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Agro-Ecosystems, 2 (1975) 127—132 127 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands THE RELATIONSHIP BETWEEN NITRATE CONCENTRATION IN THE SOUTHERN APPALACHIAN MOUNTAIN STREAMS AND TERRESTRIAL NITR1FIERS R.L. TODD, W.T. SWANK,* J.B. DOUGLASS,* P.O. KERR,** D.L. BROCKWAY^" and C.D. MONKtt Department of Agronomy and Institute of Ecology, University of Georgia, Athens, Ga. 30102 (U.S.A.) *Coweeta Hydrologic Laboratory, U.S. Forest Service, Franklinf, N.C. 28734 (U.S.A.) **Department of Microbiology, University of Georgia, Athens, Ga. 30602 (U.S. A) 'U.S. Environmental Protection Agency, Southeast Environmental Research Laboratory, Athens, Ga. 30601 (U.S.A.) ft Department of Botany, University of Georgia, Athens, Ga. 30602 (U.S.A.) Contribution No. 243 from the Deciduous Forest Biome, US-IBP. (Received April 8th, 1975) ABSTRACT Todd, R.L., Swank, W.T., Douglass, J.E., Kerr, P.O., Brockway, D.L. and Monk, C.D., 1975. The relationship between nitrate concentration in the Southern Appalachian mountain streams and terrestrial nitrifiers. Agro-Ecosystems, 2: 127—132. The nitrate content of stream water and the nitrifying bacterial population of the terrestrial horizon were measured in three southern Appalachian watersheds over a 22-month period. The watersheds studied were a fescue grass catchment, a 15-year old white pine plantation, and a mature undisturbed hardwood forest. Monthly averages of nitrate-nitrogen in stream water from the three watersheds were 730, 190, and 3 ppb respectively; the respective nitrifying populations averaged 16,000, 175 and 22 per gram of dry weight for each 40 cm soil profile. These populations were concentrated in the upper 10 cm of the profile (grass = 98%, white pine = 90%, and hardwood = 88%). A correlation is evident between the number of nitrifying bacteria in the soil from gaged watersheds and the NO3 content of the streams. Nitrifying activity appears to be dependent on vegetation type and suceessional stage. INTRODUCTION Nitrogen transformations occurring within the biosphere are regulated almost completely by terrestrial and aquatic microorganisms. The biological formation of nitrate and/or nitrite from compounds containing reduced nitrogen (ammonia) is a process termed nitrification. Nitrification in forest ecosystems was reviewed by Chase et al. (1968). Proposals for the importance of the nitrifying process on the functioning of forested ecosystems have included an increase of nitrifying populations (Smith et al., 128 1968) and nitrate losses (Likens et al., 1969) from a cutover New Hampshire, watershed. Rice and Pancholy (1972) suggest such increases are not due to elimination of the competition for nitrate by the vegetation but result from a de-repression of the nitrification process. They propose that the suppression of nitrification is a prime factor in the establishment of a climax forest vegetation. Recent evidence (Rice and Pancholy, 1973; 1974) has been presented for the role of tannins and other polyaromatic compounds as natural occurring inhibitors of nitrification. The collation of several investigators' efforts at the US- IBP Eastern Deciduous Forest Biome study site, located at Coweeta Hydrologic Laboratory in western North Carolina, allows an in-depth assessment of the role nitrification plays in nutrient cycling within southern Appalachian forested watersheds. In this communication the terrestrial nitrifying populations are correlated with the nitrate content in the streams for a 22-month period from an area manifesting a grassto-forest serai stage after 4 years, a 15-year-old white pine plantation, and a mature undisturbed hardwood forest. The treatment histories and physical properties of these adjacent catchments with similar soil types under differing vegetative regimes have been described in detail by Johnson and Swank (1973). MATERIALS AND METHODS Composite samples were collected at biweekly intervals over a 22-month period from the soil solum from at least two locations on each catchment. The samples were transported back to the laboratory at 5°C and processed within 24 h. The number of chemoautotrophic nitrifying microorganisms was determined using a modification of the procedure described by Alexander and Clark (1965). The chemoautotrophic nitrifying population is defined in this investigation as one forming nitrate (NO3~) in a chemically defined medium in which ammonium (NH 4 + ) is the sole nitrogen source and calcium carbonate the only added energy source. Nitrate formation in the incubation medium was detected following 3 weeks incubation at 25°C (Alexander and Clark, 1965). Population density was estimated by the most probable number (MPN) method outlined by Alexander (1965). Results are expressed as numbers per gram of dry weight soil, and reported values reflect the mean of each month's set of determinations. Nitrate was quantified in the stream water from each watershed on a twiceweekly basis. Samples, collected just above the weir settling basin, were filtered within 2 h after their collection (0.45 p.m, Millipore) and stored at low (5°C) temperature in sterile polyethylene or polypropylene bottles. Nitrate was analyzed by the automated cadmium reduction method (Environmental Protection Agency, 1971). Reproducibility of the nitrate determination at 100 ppb was ± 1 ppb (McSwain, 1973). Average concentration values were weighted for streamflow for each drainage basin. 129 RESULTS The monthly mean of nitrifying bacteria (number/g of soil) and the average NO3-N concentration of stream water are illustrated in Figs 1—3. Data in Fig. 1 are taken from a watershed in the fourth and fifth year of a grass-to-forest succession. Similar activities for a 15-year white pine plantation and a mature mixed hardwood catchment are represented in Figs 2 and 3, respectively. The monthly plottings generally show low levels of NO3-N and nitrifying bacteria throughout the year for the mature hardwood cover. In contrast, the pine and grass covered watersheds show large seasonal fluctuations in bacteria and NO3-N levels; the annual maximum and minimum levels for the two variables tend to coincide. In Table I the distribution of the nitrifying populations in the soil profiles is compared to the mean annual NO3-N content of the stream water. For a 40- cm profile 98% of the microbial nitrifying population is located within the upper 10 cm of the grass watershed profile; for the pine the value is 90% and for the hardwood 88%. DISCUSSION A correlation of nitrate content in first order mountain streams and terrestrial nitrifying bacterial populations from their respective drainage watersheds is observed. The distribution of these populations occurs in the upper division of the e ?5 O CD O 4 1.4 Gross • Nitrifying Bacteria 1.2 1.0 0.8 < K. UJ CD CD Z! I u_ E0 I— 0.6 0.4 0.2 6 8 10 12 2 4 10 0 12 1972 1971 TIME (MONTHS) Fig. 1. Comparison of the terrestrial nitrifying bacteria and stream nitrate (NO3) content for the fourth and fifth year of a grass-to-forest successional vegetation. 130 1.4 Pine • Nitrifying Bacteria • N0 3 -N o o O 4 1.2 1.0 0.8 0.6 QQ 0.4 0.2 U. 4 6 8 10 12 6 1971 8 10 0 12 1972 TIME (MONTHS) Fig.2. Comparison of the terrestrial nitrifying bacteria and stream nitrate (NO 3 ) content for the fifteenth and sixteenth year of a white pine successional vegetation. O CO o o O 4 1.4 Hardwood • Nitrifying Bacteria • N0 3 -N 1.2 1.0 0.8 1' UJ 0.6 §2 CD O 0.4 0.2 10 12 2 4 1971 10 12 1972 TIME (MONTHS) Fig. 3. Comparison of the terrestrial nitrifying bacteria and stream nitrate (NO 3 ) content for a mature hardwood vegetation. 131 TABLE I Comparison of NO3-N content (weighted annual mean) in stream run-off with mean number of nitrifying bacteria in the soil Forest type NO3-N (ppm) Nitrifying bacteria (number/g oven-dry soil) by soil depth Total 0—10 cm 20—40 cm 10—20 cm Grass- to-forest succession White pine 0.792 0.190 15,750 160 90 4 250 13 16,000 175 Mature hardwood 0.003 15 2 5 22 profile as would be expected for these organisms. The observation that nitrate loss and nitrifying populations decrease as vegetation approaches climax supports the hypothesis proposed by Rice and Pancholy (1972). They propose that the nitrifiers are inhibited in the climax so that ammonium nitrogen is not oxidized to nitrate as readily as in the successional stages. Climax vegetation can either inhibit the nitrification process (Rice and Pancholy, 1973) or can better utilize ammonium-nitrogen directly than can the preceding serai stages. In spite of the positive correlation between lower nitrate losses with decline in nitrifying populations, it may be premature to speculate that these activities are responsible for nitrate loss from the terrestrial ecosystems. However, it certainly appears that the nitrate content of these streams could be largely due to biological activity in the terrestrial environment and not to biological activity within the stream. ACKNOWLEDGEMENT Research supported by the Eastern Deciduous Forest Biome, US-IBP, funded by the National Science Foundation under Interagency Agreement AG-199, BMS69-01147 A09 with the Energy Research and Development Administration — Oak Ridge National Laboratory. REFERENCES Alexander, M., 1965. Most-probable-number method for microbial populations. In: C.A. Black et al. (Editors), Methods of Soil Analysis, Part 2. Am. Soc. Agron.,Madison, Wise., pp. 1467—1472. Alexander, M. and Clark, F.E., 1965. Nitrifying bacteria. In: C.A. Black et al. (Editors), Methods of Soil Analysis, Part 2. Am. Soc. Agron., Madison, Wise., pp. 1477—1483. Chase, F.E., Corke, C.T. and Robinson, J.B., 1968. Nitrifying bacteria in soil. In: T.R.G. Gray and D. Parkinson (Editors), The Ecology of Soil and Bacteria. Univ. of Toronto Press, Toronto, Ont, pp. 593—611. Environmental Protection Agency, 1971. Methods for chemical analysis of water and wastes. Environm. Prot. Agency, 16020, pp. 175—183. 132 Johnson, P.L. and Swank, W.T., 1973. Studies on cation budgets in the southern Appalachians on four experimental watersheds with contrasting vegetation. Ecology, 54: 70—80. Likens, G.E., Bormann, F.H. and Johnson, N.M. 1969. Nitrification: Importance to nutrient losses from a cutover forested ecosystem. Science, 163: 1205—1206. McSwain, M.R., 1973. Procedures for chemical analysis of streamflow and precipitation at the Coweeta Hydrologic Laboratory. Eastern Deciduous Forest Biome Rep. No. 73, p.12. Rice, E.L. and Pancholy, S.K., 1972. Inhibition of nitrification by climax ecosystems. Am. J. Bot., 59: 1033—1040. Rice, E.L. and Pancholy, S.K., 1973. Inhibition of nitrification by climax ecosystems. II. Additional evidence and possible role of tannins. Am. J. Bot., 60: 691—702. Rice, E.L. and Pancholy, S.K., 1974. Inhibition of nitrification by climax ecosystems. III. Inhibitors other than tannins. Am. J. Bot, 81: 1095—1103. • Smith, W., Bormann, F.H., and Likens, G.E., 1968. Response of chemoautotrophic nitrifiers to forest cutting. Soil Sci., 106: 471—473.