The combined controls of land use legacy and earthworm activity on soil organic matter stabilization and dynamics in temperate deciduous forests

Yini Ma, Purdue University

Abstract

Soil organic matter (SOM) represents terrestrial biosphere's largest pool of organic carbon (OC) and an integral part of the global C cycle. Within forests of the Smithsonian Environmental Research Center (SERC) (Edgewater, MD), we established eight plots for long-term field manipulation experiment as well as soil and earthworm monitoring. Our objective was to compare sites with no record of significant agricultural disturbance or earthworm activity to successional forests recovering from past agriculture (60-132y) that contained both native and non-native earthworms to assess dominant controlling factors for soil carbon stabilization in this complex system. Combining soil physical fractionation, organic chemical extraction techniques, and long-term microcosm laboratory incubations we were able to assess the distribution and also track the chemical fate of added plant litter into soil organic matter pools with different levels of physical and chemical protection and assess the relative importance of different SOM stabilization mechanisms of physical protection and biochemical recalcitrance to the overall stability, microbial accessibility, and temperature sensitivity of soil organic carbon (SOC) at these sites. We found that the presence of invasive earthworms at SERC prolonged the disturbance legacy of past agriculture activities by dramatically altering the nature of forest floor and O-horizon recovery, transporting annual input of C and N from forest floor to lower depth in the soil profile as well as increasing C and N within microaggregates. In addition, earthworms, through selective feeding habits, soil mixing dynamics, and presumed disruption of lignin degrading fungal networks, altered the SOC chemistry by promoting lignin enrichment in particulate organic matter (POM) soil pools. However, the overall stability of SOC at SERC forests, as assessed by long term soil incubations could not be attributed to any one protection or stabilization factor including chemical protection through binding with minerals, physical protection through aggregation and biochemical recalcitrance with increasing lignin concentration. In addition, other factors like soil pH and N concentration, which may also be influenced by earthworm activity or forest succession, may play an important role in driving SOC mineralization during incubation. A 5-year aboveground litter amendment experiment with both wood and leaf tissue conducted in this work illustrated how litter type interacted with earthworm abundance and species composition to control the capacity of forests to incorporate surface litter into soil fractions. For example, in young successional forests with relatively higher soil feeding endogeic earthworm species as well as higher overall earthworm abundances, amended woody tissue C was incorporated deeper depths and predominantly into POM fractions with respect to older successional sites. In addition, lignin phenols, predominantly derived from wood amendments, were selectively incorporated in both POM and silt+clay (SC) fractions in young forests but only incorporated into POM fractions in old forests. However, even with more than 2.5 times background annual leaf litter input added annually over five years, neither total C content nor leaf-derived substituted fatty acids (SFA) concentrations increased in the old or young successional sites. It was anticipated that in sites where wood amendments shifted the chemical trajectory of soil fractions to higher overall C and greater lignin soils would exhibit suppressed respiration rates and greater temperature sensitivity as compared to controls. Long-term soil incubation studies were used to assess the accessibility of and temperature sensitivity. In agreement with this hypothesis, it was found that with the incorporation of woody amendments, SOC mineralization was suppressed in both young and old forest soils, but with active pool being mostly affected in old forests while slow pool being mostly affected in young forest soils. This difference between young and old forest soils suggests that the abundant soil mixing earthworms in young sites partitioned woody carbon into more protected soil carbon pools. However, in contrast to the initial hypothesis, increased leaf litter input also had significant effect in stabilizing SOC. Specifically, with leaf amendments, C pool of old successional forests shifted to a proportionately larger active pool but with an much slower cycled slow pool, and in young forests, leaf amendments shifted soil C to a larger slow pool with similar MRT. Surprisingly, is in consistent with the result we obtained from the incubation study of background soil among different successional stages the temperature sensitivity was not affected by either type of amendments with distinct biochemical trajectory indicating biochemical recalcitrance is not the controlling factor on temperature sensitivity at SERC forests. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Filley, Purdue University.

Subject Area

Ecology|Biogeochemistry|Soil sciences

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