Soil organic matter fractions as early indicators for carbon stock changes under different land-use? |
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| Institution: | 1. State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, China;2. Graduate University of Chinese Academy of Sciences, Beijing 100049, China;3. Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20740, USA;1. Department of Natural Resource Ecology and Management, Iowa State University, Ames, IA 50011, USA;2. National Laboratory for Agriculture and the Environment, USDA Agriculture Research Service, Ames, IA 50011, USA;3. Northern Research Station, USDA Forest Service, Grand Rapids, MN 55744, USA;1. State Environmental Protection Key Laboratory of Wetland Ecology and Vegetation Restoration, Institute for peat & mire Research, Northeast Normal University, Changchun, Jilin, China;2. College of Urban and Environmental Sciences, Northwest University, Xi''an, Shanxi, China;3. College of Geography and Environmental Science, Hainan Normal University, Haikou, Hainan, China |
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| Abstract: | With respect to carbon sequestration in soil, attempts have been made to identify soil organic matter (SOM) fractions that respond more rapidly to changes in land-use than bulk SOM, which could thus serve as early indicators for the overall stock change. We used a combination of physical fractionation (size and density separation) and chemical characterisation (C-to-N ratios, CuO lignin signature, 13C NMR spectroscopy) to identify sensitive SOM fractions in an agricultural system with sandy dystric cambisols in Bavaria, Germany, 7 years after a land-use change. Land-use types included long-term arable land and grassland, and conversion from one system to the other. Soil carbon and nitrogen contents in 0–3 cm increased from 14 to 39 mg organic carbon g?1 soil, and from 1.7 to 3.9 mg nitrogen g?1 soil in the following order: permanent arable, conversion grassland to arable, conversion arable to grassland, and permanent grassland. Wet sieving and ultrasonic dispersion with 22 J ml?1 released <5% and 60% to 80%, respectively, of the amount of particles >20 μm relative to complete dispersion. The most sensitive fraction, with respect to land-use, was SOM in the fraction >20 μm not released after sequential wet sieving and ultrasonic dispersion. In contrast, the proportion of free light (wet sieving, density <1.8 g cm?3) and occluded light (ultrasonic dispersion with 22 J ml?1, <1.8 g cm?3) particulate organic matter (POM) showed no clear response to land-use. The structural composition of POM indicated its vegetation origin with a selective enrichment of lignin and a loss of O-alkyl C relative to its plant precursors. Decomposition of the occluded light POM was only slightly advanced relative to the free light POM. In mineral fractions <20 μm, SOM was significantly more transformed than in the coarse fractions, as shown by NMR spectroscopy; however, it revealed no specific land-use pattern. An exception to this was the proportion of O-alkyl C in the clay fraction, which increased with SOC content. Ratios of alkyl to O-alkyl C in mineral fractions <20 μm differentiated samples gave a better differentiation of samples than the C-to-N ratios. We conclude that neither free nor occluded light POM are appropriate early indicators for changes in land-use at the investigated sites; however, total SOM, its distribution with depth, and SOM allocated in stable aggregates >20 μm were more sensitive. |
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