The responses of NO2-- and N2O-reducing bacteria to maize inoculation by the PGPR Azospirillum lipoferum CRT1 depend on carbon availability and determine soil gross and net N2O production

Seed inoculation by plant growth promoting rhizobacteria (PGPRs) is an agronomic practice that stimulates root carbon (C) exudation and nitrogen (N) uptake. Inoculation thus increases and decreases C and N availabilities to denitrifiers in the rhizosphere, respectively. Hence, denitrification rates in the rhizosphere can be positively or negatively influenced by root activity depending on the balance between these two processes. We assumed that inoculation effect on denitrifiers could strongly differ according to soil conditions. Would denitrifiers be mostly limited by C, inoculation would increase denitrifier abundance and activity through increased labile C availability. Would denitrifiers be limited by N rather than C, inoculation would decrease denitrifier abundance and activity through increased competition for N. Here we manipulated denitrification limitation by C and N (i) in a field trial through the use of different fertilization levels, and (ii) in a growth chamber experiment by mimicking root exudate inputs. We analyzed how the effects of maize inoculation by the PGPR Azospirillum lipoferum CRT1 on potential gross and net N2O production rates and NO2- and N2O-reducer abundances were related to C and N limitation levels. An increase in potential gross (up to + 113%) and to a lesser extent net (+37%) N2O production was observed for soils where denitrification was highly limited by C. This was explained by strong and moderate increases in the abundances of NO2- and N2O-reducers, respectively. In contrast, when denitrification was weakly limited by C, gross and net N2O productions were negatively affected by inoculation (-15 and -40%, respectively). Our results show that the inoculation practice should be evaluated in term of possible increased crop yield but also possible modified N2O emission, paying much attention to cropland soils where denitrifiers are highly limited by C.