Biochar amendment improves soil physico-chemical properties and alters root biomass and the soil food web in grazed pastures

Biochar application has been recognised as an effective way to improve soil functions. In this study, we investigated how adding biochar as an amendment affects soil biological and physico-chemical properties in grazed pastures in a one-year field-based mesocosm experiment conducted on two contrasting soils – a sil-andic Andosol and a dystric Cambisol. Each site had two pastures managed under different nutrient and livestock practices: with and without effluent under dairy cow grazing on the Andosol, and with either nil or high phosphorus (P) fertiliser input under sheep grazing on the Cambisol. The soil amendment treatments were: (i) willow biochar produced at 350 °C (1% w/w); (ii) lime, added at the liming equivalence of the biochar application (positive control); (iii) no amendments (negative control). With few exceptions, after 12 months and compared with initial values and controls, biochar caused (i) an increase in soil total nitrogen (N) and Olsen P content (all P < 0.005) beyond those expected from the N and P added with biochar; (ii) decrease in soil bulk density (P < 0.005), on average by 7%, more than can be explained by the reduced density of the biochar added; (iii) increase in microbial biomass C (P < 0.005), and (iv) increase in root biomass by 6.9 Mg ha−1 (P < 0.005). Lime addition increased pH values in both soils under all four management practices, and increased earthworm abundance compared with controls across all sites. In biochar-amended soils, earthworm abundance was higher only in the Andosol without effluent (P < 0.05), while Collembola abundance was significantly higher (P < 0.005) than the controls in all pastures; there was no effect on soil Oribatida and Gamasina populations. Adding biochar made from willow wood produced at low temperature to these pasture soils can positively affect the soil structure, nutrient cycling, and stimulate root growth, a key contributor to organic C input to the soil and the soil food web.