Short Communication Constraining the conditions conducive to dissimilatory nitrate reduction to ammonium in temperate arable soils Christoph S. Schmidt a, 1 , David J. Richardson b , Elizabeth M. Baggs a, * a Institute of Biological and Environmental Sciences, University of Aberdeen, Cruickshank Building, St Machar Drive, Aberdeen AB24 3UU, UK b Centre for Molecular Structure and Biochemistry (CMSB), School of Biological Sciences, University of East Anglia, Norwich, NR4 7TJ, UK article info Article history: Received 2 June 2010 Received in revised form 14 February 2011 Accepted 18 February 2011 Available online 4 March 2011 Keywords: Dissimilatory nitrate reduction to ammonium Nitrate ammonification Nitrous oxide Stable isotopes abstract Here we offer the first assessment of conditions conducive to dissimilatory nitrate reduction to ammonium (DNRA) in temperate arable soils, through an examination of the potential for this process to occur in a range of soils of contrasting characteristics. NH 4 15 NO 3 (6.2 g N m 2 , 25 atom % excess 15 N) was applied, and recovery of 15 N in the NH þ 4 pool taken as indicative of occurrence of DNRA. Up to 5% of applied 15 N was recovered in the NH þ 4 pool 2 d after addition of N, glucose (44.6 g C m 2 ) and L-cysteine (7.7 g m 2 , 0.9 g N m 2 , 2.3 g C m 2 ). 15 NeNH þ 4 concentrations were positively correlated with soil pH, C-to-NO  3 ratio, bulk density, sand content and NO  2 concentration, but negatively correlated with soil C and organic N content. Our results demonstrate the potential for DNRA to contribute to N cycling in temperate arable soils, but its detection and significance is likely to depend on the provision of a low molecular weight C source. Ó 2011 Elsevier Ltd. All rights reserved. Dissimilatory nitrate reduction to ammonium (DNRA), or nitrate ammonification, is one of the least well characterised pathways of the soil N cycle, and is often ignored in N budgets. During this process NO  3 is reduced to NO  2 and NH þ 4 thereby providing a ‘short circuit’ in the N cycle by-passing denitrification and N 2 fixation (Mohan et al., 2004), with N 2 O produced at the NO  2 reduction stage (Costa et al., 1990; Kelso et al., 1997). Traditionally, NO  3 reduction in DNRA is thought to be favoured over denitrification only in intensively reduced and C-rich environments (C-to-N > 4) (Buresh and Patrick, 1978; Fazzolari et al., 1998; Tiedje, 1988; Tiedje et al., 1982). However, there is evidence for significant DNRA (up to 0.6 mg N g 1 day 1 ; up to >99% of NO  3 consumption) in forest soils (Bengtsson and Bergwall, 2000; Huygens et al., 2007; Pett-Ridge et al., 2006; Rutting et al., 2008; Silver et al., 2001, 2005; Templer et al., 2008), rice paddies (up to 21% of NO  3 consumption) (Chen et al., 1995a,b; Yin et al., 2002) and calcareous agricultural soils following glucose addition (Wan et al., 2009), suggesting DNRA may not be restricted to highly reducing conditions, and necessitating a re-assessment of the conditions under which this process occurs. Here we investigated the potential for DNRA in temperate arable soils ranging in their chemical and textural characteristics, in order to better constrain the conditions condu- cive for DNRA. We examined the recovery of 15 N, applied as 15 NeNO  3 , into the soil NH þ 4 pool as being indicative of DNRA. Our hypothesis was that DNRA would increase with soil organic matter content, C-to-NO  3 ratio, and clay content, optimising the potential for co-occurrence of sub-oxic microsites and reductant availability. Our experiment was established under controlled environment conditions (water-filled pore space (WFPS) 80%, temperature 20  C, ambient atmosphere). Bulk top-soils (0e20 cm depth) were collected from agricultural sites listed in Table 1 . To create repro- ducible and uniform conditions, and accounting for the frequent mechanical disturbance occurring at agricultural sites, soils were mixed, ground (<3 mm) and pre-wet 7 d prior to the start of the experiment, and 200  BD g of soil lightly packed in 500 ml Kilner jars to the bulk densities (BD) presented in Table 1 . Nitrogen was applied in solution as NH 4 15 NO 3 (25 atom % excess 15 N) at a concentration of 220  BD 1 mg N kg 1 soil dry weight (6.2 g N m 2 for each soil). Glucose (44.6 g C m 2 ,1.4 g C kg 1 at BD ¼ 1.1 g cm 3 ) and the reducing agent L-cysteine (7.7 g m 2 ; 0.9 g N m 2 and 2.3 g C m 2 ; translating into 0.029 g N kg 1 soil, 0.079 g C kg 1 soil respectively, at BD ¼ 1.1 g cm 3 ) were also applied to create conditions conductive for DNRA, as described by Yin et al. (2002).C 2 H 2 (0.01%) was added to the closed Kilner jar head space at the start of the experiment and after each sampling occasion, to inhibit oxidation of 15 NeNH þ 4 produced during DNRA * Corresponding author. Tel.: þ44 (0) 1224 272691; fax: þ44 (0) 1224 272703. E-mail address: e.baggs@abdn.ac.uk (E.M. Baggs). 1 Current address: Julius Kühn Institute for Biological Control, 64287 Darmstadt, Germany. Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2011.02.015 Soil Biology & Biochemistry 43 (2011) 1607e1611