Abstract:
Chemical and microbial analyses were made at four seasonal intervals on soil horizons under red alder, conifers, and mixed
alder-conifers at the Cascade Head Experimental Forest, established in 1937 by the U. S. Forest Service near Otis, Oregon.
Microbial analyses showed that although plate counts of molds and bacteria fluctuated with season, trends were associated with forest type. Molds were least abundant in both the F- and A-horizons under alder, and were always most numerous in the conifer A-horizon. Bacteria in F-horizons always exceed those in A-horizons. The conifer A-horizon invariably, gave highest counts of bacteria; mixed stand A-horizon_always showed lowest numbers. The percentage of Streptomyces was greatest under the mixed stand and generally was lowest under conifers. Chemical analyses showed the lowest pH values occurred in soil under alder. Kjeldahl nitrogen was consistantly lowest in
conifer soil. Ammonium nitrogen in F- and A-horizons was generally lowest in soil under conifers. Nitrite nitrogen was very low in
all samples. More nitrate nitrogen was found under the alder and the mixed stands than under conifers. Sulfate concentrations in
both F- and A-horizons were greater under alder and mixed stands than under conifers, but available phosphate was higher under
conifers. The exchange capacity of A-horizon soil was parallel with loss on ignition, water holding capacity and total carbon. Exchangeable hydrogen decreased in order under the alder, mixed and conifer stands; this agreed with the order of soil acidity
as indicated by pH values. Exchangeable cations were highest in the conifer soil.
Chemical and microbial analyses of 1-foot square subsamples taken within an area six feet in diameter atcardinal directions around a representative alder tree showed variation on the basis of both direction and distance. The variations indicate effects of tree, slope, and wind direction. Although ammonium nitrogen was lowest in both F- and A-horizons
of the conifer soil, ammonification was higher than in corresponding horizons of the alder and mixed stands. Nitrification of ammonium sulfate in the alder F-horizon was three times as great as under conifer, while mixed stand horizons showed little nitrification. Nitrification in the A-horizon was lowest for the alder and mixed soils; it was seven times as much in the conifer soil.
Nitrification of nitrite nitrogen was appreciable only in the alder F-horizon; in other cases it was less, generally being much below that of ammonium sulfate, Although added calcium carbonate greatly increased nitrification in the alder and mixed stand soils it had little influence with the conifer soil. Greater response of the alder and the mixed soils is attributible to their initially higher acidity than that of the conifer plot. Sulfur oxidizing power was greatest in F-horizon samples but the oxidation of elemental sulfur was low in all cases. Polysulfide was converted to sulfate more rapidly than flour sulfur. Mineralization of rock phosphate was increased by sulfur oxidation only in the alder F-horizon. Dry matter, Ammonium, nitrate, and Kjeldahl nitrogen in canopy rainfall were highest in alder soil and lowest under conifers; mixed stands values were intermediate. Corresponding values in stem-flow were lowest for alder; the order varied for the other stands.
