Modelling microbial exchanges between forms of soil nitrogen in contrasting ecosystems

[摘要] Although nitrogen (N) is often combined with carbon (C) in organicmolecules, C passes from the air to the soil through plant photosynthesis,whereas N passes from the soil to plants through a chain of microbialconversions. However, dynamic models do not fully consider themicroorganisms at the centre of exchange processes between organic andmineral forms of N. This study monitored the transfer of ¹⁴C and¹⁵N between plant materials, microorganisms, humified compartments, andinorganic forms in six very different ecosystems along an altitudinaltransect. The microbial conversions of the ¹⁵N forms appear to bestrongly linked to the previously modelled C cycle, and the same equationsand parameters can be used to model both C and N cycles. The onlydifference is in the modelling of the flows between microbial and inorganicforms. The processes of mineralization and immobilization of N appear to beregulated by a two-way microbial exchange depending on the C : N ratios ofmicroorganisms and available substrates. The MOMOS (Modelling of Organic Matter of Soils) model has already beenvalidated for the C cycle and also appears to be valid for the prediction ofmicrobial transformations of N forms. This study shows that the hypothesisof microbial homeostasis can give robust predictions at global scale.However, the microbial populations did not appear to always be independentof the external constraints. At some altitudes their C : N ratio could bebetter modelled as decreasing during incubation and increasing withincreasing C storage in cold conditions. The ratio of potentiallymineralizable-¹⁵N/inorganic-¹⁵N and the ¹⁵N stock in theplant debris and the microorganisms was modelled as increasing with altitude,whereas the ¹⁵N storage in stable humus was modelled as decreasing withaltitude. This predicts that there is a risk that mineralization of organicreserves in cold areas may increase global warming.

[学科分类] 地球化学与岩石