The Internal Element Cycles of an Old‐Growth Douglas‐Fir Ecosystem in Western Oregon

Abstract

Information on primary production, decomposition, hydrology, and element cycling was integrated in annual budgets of accumulation and flux among components of a mature Douglas—fir forest ecosystem. Annual N input in precipitation and dust was 2.0 kg/ha, and an estimated 2.8 kg/ha were fixed by cyanophycophilous lichens in the canopy. Annual N loss to groundwater was 1.5 kg/ha. N appeared to be accumulating in the ecosystem. An annual decrease of ~ 2.8 kg/ha in vegetation was offset by estimated increases of 5.0 kg/ha in fallen logs, and 2.8 kg/ha in soil organic matter. Microparticulate litterfall provided a large input of N to the forest floor (3.3 kg°ha—1°yr—1). Annual input of metallic cations in precipitation was only 545 eq/ha, whereas weathering input (net release of cations to solution from primary and secondary minerals) was estimated by difference at °9000 eq/ha. Total annual loss to groundwater was 9400 eq/ha and, because of little cation accumulation, loss almost exactly balanced input. Net transfers of P were small. Total annual input was 0.5 kg/ha, total loss was 0.7 kg/ha, and net accumulation was —0.2 kg/ha. Input of elements in precipitation and dryfall was small compared with that in the eastern United States. Water chemistry profiles showed that the biologically important elements N, P, and K increased in concentration as water passed through the canopy and litter layer but decreased as water passed through the rooted part of the mineral soil. In contrast, Na increased by a factor of 20 as water passed through the rooted soil. Concentrations of all elements except Mg were lower in the stream water than in solution at 2.0—m depth in the subsoil. At our site, unlike some eastern forests, Kjeldahl N (organic N plus NH4+) accounted for most of the measured N in solution. Nitrate levels were low, averaging @<20 mg/L NO3——N at all points in the profile. Titratable alkalinity dominated anion chemistry in the mineral soil, but in the upper parts of the water chemistry profile (precipitation, throughfall, and litter leachate) Cl— and SO4= together accounted for 30—40% of the negative charge. Total return to the forest floor in litterfall was greater than that reported for other Douglas—fir stands mainly because of plentiful microparticulate forms and coarse woody debris. Leaf fall accounted for less than half of the total litterfall input of N to the forest floor. Element accumulations in coarse woody debris almost cancelled the negative net annual increments in the living vegetation compartments. Overall cycling patterns show that only the biologically limiting element, N, was tightly conserved. For other elements, losses nearly equaled or even exceeded inputs. Redistribution from old to new foliage was also more important for N, P, and K than for Ca, Mg, and Na. Solution transport processes were important for all elements and dominated the cycling patterns of biologically less important elements such as Ca and Na. Vegetation absorbed metallic cations mainly from the mineral soil. However, much N and P were absorbed by roots penetrating up to or into the litter layer. Fluxes of hydrogen ions (H+) resulting from water flow were negligible (@<102 eq°ha—1°yr—1) compared with H+ release during carbonic acid dissociation and H+ removal accompanying cation release in weathering (both °104 eq°ha—1°yr—1). Uptake of metallic cations by vegetation and release during decomposition exceeded uptake and release of sulfur and phosphorus anions, resulting in a net H+ flux of °3 x 103 eq°ha—1°yr—1. An increase in acidity of the rainfall to pH 4.0 would increase H+ input only °3 x 102 eq°ha—1°yr—1.