Upland soil functions under organic grazing systems

Upland soils provide a wide range of key ecosystem services such as carbon storage, water quality regulation and flood mitigation, and supporting important trophic interactions. UK upland areas are largely used for low intensity livestock grazing due to low grassland productivity. However, it is not fully understood whether organic grazing management has any impact on key soil functions in upland areas, compared to conventional management. This thesis uses a paired case study approach to investigate the key differences in soil organic matter (OM), bulk density (BD), saturated hydraulic conductivity (Ks), effective porosity, pH, moisture content available N, exchangeable cations, and earthworm populations, between the two management systems. Each pair of field sites comprised of one organic and one conventional farm; site pairs were located in the Forest of Bowland, Yorkshire Dales, and North York Moors, and chosen for their similarity in size and proximity. All sites had a long history of sheep grazing, organic sites had all been under organic management for at least 10 years. This was then followed by an avoidance behaviour experiment to investigate the impact of the veterinary medicines used in conventional grazing management (albendazole, ivermectin, levamisole and moxidectin) on the endogeic earthworm A. chlorotica, an earthworm commonly found in upland grasslands. Organically manged sites had less OM than conventional sites, particularly at 0-5 cm depth (means of 14% under organic and 39% under conventional) but differences in OM content were attributed to differences in soil type. Saturated hydraulic conductivity was higher under organic management (medians of 65 mm hr-1 for organic and 33 mm hr-1 for conventional) while conventional sites had a higher proportion of flow through macropores. Abundance of A. chlorotica was significantly higher under organic management (33% of the total earthworm population, 16% under conventional). However, earthworm density was positively correlated with pH across all sites, most likely as a function of soil type. Avoidance behaviour was observed in A. chlorotica under exposure of environmentally realistic concentrations of albendazole and ivermectin. Calculations of carbon balance show both management systems to be carbon sinks, with conventional farming having slightly bigger SOC stocks, especially under manure application. Overall, it was difficult to disentangle the treatment effect from environmental factors due to differences in soil type and topography between field sites, but findings suggest that conventional grazing may be slightly better for carbon storage, and that anthelmintics used in livestock farming may be negatively impacting earthworm behaviour.