Sedgwick Reserve

01 Jul, 03 AM - 30 Jun, 11 AM

INTRODUCTION: In most soil profiles, the concentration of organic carbon in soil decreases with soil depth and the carbon residing in deeper soil horizons is mineralized very slowly (Brady and Weil 2002)(Trumbore 2000). It is not clear why the rates of mineralization of subsurface soil organic matter (SOM) are so low and why the residence times of C in subsurface soils are so long. Is the C physically protected, rendering it physically inaccessible to microbial mineralization? Or is subsurface SOM so chemically complex that microbial mineralization is inhibited? Or could the accumulation and persistence of C in these subsurface horizons be due to nutrient imbalances that limit microbial activity? While a large number of studies have looked at organic C concentrations through the profile, we have a very poor understanding of the controls on microbial mineralization of organic C through the profile. This gap in our knowledge is particularly glaring considering that the total amount of carbon residing in subsurface horizons can be significant. Although C concentrations decline sharply with depth, the volume of soil residing in subsurface horizons is large; over 50% of the carbon in 1m deep soil profiles is found below 30 cm in depth (Batjes 1996). Understanding the dynamics of subsurface C pools is especially important if we want to understand how anthropogenic disturbances ? including increased nutrient loading rates, soil acidification, and the physical disturbance of subsurface soils will affect the large reservoir of organic C contained in subsurface soil horizons. Existing literature has primarily focused on the effects of anthropogenic disturbance on soil processes and carbon fluxes in the surface soil horizons, largely ignoring the deeper soil carbon pools. The goal of this study is to understand the mechanisms and controls on microbial C mineralization of SOM through the soil profile. There are four possible explanations for the decrease in C turnover rates (the decrease in microbial mineralization of SOM) with soil profile depth: H1: Physical protection afforded by soil structure (i.e. protection in aggregates) limits decomposition. H2: Physical protection afforded by organo-mineral complexation limits decomposition. H3: Chemical protection due to recalcitrance of organic matter limits decomposition. H4: Nutrient availability limits decomposition. This project has been designed to explicitly test these 4 hypotheses in order to better understand organic C dynamics throughout the soil profile. Soils will be collected from various different depths throughout representative soil profiles at three annual grassland sites in California: Sedgwick Reserve, north of Santa Barbara; Hastings Reserve, south of Monterrey; and the McLaughlin Reserve, north of Davis. The three sites lie along a precipitation gradient and we expect the three sites to have quantitatively different soil C distributions through the soil profiles. As UC Reserves, these three locations have the documentation of land-use history essential for finding sites that have been subjected to little or no disturbance. I will dig 1-4 soil pits, roughly 1.5x1.5m by 2m deep. I will describe the soil profile, then take 500-1000g soil from several different depths/horizons/subhorizons in the soil profile. I will take the soil back to the Schimel lab in Santa Barbara for the following tests of the hypotheses presented above. When I am done at the reserve, I can fence off but leave open the pits for other researchers, or I can fill them back in. I am hoping to locate the pit(s) in a relatively undisturbed (i.e. free of grazing, plowing, harvesting) annual grassland, and will work with the Reserve Director to find suitable sampling locatioins. TESTS OF SPECIFIC HYPOTHESES: H1: Physical protection afforded by soil structure (i.e. protection in aggregates) limits decomposition. The physical protection afforded by soil structure is readily disrupted by grinding. Soils will be separated into three treatments corresponding to different amounts of grinding. Soils will be ground using a Ball Mill to pass through either a #20, #60, or #100 mesh, then incubated at constant moisture. Respiration measurements under controlled incubation conditions will provide an index of the rates and extent of mineralization of carbon in the three different treatments. H2: Physical protection afforded by organo-mineral complexation limits decomposition. I propose to break down the organic-mineral associations in soil with hydrofluoric acid (HF), a weak acid commonly used to remove mineral matter in soil without affecting the structure of the SOM (K?gel- Knabner 1997). Soil from the various soil horizons would first be sterilized using g-irradiation to minimize microbial activity during the separation procedure. The sterilized soil would be treated in a slurry with HF, filtered, and then rinsed with deionized water (DI) three times to remove residual HF. These solutions will all be combined, and the DOM they contain will be isolated from the HF through dialysis using a 500 molecular weight cut off filter, which should retain almost all of the organic matter (Qualls 2000). Some of the DOM may flocculate during this procedure due to changes in pH and concentration, but this should not affect the quality of the organic matter retained, just its status as dissolved or particulate organic matter. This mix of soluble and insoluble organic matter will then be recombined with the solid organic matter. This recombined soil will then be slurried, re-inoculated with soil microorganisms, and incubated to examine the mineralization of carbon. A parallel incubation will be established with identical soil that has been sterilized, slurried, and re-inoculated. The incubations will be run until C mineralization is no longer detectable. Differences in mineralization between these two treatments will provide an index of the degree to which physical protection in organomineral complexes limits decomposition. H3: Chemical protection due to recalcitrance of subsurface SOM limits decomposition. The experiment testing H2 will also yield information on the chemical protection of subsurface SOM. The equilibrium rate of mineralization of the non-mineral associated carbon can be used to infer the degree of chemical protection of SOM. The initial oxidation of high molecular weight organic matter into lower molecular weight organic matter may be the limiting step in microbial mineralization of organic matter, preventing the production of CO2 from recalcitrant OM. I will test this hypothesis by using hydrogen peroxide to chemically oxidize the SOM of sterile soils, effectively reducing organic matter into components with lower molecular weight. I will then re-inoculate the hydrogen peroxide treated soils, and follow the time course of microbial mineralization. Sterile soils will also be inoculated without the peroxide treatment, and the difference between these samples and the peroxide treated samples will provide some insight into the mechanisms protecting SOM from microbial mineralization. H4: Nutrient availability to microbes limits microbial mineralization of SOM. Previous work in the Schimel lab has found that even in some subsurface soils microbes exhibit the classic signs of nutrient limitation, as assessed by nutrient addition experiments (Fierer 2003). I will add N and P to soil samples through the profiles of the three different soils, and look at the effect on microbial mineralization rates. If mineralization rates are limited by limitations on their ability to produce the physiological machinery to produce the exoenzymes and other components necessary for breaking down SOM, then addition of that nutrient should lead to an increase in cumulative C mineralization and mineralization rates. References: Batjes, N. H. (1996). ?Total carbon and nitrogen in the soils of the world.? European journal of soil science. 47: 151-163. Brady, N. C. and R. R. Weil (2002). The Nature and Properties of Soils. Upper Saddle River, NJ, Prentice Hall. Fierer, N. (2003). Stress ecology and the dynamics of microbial communities and processes in soil. Ecology, Evolution, and Marine Biology. Santa Barbara, University of California, Santa Barbara: 226. K?gel-Knabner, I. (1997). ?13C and 15N NMR spectroscopy as a tool in soil organic matter studies.? Geoderma 80: 243-270. Qualls, R. G. (2000). ?Comparison of the behavior of soluble organic and inorganic nutrients in forest soils.? Forest Ecology and Management 138(1-3): 29-50. Trumbore, S. E. (2000). ?Age of soil organic matter and soil respiration: Radiocarbon constraints on belowground C dynamics.? Ecological Applications 10: 399-411.

Visit #2116 @Sedgwick Reserve

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Under Project # 1692 | Research

Controls on carbon mineralization in subsurface soils in California annual grasslands

graduate_student - University of California, Santa Barbara

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Group of 2 Graduate Student Jul 1, 2003 - Jun 30, 2004 (366 days)

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Day Use Only 2 Jul 1, 2003 - Jun 30, 2004