A multi-level meta-analysis on the effects of grazing on soil hydraulic properties under variable grazing management, climate and clay properties

Various studies have investigated the effects of grazing on soil hydraulic properties (SHPs) under different soil and environmental conditions, and grazing management practices, across different regions of the world. However, despite a relatively large body of research on this topic, the overall influence of grazing on SHPs across diverse contexts remains ambiguous due to the complex interplay of factors that moderate these effects. This study adopts a multi-level meta-analytic model to systematically collate and analyse global field data, obtained from the literature (comprising 74 papers), to investigate the magnitude of changes in SHP as influenced by grazing, moderated by 17 factors relating to management (grazing intensity, duration, strategy, livestock type, rooting depth), climate, and intrinsic soil physical properties (texture, clay content, clay type fraction and related mechanical properties). The moderating factors were obtained from details reported in the publications, as well as from independent globally distributed databases (the clay property database by Ito and Wagai (2017), with clay mechanical properties derived from equations provided in Lehmann et al. (2021)); the WorldClim 2.1 dataset (Fick and Hijmans, 2017) for mean annual rainfall and temperatures; germplasm databases for individual species listed in the publications to obtain rooting depth). Our findings showed that grazing significantly affects soil structure, causing decreased saturated hydraulic conductivity, Ksat (56%), mean infiltration rates, MIR (38%), and macroporosity, MP (10%), and an increase in bulk density, BD (28%). The meta-analysis reveals that the impact of grazing on SHPs is significantly greater under heavy grazing (for MIR, BD), long-term grazing (Ksat, BD), in areas dominated by shallow-rooted pasture compared to mixed or deep-rooted systems (BD, MP), and in cattle dominated grazing systems as opposed to sheep or mixed grazing systems (Ksat, BD, MP). Additionally, the negative effects of grazing increase with increases in mean annual precipitation (all SHP) and temperature (all, but not BD). It is also notable that clay type properties, specifically derived mechanical properties, also showed significant relationships with grazing effects, across all SHPs. The findings suggest that future research should be focused on the long-term effects of cattle grazing on soils with large fractions of active to moderately active clay types in climates with high precipitation to help develop grazing management and planting strategies that support sustainable grazing while mitigating negative soil hydrological impacts.Fick and Hijmans (2017), DOI: 10.1002/joc.5086; Ito and Wagai (2017), DOI: 10.1038/sdata.2017.103; Lehmann et al. (2021), DOI: 10.1029/2021GL0953