Nitrogen and carbon solution chemistry of an old growth coniferous forest watershed before and after cutting

Abstract

Dissolved Kjeldahl N and nitrate concentrations were monitored for 1 to 2 years before and for 3 years after clearcutting of a 10.2‐ha, Douglas‐fir dominated watershed in the Oregon Cascade Mountains. Dissolved organic C (DOC) and ammonium concentrations were monitored after cutting. We sampled throughfall, litter leachate, and soil solution (at four depths in and below the rooting zone) at four locations along a transect up and down the slope. We also sampled precipitation, streamwater, and seepage water. A site just off of the watershed served as the control for all positions in the water profile except for streamwater, for which a similar, nearby watershed was the control. Objectives were (1) to document effects of clearcutting on solution chemistry and (2) to provide data from which to calculate ecosystem nutrient cycling budgets before and after cutting. Beginning 7–18 months after clearcutting, nitrate concentrations increased 3–100 times on the cut watershed but remained nearly unchanged on an adjacent, uncut, control area. Increases were greater at 2.0‐m depth than at other positions along the water profile and least in seepage and streamwater. Greatest nitrate levels were detected near the bottom of the slope. At the uppermost site, about 50 m below the ridgetop, nitrate values remained the same as at the control site. Concentrations of Kjeldahl N (organic N plus ammonium) increased much less than those of nitrate (only 50–100% over precut levels) but were greater than nitrate concentrations at most profile positions both before and after cutting. Even after cutting, nitrate dominated only at 2.0‐m depth and in seepage water. DOC concentrations were slightly greater on the cutover area than on the control. However, they increased as much as 3 times from year 2 to year 3 after cutting at both treated and control sites, whereas nitrate decreased from year 2 to 3, and Kjeldahl N increased little or remained the same. DOC concentrations decreased consistently with depth in the soil profile. The study showed (1) that dissolved organic N must be measured if total N transfers are to be calculated at most sites, (2) that patterns of nitrate appearance can be explained in part on the basis of decreased availability of oxidizable C, (3) that patterns of nitrate appearance after devegetation vary greatly with depth in the soil and with slope position, and (4) that loss of dissolved N after cutting was small (<2.0 kg ha−1 yr−1) compared with nearly 400 kg N ha−1 removed in tree boles.