Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma Efect of aggregate size distribution on soil moisture, soil-gas difusivity, and N 2 O emissions from a pasture soil J.R.R.N. Jayarathne a , T.K.K. Chamindu Deepagoda a,b , Timothy J. Clough b, ⁎ , Steve Thomas c , Bo Elberling d , Kathleen M. Smits e a Dept. of Civil Engineering, Faculty of Engineering, University of Peradeniya, 20400 Peradeniya, Sri Lanka b Dept. of Soil and Physical Sciences, Lincoln University, P.O. Box 85084, Lincoln 7647, New Zealand c Plant & Food Research Ltd. Gerald St, Lincoln 7608, New Zealand d Dept. of Geosciences and Natural Resource Management, Center for Permafrost (CENPERM), University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen, Denmark e Dept. of Civil Engineering, The University of Texas at Arlington, Arlington, Texas 76019, USA ARTICLE INFO Handling Editor: Yvan Capowiez Keywords: Soil aggregation Soil-gas difusivity Soil moisture N 2 O fuxes ABSTRACT Grazed pastures rich in nitrogen (N) from ruminant urine and fertilizer inputs are signifcant sources of nitrous oxide (N 2 O), a highly potent greenhouse gas. Difusion-controlled emission ofN 2 O from pasture systems can be described by soil-gas difusivity (D p /D o ), and its dependency on soil physical properties and soil moisture dy- namics. But studies linking soil aggregation, soil moisture variation, D p /D o and N 2 O emissions are lacking. Using coarse (2–4 mm) and fne (< 0.2 mm) aggregates, and seven diferent combinations thereof, the efect of soil aggregate size distribution on soil–water characteristic (SWC), D p /D o and N 2 O fuxes in a pastoral soil were investigated. Sieved-repacked samples, with varying fne aggregate fractions (F = 0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 1.0) were saturated with KNO 3 (1800 μg mL −1 ) solution and systematically drained to nine diferent matric potentials (−1 kPa to −10 kPa), followed by an air-dry step (−30 kPa). At each potential, D p /D o and N 2 O fuxes were measured. The measured SWC and SWC-derived pore-size distributions showed clear bimodal pore structures in all combinations. The highest and lowest total porosities were observed with F = 0 and 0.7, respectively. The lowest N 2 O peak fux was observed with F = 0.7 which also had the lowest D p /D o , while the highest fux among all combinations was observed in F = 1.0 at D p /D o = 0.002. Peak N 2 O fux varied with D p /D o dynamics that were in turn a function of inter-aggregate pore drainage. Initially increasing the fne fraction is speculated to have enhanced nitrifer-denitrifcation while further increases in the fne fraction, which lowered N 2 O peak emissions, were likely due to a shift from nitrifer-denitrifcation to denitrifcation and associated N 2 O consumption or entrapment. 1. Introduction Nitrous oxide (N 2 O) is the third most potent greenhouse gas (GHG) after carbon dioxide (CO 2 ) and methane (CH 4 ) with a global warming potential 298 times that of CO 2 over a 100-year horizon (Myhre et al., 2013). It is also considered as the single most important stratospheric ozone depleting substance (Ravishankara et al., 2009). Grazed pasture systems are typically rich in N due to ruminant urine and fertilizer in- puts, creating ‘hot-spots’ for N 2 O formation. The generation of N 2 O in these hot spots may result from denitrifcation activity that can be as- sociated with particulate organic matter in the soil (Parkin 1987). Re- lease of N 2 O to the atmosphere occurs primarily via direct emissions from urine afected soil or fertiliser afected pasture soil (Davidson, 2009; Oenema et al., 2005). For example, in New Zealand, where grazed pasture occupies 41% of the total land area, pasture grazing contributes to 95% of the national N 2 O emissions footprint (Kelliher et al., 2014; Ministry for the Environment, 2018). Formation of N 2 O as a consequence of N inputs to a pasture soil may occur via a range of microbial transformation pathways including ni- trifcation, nitrifer-denitrifcation, and denitrifcation (Kool et al., 2010; Clough et al., 2017; Wrage-Mönnig et al., 2018). Oxygen (O 2 ) supply is a key determinant of the biological pathways producing and consuming N 2 O in soils (Wrage-Mönnig et al., 2018). Under oxic con- ditions nitrifcation sequentially converts ammonia to hydroxylamine, nitric oxide, nitrite and nitrate, with N 2 O produced as a result of abiotic or biotic transformations of the intermediaries (Stein, 2019). If https://doi.org/10.1016/j.geoderma.2020.114737 Received 9 June 2020; Received in revised form 7 September 2020; Accepted 14 September 2020 ⁎ Corresponding author at: Dept. of Soil and Physical Sciences, Lincoln University, P.O. Box 85084, Lincoln 7647, New Zealand. E-mail address: Timothy.Clough@lincoln.ac.nz (T.J. Clough). Geoderma 383 (2021) 114737 0016-7061/ © 2020 Elsevier B.V. All rights reserved. T