Soil organic matter quantity, quality and microbial activity in deep soil profiles

Project Description: 

1) How does the quantity and quality of soil organic matter change as a function of soil profile depth:
-in old, highly weathered oxisol profiles versus younger, granitic inceptisol profiles?
-in three different forest types: Tabunuco, Colorado and sierra palm?
2) How do soil microbial community structure and functional characteristics change with depth across contrasting parent materials and forest types?
3) Can we relate microbial community characteristics to SOM quantity and quality?

The “critical zone” is defined as the region of the Earth where rock meets life. Soils, consisting of a heterogeneous mixture of dead organic matter, live microbial decomposers and inorganic rock-derived minerals, represent a part of the critical zone where rock and life directly interact. From the forest floor down, a soil profile represents a continuum of actively cycling energy in the form of organic carbon (C) and nutrients that supports the most diverse communities of microorganisms on earth. Despite the importance of deep (1-2 m) SOM as a long-term sink of C from the biosphere, most studies that aim to characterize SOM focus on the upper 20-30 cm (Jobbagy & Jackson 2000). In addition, little is known about how microbial communities change with depth (Fierer et al. 2003), and few studies attempt to explicitly link SOM properties with microbial community characteristics.
This study aims to describe how SOM and microbial community characteristics are related and how they change with depth in across the two parent materials and three forest types that comprise the Luquillo CZO. Due to rapid decomposition of surface litter in these tropical soils, we expect the chemical composition of SOM to be relatively decoupled from overlying vegetation and that SOM residence time will increase with depth. We expect deep soil microbial communities to be less diverse than surface communities, possibly exhibiting greater substrate specialization and different metabolic strategies.
Experimental Approach
We collected soil at six sites at similar elevations that represent two parent materials (sedimentary volcaniclastic and igneous quartz-diorite) and three forest types (Tabunuco, Colorado and sierra Palm) that characterize the Luquillo CZO. Five soil profiles were dug along catenas such that for each site, one ridge, one valley and three slope profiles were dug. Approximately 70% of Luquillo soils exist on sloping terrain (Scatena & Lugo 1995) and we therefore biased our sampling design to incorporate more slope profiles than ridges or valleys. Each soil profile was dug down to 140 cm (or bedrock) and sampled aseptically at 10 cm intervals. Aliquots of each sample were frozen for microbial analysis. The rest of the soil was air dried and sieved to 2mm.
A suite of biological, chemical and physical techniques will be used to compare and contrast SOM composition and stability with depth across the different parent materials and forest types. Microbial communities will be examined using fingerprinting approaches, including DGGE and qPCR, to broadly assess community richness and evenness, and GeoChip microarray analysis to measure the presence and metabolic activity of ecologically functional organisms. A suite of hydrolytic and oxidative C-acquiring enzymes, as well as several nitrogen and phosphorus acquiring enzymes will also be measured in order to link community structure to carbon and nutrient cycling.

Additional funding sources: National Science Foundation Graduate Research Fellowship to M. Stone.

Research Location: 
Core Area(s) and/or Keywords: 

Microbial ecology, biogeochemistry, rhizosphere

Source of Funding: 
Mellon Foundation
Relevant Publications: 

Fierer N., J.P. Schimel, and P.A. Holden 2003. Variations in microbial communities through two soil depth profiles. Soil Biology Biochemistry, 35, 167-176.
Jobbagy, E. G., and R. B. Jackson. 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecological Applications,10, 423-436.
Scatena F.N, and Lugo A.E. 1995. Geomorphology, disturbance, and the soil and vegetation of two subtropical wet steepland watersheds of Puerto Rico. Geomorphology, 13, 199-213.

Contact Information
Person(s) Completing This Form: 
Maddie Stone
Maddie Stone (301)-785-1239
Investigator E-mail Addresses: