Water availability - A physiological constraint on nitrate utilization in plants of Australian semi-arid mulga woodlands

Nitrate was found to be the predominant form of available nitrogen in mulga soils. Nitrate reductase activities on a fresh mass basis of a range of plants from eastern (Queensland) mulga ecosystems 2 weeks after partial relief from drought were uniformly low for both herbaceous species (165 +/- 25 pkat g(-1)) and woody perennials (77 +/- 14 pkat g(-1)). Supply of nitrate for 24 h to cut transpiring shoots of woody species or application of nitrate solution to the rooting zone of herbaceous species promoted little further increase in mean shoot nitrate reductase activities. Most species exhibited high tissue nitrate concentrations during water stress and soluble organic N profiles were in many cases dominated by the osmoprotective compounds, proline or glycine betaine. Species with low levels of proline or glycine betaine showed high foliar concentrations of other compatible osmotica such as polyols or sugars. Effects of relieving water stress on nitrate reductase activity, proline, glycine betaine and nitrate levels were followed over 3 d of irrigation. Available soil nitrate rose 10-fold immediately and, following rapid restoration of leaf water status of the eight study species, a 4-fold increase occurred in mean nitrate reductase activity together with progressive decreases in mean tissue concentrations of nitrate, proline and glycine betaine over the 3 d period. Similar changes in soil nitrate, nitrate reductase activity, proline and tissue nitrate were observed in the same ecosystem following a natural rainfall event and in western (S.W. Australia) mulga following irrigation. It is concluded that, although nitrate nitrogen is present at high concentrations and is the predominant inorganic nitrogen source in soils of the mulga biogeographic region, its assimilation by perennial and ephemeral vegetation is limited primarily by water availability. A scheme is presented depicting interrelated physiological and biochemical events in typical mulga species following a rain event and subsequent drying out of the habitat

Reed die-back related to increased sulfide concentration in a coastal mire in eastern Hokkaido, Japan

A drastic decline of Phragmites australis was observed along the middle reaches of Ichibangawa River in Kiritappu Mire, eastern Hokkaido, Japan, during the last 50 years. In an area of ~30 ha, reed-sedge vegetation and alder forest have been replaced by bare soil and patches of salt marsh vegetation. A gradual increase in frequency of flooding by brackish water probably was the ultimate cause of the vegetation change. We measured redox potentials and oxygen and sulfide concentrations in soil profiles using needle electrodes. Measurements were carried out in areas where reed has disappeared and in sites where reed stands were still healthy. The concentration of selected ions in the surface water was also measured at various sites. Surface water in low-lying areas was clearly influenced by seawater. Very high sulfide concentrations were measured in bare peat sites (more than 600 μmol l⁻¹), which exceeded P. australis tolerance 2 – 3 times. In a healthy reed zone adjacent to an area with poor fen vegetation, sulfide concentration in the rooting zone of Phragmites was also high (300–400 μmol l⁻¹), particularly during the night. The fact that Phragmites in this zone was still healthy indicates that sulfide did not reach toxic levels in the direct vicinity of the roots. Sulfide that is produced in this area is probably fixed by iron, which is supplied through a continuous discharge of iron-rich groundwater. An increase in frequency of flooding by brackish water could be related to ongoing subsidence of this part of the Pacific coast which is located at the Kuril subduction zone. Sea level rise could also contribute to a stronger inflow of seawater into the mire system