Residual effect of BNI by Brachiaria humidicola pasture on nitrogen recovery and grain yield of subsequent maize

The forage grass Brachiaria humidicola (Bh) has been shown to reduce soil microbial nitrification. However, it is not known if biological nitrification inhibition (BNI) also has an effect on nitrogen (N) cycling during cultivation of subsequent crops. Therefore, the objective of this study was to investigate the residual BNI effect of a converted long-term Bh pasture on subsequent maize (Zea mays L.) cropping, where a long-term maize monocrop field (M) served as control. Four levels of N fertilizer rates (0, 60, 120 and 240 kg N ha−1) and synthetic nitrification inhibitor (dicyandiamide) treatments allowed for comparison of BNI effects, while 15N labelled micro-plots were used to trace the fate of applied fertilizer N. Soil was incubated to investigate N dynamics. A significant maize yield increase after Bh was evident in the first year compared to the M treatment. The second cropping season showed an eased residual effect of the Bh pasture. Soil incubation studies suggested that nitrification was significantly lower in Bh soil but this BNI declined one year after pasture conversion. Plant N uptake was markedly greater under previous Bh compared with M. The N balance of the 15N micro-plots revealed that N was derived mainly (68–86%) from the mineralized soil organic N pool in Bh while plant fertilizer N recovery (18–24%) was not enhanced. Applied N was strongly immobilized due to long-term root turnover effects, while a significant residual BNI effect from Bh prevented re-mineralized N from nitrification resulting in improved maize performance. However, a significant residual Bh BNI effect was evident for less than one year only.