Short-Lived Effects of Walnut Shell Biochar in a Long-Term Field Experiment.

Short-term effects of biochar applications on soil fertility and plant productivity have been well studied, but meta-analyses and reviews on this topic continually note the lack of experiments investigating biochars’ effects over multiple seasons and call for validation of mesocosm results in field studies. Therefore, we investigated the effects of a high temperature (900 °C) walnut shell (WS) biochar on crop yields and soil nutrient cycling and availability over four seasons in a field experiment. Previously, this WS biochar was found to increase net nitrification rates in a short-term mesocosm experiment; therefore, we hypothesized that each year, plots with biochar would have higher NO₃^--N concentrations early in the growing season, but lower concentrations late in the season due to increased losses, causing negative effects on crop yields due to decreased N availability. Long-term plots of a tomato-corn rotation were established in a 2x2 factorial design of treatments i) with or without WS biochar amendment and ii) fertilized with mineral fertilizer or composted poultry manure. Biochar was applied once in 2012 at a rate of 10 t ha⁻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. WS biochar had an effect only in 2013, one year after biochar application, when it increased corn yields by ~8% in both 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 nitrogen transformations.