The effect of soil aggregate size on pore structure and its consequence on emission of greenhouse gases S. Mangalassery, S. Sjo ¨ gersten, D.L. Sparkes, C. J. Sturrock, S.J. Mooney * School of Biosciences, Sutton Bonington Campus, University of Nottingham, Sutton Bonington, Loughborough, Leicestershire LE12 5RD, UK 1. Introduction Greenhouse gas (GHG) emissions from agricultural soils are a substantial contributor to climate change (Smith et al., 2008) and developing agricultural practices that bring mitigation of GHG emissions from agricultural soils is important. Currently, several different soil management strategies have been considered with regard to their potential to reduce the release of GHG from agriculture e.g. no-till practices (Ugalde et al., 2007; Uri, 2000), cover crops (Tubiello and Ewert, 2002) and agroforestry (Calfapie- tra et al., 2010; Pandey, 2002). Studies investigating the impact of such changes in practice on GHG emissions and soil C storage have illustrated wide-ranging results, possibly due to differences between studies in climatic zones, soil types, length of manage- ment practice and cropping systems. This highlights the impor- tance of developing a mechanistic understanding of how soil management directly impacts on GHG release and C storage through changes in soil biophysical properties in particular. It is well known that crop management activities, such as tillage, exert significant influence on soil physical properties. For example, tillage brings about the disruption of soil aggregates especially at the zone of disturbance and potentially the creation of hard pans at lower depths (Zotarelli et al., 2007). Soil micro aggregates are typically formed by binding microbial poly- saccharides with smaller soil particles such as silt and clay whereas macro aggregates are typically formed around plant roots and coarse organic fragments (Ian, 2011). Also, stable micro aggregates (<250 mm) can reorient themselves into macro aggregates with the help of newly formed particulate organic matter (Jastrow et al., 1996). The protection of soil aggregates depends on their stability on contact with water and responses to mechanical stresses like tillage. Tisdall and Oades (1980) showed conventional tillage leads to oxidation of soil organic matter, which act as binding agents for macro aggregates, and hence water stable aggregates (>250 mm) become less stable under intensive tillage systems. Kasper et al. (2009) observed reduced stability in aggregates under conventional tillage (18.2%, compared with 37.6% under mini- mum tillage). Different sized aggregates exert varying contribu- tions on the soil porous system and this in turn governs water and gas movement in soil (Perret et al., 1999). Soil & Tillage Research 132 (2013) 39–46 A R T I C L E I N F O Article history: Received 17 January 2013 Received in revised form 10 May 2013 Accepted 13 May 2013 Keywords: X-ray CT Saturated hydraulic conductivity Soil aggregates Pore characteristics Greenhouse gases A B S T R A C T Soil aggregation is an important physical property that influences the physico-chemical and biological properties of soil. Soil disturbances such as tillage can have a significant effect on soil aggregation. This study sought to examine the effect of soil aggregate size on soil pore characteristics and the subsequent effect on emission of greenhouse gases (GHGs) for both sandy loam and clay loam soils. Columns of aggregates in the size ranges of 2–4 mm, 1–2 mm, 0.5–1 mm and <0.5 mm were tested along with a field structured soil (i.e. aggregates <4 mm). Soil pore characteristics were quantified using X-ray Computed Tomography (CT). The average porosity in the soil columns ranged from 38.7 to 50.7%. Aggregate size influenced the total soil organic matter content with average values ranging from 7.5 to 8.6% in the clay loam soil and 2.8 to 5.2% in the sandy loam soil. CO 2 and CH 4 flux was significantly affected by size of aggregates. Clay loam soils emitted the most CO 2 from the small sized aggregates, whereas in sandy loam soils the larger aggregates produced the maximum CO 2 flux. Smaller aggregates produced higher CH 4 flux in both soil textures. No significant difference between aggregate sizes and soil textures was found for N 2 O fluxes. Soil pore characteristics such as porosity and pore size significantly affected fluxes of GHGs such as CO 2 and CH 4 . These results indicate that management practices such as tillage that heavily influence soil aggregation and pore characteristic development can have a direct impact on emission of greenhouse gases and subsequently have implications for global warming. ß 2013 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +44 115 9516257. E-mail addresses: sacha.mooney@nottingham.ac.uk, shamsudheenm@gmail.com (S.J. Mooney). Contents lists available at SciVerse ScienceDirect Soil & Tillage Research jou r nal h o mep age: w ww.els evier .co m/lo c ate/s till 0167-1987/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.still.2013.05.003