Soil aggregation and soil fraction associated carbon under different vegetation types in a complex landscape Xiaoqing Li A,B,E , Iris Vogeler C,D , and Luitgard Schwendenmann A A School of Environment, University of Auckland, Private Bag 92019, Auckland 1010, New Zealand. B Qinghai University, 251 Ningda Road, Chengbei District, Xining, Qinghai, China. C Plant & Food Research, Private Bag 92169, Auckland 1142, New Zealand. D Aarhus University, Blichers Alle 20, 8830 Tjele, Denmark. E Corresponding author. Email: xli679@aucklanduni.ac.nz Abstract. Land cover change has been shown to affect soil characteristics and soil organic carbon (SOC) storage. However, little is known about the driving factors associated with soil carbon (C) stabilisation in complex landscapes. This study was conducted to investigate the effects of both inherent soil characteristics and recent vegetation cover change on soil aggregation and soil fraction associated C in a complex landscape. The specific objectives were: (1) determine bulk soil properties and SOC saturation deficit; (2) quantify soil aggregation, soil size-density fractions, and soil fraction associated C; and (3) identify the factors that influence soil fractions and soil fraction associated C in two adjacent catchments differing in vegetation cover, Central Otago, New Zealand. Catchment GH1 (n = 17 plots) was dominated by tussock grassland and native shrubs. Catchment GH2 (n = 21 plots) was converted from tussock grassland into a pine forest in 1981. The catchments differed in soil texture (e.g. sand content GH1: 62.9%, GH2: 50.7%, P = 0.007), soil SOC stocks (GH1: 5.0 kg C m À2 , GH2: 4.3 kg C m À2 , P = 0.04), mean weight diameter (MWD) (GH1: 782.3 mm, GH2: 736.5 mm, P = 0.002), and proportion of the macroaggregate heavy fraction (macro_HF) (GH1: 72%, GH2: 55%, P = 0.01). No significant differences were found in SOC saturation deficit (GH1: 14.8 mg g À1 , GH2: 13.1 mg g À1 , P = 0.13). Dominant vegetation cover explained 21% of the variation in MWD in GH1, whereas silt+clay C content explained 31.6% of the MWD variation in GH2. The macro_HF fraction was negatively correlated with the proportion of silt+clay. Our findings illustrate that physical and chemical soil characteristics are important drivers in such a complex landscape and may have masked the effect of recent vegetation change on soil aggregation, and soil fraction associated C. Additional keywords: land cover change, New Zealand, pine forest, shrub encroachment, size-density fractionation, soil aggregation, soil organic carbon saturation. Received 9 July 2018, accepted 1 February 2019, published online 7 March 2019 Introduction Soil is the largest terrestrial carbon (C) pool, storing four times more C than the biosphere and over three times more C than the atmosphere (Lal 2008; Stockmann et al. 2015). Maintaining and increasing soil organic carbon (SOC) can aid reduction in the concentration of CO 2 in the atmosphere, and is thus critical for mitigating climate change (Lal 2004; Paustian et al. 2016). Soil organic C is also the basis of soil fertility and plant productivity and plays an important role in soil aggregate stability (Jackson et al. 2017). Soil C storage has been intensively studied across ecosystems (Jones and Donnelly 2004; Jandl et al. 2007; Lal 2008; Lal et al. 2015). Differences in the magnitude of SOC stocks across sites have been attributed to climate, soil physical- chemical properties, topography, and vegetation cover (Xiong et al. 2014; Jackson et al. 2017; Rabot et al. 2018). However, the quantification of SOC stocks and the assessment of its driving factors in landscapes with complex topography and vegetation patterns remains understudied but is critical to improve our understanding of soil C dynamics (Conforti et al. 2016; Román-Sánchez et al. 2018). Changes in vegetation cover have been shown to modify the amount and quality of the C input, soil structure, and microbial communities, all of which affect SOC stocks (Guo et al. 2007; Dlamini et al. 2016; Delelegn et al. 2017). Worldwide, large areas of grass-dominated systems are converted into shrub or tree-dominated systems through reforestation and afforestation (Jackson et al. 2002; Throop and Archer 2008; Berthrong et al. 2012). Furthermore, shrub and woody plant encroachment into grasslands has been observed globally (Naito and Cairns 2011). Some studies found an increase in SOC stocks following afforestation and shrub encroachment (Throop and Archer 2008; Deng et al. 2014; Li et al. 2016; Wang et al. 2016; Hunziker et al. 2017), whereas others reported decreases (Jackson et al. 2002; Paul et al. 2002; Farley et al. 2004; Journal compilation Ó CSIRO 2019 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2019, 57, 215–227 https://doi.org/10.1071/SR18193