Forest calcium depletion and biotic retention along a soil nitrogen gradient

• Published in: Ecological applications 2013-12, Vol.23 (8), p.1947-1961
• Main Authors: Perakis, Steven S, Sinkhorn, Emily R, Catricala, Christina E, Bullen, Thomas D, Fitzpatrick, John A, Hynicka, Justin D, Cromack, Kermit
• Format: Article
• Language: English
• Subjects: Acid soils; aluminum; Animal and plant ecology; Animal, plant and microbial ecology; base cation depletion; Biological and medical sciences; Biomass; calcium; Calcium - chemistry; Calcium - metabolism; Chemical bases; Douglas-fir; Ecosystem; Environmental Monitoring; Forest ecosystems; Forest soils; Fundamental and applied biological sciences. Psychology; Leaching; magnesium; Mineral soils; nitrate leaching; Nitrogen - chemistry; nitrogen saturation; phosphorus; potassium; Soil - chemistry; Soil depth; Soil ecology; Soil nutrients; Soil water; Synecology; temperate forest; Terrestrial ecosystems; Trees
• ISSN: 1051-0761; 1939-5582
• DOI: 10.1890/12-2204.1

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.