Release of non-exchangeable {}^{{{text{15}}}}{text{NH}}^{{text{ + }}}_{{text{4}}} from subgrade, decomposed granite substrates and uptake by non-mycorrhizal and mycorrhizal California native annual grass, Vulpia microstachys

Release rates of recently fixed $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ from non-exchangeable interlayer sites in 2:1 silicate minerals were determined for decomposed granite (DG) saprolites from three locations in California, USA. Recently-fixed $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ release from the DG substrate was quantified by extracting diffused $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ with H-resin, as well as a native, annual grass Vulpia microstachys. The $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ release data varied with via the method of extraction, which included H-resin pre-treatments (Na⁺ or H⁺) and V. microstachys uptake (mycorrhizal inoculated or uninoculated). After 6 weeks (1008 h), more $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ was recovered from fixed interlayer positions by the H-resins as compared to uptake by V. microstachys. The H⁺ treated H-resins recovered more released $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ (≈94 mg ${text{NH}}^{{text{ + }}}_{{text{4}}} - {text{N}};{text{kg}}^{1} $ or (12%) of total fixed $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ ) in two of the three DG samples as compared to the Na⁺ treated resins, (which recovered ≈70–78 mg ${text{NH}}^{{text{ + }}}_{{text{4}}} - {text{N}};{text{kg}}^{{{text{ - 1}}}} $ (or 9–10%) of the total fixed $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ ). The V. microstachys assimilated 8–9% of the total fixed $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ with mycorrhizal inoculum as compared to only 2% without a mycorrhizal inoculum, over the same time period. The fixed $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ release kinetics from the H-resin experiments were most accurately described by first order and power function models, and can be characterized as biphasic using a heterogeneous diffusion model. Uptake of both the ¹⁵N and ambient, unlabelled N from the soils was closely related to plant biomass. There was no significant difference in percent of N per unit of biomass between the control and mycorrhizal treatments. The findings presented here indicate that observed, long-term $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ release rates from DG in studies utilizing resins, may overestimate the levels of fixed $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ made available to plants and microorganisms. Additionally, the study suggested that mycorrhizae facilitate the acquisition and plant uptake of fixed $ {text{NH}}^{{text{ + }}}_{{text{4}}} $ , resulting in markedly increased plant biomass production.