Effects of organic amendments on aggregation and microbial community dynamics in soils

Project Overview

Project Type: Graduate Student
Funds awarded in 2008: $10,000.00
Projected End Date: 12/31/2011
Grant Recipient: University of Kentucky
Region: Southern
State: Kentucky
Graduate Student:
Major Professor:
Elisa D'Angelo
University of Kentucky
Major Professor:
Mark Williams
University of Kentucky

Annual Reports


  • Agronomic: vetches


  • Crop Production: cover crops, organic fertilizers
  • Natural Resources/Environment: biodiversity, soil stabilization
  • Soil Management: green manures, organic matter, soil analysis, soil microbiology, soil quality/health


    This project evaluated the use of organic amendments to foster a soil microbial community profile that favors improved soil structure. The effects of soil amendments on microbial community dynamics and aggregate formation were tested in three different agricultural soils. The native soil structure of experimental soils was destroyed by forcing soils through a 250µm sieve. Soils were treated with organic amendments based on amendment C content at a rate of 0.01g-amendment C/g soil. Amendment treatments included hairy vetch residue, dairy manure, compost, or no-amendment (control). Soils were then incubated at 25°C for 82 days. Soils were destructively sampled on incubation days 0, 5, 12, 30, and 82. A wet sieving method was used to assess the formation of large macroaggregates (LMacAg) which were defined as aggregates >2000 µm. Differences in microbial community structure were assessed through analysis of microbial fatty acid methyl esters (FAME). Incubated soils were also analyzed for the fungal biomarker ergosterol. Relationships between amendment treatments, microbial parameters, and aggregate formation through time were investigated in order to test the hypothesis that amendments that stimulate greater fungal activity will also promote greater macroaggregate formation. In general, the vetch treatment resulted in the greatest formation of LMacAg followed by the manure, control, and compost treatments, respectively. In two of the three soils tested, the vetch treatment showed the greatest fungal/bacterial ratio (F:B) after 82 days of incubation, while the manure treatment resulted in the highest 82 day F:B in the much sandier Yeager soil. Ergosterol content was greatest in the vetch treatment in all soils. Large macroaggregate formation was most positively related to the relative abundance of 18:2?6c, a signature FAME for fungi. Formation of LMacAg was also strongly positively related to soil ergosterol. We conclude that the compost used neither enhanced soil structure nor did it significantly alter the microbial community relative to non-amended soils. In general, vetch is a good amendment for enhancing soil structure because it promotes higher relative abundances of fungi in most soils. Manure also promotes high relative abundance of fungi and may possibly be as effective as vetch in coarser textured soils.


    Soil aggregation is the foundation for soil structure in surface horizons, affecting processes such as water infiltration and movement, oxygen diffusion, and plant nutrient availability. Microorganisms play major roles in formation and stabilization of soil aggregates (Oades, 1993). Fungi are thought to be the dominant microbes involved in macroaggregate formation while bacterial activity facilitates microaggregate (< 250µm diameter) formation (Oades, 1993). Fungal hyphae bind macroaggregates by physically enmeshing microaggregates and soil particles (Tisdall and Oades, 1982). Fungi also exude polysaccharides that act as aggregate binding agents (Oades, 1993). Bacteria thrive in micropore zones where they are protected from size-excluded bactivores (Monreal and Kodama, 1997). In these zones, bacteria also facilitate aggregation on a microscopic level by excreting polysaccharides which bind silt and clay particles into microaggregates (Oades, 1993). Monreal and Kodama (1997) found proportions of fungi and bacteria to differ by aggregate size class, with macroaggregates having higher fungi to bacteria ratios than microaggregates. Beare et al. (1997) showed reduced macroaggregate stability when fungicides were applied.

    Organic amendments can alter the microbial community (Schutter et al. 2001) and they generally have a positive effect on soil aggregation (Bronik and Lal, 2005). Organic amendments are a carbon source for microbes and their composition can affect microbial community dynamics. Larkin et al. (2006) found that manure inputs caused increased bacterial populations. Schutter et al. (2001) found that cover crops increased molecular biomarkers for fungi. Amendment carbon complexity can also affect the soil microbial community. Schutter and Dick (2001) used fatty acid biomarkers to assess soils amended with cellulose and found these soils had elevated levels of fungal biomarkers relative to soils amended with simpler carbon substrates such as glucose or gelatin. Thus, given their influence on soil microbial community profiles, different amendments may stimulate different microbial aggregation processes. In the interest of building and maintaining good soil structure, understanding which amendments promote aggregation by fostering a favorable microbial community would be useful in developing soil amendment plans for producers.

    This research examined changes in microbial community dynamics and aggregate dynamics in response to soil amendment with different organic materials. The primary hypothesis being investigated is that, relative to other amendments, amendments that stimulate greater fungal activity will also lead to greater levels of stable macroaggregates.

    Project objectives:

    1. 1) Determine the effects of organic soil amendments on the dynamics of microbial community composition and on soil aggregate formation. 2) Determine if amendment input influenced microbial community composition can be related to soil aggregate formation.
    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.