Microstructural and biochemical diversity of forest soil organic surface layers revealed by density fractionation

Abstract

Particularly on sites with shallow, nutrient-poor soils, forest soil organic surface layers (O layers) are important for the storage of plant-available nutrients and water, and thus for forest vitality and productivity. O layers can contain up to 70 mass percent of inorganic compounds (minerals) which may be spatially separated from or closely associated with soil organic matter (SOM). O layer SOM may differ in biochemical properties and resistance against mineralization, depending on the extent of organo-mineral association. Moreover, type and intensity of SOM-mineral mixing in O layers conveys information about biological activity. Here, for the first time we present detailed density fractionation results to distinguish SOM constituents in O layers with different chemical properties and likely also different degrees of organo-mineral association. Additionally, we characterized spatial association patterns of the SOM fractions. We investigated samples of Of (Oe) and Oh (Oa) horizons of temperate forest soils with different parent materials (basalt, gneiss, andesite, Pleistocene gravel, dolomite). The samples were distinguished into six density fractions (ρ < 0.8 g cm−3; 0.8–1.0 g cm−3; 1.0–1.2 g cm−3; 1.2–1.4 g cm−3; 1.4–1.6 g cm−3; >1.6 g cm−3) by sequential treatment with deionized H2O (ρ = 1.0 g cm−3), 1-propanol (ρ = 0.8 g cm−3), and sodium polytungstate (SPT) solution of increasing density (ρ = 1.2, 1.4, 1.6 g cm−3). The procedure is characterized by a mass recovery of 93 ± 5% and an organic carbon (OC) recovery of 90 ± 9%. In each fraction, we analyzed the concentrations of C, N, metal cations (Fe, Al, Ca, Mg), C speciation (13C NMR spectroscopy), as well as abundances of 13C and 15N. For selected fractions, we additionally determined the radiocarbon age and acquired microscopic images. From the Of to the Oh horizons, the contribution of heavy fractions (>1.4 g cm−3) to total soil mass and total SOC increased, indicating augmented organo–mineral association with progressive SOM decomposition state. Compared to bulk soil, heavier fractions >1.4 g cm−3 were characterized by smaller SOC concentrations, whereas concentrations of Al and Fe were increased for fractions >1.4 g cm−3 in the of and >1.6 g cm−3 in the Oh horizon. The SOM in the heavy fractions was enriched in N, carboxyl C, and alkyl C, but depleted in O/N-alkyl C and aryl C. Smaller mean particle sizes and C/N ratios as well as increased alkyl C / O/N-alkyl C ratios and 13C and 15N abundances indicate an advanced SOM decomposition state and enrichment of microbial-derived SOM in heavier fractions. However, according to its 14C signature, in contrast to high-density (ρ > 1.6 g cm−3) SOM in the mineral soil, forest floor SOM with high density is a rapidly cycling SOM pool with a turnover time <50 years.