Variation in nitrogen cycling activity in soil supporting rice (Oryza sativa) cultivars

Nitrogen use efficiency in paddy rice production is estimated to be as low as 40%. The remaining
added nitrogen is lost through leaching or gaseous loss, including the potent greenhouse gas nitrous
oxide (N2O). Most Nitrogen transformations in soil are microbial, with four critical processes in
paddy systems: nitrification, denitrification, DNRA, and anammox. Rice plays a vital role in
manipulating the soil ecology of its rhizosphere through, for example, nitrogen assimilation,
respiration, and root exudation that shape the rhizosphere community and affect microbial activity.
These effects may be directly linked with rice genetics; I hypothesised that rice-based nitrogen
modulation. cycling is genotype-specific and thus linked to rice variety. I screened ~100 rice
genotypes using a single optimised microcosm experiment with paddy soil supporting rice growth.
In this system, I simultaneously measured actual nitrification, denitrification and anammox process
rates and linked these results to the rice line using GWAS. I observed significant variation in
activity with all processes. The results suggest low nitrification-supporting rice genotypes
limited nitrification by secreting BNI-like secondary metabolites. Whilst emission of N2O
significantly varied among the tested rice genotypes mainly due to variations in complete
denitrification, the degree of partial denitrification was a vital factor droving the variation; this was
found to be negatively correlated with the carbon: nitrate ratio. However, speculative at this stage,
secondary root development genes were found to be associated with N2O emission variation.
Although links were discovered between rice genotypes and anammox, these were thought to be
indirect through the regulation of denitrification. These experimental results suggest that rice
breeding could be used to improve agricultural sustainability through microbial manipulation to
reduce nitrification and mitigate N2O emissions from fertilised flooded paddy soil.