FOREST SOIL STUDIES

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Abstract
31 representative samples of mature undecomposed leaves and needles, representing 19 spp., were studied, in regard to (1) differences in certain chem. properties[long dash]pH, total N, water-soluble organic matter, ash, Ca, and excess base[long dash]and (2) their rates of decomposition under controlled conditions, as measured by loss of original material detd. after periods of 2, 4, and 6 months. Studies were made of the relation between the chem. properties of the litter and the rate of decomposition. The pH of the samples varied from 3.46 to 6.68, with most of the values below 5. The N content varied between 0.45% for a sample of Acer saccharum and to 2.06% for Robinia pseudoacacia; the ash content, from 1.94% for Pinus rigida to 11.54% for Juglans nigra; the Ca, from 19.1 m.e. per 100 g. for P. virginiana to 162.8 m.e. for Cornus florida; the water-soluble organic matter, from 4.7% for J. nigra to 26.4% for Acer rubrum; and the excess-base content varied from 38.2 m.e. per 100 g. for P. rigida to 214.7 m.e. for J. nigra. The values for excess base are of particular interest, since they represent the amt. of base available for neutralizing soil acidity upon complete decomposition of the litter. Although some variation exists in the chem. composition of different samples from the same species, more marked differences were found between different spp. and genera with respect to total N, ash, Ca, and excess base. Variations in water-soluble organic matter between different samples of the same sp. are considerable and may be greater than those between different spp. This is probably to be expected: a slight variation in the maturity of the samples when collected would affect the content of water-soluble organic constituents. The pH of the residual material, in general, increased progressively as decomposition proceeded. There was no definite relation between the pH of the original material and the pH of the residual organic matter, but a significant relation between the latter and the excess-base content of the original litter. Leaves high in excess base generally gave, upon decomposition, a residue of high pH; and those low in excess base, a low pH. There was a marked difference in the rate of decomposition of fresh leaf litter from various forest-tree spp. when maintained under controlled conditions and inoculated uniformly with a forest humus suspension: after 6-months the decomposition averaged less than 25% for Pinus rigida, P. virginiana, P. strobus, Fagus grandifolia, and Platanus occidentalis; it averaged over 45% for Liriodendron tulipifera, Rhus hirta, Robinia pseudoacacia, and Juglans nigra. This variation is explained largely by variation in the chem. properties of the litter. Of the properties of the litter studied, those most closely associated with rate of decomposition are water-soluble organic matter, % of N, and excess-base content. During the first 2 months the first 2 were the controlling factors, the multiple coefficient of correlation being 0.85; during the 2-6-month period the dominant factor was the excess-base content of the litter, the simple coefficient of correlation being 0.77. The multiple coefficient of correlation between % decomposition during 6-months and the 3 independent variables[long dash]N, water-soluble organic matter, and excess base[long dash]was 0.862.