The effect of biochar addition on N 2 O and CO 2 emissions from a sandy loam soil e The role of soil aeration Sean D.C. Case a, b, * , Niall P. McNamara a , David S. Reay b , Jeanette Whitaker a a Centre for Ecology and Hydrology, Lancaster Environment Centre, Library Avenue, Bailrigg LA1 4AP, UK b School of Geosciences, The University of Edinburgh, High School Yards, Edinburgh EH8 9XP, UK article info Article history: Received 1 August 2011 Received in revised form 17 March 2012 Accepted 19 March 2012 Available online 17 April 2012 Keywords: Biochar Charcoal Climate change Water holding capacity abstract Biochar application to soil has significant potential as a climate change mitigation strategy, due to its recalcitrant C content and observed effect to suppress soil greenhouse gas emissions such as nitrous oxide (N 2 O). Increased soil aeration following biochar amendment may contribute to this suppression. Soil cores from a Miscanthus X. giganteus plantation were amended with hardwood biochar at a rate of 2% dry soil weight (22 t ha 1 ). The cores were incubated at three different temperatures (4, 10 and 16  C) for 126 days, maintained field moist and half subjected to periodic wetting events. Cumulative N 2 O production was consistently suppressed by at least 49% with biochar amendment within 48 h of wetting at 10 and 16  C. We concluded that hardwood biochar suppressed soil N 2 O emissions following wetting at a range of field-relevant temperatures over four months. We hypothesised that this was due to biochar increasing soil aeration at relatively high moisture contents by increasing the water holding capacity (WHC) of the soil; however, this hypothesis was rejected. We found that 5% and 10% biochar amendment increased soil WHC. Also, 10% biochar amendment decreased bulk density of the soil. Sealed incubations were performed with biochar added at 0e10 % of dry soil weight and wetted to a uniform 87% WHC (78% WFPS). Cumulative N 2 O production within 60 h of wetting was 19, 19, 73 and 98% lower than the biochar-free control in the 1, 2, 5 and 10% biochar treatments respectively. We conclude that high levels of biochar amendment may change soil physical properties, but that the enhancement of soil aeration by biochar incorporation makes only a minimal contribution to the suppression of N 2 O emissions from a sandy loam soil. We suggest that microbial or physical immobilisation of NO 3  in soil following biochar addition may significantly contribute to the suppression of soil N 2 O emissions. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Greenhouse gas emissions from soils Nitrous oxide (N 2 O) is a greenhouse gas of high importance, with emissions accounting for approximately 6% of total anthro- pogenic radiative forcing (Davidson, 2009). Agriculture accounts for 58% of anthropogenic emissions of N 2 O(Solomon et al., 2007). A large proportion of N 2 O from agriculture comes from the inefficient use of N-based fertiliser, particularly from incomplete denitrifica- tion in wet or saturated soils (Davidson, 2009). N 2 O is produced in soils primarily via microbial activity through nitrification (Khalil et al., 2004), nitrifier denitrification (Wrage et al., 2005) and denitrification (Gillam et al., 2008). At high moisture contents, N 2 O production from denitrification is thought to be the dominant source (Bateman and Baggs, 2005). Denitrifi- cation is known to be strongly affected by soil temperature, nitrate (NO 3  ) content, organic matter availability and lability, redox potential and pH (Hofstra and Bouwman, 2005). Both nitrification and denitrification are highly moisture sensi- tive, as increased moisture content reduces oxygen availability to soil microorganisms (Barnard et al., 2005; Gillam et al., 2008). Across soil types, nitrifier activity peaks at around 60% of water holding capacity (WHC) and decreases above this when oxygen becomes more limiting. Denitrifier activity increases above 70% WHC (Linn and Doran, 1984). Considering instead a measure of soil aeration e water filled pore space (WFPS) e nitrifier activity has been found to peak at 60% WFPS and denitrifier activity increases above 70% WFPS (Bateman and Baggs, 2005). In soils approaching fully waterlogged conditions (and thus fully anoxic conditions) complete denitrification to N 2 may occur resulting in decreased * Corresponding author. Centre for Ecology and Hydrology, Lancaster Environ- ment Centre, Library Avenue, Bailrigg LA1 4AP, UK. Tel.: þ44 (0) 1524 595800; fax: þ44 (0) 1524 61536. E-mail address: secase@ceh.ac.uk (S.D.C. Case). Contents lists available at SciVerse ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2012.03.017 Soil Biology & Biochemistry 51 (2012) 125e134