Graduate Thesis Or Dissertation
 

The legacy of Dr. Francis Hole : examining soil organic carbon after 50 years of detrital manipulation

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  • Models of ecosystem carbon (C) balance generally assume a strong relationship between net primary productivity (NPP), litter inputs, and soil C accumulation, but there is little direct evidence for such a coupled relationship. Using a unique 50-year detrital manipulation experiment in a in a mixed deciduous forest and restored prairie grasslands in Wisconsin, combined with sequential density fractionation, isotopic analysis, and short-term incubation, we examined the effects of detrital inputs and removals on soil C stabilization, destabilization, and quality. Both forested sites showed greater decline in bulk soil C content in litter removal plots (55 and 66% decrease in surface soils compared to controls) compared to litter addition plots (27 and 38% increase in surface soils compared to controls). No accumulation in the mineral fraction C was observed after 50 years of litter addition of the two forest plots, thus increases in the light density fraction pool drove patterns in total C content. Litter removal across both ecosystem types resulted in a decline in both free light fraction and mineral C content, with an overall 51% (7%) decline in mineral-associated carbon in the intermediate (1.85 - 2.4 g cm⁻³) density pool; isotopic data suggest that it was preferentially younger C that was lost. The carbon content response in the >2.4 g cm⁻³ mineral fraction was variable. In contrast to results from other, but younger litter manipulation sites, there was with no evidence of priming even in soils collected after 28 years of treatment. In prairie soils, aboveground litter exclusion had an effect on C levels similar to that of root exclusion, thus we did not see evidence that root-derived C is more critical to soil C sequestration. There was no clear evidence that soil C quality changed in litter addition plots in the forested sites; δ¹³C and ∆ ¹⁴C values, and incubation estimates of labile C were similar between control and litter addition soils. C quality appeared to change in litter removal plots; soils with litter excluded had ∆ ¹⁴C values indicative of longer mean residence times, δ¹³C values indicative of loss of fresh plant-derived C, and decreases in all light fraction C pools, although incubation estimates of C quality did not change. In prairie soils, δ¹³C values suggest a loss of recent C4-derived soil C in litter removal plots along with significant increases in mean residence time, especially in plots with removal of roots. Our results suggest surface mineral soils may be more vulnerable to loss than to gain, in association with disturbance, land use change, or perhaps even climate change over century - decadal timescales, and also highlight the need for longer-term experimental manipulations to study soil organic matter dynamics.
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