Grandy, A. S. 2005. Ecosystem consequences of soil aggregation following soil disturbance. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

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Carbon sequestration and greenhouse gas abatement in soils are two of a limited number of rapidly-deployable, high impact CO2 stabilization options now available to policy makers. Among agricultural management options, eliminating tillage may have the broadest mitigation potential because it alters emissions of all three biogenic greenhouse gases. No-till soil management in the U.S, although increasing, however, continues to be rotated with tillage because of perceptions that no-till limits N availability and decreases yields. This raises questions about the permanence of stored C, which has been identified as the fundamental challenge to terrestrial C sequestration. In this dissertation I address the ecological processes underlying soil organic matter permanence and the ecosystem and agronomic consequences of long-term no-till.

In the first series of experiments, reported in Chapter 2, I determined aggregate-associated soil C pools in ten ecosystems on the same soil series along a management intensity gradient. I also assess the degree to which C is protected by aggregates using size and density fractionation techniques coupled with long-term mineralization assays of crushed and intact aggregates. Active pool C increases when 2000-8000 pm aggregates were broken into microaggregates (<250 p.m) ranged from 18% in conventional agriculture to 59% in alfalfa. Potential release of whole-soil labile C from physical protection following macroaggregate destruction was seven to nine-fold greater in successional systems than conventional agriculture.

In the second set of experiments, reported in Chapter 3-5, I cultivated a never-previously cultivated field and minimized plant community changes to look at soil disturbance free from the influence of other agricultural management practices. I infer soil C permanence from responses of aggregate-protected soil organic matter, enzyme activities that reflect soil microbial activity, and trace gas fluxes. I, therefore, attempt to advance our ability to predict tillage effects on soils by understanding the basic ecological processes controlling soil’s response to disturbance. Cultivation immediately reduced 2000-8000 pm aggregates to levels commonly found in agricultural soils tilled > 50 years. The destruction of aggregates released particulate C from protected microsites and limited the incorporation of aboveground C into new aggregates. This lead to a series of biogeochemical transformations I term an aggregate cascade. Microbial activity and N cycling increase, substantially increasing fluxes of both nitrous oxide and carbon dioxide. Growing plants can modify the effects of tillage on N availability and N20 flux but have little effect on aggregation or CO2 emissions. This cascade is accelerated by increased soil temperature and other environmental changes following decomposition that promote decomposition.

In chapter 6, I report on an analysis of data from the till and no-till treatments of the KBS LTER and conclude that over 12 years there were no yield declines or environmental consequences (e.g. increased N20 emissions) associated with no-till. This work was performed in collaboration with classmates in a graduate seminar.

My results show that cultivation immediately destabilizes physical and microbial processes related to C and N retention in soils. I also demonstrate that no-till cropping can be practiced with no yield or environmental trade-offs. Together, these results demonstrate the importance and feasibility of protecting no-till soils from periodic cultivation

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