Effect of biochar on nutrient retention and nectarine tree performance: A three-year field trial

We performed a series of experiments in controlled conditions to assess the potential of hardwood- derived biochar either as a source or as a removing additive of macronutrients [nitrate-nitrogen (NO3-N), ammonium-N (NH4-N), potassium (K), phosphorus (P), and magnesium (Mg)] in solution. In addition, a 3-year field trial was carried out in a commercial nectarine orchard to evaluate the effect of increasing soil-applied biochar rates on tree nutritional status, yield, fruit quality, soil pH, soil NO3-N, and NH4-N concentration and soil water content. In controlled conditions, the concentrations of K, P, Mg, and NH4-N in solution were significantly increased and positively correlated with biochar rates. Biochar was ineffective in removing NO3-N, K, P, and Mg from enriched solutions, while at the rate of 40 g L–1 biochar removed almost 52% of the initial NH4-N concentration. In a mature, irrigated, fertilized, commercial nectarine orchard (Big Top/GF677) on a sandy-loam soil in the Italian Po Valley, soil-applied biochar at the rates of 5, 15, and 30 t ha–1 were effective in reducing the leached amount of NH4-N in the top 0.25 m soil layer over 13 months, as estimated by ion exchange resin lysimeters. Nevertheless, independent of the rate, biochar did not affect soil pH, soil N mineral availability, soil moisture, tree nutritional status, yield, and fruit quality. We conclude that, unless an evident constraint is identified, in non-limiting conditions (e.g., water availability and soil fertility), potential benefits from biochar application in commercial orchards are hidden or negligible