Updated: December 2012
Understanding Soil Bacterial, Fungal, and Nitrate Responses to Sugar Amendment in the Puget Lowland Prairies
Plants and their soil microbial communities influence each other, and these interactions may contribute to plant community diversity, invasion success, and success of restoration. Bacteria and fungi compete for carbon sources in the soil, and fungi predominate in soils of mature plant communities forming mycorrhizal partnerships with native plants. I previously hypothesized that sugar amendment in soils would enhance bacterial populations, which would lead to reductions in beneficial plant-fungal partnerships. However, I recently demonstrated that, in the short term, sugar suppressed bacterial activity, enhanced fungal activity, and boosted soil nitrate. Therefore, I propose to further investigate these unexpected responses.
With the notable exceptions of mycorrhizae and nitrogen-fixing bacteria, plant ecologists have traditionally viewed soil simply as the medium from which plants gain their water and mineral nutrients and have focused their attention on interactions among species they could see aboveground. However, it is becoming increasingly clear that to understand what happens in aboveground plant communities, we need to understand the relationship between plants and the broader soil microbial communities. Recent evidence has shown that plants influence the microbial communities around their roots (Bever 1994, Stephan et al. 2000, Kowalchuk et al. 2002, Zak et al. 2003, Hartmann et al. 2009), that the microbial communities around their roots can affect the success of plants (Bever et al. 1997, Bever 1999, Bever 2002), and that direct and indirect interactions between plants and microbes are likely to contribute to larger scale patterns such as plant community diversity (Broughton & Gross 2000, Westover & Bever 2001, Reynolds et al. 2003, Wardle et al. 2004, Vogelsang et al. 2006, van der Heidjen et al. 2008), success of invasive species (Belnap & Phillips 2001, Klironomos 2002, Callaway et al. 2004, Xingjun et al. 2005, Hawkes et al. 2006, Rout & Callaway 2009) and potential success of restoration efforts (Wolfe & Klironomos 2005).
For the last several years, I have been exploring mechanisms to restore the South Puget Lowland prairie ecosystem. The prairies of western Washington are one of the most threatened habitats west of the Cascades, and currently occupy less than 3% of their historical range (Chappell et al. 2001). A number of native animal species depend on native prairie plant diversity (e.g., the streaked horned lark, Mazama pocket gopher, Mardon skipper butterfly), but many native plant species populations declined markedly after the prairies were invaded by Scotch broom [Cytisus scoparius (Haubensak & Parker 2004)]. Although the broom has been controlled, its suppression was followed by the establishment of many invasive weedy plants, particularly grasses. I have previously used soil sugar addition to successfully reduce the competition from these grasses on the native species, but the slow-growing native species were unable to take advantage of this temporary window of opportunity, and by five years post treatment, those areas in which sugar was added had significantly more non-native species than the controls. This outcome suggests that sugar addition altered the long-term composition of the microbial community in a detrimental way, as has been postulated previously by Cleveland et al. (2007). Last summer, data I collected showed clearly that in our prairie soils, bacteria and fungi suppress each other (Figure 1). Given the assumption this would be true, I had hypothesized that adding a readily-available simple sugar to soils would enhance the populations of opportunistic, non-specific bacteria which might then compete with more specialized symbiotic microbes, particularly fungi, on which native prairie plant diversity depends. However, the data (in very short-term responses -two weeks post-sugar addition) clearly showed the opposite result: sugar addition suppressed bacterial activity and enhanced fungal activity (Figure 2). Moreover, in this very short-term, soil nitrate concentrations jumped by 600% in sugar treated prairie soils as compared to two weeks earlier and as compared to the control soils tested simultaneous (Figure 3). This nitrate result was also completely unexpected given the decrease in nitrate in response to sugar addition that I’ve seen in my previous work. The experiments generating these results differ from earlier experiments in both duration (2 weeks vs. 6 weeks to 5 years) and seasonal timing of the sugar addition (mid-June vs. mid-March). Both of these procedural differences might have had significant effects, and in sum indicate that there is a great deal we don’t understand about the soil community interactions and our ability to manipulate soil properties to enhance native plant species.
Understanding the prairie soil microbial community is a dauntingly complex objective. Over the last three years, my students and I have successfully used two methods to investigate soil microbial community responses to sugar addition: community-level physiological profiling (CLPP; Wolfe & Klironomos 2005), and quantitative real-time PCR (qPCR). I used Biolog® Ecoplates, commercially available 96-well plates set up to do soil microbial CLPP analysis, to generate the data presented in Figures 1-3. However, since CLPP assess only microbial activity, this method alone cannot whether or how bacterial and fungal populations are changing. Therefore, we have employed qPCR to assess changes in the relative populations of soil fungi and bacteria. My current student, Jessica Wong, is about to finish collecting qPCR data on the same soils for which I generated the CLPP data. Her results should give us some insight into the causes of these functional differences. Moreover, qPCR can be used to separate the changes in populations of different kinds of fungi and/or bacteria if we use primers specific for subgroups of fungi or bacteria (e.g., mycorrhizal vs. opportunistic fungi or nitrogen-fixing bacteria vs. other bacteria). So the combination of these two methodologies is much more powerful than either is alone. In addition, I test all the soils for nitrate (and I hope, ammonium this year) in my lab, but to do so, I need a set of professionally analyzed soils for calibration of my standard curves. I need to refresh my supply of reference soils this year.
My specific objectives for this spring and summer are to compare bacterial and fungal responses to sugar addition between early spring (when we’ve done the sugar additions previously) with late spring (when we did the sugar addition last year) and to compare short term responses from last year with longer term responses on those same plots. To accomplish these goals, I will establish a set of plots replicating those from last summer, but add sugar in mid-March and follow the short term responses of fungi and bacteria. I will then do the same experiment in mid-June to see if the responses are similar to those we got last year. Just as Jessica and I did this year, we will use the same soil samples for CLPP and qPCR to get both kinds of data. In addition, I will retest the plots we set up last year to look at the longer term responses of soil bacteria and fungi. Because I found relatively low variances among plots last year, and because I will be more than doubling the size of the experiment, I can and must decrease the number of replicate samples within each experimental treatment to keep the project within the budgetary constraints. These data will then allow me to assess 1) whether last year’s data are anomalous or representative, 2) whether the timing of sugar addition fundamentally changes the interactions between soil bacteria and fungi, and 3) whether the long-term responses of soil bacteria and fungi to sugar addition are fundamentally similar or different from the short-term responses.
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