title: Forest calcium depletion and biotic retention along a soil nitrogen gradient.
abstract: High nitrogen (N) accumulation in terrestrial ecosystems can shift patterns of nutrient limitation and deficiency beyond N toward other nutrients, most notably phosphorus (P) and base cations (calcium [Ca], magnesium [Mg], and potassium [K]). We examined how naturally high N accumulation from a legacy of symbiotic N fixation shaped P and base cation cycling across a gradient of nine temperate conifer forests in the Oregon Coast Range. We were particularly interested in whether long-term legacies of symbiotic N fixation promoted coupled N and organic P accumulation in soils, and whether biotic demands by non-fixing vegetation could conserve ecosystem base cations as N accumulated. Total soil N (0-100 cm) pools increased nearly threefold across the N gradient, leading to increased nitrate leaching, declines in soil pH from 5.8 to 4.2, 10-fold declines in soil exchangeable Ca, Mg, and K, and increased mobilization of aluminum. These results suggest that long-term N enrichment had acidified soils and depleted much of the readily weatherable base cation pool. Soil organic P increased with both soil N and C across the gradient, but soil inorganic P, biomass P, and P leaching loss did not vary with N, implying that historic symbiotic N fixation promoted soil organic P accumulation and P sufficiency for non-fixers. Even though soil pools of Ca, Mg, and K all declined as soil N increased, only Ca declined in biomass pools, suggesting the emergence of Ca deficiency at high N. Biotic conservation and tight recycling of Ca increased in response to whole-ecosystem Ca depletion, as indicated by preferential accumulation of Ca in biomass and surface soil. Our findings support a hierarchical model of coupled N-Ca cycling under long-term soil N enrichment, whereby ecosystem-level N saturation and nitrate leaching deplete readily available soil Ca, stimulating biotic Ca conservation as overall supply diminishes. We conclude that a legacy of biological N fixation can increase N and P accumulation in soil organic matter to the point that neither nutrient is limiting to subsequent non-fixers, while also resulting in natural N saturation that intensifies base cation depletion and deficiency.
authors.affiliation: U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, Oregon 97331, USA. steven.perakis@oregonstate.edu
authors.forename: Steven S
authors.initials: SS
authors.lastname: Perakis
authors.affiliation: Oregon State University, Department of Forest Ecosystems and Society, Corvallis, Oregon 97331, USA.
authors.forename: Emily R
authors.initials: ER
authors.lastname: Sinkhorn
authors.affiliation: U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, Oregon 97331, USA.
authors.forename: Christina E
authors.initials: CE
authors.lastname: Catricala
authors.affiliation: U.S. Geological Survey, National Research Program, Menlo Park, California 94025, USA.
authors.forename: Thomas D
authors.initials: TD
authors.lastname: Bullen
authors.affiliation: U.S. Geological Survey, National Research Program, Menlo Park, California 94025, USA.
authors.forename: John A
authors.initials: JA
authors.lastname: Fitzpatrick
authors.affiliation: Oregon State University, Department of Forest Ecosystems and Society, Corvallis, Oregon 97331, USA.
authors.forename: Justin D
authors.initials: JD
authors.lastname: Hynicka
authors.affiliation: Oregon State University, Department of Forest Ecosystems and Society, Corvallis, Oregon 97331, USA.
authors.forename: Kermit
authors.initials: K
authors.lastname: Cromack
mesh_terms: D018533:Biomass; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D017753:Ecosystem; D004784:Environmental Monitoring; D009584:Nitrogen / Q000737:chemistry*; D012987:Soil / Q000737:chemistry*; D014197:Trees
chemical_list: D012987:Soil; D009584:Nitrogen; D002118:Calcium