Watershed responses to clear-cutting: Effects on soil solutions and stream water discharge in central New Brunswick

Abstract

Elemental concentrations (H, Ca, Mg, K, Na, NH4-N, NO3-N, P) and water flux data for precipitation and stream discharge as well as ion concentration data for soil solutions were collected and summarized for the Hayden Brook (HB) and Narrows Mountain Brook (NMB) watersheds of the Nashwaak Experimental Watershed Project (NEWP) in Central New Brunswick. Elemental concentrations, fluxes and stream discharge from both watersheds were compared for pre- and post-harvest periods (1972–1978 and 1978–1984, respectively). For soil and streamwater solutions, elemental concentrations were typically highest in late summer to fall, and continued to be high throughout the dormant season. For the soil solution, concentrations of NO3-N, NH4-N, Ca and K peaked in midsummer. Highest NO3-N concentrations were found in post-harvest soil solutions taken from hardwood sites. Nitrate levels were low in soil solutions taken from conifer sites, with post-harvest levels slightly higher than pre-harvest levels. Soil solution concentrations were found to vary with soil depth: pH values were lowest at the surface, and increased uniformly with depth; bases (Ca, Mg, K, Na) and NO3-N tended to be lowest at intermediate soil depth. Seasonally divergent trends were observed for post-harvest NO3-N in soil solutions and in streamwater: midsummer levels were high in the former, but low for the latter. Several aspects likely contributed to this divergence: (1) enhanced rates of N mineralization and nitrification in upland soils during post-harvest midsummers, (2) reduced post-harvest vegetational N uptake, (3) possibly accelerated N absorption by microbes and vegetation in the wet areas of the cut watershed. Altogether, post-harvest effects on stream discharge and streamwater chemistry were short-term: differences for elemental concentrations and stream discharge became insignificant after about 5 and 10–12 yr, respectively. Vegetation, especially tolerant hardwoods, recovered rapidly from stump and root sprouts. Key words: Clearcutting, stream discharge, soil solution, pH, Ca, Mg, K, Na, P, NO3-N, NH4-N, seasonal trends