Nitrogen turnover and N$_{2}$O/N$_{2}$ ratio of three contrasting tropical soils amended with biochar

Abstract

Biochar has been reported to reduce emission of nitrous oxide (N$_{2}$O) from soils, but the mechanisms responsible remain fragmentary. For example, it is unclear how biochar effects on N$_{2}$O emissions are mediated through biochar effects on soil gross N turnover rates. Hence, we conducted an incubation study with three contrasting agricultural soils from Kenya (an Acrisol cultivated for 10-years (Acrisol10); an Acrisol cultivated for over 100-years (Acrisol100); a Ferralsol cultivated for over 100 years (Ferralsol)). The soils were amended with biochar at either 2% or 4% w/w. The $^{15}$N pool dilution technique was used to quantify gross N mineralization and nitrification and microbial consumption of extractable N over a 20-day incubation period at 25 °C and 70% water holding capacity of the soil, accompanied by N$_{2}$O emissions measurements. Direct measurements of N$_{2}$ emissions were conducted using the helium gas flow soil core method. N$_{2}$O emissions varied across soils with higher emissions in Acrisols than in Ferralsols. Addition of 2% biochar reduced N$_{2}$O emissions in all soils by 53 to 78% with no significant further reduction induced by addition at 4%. Biochar effects on soil nitrate concentrations were highly variable across soils, ranging from a reduction, no effect and an increase. Biochar addition stimulated gross N mineralization in Acrisol-10 and Acrisol-100 soils at both addition rates with no effect observed for the Ferralsol. In contrast, gross nitrification was stimulated in only one soil but only at a 4% application rate. Also, biochar effects on increased NH$_{4}$$^{+}$ immobilization and NO$_{3}$$^{-}$−consumption strongly varied across the three investigated soils. The variable and bidirectional biochar effects on gross N turnover in conjunction with the unambiguous and consistent reduction of N$_{2}$O emissions suggested that the inhibiting effect of biochar on soil N$_{2}$O emission seemed to be decoupled from gross microbial N turnover processes. With biochar application, N$_{2}$ emissions were about an order of magnitude higher for Acrisol-10 soils compared to Acrisol-100 and Ferralsol-100 soils. Our N$_{2}$O and N$_{2}$ flux data thus support an explanation of direct promotion of gross N2O reduction by biochar rather than effects on soil extractable N dynamics. Effects of biochar on soil extractable N and gross N turnover, however, might be highly variable across different soils as found here for three typical agricultural soils of Kenya