Short-lived effects of walnut shell biochar on soils and crop yields in a long-term field experiment

•Biochar increased crop productivity in only the second out of four seasons.•Increased yield aligned with higher soil K+, Ca2+, and P contributed by biochar.•Nitrogen transformations were not effected by biochar under field conditions.•Biochar’s effects were short-lived in this high fertility, temperate agroecosystem. Many field studies exploring biochars’ effects on plant productivity and soil quality have been limited to just one or two seasons, particularly in temperate agroecosystems, and have not shown how such impacts change as biochars age in the soil. Therefore, we investigated the lasting effects of a walnut shell (WS) biochar on crop yields and soil nutrient cycling and availability over four years in a field experiment. Long-term plots of a tomato-corn rotation were established in a 2×2 factorial design of treatments i) with or without WS biochar amendment and ii) fertilized with mineral fertilizer (MF) or composted poultry manure (CP). Biochar was applied once in 2012 (Year 1) at a rate of 10tha−1. Crop yields were measured over four seasons, and soil samples were analyzed for ammonium (NH4+-N) and nitrate (NO3−-N) concentrations and for other nutrient parameters, including exchangeable K+, Ca2+, PO4-P, SO4-S, each year. Walnut shell biochar had an effect only in Year 2 when it increased corn yields by ∼8% in both MF and CP fertilizer systems and increased exchangeable K+, PO4-P, and Ca2+ in soil through direct additions of these nutrients. These impacts were not observed until a year after application and faded in subsequent years. Inorganic N pools were not significantly affected by the biochar in any season. The WS biochar has a delayed yet short-lived effect on plant-available nutrient concentrations and crop productivity but does not significantly alter nutrient transformations.