Pergamon Soil Biol. Biochem. Vol. 26, No. 6, 799-800, 1994 pp. Copyright 0 1994 Else&r Science Ltd 00384717(93)EOO26-I Printed in Great Britain. All rights reserved 0038-0717/94 $7.00 + 0.00 SHORT COMMUNICATION AN INVERSE RELATIONSHIP BETWEEN NITRATE AND AMMONIUM IN AN ORGANIC RIPARIAN SOIL L. A. SCHIPPER,‘* A. B. COOPER,’ C. G. HARFOOT~ and W. J. DYCK~ ‘Landcare Research NZ Ltd, Private Bag 3127, Hamilton, rEcosystems Division, National Institute of Water and Atmospheric Research, P.O. Box 11115, Hamilton, ‘Department of Biological Sciences, University of Waikato, Hamilton and “New Zealand Forest Research Institute, Rotorua, New Zealand (Accepted 8 November 1993) In anaerobic environments, there are two main microbial pathways where nitrate is used as a terminal electron acceptor: denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Of these two pathways, denitri- fication is more fully understood (Tiedje, 1988). There is little information regarding the ecology of bacteria that use DNRA for energy production and even less information about competition between these bacteria and denitrifiers. Results are presented here that indicate competition between these two groups of bacteria in an organic riparian soil. Our study was part of a more detailed investigation of the regulators and rates of denitrification in an organic riparian zone (Schipper et al., 1993). The riparian zone received high concentrations of nitrate (ca 10 mg I-’ nitrate-N) in ground- water from an upslope land application of secondary- treated domestic effluent. A 2 x 2 m section of the riparian zone was divided into a grid of 32 sampling sites from which soil cores (30 cm deep by 2 cm dia) were collected. The site was sampled five times during a 2-yr period giving a total of 160 samples. Soil subsamples (co 20g wet wt) were individually placed into bottles containing 40 ml of distilled water and lOOmgl_ HgCl, as a preservative. The bottles were briefly shaken and returned to the laboratory on ice. The extract was filtered (0.45 pm Millipore), and the filtrate was analysed for nitrate, using the cadmium reduction method (United States Environmental Protection Agency, 1979), and for ammonium, using the method of Searle (1974). Denitrifying enzyme activity (DEA) was measured in subsamples of soil cores using methods described in Schipper er al. (1993). The cation-exchange capacity of the soil was 50.3 cmol kg-’ determined by the method of Nicholson (1984). Percent organic matter was determined on a soil subsample from each core by loss on ignition (Hesse, 1971). Three dimensional plots were constructed, using the average value for each parameter at each sampling location over time. An inverse relationship was observed between the ratio of extracted nitrate-to-percent organic matter and the ex- tracted ammonium concentration (Fig. 1). The nitrate-to- percent organic matter ratio was largely determined by the nitrate concentration as the percent organic matter (average 26%, SD 4.3%) was evenly distributed throughout the riparian zone. The relatively high organic matter content and the water-saturated nature of this soil suggested that the soil was predominantly anaerobic, which agreed with the *Author for correspondence. high rates of in situ denitrification measured at this site (Schipper et al., 1993). Because of the anaerobic nature of these soils, coupled nitrification-denitrification would have been restricted to the top few millimetres of the soil (e.g. Jensen et al., 1993). Since soil to 30 cm deep was extracted for nitrate and ammonium, the influence of nitrification on the observed inverse relationship was thought to be small. The predominant source of nitrate in the soil water was from inflowing groundwater. Anaerobic mineralization of organic N to ammonium would have contributed to the formation of ammonium. However, mineralization would not account for the observed spatial patterns of ammonium (Fig. 2) or the inverse relationship between the ammonium and nitrate-to-carbon ratios. An explanation for the inverse relationship might be found by considering the factors which regulate anaerobic nitrate reduction to nitrogen gases (by denitrification) or to ammonium (by DNRA). It has been hypothesized that the partitioning of nitrate reduction between DNRA and denitrification is dependent on the nitrate-to-carbon ratio of the environment (Tiedje et al., 1982; Tiedje, 1988) where DNRA is favoured when the ratio is low and denitrification is favoured when the nitrate-to-carbon ratio is high. In the riparian zone studied, DNRA might be favoured over denitrification where the nitrate-to-percent organic matter ratio was low, resulting in a relatively higher rate of ammonium formation. Where the 8 40 * . ‘4 9 = 30 . 3 i 5 . rho ’ Ii i E 10 .2 E h. E Oo . . . . . 0.4 0.8 1.2 6 Nltrate:%crganlc matter ratio Fig. 1. Relationship between the nitrate-to-percent organic matter ratio and the ammonium concentration in the ripar- ian zone. 799