The source of nitrogen (NH4+ or NO3-) affects the concentration of oxalate in the shoots and the growth of Atriplex nummularia (oldman saltbush)

Abstract

Atriplex nummularia Lindl. (oldman saltbush) is a halophytic shrub used widely as a forage for ruminant production in saline farming systems. However, it can contain high concentrations of oxalate in the leaves, which may cause calcium deficiency in grazing animals. We hypothesised that supplying NH4+ instead of NO3- to a clone of this species would decrease oxalate concentrations in the shoots, and also decrease plant growth. Oxalate concentrations were measured in plants in the field, and a glasshouse experiment was conducted in which plants were grown with 10mM NO3- or NH4+, with 50, 200 or 500mM NaCl. The field survey showed effects of site (P<0.001), with average oxalate concentrations in shoots varying between 2.4 and 6.4% dry mass (DM). In the glasshouse, oxalate concentrations and plant growth were both affected by N-source and salinity (P<0.001). Averaged across salinities, plants grown with NH4+ for 24 days had only 43% of the shoot DM but 25% of the oxalate concentration of plants grown with NO3-. We discuss the effects of N-source on oxalate concentrations, the implications of this for halophyte growth, and the opportunity to select halophytes with lower oxalate and higher nutritive value for livestock.