NITROGEN CYCLING IN POST-FIRE SOILS ACROSS A BURN INTENSITY GRADIENT

Wildfire frequency and intensity has increased over the last decade and will significantly impact soil nitrogen pools. Although previous studies document that inorganic nitrogen pools increase after fire, the mechanisms controlling this increase and subsequent nitrogen transformations are poorly understood.

We collected soils across burn intensity gradients in two different vegetation types in the Jemez River Basin, NM. Soils were also collected down to one meter from six pits along an elevation transect. We hypothesized that burn intensity would increase pools of inorganic nitrogen and alter cycling rates of inorganic nitrogen. We used aerobic incubations and potassium chloride extractions to quantify N pools and rates of net N-mineralization and nitrification.

Soil extractable ammonium increased immediately following the fire and varied with burn intensity. Three months after the fire, ammonium pools decreased and nitrate pools increased in severely burned plots. Rates of N-mineralization differed across vegetation types. Specifically, soils in mixed conifer stands showed net N-mineralization, whereas soils in ponderosa pine stands showed net N-immobilization. Along the elevation gradient, we found greater (larger?) pools of ammonium in surface soils at higher elevations than at lower elevations likely due to moisture and substrate controls. In contrast, surface soils at low elevations showed greater pools of extractable nitrate. Collectively, our results demonstrate that N dynamics and the controls on these dynamics vary with vegetation, burn intensity and landscape position.