Soil aggregation and the stabilization of organic carbon as affected by erosion and deposition

The importance of soil aggregation in determining the dynamics of soil organic carbon (SOC) during erosion, transportation and deposition is poorly understood. Particularly, we do not know how aggregation contributes to the often-observed accumulation of SOC at depositional sites. Our objective was to assess how aggregation affects SOC stabilization in comparison to interactions of SOC with minerals. We determined and compared aggregate size distributions, SOC distribution in density fractions, and ligninderived phenols from aggregated soil samples at both eroding and depositional sites. The stabilization effect of aggregation was quantified by comparing mineralization from intact and crushed macroaggregates. Deposition of eroded soil material resulted in carbon (C) enrichment throughout the soil profile. Both macro-aggregate associated SOC and C associated with minerals (heavy fraction) increased in their importance from the eroding to the depositional site. In the uppermost topsoil (0e5 cm), SOC mineralization from intact aggregates was larger at the depositional site than at the eroding site, reflecting the large input of labile organic matter (plant residues) promoting aggregation. Contrastingly, in the subsoil, mineralization rates were lower at the depositional site because of effective stabilization by interactions with soil minerals. Aggregate crushing increased SOC mineralization by 10e80% at the eroding site, but not at the depositional site. The content of lignin-derived phenols did not differ between eroding and depositional sites in the topsoil (24.6e30.9 mg per g C) but was larger in the subsoil of the eroding site, which was accompanied by higher lignin oxidation. Lignin data indicated minor effects of soil erosion and deposition on the composition of SOC. We conclude that SOC is better protected in aggregates at the eroding than at the depositional site. During transport disaggregation and consequently SOC mineralization took place, while at the depositional site re-aggregation occurred mainly in the form of macro-aggregates. However, this macro-aggregation did not result in a direct stabilization of SOC. We propose that the occlusion of C inside aggregates serves as a pathway for the eroded C to be later stabilized by organo-mineral interaction

Soil Organic Carbon Redistribution by Water Erosion - The Role of CO2 Emissions for the Carbon Budget

A better process understanding of how water erosion influences the redistribution of soil organic carbon (SOC) is sorely needed to unravel the role of soil erosion for the carbon (C) budget from local to global scales. The main objective of this study was to determine SOC redistribution and the complete C budget of a loess soil affected by water erosion. We measured fluxes of SOC, dissolved organic C (DOC) and CO2 in a pseudo-replicated rainfall-simulation experiment. We characterized different C fractions in soils and redistributed sediments using density fractionation and determined C enrichment ratios (CER) in the transported sediments. Erosion, transport and subsequent deposition resulted in significantly higher CER of the sediments exported ranging between 1.3 and 4.0. In the exported sediments, C contents (mg per g soil) of particulate organic C (POC, C not bound to soil minerals) and mineral-associated organic C (MOC) were both significantly higher than those of non-eroded soils indicating that water erosion resulted in losses of C-enriched material both in forms of POC and MOC. The averaged SOC fluxes as particles (4.7 g C m−2 yr−1) were 18 times larger than DOC fluxes. Cumulative emission of soil CO2 slightly decreased at the erosion zone while increased by 56% and 27% at the transport and depositional zone, respectively, in comparison to non-eroded soil. Overall, CO2 emission is the predominant form of C loss contributing to about 90.5% of total erosion-induced C losses in our 4-month experiment, which were equal to 18 g C m−2. Nevertheless, only 1.5% of the total redistributed C was mineralized to CO2 indicating a large stabilization after deposition. Our study also underlines the importance of C losses by particles and as DOC for understanding the effects of water erosion on the C balance at the interface of terrestrial and aquatic ecosystems

Mineralization of Eroded Organic Carbon Transported from a Loess Soil into Water

The fate of soil-derived organic C (SOC) transported during erosion is a large uncertainty in assessing the impact of soil erosion on aquatic environments and in balancing C budgets. In our study, we determined C mineralization from solid soil organic C and dissolved organic C (DOC) translocated from a loess soil into surface water. We used runoff generated during rainfall simulation experiments. Both total runoff C and DOC were incubated to measure CO2 evolution during 28-d experiments. Cumulative CO2 emissions from runoff accounted for 3.9 to 4.8% of initial runoff C. It was estimated that 3.3 to 3.7% of initial solid SOC was mineralized contributing to 69 to 80% of total C mineralization from runoff. Mineralization of DOC was larger (7.3-30.2% of initial DOC) and showed a much larger variability than mineralization from solid SOC. However, DOC mineralization contributed to 20 to 31% of total C mineralization from runoff only because of the much smaller amounts of DOC than solid SOC. We could confirm a preferential removal of labile C from soils by water erosion. Nevertheless, the majority of this C will contribute to an aquatic C sink with less than 5% being potentially mineralizable. Our results indicated that the base level of C mineralization from translocated C was derived from the solid phase whereas the variability depends largely on DOC

Predictors of the incidence of all-cause mortality and deaths due to diabetes and renal diseases among patients newly prescribed antihypertensive agents: A cohort study

<b>Background</b> Randomized trials have shown that the major antihypertensive drug classes are similarly effective to reduce mortality, but whether these drug class difference exists in clinical practice has been scarcely explored. This study evaluated the association between antihypertensive drug class, all-cause mortality and deaths due to diabetes or renal disease in real-life clinical settings.<p></p> <b>Methods</b> A clinical database in Hong Kong included all patients who were prescribed their first-ever antihypertensive agents between 2001 and 2005 from the public healthcare sector. All patients were followed up for five years, and grouped according to the initial antihypertensive prescription. The associations between antihypertensive drug class, all-cause mortality or combined diabetes and renal mortality, respectively, were evaluated by Cox proportional hazard models.<p></p> <b>Results</b> From 218,047 eligible patients, 33,288 (15.3%) died within five years after their first-ever antihypertensive prescription and among which 1055 patients (0.48%) died of diabetes or renal disease. After adjusted for age, gender, socioeconomic status, service settings, district of residence, medication adherence, and the number of comorbidities, each drug class was similarly likely to be associated with mortality due to diabetes or renal disease [Adjusted Hazard Ratios (AHR) ranged from 0.92 to 1.73, p = 0.287–0.939] and all-cause mortality (AHR ranged from 0.83 to 1.02) except for beta-blockers (AHR = 0.815, 95% C.I. 0.68–0.87, p = 0.024) when ACEI was used as a reference group in propensity score-adjusted analysis.<p></p> <b>Conclusions</b> These findings provide real-life evidence reinforcing that any major antihypertensive drug class is suitable as a first-line agent for management of hypertension as recommended by international guidelines

Predictors of the incidence of all-cause mortality and deaths due to diabetes and renal diseases among patients newly prescribed antihypertensive agents: a cohort study

<b>Background</b> Randomized trials have shown that the major antihypertensive drug classes are similarly effective to reduce mortality, but whether these drug class difference exists in clinical practice has been scarcely explored. This study evaluated the association between antihypertensive drug class, all-cause mortality and deaths due to diabetes or renal disease in real-life clinical settings.<p></p> <b>Methods</b> A clinical database in Hong Kong included all patients who were prescribed their first-ever antihypertensive agents between 2001 and 2005 from the public healthcare sector. All patients were followed up for five years, and grouped according to the initial antihypertensive prescription. The associations between antihypertensive drug class, all-cause mortality or combined diabetes and renal mortality, respectively, were evaluated by Cox proportional hazard models.<p></p> <b>Results</b> From 218,047 eligible patients, 33,288 (15.3%) died within five years after their first-ever antihypertensive prescription and among which 1055 patients (0.48%) died of diabetes or renal disease. After adjusted for age, gender, socioeconomic status, service settings, district of residence, medication adherence, and the number of comorbidities, each drug class was similarly likely to be associated with mortality due to diabetes or renal disease [Adjusted Hazard Ratios (AHR) ranged from 0.92 to 1.73, p = 0.287–0.939] and all-cause mortality (AHR ranged from 0.83 to 1.02) except for beta-blockers (AHR = 0.815, 95% C.I. 0.68–0.87, p = 0.024) when ACEI was used as a reference group in propensity score-adjusted analysis.<p></p> <b>Conclusions</b> These findings provide real-life evidence reinforcing that any major antihypertensive drug class is suitable as a first-line agent for management of hypertension as recommended by international guidelines