Abstract
Manipulative experiments—characterized by the comparing treatments to controls—are widespread in scientific investigations. This study uses experimental micropedology to investigate whether soil microbes precipitate carbonate if a liquid growth-medium is applied to soil in situ. This was undertaken using apparatuses designed to (1) obtain micromorphological images of biogenic carbonate on microscope slides, (2) to quantify carbonate formation in fiberglass cloths, and (3) to measure associated carbon-isotope fractionations. The apparatuses were buried and harvested at monthly intervals from December 2010 to June 2011. The study was conducted along an ecological transect in New Mexico, USA, at three sites: a low-elevation desert (C3 shrubs), an intermediate-elevation steppe (C4 grasses), and a high-elevation forest (C3 conifers). In addition to comparing bioclimatic zones, the effect of parent material was also tested using paired limestone and igneous soils at each site. Microscope slides were analyzed with binocular, petrographic, and scanning electron microscopy equipped with an x-ray microanalyser (EDS), and the fiberglass traps were analyzed with x-ray diffraction and a mass spectrometer for carbon concentrations and isotope ratios. Naturally occurring calcified microbes were found at each site in the form of calcified hyphae, needle fiber, and calcified root hairs, with the exception of the forest site on igneous parent material. Liquid growth medium induced microbial calcification regardless of whether the vegetation was desert shrubs, grassland, or forest, and regardless of whether the parent material was igneous or limestone. Thus, the ability of soil microorganisms to biomineralize carbonate when supplied with liquid growth medium in situ is a phenomenon that crosses biomes and is not limited to microbes endemic to either limestone or igneous parent material.
Highlights
Micropedology, like pedology, studies soil as a natural phenomenon, taking into account its composition, distribution and method of formation (FitzPatrick 1972, 1993)
This study addresses questions concerned with how widespread carbonate biomineralization is in different, but adjacent, bioclimatic zones and parent materials
Soil micromorphology studies of soil carbonate have been observational and based on thin section petrography, similar to mineralogy studies conducted by geologists (e.g., Drees and Wilding 1987; Khormali et al 2006)
Summary
Micropedology, like pedology, studies soil as a natural phenomenon, taking into account its composition, distribution and method of formation (FitzPatrick 1972, 1993). Micropedology and pedology differ from other disciplines of soil science because they work with undisturbed soil in its natural environment. Soil samples must be taken to the lab and “a soil sample really is not soil” (Kubiëna 1970). Fabric analysis using petrographic methods, like profile analysis using field methods, is a descriptive science (Brewer 1964; Bullock et al 1985; Stoops 2003). This study takes a manipulative-experimental approach used in many agricultural, forestry, and environmental studies by comparing treatments with controls. Rather than taking samples to the lab, it brings the microscope slides to the natural soil in the field. The focus of this study (i.e., the response variable of this manipulative experiment) is soil carbonate
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