The pattern of in situ summer nitrogenase activity in terrestrial Nostoc commune from Stipa–Bouteloa grassland, southern Alberta

Abstract

Field rates of nitrogenase activity by N. commune were followed over a 1-month summer period in Stipa–Bouteloa grassland of southern Alberta, using repeated short-term acetylene-reduction assays. Concurrent patterns of temperature, moisture, and light at the soil surface were continuously monitored. There is a rapid response of nitrogenase activity to both small precipitation events and dewfall hydration periods. Typically, thalli wet by thundershower activity in the previous evening showed a midday maximum of acetylene reduction of up to 5 nmol C2H4 mg−1 h−1, followed by a rapid cessation of activity as thalli desiccated below critical moisture levels. Over extended precipitation periods rates continue to climb throughout the day, gradually falling over the night period. Dry thalli frequently experience surface temperatures of up to 65 °C, without any discernable effects on rehydration recovery of nitrogenase activity. Equally, ground frosts early in the study period did not affect subsequent activity. Surface densities of up to 23.9 g m−2 were found for N. commune, with a mean cover of 4.3 g m−2. Mean cumulative nitrogen fixation over the 31-day field period was estimated to be 10.0 mg N m−2, although up to 77.3 mg N m−2 might be expected in localized microsites. The utilization of small rainfall events in large part accounts for the magnitude of observed nitrogenase activity. Although subsequent losses of fixed nitrogen through denitrification and volatilization are yet unknown, a significant proportion of nitrogen input to this grassland site must originate from asymbiotic biological fixation.