Sugarcane root exudate impact on the potential nitrification rate and N dynamics in the rhizosphere

Soil nitrification is a key process in regulating nitrogen transformation of various inorganic N forms for plant uptake and reducing N loss (N2O and NO3−). Nitrification inhibition by plant-released root exudates is referred to as biological nitrification inhibition (BNI). This study aimed to detect the BNI capacity of sugarcane (Saccharum spp.) root exudates and determine the influence of BNI compounds (BNIs) on the net nitrification rate in rhizosphere soil during sugarcane growth. Three sugarcane varieties were considered in this study, i.e., wild cane, UT13 and KK3. This study included 3 experiments, namely, Experiment I, determination of the potential nitrification rate (PNR) via a soil bioassay and assessment of the effect of dicyandiamide (DCD) on the PNR via a microcosm incubation study; Experiment II, assessment of the effect of sugarcane cultivation on soil N dynamics and the net nitrification rate in rhizosphere soil (in situ); and Experiment III, assessment of the PNR and immobilization in soil via microcosm incubation (ex situ). Sugarcane root exudates were collected under hydroponics, and the BNI capacity was obtained through a soil bioassay. Pot experiments were conducted to determine the influence of sugarcane growth on the net nitrification rate relative to unplanted soil. The results of Experiment I indicated that sugarcane root exudates significantly inhibited the nitrification rate via a soil bioassay (p < 0.05), with a high variation in the BNI capacity according to the sequence of Spone (84.29% inhibition) > UT13 (80.94% inhibition) > KK3 (45.4% inhibition). In addition, Experiment II confirmed a reduced net nitrification rate (p < 0.05) among all three sugarcane varieties. However, net nitrification rate reduction was more notably the result of heterotrophic N immobilization and plant N uptake than the result of BNIs. Evidence of BNI-induced reduction in gross nitrification among all sugarcane varieties was confirmed via soil incubation (Experiment III), and a notable difference in the BNI capacity was found among all sugarcane varieties. Based on the difference in plant mechanisms controlling the net nitrification rate, our study demonstrates that the dominant mechanisms are determined by the soil N availability.