Molecular biogeochemistry of soil organic matter after twenty years of litter additions and exclusions in a coniferous old-growth forest

Abstract:

Soil carbon storage and soil organic matter (SOM) composition are sensitive to environmental changes. Shifts in moisture, altered net primary productivity, species composition, and rising temperatures may alter the proportions of above and belowground plant litter entering soil. Previous studies at deciduous temperate forests have shown that twenty years of doubling litter did not sequester more soil carbon but accelerated the degradation of SOM. Litter exclusions (no roots, no litter, and no inputs) also resulted in more degraded SOM. To further ascertain how changes in plant litter quality and quantity alter soil carbon and SOM composition, molecular biogeochemical methods were used to analyze forest floor and mineral soil samples from a twenty year litter manipulation study in a conifer-dominated old-growth forest in the Pacific Northwest. Targeted SOM analyses (solvent extraction, base hydrolysis and cupric (II) oxide oxidation) and solid-state nuclear magnetic resonance (NMR) spectroscopy were used to identify changes in SOM composition and stage of degradation. Microbial activity and community composition were assessed using phospholipid fatty acid (PLFA) analysis. Doubling aboveground litter did not enhance soil carbon content in either the forest floor or mineral soil. Double wood inputs did increase soil C in the forest floor but not in the mineral horizon. Exclusion of above and belowground litter reduced soil carbon. NMR analysis revealed a shift to more degraded SOM with all exclusion treatments as well as with double litter. Double wood inputs changed the SOM composition in the forest floor and A horizons uniquely (advanced degradation in the mineral soil relative to the control). PLFAs also demonstrate changes in total microbial biomass and community structure suggesting the accelerated processing of specific SOM components with litter manipulation. Compound-specific analysis revealed that lignin-derived SOM was not altered significantly. Cutin and suberin compounds varied in response to input changes. These results collectively reveal that changes in the quantity and quality of plant litter inputs alters the molecular-level composition of SOM. Our study also suggests that increased litterfall is unlikely to enhance soil carbon storage over the long-term in temperate forests.