Microbial responses to the erosional redistribution of soil organic carbon in arable fields Jennifer A.J. Dungait a, * , Claire Ghee b, c , John S. Rowan d , Blair M. McKenzie e , Cathy Hawes e , Elizabeth R. Dixon a , Eric Paterson c , David W. Hopkins f a Department of Sustainable Soils and Grassland Systems, Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK b School of Biological Sciences, University of Aberdeen, School of Biological Sciences, St. Machar Drive, Aberdeen AB24 3UU, UK c Ecological Sciences, The James Hutton Institute, Craigiebuckler, Aberdeen AB15 8QH, UK d School of the Environment, University of Dundee, Dundee DD1 4HN, UK e Environmental and Biogeochemical Sciences, The James Hutton Institute, Invergowrie, Dundee DD2 5DA, UK f School of Life Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK article info Article history: Received 17 September 2012 Received in revised form 7 December 2012 Accepted 29 January 2013 Available online 16 February 2013 Keywords: Soil erosion Agriculture Soil microbial biomass Carbon cycling Nitrogen cycling abstract Quantifying the potential for eroding agricultural soils to act as sinks or sources of atmospheric carbon relies on accounting for the pools and fluxes of soil organic carbon (SOC) and nutrients, e.g. nitrogen (N), affected by erosion. Herein, we report the outcomes of an experiment where a C 4 maize (Zea mays) crop (d 13 C ¼12.1&) was cultivated and incorporated for 2 years to introduce a ‘pulse’ of 13 C-enriched SOC to aC 3 arable soil (d 13 C ¼27.4&). Soils were sampled at eroding (top slope and upper slope) and dep- ositional (lower slope and slope foot) positions of an accelerated erosion pathway that were confirmed using 137 Cs measurements. The sand particle-sized fraction (63e2000 mm) was predominant and increased in the depositional slope positions due to selective loss of fine particles and preferential deposition of the coarsest fraction of transported sediment. There was a significant isometric relationship between the percentage SOC and total N: top slope > upper slope > lower slope, with similar values in the slope foot to the top slope. The d 15 N values of the soils were enriched (7.3&) at the slope foot, compared with the other slope positions (average 6.3&), suggesting increased denitrification rates. The d 13 C values of the soil microbial biomass C extracted from surface soils (0e5 cm) at each slope position showed that the proportion of maize C being incorporated into the soil microbial biomass declined in the downslope direction from 54% (top slope) to 43% (upper slope) to 18% (lower slope) in inverse proportion to the size of the soil microbial biomass, and increased to 41% at the slope foot. This suggests dynamic replacement of the SOC with crop C in the eroding slope positions and dilution of the transported C by C3-SOC in the depositional slope posi- tions. This paper is evidence that erosional distribution of soil carbon leads to differential microbial utilisation of SOC between eroding and depositional sites. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Terrestrial ecosystems hold the potential to capture and store substantially more carbon (C) in soil organic matter (SOM) through changes in management that are also of benefit to the multitude of ecosystem services that soils provide (Dungait et al., 2012a). There is an opportunity to store more C in agricultural soils by replen- ishing the 55e78 Pg soil organic carbon (SOC) lost from the global terrestrial C pool after land use change from native vegetation to crop production (Lal, 2004; Smith et al., 2008). Full exploitation of this potential relies on accounting for the pools and fluxes of SOC affected by erosion (Smith et al., 2010). The magnitude of the error associated with the calculations of the global erosion flux in agri- cultural soils is very large (0.5  1.5 Pg a 1 ; Quinton et al., 2010) and will be compounded by the effects of climate change wherein soil erosion rates are expected to increase by 1.7% for each 1% increase in total precipitation (Nearing et al., 2004). Soil erosion is the most widespread form of soil degradation, accounting for up to 70% of C loss from cultivated soils (Gregorich et al., 1998), and is a source of CO 2 that contributes 0.8e1.2 Pg C a 1 to the atmospheric C pool (Lal, 2008). ‘Critical’ concentrations of less than 1e2% SOC in soil are considered to impair soil function leading to lower crop yields * Corresponding author. Tel.: þ44 (0)1837 883500. E-mail address: jennifer.dungait@rothamsted.ac.uk (J.A.J. Dungait). Contents lists available at SciVerse ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.soilbio.2013.01.027 Soil Biology & Biochemistry 60 (2013) 195e201