because under such uniform and controlled conditions, the possibility that δ15N values will yield results comparable to the 15N labeled fertilizer is optimal. If results indicate δ15N values are a valid means of tracing fertilizer NO3-, it would be a most useful methodology to develop for environmental investigations, such as continued monitoring of drinking water wells and other ground water in Illinois.
Private Drinking Wells
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 615K 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 percent of the area being planted to corn and soybeans. Most o£ 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 percent 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 here range from 60 to 195 feet. No irrigation occurs in the study area.
Effingham County Soils here are exclusively Alfisols mostly under non-irrigated corn and soybean production with about 30 percent of the land under forest. The depth to the aquifer is greater than 50 feet. Well depth varies from 20 to 70 feet.
Detailed 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. Wells are being sampled biweekly for a one-year period. During. sampling, a one liter subsample is being retained for 15N analysis by the University of Illinois Agronomy Department.
The 15N content of the NO3- in the water samples is expressed as:
δ15N = %15N(sample) - %15N (standard) / %15N(standard) X 1000
The range of δ15N values in water is generally about 0-22. According to the literature on this methodology, as he value decreases, the level ofNO3- originating from commercial fertilizer increases.
Preliminary results from the field lysimeters (not given here but to be presented) indicate that where 15N-labeled fertilizer is not used the presence of fertilizer N cannot be detected using δ15N values. Before the fertilizer front reached the drainage lines, considerable variability existed for the δ15N values of drainage water, indicating a lessened chance of their usefulness for identifying NO3- sources. Over a three month period the 615N values ranged to about 6.5 units. In addition, when 15N was detected in the labeled lysimeters, fertilizer could not be detected in the unlabeled lysimeters, indicating 15N-labeled fertilizer is the only legitimate! way to distinguish fertilizer N sources from other sources.
For the Illinois study, approximately 40 percent (100) of the samples from private water wells have been analyzed. Of these, about half had enough N for isotope analysis. These data are presented in Figure 1. The data given is for all counties. The range in concentrations of NO3--N is 0.5-57.5 ppm and the range in δ15N values about 1-30. The R2 value of 0.0043 indicates an extremely poor relationship between NO3--N concentration and δ15N value. These data indicate there is little possibility of using δ15N values to determine the source of nitrate contaminating water and that-use of this methodology in 'environmental monitoring programs in broad regions is not warranted. These results conflict with the results of other studies where the methodology has shown promise as an environmental monitoring tool. It is of particular interest to note that in one state this methodology has been recommended as a means to identify the source of NO3--N in groundwater.
Our preliminary results suggest that the use of natural variations in the 15N content of NO3-N (δ15N values) to identify and quantify NO--N sources in water samples is not justified. It is hoped that when our results are complete we will have a better understanding of the limits of the application of this methodology to environmental monitoring, or if it has any promise for application at all.
Figure 1. Nitrate-N concentration of sample (ppm)
Bremmer, J.M., and M. A. Tabatabai. 1973. Nitrogen-15 enrichment of soils and soilderived nitrate. J. Environ. Qual. 2:363-265.
Broadbent,F.E., R. S. Rauschkolb, K. A. Lewis, and G. Y. Chang. 1980. Spatial variability of nitrogen-15 and total nitrogen in some virgin and cu4ivated soils. Soil Sci. Soc. Am. J. 44:524-527.
Edwards, A. P. 1973. Isotopic tracer techniques for identification of sources of nitrate pollution. J. Environ. Qual. 2:382-387.
Edwards, A. P. 1975. Isotope effects in relation to the interpretation of 15N/14N ratios in tracer studies. p. 455-468. In- Isotope ratios as pollutant source and behavior indicators. International Atomic Energy Agency, Vienna.
Gormly, J. R., and R. F. Spalding. 1979. Sources and concentrations of nitratenitrogen in ground water of the Central Platte Region, Nebraska. Ground Water 17.291301.
Hauck, R. D., W. V. Bartholomew, JJ. M. Bremmer, F. E. Broadbent, H. H. Cheng, A. P. Edwards, D.R. Keeney, L. 0. Legg, S. R. Olsen, and L. K. Porter. 1972. Use of variation in natural nitrogen isotope abundance for environmental studies: a questionable approach. Science 177:453-454.
Kohl, D. H., G. B. Shearer, and B. Commoner. 1971. Fertilizer nitrogen: contribution to nitrate in surface water in a corn belt watershed. Science 174:1333-1334.
Karamanos, R. E., R. P. Voroney, and D. Via. Rennie. 1981. Variation of natural N-15 abundance of Central Saskatchewan soils. Soil Sci. Soc. Am. J. 45:826-828.
Kreitler, C. W., and D. G. Jones. 1975,. Natural soil nitrate: the cause of the nitrate contamination of ground water in. Runnels County, Texas. Ground Water 13:53-61.
Kreitler, C. W., S. E. Ragone, and B. G. Katy, 1978. N15/N14 ratios of groundwater nitrate, Long Island, New York. Ground Water 16:404-409.
Meints, V. W., L. V. Boone, and L. T. Kurtz. 1975. Natural 15N abundance in soils, leaves, and grain as influenced by long term additions of fertilizer N at several rates. J. Environ. Quay. 4:486-490:
Montgomery, B. R., L. Prunty, and J. W. Bauder. 1987. Vacuum trough extractors for measuring drainage and nitrate flux through sandy soils. Soil Sci. Soc. Am. J. 51:271-276
Selles, F., R. E. Karamanos, and R. G. Kachanoski. 1986. The spatial variability of nitrogen-15 and its relation to the variability of other soil properties. Soil Sci. Soc. Am. J. 50:105-110.
