Spectroscopic and Chemical Evidence of Variability across a Pine Litter Layer

Abstract

AbstractChemical‐composition data and spectroscopic analyses are reported for three layers in a ponderosa pine (Pinus ponderosa Dougl. ex P. Lawson & Lawson) litter sample (O horizon) and for fresh needles sampled from a plantation site. The data were taken to obtain evidence for variability in chemical properties accompanying well‐known changes in the visual characteristics of a litter layer that result from the gradual humification of its organic matter. Composition data for Mg, Al, Si, P, S, Ca, Mn, Fe, Cu, and Pb showed that all but Mg, P, S, and Pb increased in content in the litter relative to the needles, and that the content of Al, S, Mn, and Fe increased, while that of Mg decreased, in passing from the Oi to the Oa layer. The increases for Al and Fe were especially large. Fourier‐transform infrared and electron spin resonance spectra of the litter samples indicated, respectively, the presence of a variety of aromatic and aliphatic structures with carboxyl and hydroxyl functionalities and an increase in organic free‐radical content with depth in the litter layer. The electron spin resonance spectra further indicated that Fe(III) was bound in inner‐sphere complexes uniformly down the litter layer, whereas Mn(II) was bound in outer‐sphere complexes. Aqueous extracts of the litter samples showed increases in pH and decreases in the concentration of organic C, carboxyl groups, and free organic acids with depth, some of the changes being as large as an order of magnitude. Composition data for aqueous extracts of the needle and litter samples showed that the water extractability of Mg, Al, P, Ca, and Mn was greater in the needles than in the litter, and that extractability decreased with increasing depth for all of the elements examined except Si and Pb. The largest relative decrease in water extractability (20‐fold) was for Al. These trends, which require confirmation for other Pinus species litter samples, were interpreted as an effect of the variability in organic structures in the litter that develop from differing degrees of humification, bioassimilation, and leaching/retention processes.