Soil Nitrogen Changes during Primary Succession on a Floodplain in Alaska, U.S.A.

Abstract

Surface soil nitrogen changes along a subarctic vegetation chronosequence on a floodplain in central Alaska resulted from interactions between stochastic flooding and the influence of vegetation. River alluvium initially contributed 400 kg ha-1 of nitrogen to the top 200 mm of time-zero soils. Subsequent nitrogen accumulations were in part due to nitrogen fixers such as alder (Alnus tenuifolia). Kjeldahl nitrogen levels reached 1696 kg ha-~ in surface mineral soils of 30-yr-old alder stands. Extractable forms of nitrogen also increased four-fold in 30 yr. However, soil nitrogen increases also resulted from frequent floods that deposited additional nitrogen-rich alluvium. Frequently flooded low terraces had high silt content and relatively high nitrogen levels. Frequent flooding of some forested upper terraces resulted in poorer development of forest floors, higher sand content, and dilution of nitrogen levels compared with upper terraces that were less often flooded. Although nitrogen levels were primarily determined by biotic factors, periodic flooding and associated changes in particle size also affected concentration and pool sizes of nitrogen in soils of the vegetation chronosequence.