Soil protease: Bottleneck of the terrestrial nitrogen cycle

Nitrogen (N) availability represents one of the key regulators of primary productivity in terrestrial ecosystems(1). Emerging evidence, however, is challenging the most recent paradigms of N cycling in plant-microbial-soil systems. Specifically, new insights include the identification that (i) roots release exudates to manipulate the solubility and availability of chemically and physically protected humic-C in soil(2); (ii) that roots can stimulate soil organic matter turnover by the direct release of proteases into soil(3), (iii) that protease represents the key bottleneck in terrestrial N cycling (4), and (iv) that plants can take up proteins directly into their roots, by-passing the need to acquire oligopeptides, amino acids, NH4+ and NO3- (5). From this latest evidence we hypothesise that:
(i) Plants directly accelerate soil N cycling by releasing proteases and that this represents a new mechanism to explain the observed plant-mediated positive priming of soil organic matter (direct biotic priming).
(ii) Plants indirectly accelerate soil N cycling and N acquisition by the release of C-rich exudates which stimulates soil microbial activity and N turnover (indirect biotic priming).
(iii) Plants release organic acids that induce the solubilisation, disaggregation and bioavailability of N contained in humic substances (indirect abiotic priming).
(iv) Plant and microbial protease release is directly regulated by plant N demand and soil N availability