Nitrate Reductase Activity as Potential Indicator for Biological Nitrification Inhibition in Brachiaria humidicola
Marc-André Sparke1, Hannes Karwat1, Jacobo Arango2, Jonathan Núñez2, Danilo Moreta2, Idupulapati Rao2, Georg Cadisch1
1University of Hohenheim, Inst. of Agricultural Sciences in the Tropics (Hans-Ruthenberg-Institute), Germany
“Biological Nitrification Inhibition” (BNI) is a naturally occurring process by which soil nitrogen (N) is conserved in the less mobile form of ammonium (NH4+) resulting in a reduction of nitrate (NO3-) leaching losses and N2O emissions. The pasture grass Brachiaria humidicola (Bh) is currently the most prominent species showing potential to naturally inhibit nitrification. This research aimed to identify in situ plant physiological indicators for BNI activity and to evaluate Bh accessions for their BNI potential with a rapid and reliable methodological approach under field conditions, proposing a direct linkage between nitrification deriving NO3- in soil and nitrate reductase activity (NRA) in plants. In vivo NRA in leaf was determined after N fertilisation and synchronised with soil sampling to test the relationship between soil nitrification rates and NRA. Soil N-NO3- was determined in situ to facilitate correlation analysis. The genotypes tested included two B. humidicola CIAT germplasm accessions (CIAT 16888 and CIAT 26146) as controls and four B. humidicola hybrids that were preselected based on greenhouse evaluation of BNI activity in soil. Beforehand enzyme substrate (NO3-) inducibility was clearly verified and plant leaves turned out to be the main tissue of NO3- reduction (roots vs. leaves). The high BNI Bh accession (CIAT 16888) showed the lowest NRA, whereas the low BNI accession (CIAT 26146) showed highest NRA among all the tested materials. Previously, Bh-08-679 and Bh-08-675 were identified as high BNI and low BNI hybrids, respectively. The NRA assay confirmed these previous observations. Two Bh hybrids (Bh-08-700 and Bh-08-1149) showed an intermediate level of BNI potential. Methodological comparison (soil nitrification rates vs. in vivo NRA) resulted in discrepancies concerning ranking of BNI capacity. NRA in plant tissue correlated well with soil N-NO3- concentration (R2=0.81, P<0.05). This significant positive correlation between NRA vs. N-NO3- concentration in the soil indicates that plant tissue NRA can be a potential indicator of BNI in soil. Further research is needed to estimate to what extent the high BNI Bh hybrids can contribute to N conservation in soil to benefit resource-poor smallholders in the tropics where synthetic nitrification inhibitors are expensive and less effective.
Keywords: Forages, nitrogen cycle, nitrogen use efficiency, smallholder crop-livestock systems, tropical grasslands
Contact Address: Marc-André Sparke, University of Hohenheim, Inst. of Agricultural Sciences in the Tropics (Hans-Ruthenberg-Institute), 70593 Stuttgart, Germany, e-mail: marc.sparkesparke-wohnbau.de