Rates and controls of nitrogen fixation in postfire lodgepole pine forests

Severe, stand-replacing wildfire substantially depletes nitrogen (N) stocks in subalpine conifer forests, potentially exacerbating N limitation of net primary productivity in many forested regions where fire frequency is increasing. In lodgepole pine (Pinus contorta var. latifolia) forests in the Greater Yellowstone Ecosystem (GYE), long-term data show surface soil and biomass N stocks are replenished during the first few decades following wildfire, but the source(s) of that N are unclear. We measured acetylene reduction rates in multiple cryptic niches (i.e., lichen, moss, pine litter, dead wood, and mineral soil) in 34-year-old lodgepole pine stands in the GYE to explore the rates, temporal patterns, and climate controls on cryptic N fixation. Acetylene reduction rates were highest in late May (0.376 nmol C₂H₄ g⁻¹ h⁻¹) when moisture availability was high compared with early August and mid-October when moisture was relatively low (0.112 and 0.002 nmol C₂H₄ g⁻¹ h⁻¹, respectively). We observed modest rates of nitrogenase activity in a few niches following a mid-summer rain event, suggesting that moisture is an important factor regulating field-based N fixation rates. In a laboratory experiment, moss responded more strongly to temperature and moisture variation than all other niches. Acetylene reduction rates in dead wood increased with temperature but not moisture content. No other niches showed clear responses to either moisture or temperature manipulation. Together, the field and laboratory results suggest that frequent asynchrony between favorable temperature and moisture conditions may limit N fixation rates in the field. Overall, total annual cryptic N fixation inputs (mean: 0.26; range: 0.07–2.9 kg N ha⁻¹ year⁻¹) represented <10% of the postfire biomass and surface soil N accumulation in the same stands (39.4 kg N ha⁻¹ year⁻¹), pointing to a still unknown source of ecosystem N following fire.