CATION DISTRIBUTION, CYCLING, AND REMOVAL FROM MINERAL SOIL IN DOUGLAS-FIR AND RED ALDER FORESTS

Overstory species influence the distribution and dynamics of nutrients in forest ecosystems. Ecosystem-level estimates of Ca, Mg, and K pools and cycles in 50-year old Douglas-fir and red alder stands were used to determine the effect of overstory composition on net cation removal from the mineral soil, i.e. cation export from the soil in excess of additions. Net cation removal from Douglas-fir soil was 8 kg Ca ha-1 yr-1, 1 kg Mg ha-1 yr-1, and 0.3 kg K ha-1 yr-1. Annual cation export from soil by uptake and accumulation in live woody tissue and O horizon was of similar magnitude to leaching in soil solution. Atmospheric deposition partially off-set export by adding cations equivalent to 28-88% of cation export. Net cation removal from red alder soil was 58 kg Ca ha-1 yr-1, 9 kg Mg ha-1 yr-1, and 11 kg K ha-1 yr-1. Annual cation accumulation in live woody tissue and O horizon was three times greater than in Douglas-fir, while cation leaching in soil solution was five to eight times greater. The lack of excessive depletion of exchangeable cations in the red alder soil suggests that mineral weathering, rather than exchangeable cations. was the source of most of the removed cations. Nitric acid generated during nitrification in red alder soil led to high rates of weathering and NO3-driven cation leaching.