Contrasting effect of elevated atmospheric CO2 on the C/N ratio of faba bean and spring wheat residues exert only minor changes in the abundance and enzyme activities of soil proteolytic bacteria

- Both proteolytic genes revealed an inconsistent response to the residue types.
- No relationship between activity potentials and gene abundance was found.
- This implied the presence of inactive proteolytic populations in studied soils.
- A likely functional redundancy among proteolytic genes was noticed.

Our primary objective was to follow the effects of altered C/N ratios of two residue materials on the abundance and potential activities of soil proteolytic bacteria during the initial stages of decomposition of organic residue derived nitrogen (N), taking into account the explicit physiological responses of spring wheat (SW; increase of C/N ratio) and faba bean (FB; decrease of C/N ratio) to elevated carbon dioxide (CO2) exposure. FB and SW residues were obtained from a Mini-FACE experiment (ambient (380 μl l−1) CO2:C/N ratio FB: 100:1; SW: 75:1), elevated (550 μl l−1) CO2 (FB: 42:1; SW: 105:1)) and incubated in a stagnic luvisol (Ap-horizon, 0-10 cm). Soil samples were obtained at 7, 14, 28, and 56 days of incubation and measured for potential activities of thermolysin-like neutral metalloproteases and alanyl-aminopeptidases as well as abundances of corresponding genes (i.e., neutral metalloproteases (npr) and alanine aminopeptidases (pepN)). It was our major finding that distinct C/N ratios revealed only minor and inconsistent effects on gene abundances and potential enzyme activities. There was no relationship between activity potentials of both enzymes and abundances of corresponding genes implying the presence of inactive proteolytic bacteria. Moreover, a likely functional redundancy among proteolytic genes was noticed as explained by the omnipresence of Firmicutes (i.e., npr gene) as well as α- and β-proteobacteria (i.e., pepN gene) in studied soils. It was concluded that for prospective research, a greater variety of organic residue types from crops cultivated under elevated CO2 with their individual biochemical characteristics should be considered to get a closer understanding of the ecological significance and complex dynamics of hitherto largely overlooked proteolytic bacteria in agricultural ecosystems.