A large ephemeral release of nitrogen upon wetting of dry soil and corresponding root responses in the field

To assess changes in soil nutrients, root growth and mycrorrhizal infection in response to rain events, a water pulse was applied to a very dry soil. Wetting of a dry soil in the Great Basin of the Western United States led to a striking pulse of available soil nitrate in a field plot, but available phosphate was not affected. This is the first field demonstration of this phenomenon in the Great Basin as far as we are aware. This pulse was only apparent for a few days, probably due to microbial immobilization of the nitrogen. Root ammonium uptake capacity increased within one day of the water pulse, but new root growth was not apparent until 3 days after the water pulse. Thus, to capture this ephemeral release of nitrogen, enhanced uptake capacity of existing roots was probably more important than development of new roots. Mycorrhizal infection was not affected by the water pulse treatments. However, since the water pulse only affected nitrogen availability and mycorrhizae are generally most effective in facilitating acquisition of less mobile nutrients such as phosphate, mycorrhizae likely do not play an important role in taking advantage of this opportunity provided by the pulse of water.