Salle de Conférence de l’INRA-Nancy (Champenoux), mardi 5 février 2013, 14h00
Biogeochemical Cycling in Forests of the Sierra Nevada Mountains: The Classical Conceptual Model Does Not Fit
Dale W. Johnson
Natural Resources and Environmental Science, University of Nevada, Reno, Nevada USA
The classical conceptual models of biogeochemical cycling include inputs by precipitation, N2 fixation, and (later) dry deposition. Outputs include leaching or streamflow and sometimes erosion. The classical soil nutrient cycling sub-models portray intense competition between roots and microbes for limiting nutrients, usually nitrogen. Forests of the Sierra Nevada Mountains of Nevada and California do not fit this model. Because of extremely dry summers, roots – and therefore root-microbe competition – are absent in the forest floor (O horizons). Nutrient mineralization in O horizons is carried by water flowing over the mineral soil, which becomes hydrophobic every summer when it dries out. We find evidence that entry of these nutrient-rich waters into preferential flow paths in soils creates nutrient hotspots, which are nearly always abundant in these soils.
The most significant reason that the classical biogeochemical models are not applicable to Sierran forests is the omission of fire. These forests are naturally prone to fire, and wildfire in recent times has become more frequent and severe because of unnatural fuel buildups and climate warming. On an annualized basis, the losses of nitrogen by volatilization during fires is many orders of magnitude greater than losses by leaching. Fire can indirectly cause net increases in the nitrogen capital of these ecosystems, however, by fostering post-fire N2 fixing vegetation which inputs N to the system at a rate that is at least an order of magnitude greater than inputs by precipitation. Fire is far more important than water in controlling the fluxes nitrogen in these systems over the long term.
