Turnover Rates Of Organic Carbon Research Articles

Polysaccharide hydrolysis in the presence of oil and dispersants: Insights into potential degradation pathways of exopolymeric substances (EPS) from oil-degrading bacteria

Oceanic oil-degrading bacteria produce copious amounts of exopolymeric substances (EPS) that facilitate their access to oil. The fate of EPS in the water column is in part determined by activities of heterotrophic microbes capable of utilizing EPS compounds as carbon and energy sources. To evaluate the potential of natural microbial communities to degrade EPS produced during oil degradation, we measured potential hydrolysis rates of six structurally distinct polysaccharides in two roller bottle experiments, using water from a natural oil seep in the northern Gulf of Mexico. The suite of polysaccharides used to measure the initial step in carbon degradation is indicative of polymers within microbial EPS. The treatments included (i) unamended surface or deep waters (whole water), and water amended with (ii) a water-accommodated fraction of oil (WAF), (iii) oil dispersant Corexit 9500, and (iv) WAF chemically-enhanced with Corexit (CEWAF). The oil and Corexit treatments were employed to simulate conditions during the Deepwater Horizon oil spill. Polysaccharide hydrolysis rates in the surface-water treatments were lowest in the WAF treatment, despite elevated levels of EPS in the form of transparent exopolymer particles (TEP). In contrast, the three deep-water treatments (WAF, Corexit, CEWAF) showed enhanced hydrolysis rates and TEP levels (WAF) compared to the whole water. We also observed variations in the spectrum of polysaccharide-hydrolyzing enzyme activities among the treatments. These substrate specificities were likely driven by activities of oil-degrading bacteria, shaping the pool of EPS and TEP as well as degradation products of hydrocarbons and Corexit compounds. A model calculation of potential turnover rates of organic carbon within the TEP pool suggests extended residence times of TEP in oil-contaminated waters, making them prone to serve as the sticky matrix for oily aggregates known as marine oil snow.

Carbon cycling of subsurface organic matter recorded in speleothem 14C records: Maximizing bomb-peak model fidelity

Abstract From 1955 to 1963, atmospheric testing of nuclear weapons caused a significant rise in atmospheric radiocarbon activity. This “bomb peak” has been used to calculate turnover rates of organic carbon in soils and other recent sedimentary deposits. Some speleothems contain precise and independently dated records of radiocarbon activity. These records can be used to understand, through inverse modeling, the processes and rates of turnover of subsurface organic carbon in karst regions. This approach is complicated, however, by the contribution of radioactively “dead” carbon to the stalagmite by the dissolution of host-rock limestone and/or by the respiration of relatively old soil organic matter. Previously published inverse models of the radiocarbon bomb peak in speleothems constrain the dead carbon proportion (DCP, in percent) by comparing measurements of speleothem radiocarbon activity from before the onset of the bomb peak to measurements of coeval atmospheric radiocarbon. This approach precludes modeling of speleothems that began growing after the onset of atmospheric nuclear weapons testing in 1955. Here, we advance the inverse modeling framework to calculate DCP using the entire length of the speleothem record, allowing for the modeling of speleothems that began growing after the initiation of atmospheric nuclear weapons testing. We test the sensitivity and resolution of this model and find that it can precisely resolve the turnover times and relative contributions of subsurface organic matter pools with residence times of less than a decade. The model fails to resolve turnover times or relative contributions of organic matter pools on millennial or greater timescales. These results also hold for the previously published models from which the current model is derived. We find that imprecise estimates of slow-turnover carbon add significant uncertainty to the calculated average age of respired carbon, which is a common metric of subsurface carbon cycling. The high precision and resolution attainable between sub-decadal carbon pools will allow researchers to differentiate the (sub-)annual pool, which is likely dominated by root/rhizosphere respiration, from the 2- to 10-year pools, which are likely dominated by microbial decomposition of labile organic carbon. The high precision attainable in fast-turnover pools also suggests that when there are multiple viable chronological interpretations for the same speleothem, bomb peak models could be used to help select which chronology is most likely to be accurate. This is important for high-resolution (sub-annual) speleothem climate records, where even single-year chronological offsets can result in misleading calibrations to the instrumental record.

Organic Matter Turnover in Light Fraction and Whole Soil Under Silvopastoral Land Use in Semiarid Northeast Brazil

Trees in silvopastoral systems can accumulate carbon (C) and nutrients under their canopies. Most studies measure only net changes in organic matter and nutrients without evaluating turnover of soil organic matter . Here, the change in vegetation cover from caatinga , a semideciduous thorn forest (principally C 3 metabolism) to buffel grass ( Cenchrus ciliaris L.) pasture (C 4 metabolism) was used to quantify in situ input and turnover rates of organic carbon 14 yr after land-use changes. The accretion of C under new pasture and loss of original caatinga C was studied for whole soil (WS) and light fraction (LF). The effects of two tree species preserved during selective clearing and one species planted after complete clearing of caatinga were evaluated. All trees prevented organic matter mineralization that occurred in surrounding cleared pasture. The C mineralization under pasture was twice as high (66% loss) in LF as in WS (34% loss) over 14 yr. The C 4 –C was similar under and outside the remnant and planted tree canopies, i.e., the input of new C 4 –C did not compensate for the loss of old C 3 –C that occurred following caatinga clearing and pasture establishment. The organic matter in this tropical, semiarid region mineralized rapidly with C half lives between 9 and 16 yr for LF and between 11 and 28 yr for WS. The 13 C data indicate that elevated C contents under preserved (WS and LF) and planted (LF) trees, relative to the pasture outside the tree canopies, largely represent C 3 –C inherited from the caatinga. In this silvopastoral system, derived from land-use changes from dry forest, the islands of fertility and organic matter under the trees were not built up and represent preserved, rather than new, C inputs. Los árboles en sistemas silvopastoriles pueden acumular carbono (C) y nutrientes debajo de sus doseles. Muchos estudios miden solo cambios netos en la materia orgánica y los nutrientes asociados sin evaluar la transformación actual de la materia orgánica del suelo. En este estudio el cambio en la cobertura vegetativa de la Caatinga, un bosque espinoso semideciduos (principalmente metabolismo C 3 ), a una pastura (metabolismo C 4 ) de grama buffel ( Cenchrus ciliaris L.) fue usado para cuantificar in situ la tasa de entrada y la transformación de carbono orgánico catorce años después del cambio de uso de suelo. El incremento de C debajo de la nueva pastura y la pérdida de carbono original de la Caatinga fueron estudiados para el suelo entero (WS) y la fracción livianas (LF). Los efectos de dos especies de árboles conservados durante la poda selectiva de la Caatinga y unas especies plantadas después de completar la poda de la Caatinga fueron evaluados. Todos los árboles impidieron la mineralización de materia orgánica que ocurrió en los alrededores de la pastura podada. La mineralización de C debajo de la pastura fue dos veces tan alta (66% pérdida) en el LF que en el WS (34% pérdida) através de los 14 años. El C 4 –C fue similar debajo y fuera de los doseles de los árboles remanentes y plantados, i.e., la entrada de nuevo C 4 –C no compenso la perdida de viejo C 3 –C que había ocurrido luego de la poda de la Caatinga y el establecimiento de la pastura. La materia orgánica en esta región tropical semiárida se mineralizó muy rápidamente con medias vidas entre los 9 y 16 años para el LF, y entre 11 y 28 años para el WS. Los datos de 13 C indica que los contenidos de C elevados debajo de los árboles preservados (WS y LF) y los árboles plantados (LF) relativo a la pastura fuera de los doseles de los árboles representa ampliamente el C 3 –C heredado de la Caatinga. En este sistema silvopastoril derivado de un cambio de uso de suelo de un bosque seco, las islas de fertilidad y materia orgánica debajo de los árboles no fueron edificaciones nuevas y representan establecimientos preservados más que nuevas entradas de C.

Mineralization of detritus and oxidation of methane in acid boreal coniferous forest soils: seasonal and vertical distribution and effects of clear-cut

Rates of soil carbon mineralization and CH 4 oxidation potential were measured in the different layers of the podsolized soils (20 cm topsoil) of managed boreal and Pinus sylvestris (age 38 years) and Picea abies (stand age 120 years) forests for eight seasons during the years 1997–1999. Endogenous CO 2 evolution was highest in October both in the P. sylvestris (1800–2200 mg CO 2–C m −2 d −1) and in the P. abies (2900–3000 mg CO 2–C m −2 d −1) forest soils and located mainly in the humus layer, when measured at 7 °C, i.e. close to the annual average soil temperature. The turnover rates of organic carbon were similar in the lower part of the illuvial layer and in the humus, annual mean of 0.027–0.028% C d −1 in the P. sylvestris and P. abies forests, indicating the presence of easily available sources of carbon deeper in the soil. The cumulative annual CO 2 evolution at in situ temperature was calculated as 430–440 g CO 2–C m −2 in the P. sylvestris and 630–650 g CO 2–C m −2 in the P. abies forest soil. Nineteen to twenty-three percent of this occurred during the cold half of the year. When the P. abies forest was clear-cut in 1998 the mineralizing activities of the eluvial and illuvial layers translocated into the humus layer, but the cumulative total CO 2 evolution per m 2 remained unchanged. Clear-cutting increased soil temperature dependence on the air temperature. The sources of the annual CO 2 evolution were above-ground litter (18–26%), and roots (25%) related to the direct photosynthetic activity of the trees in the eluvial and illuvial layers. CH 4 oxidation potential (at 200 ppm CH 4) of the podsolized conifer forest soils ranged from 12 to 22 mg CH 4–C m −2 d −1 (annual mean) in the P. sylvestris and the P. abies forests. The activity mainly located (50–90%) in the illuvial horizons in all seasons. In the upper, 2–3 cm zone, of the illuvial layer, immediately below the eluvial layer, methane oxidation potential was found 5–10 fold higher than in adjacent soil above or below. CH 4 oxidation potential of (g of soil organic carbon) in the forest soils increased 100 fold ( P. abies) or 5–50 fold ( P. sylvestris) with increasing depth. Ten to twenty-four percent of the annual CH 4 oxidation potential at in situ soil temperature was contributed by the cold half of the year similarly in both forests. Clear-cutting the P. abies stand decreased the potential for CH 4 oxidation at in situ temperature by 40% (to 1.8 g CH 4–C m −2 year −1).