Nitrogenase activity of soil cores of aquatic grasses

Abstract

The nitrogenase activities of field grown aquatic grasses— Phalaris arundinacea L. and Phragmites australis (Cav.) Trin. from fresh and salt water marshes — were measured by the C 2H 2 reduction method. The acetylene reduction activity (ARA) of extruded Phalaris cores was preceded by a 6-h lag period which was eliminated by preincubation for the same length of time. Unextruded cores and potted plants showed shorter lags (2 h). It is suggested that the coring process may, partly, account for the lag. The ARA of Phragmites cores and surface litter was not preceded by any observable lag periods under laboratory and field conditions. Core and surface litter ARA varied from site to site and showed seasonal changes. Highest rates were obtained for Phalaris arundinacea in July and for Phragmites australis in October. Core ARA also showed variation with depth in each habitat. Under Phragmites australis, ARA was more vigorous in the 8–16 cm zone in fresh water marsh and appears to be related to the presence of a higher number of sulphate reducing bacteria in this zone than in the 0–8 cm and 16–24 cm zones. Under the same plant in the salt marsh, ARA increased with depth up to 32 cm and is thought to be related to decreasing amounts of ammonium nitrogen. The data from soil chemical analysis suggest that low levels of inorganic nitrogen, high C:N ratios and the characteristic low pO 2 of reed beds provide conditions for a high nitrogen-fixing potential.