- Microbial Community Indicators of Soil Development in Tropical Secondary Forests (Costa Rica)
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As land management strategies in tropical regions change from intensive agricultural use to reforestation, the development of secondary forests is becoming a potentially important restoration strategy for increasing soil carbon (C) sequestration (e.g., Guo and Gifford 2002). During earlier stages of regeneration, secondary forests in the tropics are thought to have reduced levels of soil organic carbon (SOC), which over time becomes more complex as labile pools of nutrients and organic matter accumulate on the forest floor. This is believed to stimulate complex microbial activities associated with decomposition and mineralization of organic matter (e.g., Feldpausch et al. 2004), followed by an increase of C in microbial biomass (Cmic) and the ratio of Cmic to SOC, both of which suggest more efficient use of the SOC and more C incorporation into the microbiota (Table 1).
Chazdon and colleagues (2007) discussed the rates of change in vegetation occurring in secondary tropical forests. However, little, if any, work has been conducted to identify and compare changes in the belowground microbial community structure, biomass, or potential for C sequestration, how these change when primary forest is converted into pasture, and different age classes of secondary forests. Soil microbial community structure and function are the most rapidly responding biotic component of any terrestrial habitat, significantly affecting the nutrient cycle components while actually influencing patterns of vegetation distribution. Thus microbial community changes may be the earliest indicators of habitat condition and the rate and direction of recovery following implementation of ecosystem management, restoration, and conservation strategies in different habitat types (e.g., Anderson 2003, Wardle et al. 2004).
The Northern Zone of Costa Rica has experienced a variety of extraction-based land management activities over the past 30-plus years, resulting in the loss of approximately 70% of its forests. Attempts at remediation have resulted in a variety of strategies, including an extensive array of secondary forests (Schelhas and Sánchez-Azofeifa 2006), providing a unique opportunity to study the effects that such strategies may have on soil structure and function.
We assessed indicators of soil condition based on C cycle metrics (Table 1), as well as microbial activity, biomass, and abundance and diversity, in soils from old growth forests, three age classes of secondary forests, and adjacent pasture within the La Selva Research Station area of Costa Rica (10°26′N, 84°00′W), which is operated by the Organization for Tropical Studies. Our goals were to determine if soil community complexity changes across these habitats and to identify a suite of parameters that demonstrate trends in soil development and have the potential for use in developing models to predict the recovery rate of tropical forests following disturbance or management.
Sample sites were chosen so that they shared similar topography and soil structure (previously classified as younger alluvial oxisol soils) but had been managed differently. The secondary forests were previously part of the nearby old growth forest, typical of the area, that had been cleared 30, 25, and 15 years previously and allowed to recover naturally. The pasture was created from the same old growth forest in 1955, but had not been grazed or managed since 1988. Thus we were able to study the four habitat types within close proximity, each area being at least 200 m × 100 m. Soil was randomly collected along four 50 m transects (separated by approximately 20 m) established within each habitat type.
Using the methods reviewed by Anderson (2003), we determined the Cmic, ratio of Cmic to SOC, and microbial metabolic quotients (qCO2, as a ratio of CO2 from respiration to Cmic as indicators of the amount of C being incorporated into the soil as nonplant, noninvertebrate C biomass, efficiency of utilization of...