Potential proteolytic enzyme activities modulate archaeal and bacterial nitrifier abundance in soils differing in acidity and organic residue treatment

Current mechanistic knowledge of soil nitrogen (N) cycling mediated by microorganisms lacks in understanding of the functional links between activities of proteolytic extracellular enzymes that provide substrate for nitrifying populations. This relates specifically to soils of different acidity and organic residue treatments. Our hypothesis was that organic residues of high decomposability applied to less acidic soils promote proteolytic enzyme activities modulating the abundance of nitrifiers. This was justified by the presumed benefit of available substrates to microorganisms under less acidic soil conditions. Organic inputs of high (HQR) and medium (MQR) quality differing in decomposability ((Lignin+Polyphenol)/N ratio of 5.1 (HQR) versus 8.1 (MQR)) were incubated in less acidic (S5.1) and more acidic (S4.3) soils for 60 days. Soil samples were obtained at defined time intervals and analyzed for potential activities of alanine aminopeptidases (AAP), leucine aminopeptidases (LAP), and thermolysin-like proteases (TLP), along with the abundance of nitrifying bacteria (AOB) and archaea (AOA). Analysis of covariance (ANCOVA) revealed a significant positive relationship of proteolytic enzyme activities with abundance of AOB and AOA, even though the extent of this relationship was more dependent on soil pH and time than organic residue quality. Notably, the positive relationships were pronounced at the later stages of the incubation period. Within the course of the incubation, AOB benefitted from the release of N substrates (NH4+, NO3 , DON) spurred by proteolysis in S5.1. For MQR and HQR, AOA showed comparable dynamics in S4.3, indicating a niche specialization between AOB and AOA depending on soil acidity and resource availability.