Nitrogen fixation, soil nitrogen availability, and biomass in pure and mixed plantations of alder and pine in central Korea

Rates of nitrogen (N) fixation, soil N availability, and aboveground biomass were measured in 27-year-old pure and mixed Alnus hirsuta and Pinus koraiensis plantations in central Korea. Nodule biomass and N fixation were 179.3 kg ha- 1 and 46.6 kg ha- 1yr- 1 for the pure A. hirsuta plantation (PA) and 95.2 kg ha- 1 and 41.1 kg ha- 1yr- 1 for the mixed A. hirsuta + P. koraiensis plantation (MAP), respectively. A. hirsuta seemed to provide more than two thirds of annual N requirement for P. koraiensis.Rates of acetylene reduction were significantly related to soil temperature (R2 = 0.51, P < 0.001), but not to soil moisture content. Total inorganic N [ammonium (NH4+)plus nitrate (NO3-)] availability measured using ion exchange bags were significantly higher under PA (27.91 μ g-N bag- 1) and MAP (31.34 μ g-N bag- 1) than under the pure P. koraiensis plantation (PP) (14.31 μ g-N bag- 1). Especially soils under the influence of A. hirsuta showed at least 2 fold increase in resin total inorganic N concentrations. Total aboveground biomass (Mg ha- 1) was 147.3 for PA, 145.8 for MAP, and 174.8 for PP, respectively, and was not significantly different among plantations. A. hirsuta significantly increased soil N availability; however, the influence of N fixation on aboveground biomass was not significant for the study plantations.