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.