Biological Nitrification Inhibition (BNI): Phenotyping of a core germplasm collection of the tropical forage grass Megathyrsus maximus under greenhouse conditions

Modern intensively managed pastures that receive large external nitrogen (N) inputs
account for high N losses in form of nitrate (NO3
-) leaching and emissions of the
potent greenhouse gas nitrous oxide (N2O). The natural plant capacity to shape the
soil N cycle through exudation of organic compounds can be exploited to favor N
retention without affecting productivity. In this study, we estimated the relationship
between biological nitrification inhibition (BNI), N2O emissions and plant productivity
for 119 germplasm accessions of Guineagrass (Megathyrsus maximus), an important
tropical forage crop for livestock production. This relation was tested in a greenhouse
experiment measuring BNI as (i) rates of soil nitrification; (ii) abundance of ammoniaoxidizing
bacteria (AOB) and archaea (AOA); and (iii) the capacity of root tissue extracts
to inhibit nitrification in vitro. We then measured N2O emissions, aboveground biomass
and forage nutrition quality parameters. Reductions on nitrification activity ranging
between 30 and 70% were found across the germplasm collection of M. maximus.
Accessions with low nitrification rates showed a lower abundance of AOB as well as
a reduction in N2O emissions compared to accessions of high nitrification rates. The
BNI capacity was not correlated to N uptake of plants, suggesting that there may be
intraspecific variation in the exploitation of different N sources in this grass species.
A group of accessions (cluster) with the most desirable agronomic and environmental
traits among the collection was identified for further field validation. These results provide
evidence of the ability of M. maximus to suppress soil nitrification and N2O emissions
and their relationship with productivity and forage quality, pointing a way to develop N
conservative improved forage grasses for tropical livestock production.