Influence of Biochar on Nitrogen Fractions in a Coastal Plain Soil

Interest in the use of biochar from pyrolysis of biomass to sequester C and improve soil productivity has increased; however, variability in physical and chemical characteristics raises concerns about effects on soil processes. Of particular concern is the effect of biochar on soil N dynamics. The effect of biochar on N dynamics was evaluated in a Norfolk loamy sand with and without NH4NO3. High‐temperature (HT) (≥500°C) and low‐temperature (LT) (≤400°C) biochars from peanut hull (Arachis hypogaea L.), pecan shell (Carya illinoinensis Wangenh. K. Koch), poultry litter (Gallus gallus domesticus), and switchgrass (Panicum virgatum L.) and a fast pyrolysis hardwood biochar (450–600°C) were evaluated. Changes in inorganic, mineralizable, resistant, and recalcitrant N fractions were determined after a 127‐d incubation that included four leaching events. After 127 d, little evidence of increased inorganic N retention was found for any biochar treatments. The mineralizable N fraction did not increase, indicating that biochar addition did not stimulate microbial biomass. Decreases in the resistant N fraction were associated with the high pH and high ash biochars. Unidentified losses of N were observed with HT pecan shell, HT peanut hull, and HT and LT poultry litter biochars that had high pH and ash contents. Volatilization of N as NH3 in the presence of these biochars was confirmed in a separate short‐term laboratory experiment. The observed responses to different biochars illustrate the need to characterize biochar quality and match it to soil type and land use.