Charles Rhoades

Stable soil nitrogen accumulation and flexible organic matter stoichiometry during primary floodplain succession

Large increases in nitrogen (N) inputs to terrestrial ecosystems typically have small effects on immediate N outputs because most N is sequestered in soil organic matter. We hypothesized that soil organic N storage and the asynchrony between N inputs and outputs result from rapid accumulation of N in stable soil organic pools. We used a successional sequence on floodplains of the Tanana River near Fairbanks, Alaska to assess rates of stable N accumulation in soils ranging from 1 to 500+ years old. One-year laboratory incubations with repeated leaching separated total soil N into labile (defined as inorganic N leached) and stable (defined as total minus labile N) pools. Stable N pools increased faster (∼2 g N m−2 yr−1) than labile N (∼0.4 g N m−2 yr−1) pools during the first 50 years of primary succession; labile N then plateaued while stable and total N continued to increase. Soil C pools showed similar trends, and stable N was correlated with stable C (r2 = 0.95). From 84 to 95 % of soil N was stable during our incubations. Over successional time, the labile N pool declined as a proportion of total N, but remained large on an aerial basis (up to 38 g N m−2). The stoichiometry of stable soil N changed over successional time; C:N ratios increased from 10 to 22 over 275 years (r2 = 0.69). A laboratory 15N addition experiment showed that soils had the capacity to retain much more N than accumulated naturally during succession. Our results suggest that most soil N is retained in a stable organic pool that can accumulate rapidly but is not readily accessible to microbial mineralization. Because stable soil organic matter and total ecosystem organic matter have flexible stoichiometry, net ecosystem production may be a poor predictor of N retention on annual time scales.

Parent material depth controls ecosystem composition and function on a riverside terrace in northwestern Alaska

Abstract:Many studies have focused on factors that influence ecosystem composition and function, but little is known about the influence of varying quantities of a single parent material without confounding effects of age or location. On a riverside terrace of the Agashashok River, the depth of the cap of silt and sand over the gravel floodplain strongly influenced species composition, production, and response to additions of nitrogen (N) and water. Thin siltcaps (< 0.25 m) had vegetation dominated by herbaceous species, whereas thicker siltcaps had a strong component of shrubs. The depth of the siltcap accounted for about 50% of the variation in the first principle-component of the variation in species composition and cover. In situ net N mineralization increased with increasing siltcap depth, but net nitrification declined. Production by herbs increased by about 20% with water additions but not with N additions, and the responses were strongest at the two intermediate siltcap depths. Shrub production in...

Soil nitrogen accretion along a floodplain terrace chronosequence in northwest Alaska: Influence of the nitrogen-fixing shrub Shepherdia canadensis

ABSTRACT Nitrogen enters terrestrial ecosystems through multiple pathways during primary succession. We measured accumulation of total soil nitrogen and changes in inorganic nitrogen (N) pools across a 300-y sequence of river terraces in northwest Alaska and assessed the contribution of the nitrogen-fixing shrub Shepherdia canadensis. Our work compared 5 stages of floodplain succession, progressing from a sparsely vegetated silt cap to dense shrubby vegetation, balsam poplar-dominated (Populus balsamifera) and white spruce-dominated (Picea glauca) mixed forests, and old-growth white spruce forest. Total soil N (0–30 cm depth) increased throughout the age sequence, initially by 2.4 g N·m−2·y−1 during the first 120 y of terrace development, then by 1.6 g N·m−2·y−1 during the subsequent 2 centuries. Labile soil N, measured by anaerobic incubation, increased most rapidly during the first 85 y of terrace formation, then remained relatively constant during further terrace development. On recently formed terraces, Shepherdia shrubs enriched soil N pools several-fold compared to soil beneath Salix spp. shrubs or intercanopy sites. Total and labile soil N accretion was proportional to Shepherdia cover during the first century of terrace development, and mineral soil δ15N content indicated that newly formed river terraces receive substantial N through N-fixation. About half the 600 g total N·m−2 accumulated across the river terrace chronosequence occurred during the 120 y when S. canadensis was dominant. Sediment deposited by periodic flooding continued to add N to terrace soils after the decline in Shepherdia abundance and may have contributed 25% of the total N found in the floodplain terrace soils. Nomenclature: Viereck & Little, 1986.