Biochar increased retention of 15 NH 4 + in a tropical Acrisol under rain-fed maize crop

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Abstract Biochar addition has been shown to affect important soil fertility characteristics, including soil pH and soil water retention. Such changes may have pronounced effects on nitrogen (N) cycling. In a field experiment in maize on a sandy loam, we added a small dose of 15N (added as either 15NO3− or 15NH4+), 11 weeks after planting, and traced it during ten days in soil, extractable nitrate, and maize plants. In addition, we assessed the effect of biochar on nitrous oxide (N2O) fluxes. The study was conducted in 20 m2-plots established on a farm in central Zambia, under conservation agriculture without (CA-NORM) and with 4-ton ha− 1 pigeon pea biochar (CA-BC). The results show a positive effect of biochar on soil organic carbon content, soil moisture and extractable NO3−. However, no significant change was observed in soil properties such as pH, cation exchange capacity (CEC) and extractable NH4+, and neither on N2O fluxes nor biomass production. The total recovery of 15N was significantly greater if added as 15NH4+ than as 15NO3− and more than 95% of the 15N in the soil was found in the residual N pool (viz. NH4+ and organic N). The uptake of 15N in aboveground biomass was significantly greater if added as 15NH4+ than as 15NO3−. Compared to CA-NORM, CA-BC increased the recovery of 15N in the soil residual N pool with 46%, 240 hours after addition of 15NH4+. Our results suggest that biochar may reduce losses of NH4+ in Acrisols.

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