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

• Published in: Écoscience (Sainte-Foy) 2008-01, Vol.15 (2), p.223-230
• Main Authors: Rhoades, Charles, Binkley, Dan, Oskarsson, Hlynur, Stottlemyer, Robert
• Format: Article
• Language: English
• Subjects: abondance naturelles des isotopes de N; biogééochimie riparienne; biogéochimie riparienne; chronosequences; composante terrestre du cycle de N; Flooded soils; floodplain forest; Floodplains; Forest soils; forêt de plaine inondable; forêt de plaine inondable; Mineral soils; mixed forests; natural abundance N isotopes; nitrogen; nitrogen fixation; Picea glauca; Populus balsamifera; primary succession; riparian biogeochemistry; rivers; s; Salix; sediment deposition; Sedimentary soils; sedimentation; Shepherdia; Shepherdia canadensis; Shrubs; silt; Soil depth; Soil ecology; succession primaire; Terrace soils; Terraces; terrestrial ecosystems; terrestrial N cycling
• ISSN: 1195-6860; 2376-7626
• DOI: 10.2980/15-2-3027

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.