Biochar reduced nitrate leaching and improved soil moisture content without yield improvements in a four-year field study

Abstract

The use of biochar (BC) is discussed as a strategy to sequester carbon in soils, to reduce GHG emissions and improve soil fertility. However, the responses of crop yields to biochar amendments in agricultural ecosystems, specifically under temperate field conditions, are still uncertain. Furthermore, results obtained under field conditions are often differing from laboratory studies. Therefore, the establishment of long-term studies under field conditions is mandatory to provide the base for recommendations. We carried out a two-factorial split-plot field experiment over four years (2012–2015, still in progress) to compare the effects of BC on crop yields, mineral nitrogen (NO3− and NH4+) dynamics, soil moisture and initial soil CO2 efflux. A temperate sandy soil was amended with BC (0, 15 and 30Mgha−1) with the second factor being watering regime (irrigated or rainfed). The soil CO2 efflux was increased only for a short time following BC amendments. Freshly incorporated BC (30Mgha−1) initially induced manganese (Mn) deficiency at the vegetative stage of the first crop maize (Zea maize L.). Biochar amendments significantly reduced NO3− leaching, as indicated by greater NO3− stocks in the topsoil and reduced stocks in the subsoil (0–15, BC amendment zone and 60–90cm respectively). In BC treatments a higher soil moisture and higher NO3− amount was observed, however, this did not translate into higher yields. Rather, grain yields of maize (year I) and summer barley (Hordeum vulgare L., year III, no nitrogen (N) fertilization) were significantly reduced (1–11 and 5–26% respectively) due to N deficiency with BC amendment or (non-alleviated) drought stress. A prolonged drought spell in 2015 (year IV) drastically reduced the grain yield of maize (5 and 0.7Mgha−1) and N uptake (96 and 11kgha−1) in the irrigated and rainfed treatments respectively, without any alleviating effects of biochar amendment. We conclude that application of large amounts of pure, non-nutrient-loaded biochar to temperate sandy soils may provide environmental benefits, such as carbon sequestration and reduction of nitrate leaching, but without an economic incentive for implementing biochar use, at least for the initial few years of application.