Ammonium fixation dynamics in sub-grade decomposed granite

Reestablishing vegetation on disturbed decomposed granite (DG) substrates is limited by low plant available moisture and nitrogen. These unconsolidated sub-surface soils have low levels of organic matter, low water holding capacities, and are nutrient deficient. Thus significant N amendments are needed in order to encourage plant growth. However, NH₄⁺-N amendments are also problematic because many DG substrates have a substantial capacity to fix NH₄⁺ within clay interlayers. The fixed NH₄⁺ is not readily available to plants or microbes. The fixation capacities of 11 drastically disturbed DG substrates throughout California showed that up to 78% of (124 kg NH₄⁺ ha^-1) applied ammonium was shown to be sequestered within 48 hours (Rider et al., 2006). The fixed NH₄⁺, however, may provide a slow-release source of plant available NH₄⁺ for sustained plant growth. The release kinetics of fixed NH₄⁺ from interlayer sites were determined for three DG substrates that had exhibited high NH₄⁺ fixation capacities. Recently-fixed NH₄⁺ release from the DG substrates were quantified by extracting diffused NH₄⁺ with absorbant resins, as well as in a microcosm using a native, annual grass Vulpia microstachys . After 6 weeks more NH₄⁺ was recovered from fixed interlayer positions by the resins (≈ 70-94 mg NH₄⁺-N kg^-1 or 9-12% of the total fixed NH₄⁺) as compared to uptake by Vulpia microstachys. The Vulpia microstachys assimilated 8-9% of the total fixed NH₄⁺ with mycorrhizal inoculum as compared to only 2% without a mycorrhizal inoculum. These comparisons illustrate differences in chemical assays of soil processes versus biological processes. Using the plant bioassay, the release time for 50% of the fixed ammonium at optimum conditions is estimated at 3.2 years without mycorrhizal colonization and 0.5 years with colonized roots on these soils. Fixed N is not expected to rapidly desorb and is not expected to create water quality impacts.