Natural 13C abundance as a tracer for studies of soil organic matter dynamics

A method for measuring the long- and medium-term turnover of soil organic matter is described. Its principle is based on the variations of 13C natural isotope abundance induced by the repeated cultivations of a plant with a high 13C/12C ratio (C4 photosynthetic pathway) on a soil which has never carried any such plant. The 13C/12C ratio in soil organic matter being about equal to the 13C/12C ratio of plant materials from which it is derived, changing the 13C content of the organic inputs to the soil (by altering vegetation from C3 type into C4 type) is equivalent to a true labelling in situ of the organic matter. Two cases of continuous corn cultivation (Zea mays: δ13C = −12%.) on soils whose initial organic matter average δ13C is −26%. were studied. The quantity of organic carbon originating from corn (that is the quantity which had turned-over since the beginning of continuous cultivation) was estimated using the 13C natural abundance data. After 13 yr, 22% of total organic carbon had turned-over, in the system studied. Particle size fractions coarser than 50μm on the one hand, and finer than 2μm on the other. contained the youngest organic matters. The turnover rate of silt-sized fractions was slower

Methods for physical separation and characterization of soil organic matter fractions

Turnover of soil organic matter (SOM) is coupled to the cycling of nutrients in soil through the activity of soil microorganisms. Biological availability of organic substrate in soil is related to the chemical quality of the organic material and to its degree of physical protection. SOM fractions can provide information on the turnover of organic matter (OM), provided the fractions can be related to functional or structural components in soil. Ultrasonication is commonly used to disrupt the soil structure prior to physical fractionation according to particle size, but may cause redistribution of OM among size fractions. The presence of mineral particles in size fractions can complicate estimations of OM turnover time within the fractions. Densiometric separation allows one to physically separate OM found within a specific size class from the heavier-density mineral particles. Nutrient contents and mineralization potential were determined for discrete size/density OM fractions isolated from within the macroaggregate structure of cultivated grassland soils. Eighteen percent of the total soil C and 25% of the total soil N in no-till soil was associated with fine-silt size particles having a density of 2.07-2.21 g/cm3 isolated from inside macroaggregates (enriched labile fraction or ELF). The amount of C and N sequestered in the ELF fraction decreased as the intensity of tillage increased. The specific rate of mineralization (mug net mineral N/mug total N in the fraction) for macroaggregate-derived ELF was not different for the three tillage treatments but was greater than for intact macroaggregates. The methods described here have improved our ability to quantitatively estimate SOM fractions, which in turn has increased our understanding of SOM dynamics in cultivated grassland systems.

Effects of soil pH and organic matter on distribution of thorium fractions in soil contaminated by rare-earth industries

The labilities of thorium fractions including mobility and bioavailability vary significantly with soil properties. The effects of soil pH and soil organic matter on the distribution and transfer of thorium fractions defined by a sequential extraction procedure were investigated. Decrease of soil pH could enhance the phytoavailability and the potential availability of thorium in soil. Increase of organic matter reduced the phytoavailability of thorium, but enhanced the potential availability of it.

Contrasting chemical and isotopic compositions of organic matter in Changjiang (Yangtze river) estuarine and East China Sea shelf sediments

To examine the source and preservation of organic matter in the shelf sediments of the East China Sea (ECS), we measured bulk C/N and isotopes, organic biomarkers (n-alkanes and fatty acids) and compound-specific (fatty acids) stable carbon isotope ratios in three sediment cores collected from two sites near the Changjiang Estuary and one in the ECS shelf. Contrasting chemical and isotopic compositions of organic matter were observed between the estuarine and shelf sediments. The concentrations of total n-alkanes and fatty acids in the shelf surface sediments (0-2 cm) were 5-10 times higher than those in estuarine surface sediments but they all decreased rapidly to comparable levels below the surface layer. The compositions of n-alkanes in the estuarine sediments were dominated by C-26-C-33 long-chain n-alkanes with a strong odd-to-even carbon number predominance. In contrast, the composition of n-alkanes in the shelf sediment was dominated by nC(15) to nC(22) compounds. Long-chain (> C-20) fatty acids (terrestrial biomarkers) accounted for a significantly higher fraction in the estuarine sediments compared to that in the shelf sediment, while short-chain (< C-20) saturated and unsaturated fatty acids were more abundant in the shelf surface sediments than in the estuarine sediments. Stable carbon isotopic ratios of individual fatty acids showed a general positive shift from estuarine to shelf sediments, consistent with the variations in bulk delta(CTOCTOC)-C-13. These contrasts between the estuarine and shelf sediments indicate that terrestrial organic matter was mainly deposited within the Changjiang Estuary and inner shelf of ECS. Post-depositional diagenetic processes in the surface sediments rapidly altered the chemical compositions and control the preservation of organic matter in the region.

Sources and preservation of organic matter in Plum Island salt marsh sediments (MA, USA): long-chain n-alkanes and stable carbon isotope compositions

Elemental (TOC, TN, C/N) and stable carbon isotopic (delta(13)C) compositions and n-alkane (nC(16-38)) concentrations were measured for Spartina alterniflora, a C-4 marsh grass, Typha latifolia, a C-3 marsh grass, and three sediment cores collected from middle and upper estuarine sites from the Plum Island salt marshes. Our results indicated that the organic matter preserved in the sediments was highly affected by the marsh plants that dominated the sampling sites. delta(13)C values of organic matter preserved in the upper fresh water site sediment were more negative (-23.0+/-0.3) as affected by the C-3 plants than the values of organic matter preserved in the sediments of middle (-18.9+/-0.8) and mud flat sites (-19.4+/-0.1) as influenced mainly by the C4 marsh plants. The distribution of n-alkanes measured in all sediments showed similar patterns as those determined in the marsh grasses S. alterniflora and T. latifolia, and nC(21) to nC(33) long-chain n-alkanes were the major compounds determined in all sediment samples. The strong odd-to-even carbon numbered n-alkane predominance was found in all three sediments and nC(29) was the most abundant homologue in all samples measured. Both delta(13)C compositions of organic matter and n-alkane distributions in these sediments indicate that the marsh plants could contribute significant amount of organic matter preserved in Plum Island salt marsh sediments. This suggests that salt marshes play an important role in the cycling of nutrients and organic carbon in the estuary and adjacent coastal waters. (C) 2003 Elsevier Ltd. All rights reserved.

Organic geochemical studies of sinking particulate material in China sea area(I) - Organic matter fluxes and distributional features of hydrocarbon compounds and fatty acids

Sinking particulate material collected from Nansha Yongshu reef lagoon and the continental shelf of the East China Sea by sediment traps has been analyzed and studied for the first time using organic geochemical method. The results show that about half of the sinking particulate organic matter in the two study areas are consumed before reaching the depth of 5 m to the sea floor and the degree of this consumption in Yongshu reef lagoon is larger than that in the continental shelf of the East China Sea. The distributions of hydrocarbons and fatty acids indicate that the minor difference of biological sources of sinking particulate organic matter exists between Yongshu reel lagoon and the continental shelf of the East China Sea, but they mainly come from marine plankton. Stronger biological and biochemical transformations of sinking particulate organic matter are also observed and the intensity of this transformation in Yongshu reef lagoon is greater than that in the continental shelf of the East China Sea. It is found that the occurrence of C-25 highly branched isoprenoid (HBI) diene may be related to the composition of diatom species.

Three dimensional fluorescence excitation-emission on matrix spectrum of dissolved organic substance in marine microalgaes growth process

The three-dimensional fluorescence spectrum was used to detect the changes in dissolved organic substances from the cultured Skeletonema costatum, Alexandrium tamarense, Alexandrium mimutum, Scrippsiella trochodea, Prorocentrum donghaiense and Prorocentrum micans. The result indicates that all of the microalgaes can produce FDOM in the growth courses. Diatom such as Skeletonema costatum can produce humic-like FDOM. However dinoflagellate can produce protein-like FDOM at exponential growth phase. When the algae grows into decadency phase, the intensity of humic-like and protein-like fluorescence augments rapidly, which may be due to a mass of FDOM realeased by the old or dead cell fragmentation and the degradation of bacteria by using non-FDOM. The fluorescent intensity of Alexandrium tamarense, Alexandrium mimutum, Prorocentrum donghaiense and Prorocentrum micans can reduce at anaphase of decadency phase because of the degradation of bacteria and light. The same genus of algae can produce similar FDOM, for example: Alexandrium tamarense, and Alexandrium mimutum, Prorocentrum donghaiense and Prorocentrum micans, but the positions of the fluorescence peaks are different.

Independence of alpine meadow plants from soil organic matter for nitrogen supply: evidence from stable nitrogen isotopes

Stable nitrogen isotope signatures of major sources of mineral nitrogen ( mineralization of soil organic nitrogen, biological N-2 fixation by legumes, annual precipitation and plant litter decomposition) were measured to relatively define their individual contribution to grass assimilation at the Haibei Alpine Meadow Ecosystem, Qinghai, China. The results indicated that delta N-15 values (- 2.40 parts per thousand to 0.97 parts per thousand) of all grasses were much lower than those of soil organic matter (3.4 +/- 0.18 parts per thousand) and mineral nitrogen ( ammonium and nitrate together,7.8 +/- 0.57 parts per thousand). Based on the patterns of stable nitrogen isotopes, soil organic matter (3.4 +/- 0.18 parts per thousand), biological N-2 fixation (0 parts per thousand), and precipitation (- 6.34 +/- 0.24 parts per thousand) only contributed to a small fraction of nitrogen requirements of grasses, but plant litter decomposition (- 1.31 +/- 1.01 parts per thousand) accounted for 67%.