Microorganisms in soils regulate important terrestrial ecosystem processes, such as plant productivity, nutrient availability, soil carbon sequestration, and greenhouse gas production. Much research has focused on global change impacts on plant diversity and nutrient cycling, with less attention on the bacteria and fungi that drive many ecological and biogeochemical processes. Linking soil microbial community composition with function, and understanding their response to changes in land use and land cover is important for sustaining ecosystem productivity and for predicting the impact of future disturbances. This research will combine novel genomic technologies, biomarker analyses, and measurements of microbial activity to identify relationships between soil microbial functional diversity and activity and soil organic matter cycling. The investigators will conduct this research in a well-replicated series of secondary forest sites of different ages growing on former pastures in Puerto Rico. Changes in soil carbon, forest structure, biomass, and tree species composition have already been described. This research will focus on the role of soil microbial communities in carbon cycling and their response to ecosystem recovery.
Advances in genomics are increasing our knowledge about the vast diversity of microorganisms in soils at a very fast pace, but more work is needed to link microorganism identity with ecological processes. The emergence of carbon trading markets and payments for ecosystem services has focused global attention on forest carbon sequestration. However, the soil carbon pool is still unable to be predicted. This pool, which can be twice as large as that in aboveground plant biomass, will respond to changes in vegetation. A better understanding of how microbial functional diversity changes during forest recovery, and how this affects organic matter retention or loss from soils, can improve efforts to maintain ecosystem productivity and restore soil fertility to degraded sites. Findings from this research will be incorporated into existing courses on biogeography, biogeochemistry, and microbial ecology. The investigators are actively engaged in education and mentoring activities at the University of Wisconsin, and in external programs for increasing the participation of underrepresented minorities, including women, in the environmental sciences. Presentations in English and Spanish will be developed on tropical ecology, microbiology, and global change for local schools with diverse student bodies in Wisconsin and in Puerto Rico.
The primary goal of this research was to understand how tropical land use change affected microbial communities in soils to better predict the response of soil carbon and nutrient pools to management and landscape disturbance. Overall understanding of how shifts in microbial community composition affect the cycling of biologically important elements (such as carbon, nitrogen and phosphorus) is poor, especially in tropical ecosystems. Tropical ecosystems play a major role in the global carbon cycle through high rates of plant production and microbial activity and dynamic changes in land use and land cover. Large areas of the tropics are experiencing forest regrowth after the abandonment of agricultural or pastureland. The history of disturbance of a site can have a long-lasting effect on the species composition of plants in post-agricultural forests, with unknown effects for soil microbes and nutrient cycling. Through this research, we measured changes in plant and microbial species during reforestation by using a chronosequence approach (comparing a number of sites of different ages) and through repeated sampling (measuring the same site over time). Our sites were unique in that they contained forests of ages varying from 20 to 90 years old, providing a rare opportunity to look at long-term changes in forest processes over time. Our data revealed strong temporal differences in microbial community composition during post-agricultural reforestation across two contrasting tropical soils. Successional patterns in microbial community composition persisted through changes in the makeup of the community between wet and dry seasons and from year to year. In the highly weathered low fertility tropical soil, patterns in microbial composition were not explained by soil properties. However on the younger, more fertile soil, differences in soil pH and nutrient pools were related to community composition. In the forests with higher rainfall, microbial communities followed successional shifts in plant species, with distinct communities between early and late successional forests. In the forests with a marked dry season, microbial communities also showed successional patterns but no differences between secondary forests with different land use histories, despite distinct tree composition. In both regions, enzyme activities differed between pastures and forests and showed seasonal patterns but little to no effect of forest age or past land use. The main lessons from this research are (1) strong climatic controls on soil microbial community composition and activity, and (2) different trends in forests on different soils, highlighting the need to understand variability in microbial response to land use change across a diversity of tropical environments. Observed inter- and intra-annual variability in microbial community structure and activity revealed the importance of a multiple, temporal, sampling strategy when investigating microbial community dynamics. By measuring a site repeatedly over time, we were able to capture short-term dynamics in microbial communities as one of the pastures became reforested over the duration of the study. Successional control over microbial composition with forest recovery suggests strong links between plant and soil communities and has implications for nutrient cycling with changes in vegetation cover. As post-agricultural forests become a dominant tropical land cover, understanding patterns in belowground community structure and function can improve predictions of the fate of ecosystem carbon with changes in forest cover. Our data revealed new insights into the processes that contribute to the formation of soil aggregates in tropical soils. Soil aggregates contribute to a well-developed soil structure, which is important for plant root growth, soil fertility, and carbon sequestration. Despite this, most of our understanding of soil aggregation processes comes from temperate agricultural soils, limiting our ability to predict how disturbance events can alter carbon storage in tropical soils. Ours was one of the few studies to investigate microbial composition in different soil aggregates in tropical soils. Another goal of this research was to provide training to the next generation of scientists. Three graduate students and > 10 undergraduate students were partially supported by this grant, including members from two underrepresented groups in the sciences. Students were mentored in all aspects of research, from start to finish, including fieldwork and labwork, data analysis, and oral and written presentations. Research results were presented at local meetings for land managers, field practitioners, and community organizers, at scientific conferences, through invited lectures at multiple universities, and published in peer-reviewed journals.
