An accepted paradigm of terrestrial ecosystems in temperate climates is that nitrogen (N) rather than phosphorus (P) is the dominant limiting nutrient of net primary productivity. Recent studies, however, conclude that anthropogenic releases of N have overwhelmed the N cycle, such that the bioavailability of P rather than N may now control productivity and thus carbon sequestration in many ecosystems, particularly those with low soil P. The importance of atmospheric deposition of P from both natural and anthropogenic sources has been recognized in global biogeochemical cycles, but few studies have directly documented the species of inorganic or organic airborne P deposited to ecosystems, or their bioavailability, fate after deposition, or processing pathways by microorganisms. In general, the relative importance of atmospheric P particulate deposition to local ecosystem productivity has not been examined at the level of detail needed, or with sufficient quantification, for biogeochemical model parameterization. We propose to collect exploratory data that will test the hypothesis that the amount and the chemical speciation of P from atmospheric particle deposition can have a significant impact on net ecosystem productivity in P-limited, N-replete terrestrial environments.

In this exploratory project, we aim to establish the chemical speciation of P in airborne particulate matter deposited at two mountain watershed sites, and evaluate the potential for reactivity and biogeochemical cycling of air-deposited P compared with resident soil P. Air and soil samples will be compared from two sites: one along a well-established altitudinal transect in the Southern Sierra Critical Zone Observatory (SSCZO), and one in the East River Watershed in Upper Colorado River Basin (in collaboration with Lawrence Berkeley National Laboratory (LBNL)). A combination of spectroscopic methods (P XAS, solid-state ³¹P NMR, FTIR), source characterization using stable isotopes (δ¹⁸O in PO₄), and selective chemical extractions will be used to determine labile or recalcitrant P species, and to evaluate sources and changes in P speciation with deposition to soil. Pilot experiments using field soils in controlled laboratory microcosms with the addition of isotopically labeled, model inorganic or organic P compounds will examine the feasibility of tracking microbially catalyzed P transformations. We will collaborate with scientists at LBNL for sample collection at the East River site, and draw on their unique expertise for molecular environmental microbiology characterization. Data will be synthesized and simulated in a preliminary reactive transport biogeochemical model. This project will take advantage of DOE-supported facilities (the Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory for NMR studies; the Stanford Synchrotron Radiation Lightsource for XAS). These studies will establish whether the chemical forms of P from atmospheric particle deposition have significant reactivity for incorporation into soil biogeochemical cycles, and thus may have a potentially large and disproportionate impact on net ecosystem productivity in P-limited terrestrial ecosystems.