Forest calcium depletion and biotic retention along a natural gradient of soil nitrogen

Background/Question/Methods

High nitrogen (N) accumulation and availability in terrestrial ecosystems can shift patterns of nutrient limitation and deficiency beyond N towards 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.   

Results/Conclusions

Our preliminary findings indicate that total soil N (0-100 cm) increased nearly 3-fold across the N gradient, leading to increased nitrate leaching, declines in soil pH from 5.8 to 4.2, 10-fold declines in 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.  These patterms imply that historic symbiotic N-fixation promoted soil organic P accumulation and P sufficiency for non-fixers. In contrast, Ca was the only base cation that declined in biomass and soil pools as N increased, suggesting the emergence of Ca deficiency at high N. Biotic conservation of Ca increased in response to Ca deficiency, as indicated by preferential Ca accumulation in biomass and surface soil despite whole-ecosystem Ca depletion at high N. Our findings support a hierarchical model of N-Ca couplings in response to excess N, whereby N-saturation and elevated nitrate leaching deplete readily available soil Ca, which stimulates 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, base cation depletion, and Ca deficiency.