Biophysical and biogeochemical controls on nitrogen fixation in cyanolichen and decaying wood

Abstract

Cryptic (non-symbiotic) forms of nitrogen (N) fixation are increasingly recognized as a major source of N to natural and agroecosystems, yet the mechanistic controls on cryptic N fixation remain unresolved. A recent synthesis posited that both biophysical (temperature and moisture) and biogeochemical (nutrients) factors regulate N fixation rates, but that biophysical factors exert primary control. Given this conceptual framework, we tested the hypothesis that both biophysical and biogeochemical factors regulate N fixation rates in foliose cyanolichen and decaying wood, but that biogeochemical controls would be stronger when biophysical conditions were more favorable for fixation. We conducted two experiments to explore controls on rates of nitrogenase activity among cyanolichen and decaying wood samples from a temperate lodgepole pine (Pinus contorta var. latifolia) forest. We measured the effects of moisture, temperature, N and phosphorus (P) availability on acetylene reduction rates on samples in both field and growth chamber settings. Moisture and temperature exerted predominant control, but lichen nitrogenase activity responded more to moisture while wood nitrogenase activity responded more to temperature. We found little evidence of nutrient (N and P) controls on nitrogenase activity even when biophysical conditions were favorable for N fixation. Our study suggests that the nature and strength of biophysical and/or biogeochemical controls on cryptic N fixation vary in response to a range of environmental conditions, and that controls on cryptic N fixation may not be consistent across the broad array of cryptic N-fixing niches.