Abstract:
Understanding how N availability influences base cation stores is critical for long-term ecosystem sustainability. Indices of nitrogen (N) availability and the distribution of nutrients in plant biomass, soil, and soil water were examined across ten young, unpolluted Douglas-fir (Pseudotsuga menziesii) stands in the Oregon Coast Range spanning a three-fold soil N gradient (0-10 cm: 0.21 - 0.69% N, 0-100 cm: 9.2 – 28.8 Mg N . ha⁻¹) but having similar stand age and sandstone parent material. δ¹⁵N in foliage and forest floor increased across the gradient and approached the isotopic signature of the atmosphere at high soil N stands, suggesting that variation in N accumulation across sites is related to historic site occupancy by N₂-fixing red alder (Alnus rubra). Although no longer present on these sites, red alder stands can add 100-200 kg N ha⁻¹ yr⁻¹ to an ecosystem for decades, a significantly higher N input than precipitation (0.65 kg N ha⁻¹ yr⁻¹). Annual net N mineralization and litterfall N return displayed non-linear relationships with soil N, increasing initially, and then decreasing at more N-rich sites. In contrast, nitrate leaching from deep soils increased linearly across the soil
N gradient and ranged from 0.074 to 30 kg N . ha⁻¹ . yr⁻¹. Nitrogen availability was negatively correlated with indices of Ca availability. Soil exchangeable Ca, Mg, and K pools to 1 m depth were negatively related to nitrate losses across sites. Calcium was the only base cation that decreased in both plant and soil pools across the soil N gradient, and a greater proportion of total available ecosystem Ca was sequestered in plant biomass at high N, low Ca sites. The preferential storage of Ca in aboveground biomass at high N and low Ca sites, while critical for sustaining plant productivity, may also predispose forests to Ca depletion in areas managed for intensive biomass removal. Our work supports a hierarchical model of coupled N-Ca cycles across gradients of soil N enrichment, with microbial production of mobile nitrate leading to depletion of readily available Ca at the ecosystem scale, and plant sequestration promoting Ca conservation as Ca supply diminishes. Long-term N enrichment of temperate forest soils appears capable of sustaining an open N cycle and key symptoms of N saturation for multiple decades after the cessation of elevated N inputs.
