Piping in loose sands: the importance of geometrical fixity of grains

Piping is one of the possible failure mechanism for dams and levees with a sandy foundation. Water flowing through the foundation causes the onset of grain transport, due to which shallow pipes are formed at the interface of the sandy layer and an impermeable blanket layer. In the past, the mechanism has been investigated predominantly in densely packed sands, in which the process was observed to start at the downstream side (backward erosion). Recently performed experiments in loose sand (van Beek et al. 2009) showed a different failure mechanism (forward erosion). In this article additional experiments of piping in loose sands are described for investigating the relevance of the forward process for practice. In these experiments the type of process was found to be dependent on the presence of shear resistance between sand box cover and top sand grains, that causes grains to be fixed geometrically. Without this shear resistance the process was found to be forward, whereas with this shear resistance the process was found to be backward oriented. The change in degree of fixity and relative density as a result of loading is investigated with electrical density measurements. The experiments show that the forward process is not relevant for levees in practice, in which the cohesive blanket layer causes the sand grains to be fixed properly

FeEDDHA-facilitated Fe uptake in relation to the behaviour of FeEDDHA components in the soil-plant system as a function of time and dosage.

FeEDDHA products are widely used to prevent and remedy Fe chlorosis in crops grown on calcareous soils. These products consist of a mixture of FeEDDHA components: racemic o,o-FeEDDHA, meso o,o-FeEDDHA, o,p-FeEDDHA and rest-FeEDDHA. The FeEDDHA components differ in physical and chemical properties, and as a consequence also in effectiveness as Fe fertilizer. In order to efficiently match dose, frequency and moment of FeEDDHA application with the Fe requirements of plants, it is important to understand the behaviour of the FeEDDHA components in the soil-plant system as a function of time and dosage, and to relate this behaviour to Fe uptake by plants. These issues have been examined in a pot trial study with soybean plants (Glycine max (L.) Merr. cv Mycogen 5072) grown on calcareous soil from Santomera, Spain. Four FeEDDHA treatments (two compositions, two dosages) were applied prior to the set in of chlorosis. Leaching of FeEDDHA components was prevented. Plant and soil were sampled every week, for six weeks. From one week onward the Fe concentration in the pore water was largely gouverned by racemic and meso o,o-FeEDDHA. The concentration behaviour of the o,o-FeEDDHA isomers underwent two stages: a strong decline within the first week resulting from linear adsorption, and a gradual decline from one week onward. For meso o,o-FeEDDHA, unlike racemic o,o-FeDDHA, the gradual decline could be mathematically well described with an exponential decay function. Soybean plants mainly took up Fe in the progressed vegetative stage (3rd and 4th week) and in the reproductive stage, when the pods were being filled with seeds (6th week). Fe uptake and removal of racemic o,o-FeEDDHA from the soil system display a similar time-trend, whereas the removal of meso o,o-FeEDDHA had a plant-independent character. This indicates the removal of racemic o,o-FeEDDHA is to a larger extent plant-relate

FeEDDHA-facilitated Fe uptake in relation to the behaviour of FeEDDHA components in the soil-plant system as a function of time and dosage.

FeEDDHA products are widely used to prevent and remedy Fe chlorosis in crops grown on calcareous soils. These products consist of a mixture of FeEDDHA components: racemic o,o-FeEDDHA, meso o,o-FeEDDHA, o,p-FeEDDHA and rest-FeEDDHA. The FeEDDHA components differ in physical and chemical properties, and as a consequence also in effectiveness as Fe fertilizer. In order to efficiently match dose, frequency and moment of FeEDDHA application with the Fe requirements of plants, it is important to understand the behaviour of the FeEDDHA components in the soil-plant system as a function of time and dosage, and to relate this behaviour to Fe uptake by plants. These issues have been examined in a pot trial study with soybean plants (Glycine max (L.) Merr. cv Mycogen 5072) grown on calcareous soil from Santomera, Spain. Four FeEDDHA treatments (two compositions, two dosages) were applied prior to the set in of chlorosis. Leaching of FeEDDHA components was prevented. Plant and soil were sampled every week, for six weeks. From one week onward the Fe concentration in the pore water was largely gouverned by racemic and meso o,o-FeEDDHA. The concentration behaviour of the o,o-FeEDDHA isomers underwent two stages: a strong decline within the first week resulting from linear adsorption, and a gradual decline from one week onward. For meso o,o-FeEDDHA, unlike racemic o,o-FeDDHA, the gradual decline could be mathematically well described with an exponential decay function. Soybean plants mainly took up Fe in the progressed vegetative stage (3rd and 4th week) and in the reproductive stage, when the pods were being filled with seeds (6th week). Fe uptake and removal of racemic o,o-FeEDDHA from the soil system display a similar time-trend, whereas the removal of meso o,o-FeEDDHA had a plant-independent character. This indicates the removal of racemic o,o-FeEDDHA is to a larger extent plant-relate

Iron fertilization with FeEDDHA : the fate and effectiveness of FeEDDHA chelates in soil-plant systems

Iron deficiency chlorosis is a nutritional disorder in plants which reduces crop yields both quantitatively and qualitatively, and causes large economic losses. It occurs world-wide, predominantly in plants grown on calcareous soils, as a result of a limited bioavailability of iron related to the poor solubility of iron at high soil-pH (7.5-8.5). Iron fertilizers based on FeEDDHA (iron ethylene diamine-N,N'-bis(hydroxy phenyl acetic acid)) chelates are among the most efficient in preventing and remedying iron deficiency in soil-grown plants. FeEDDHA based fertilizers comprise mixtures of FeEDDHA components which can be divided into racemic o,o-FeEDDHA, meso o,o-FeEDDHA, o,p-FeEDDHA and rest-FeEDDHA. Both the composition of FeEDDHA based fertilizers and the properties of the FeEDDHA components differ considerably. The fate and effectiveness of FeEDDHA chelates in soil-plant systems were examined in order to facilitate a more efficient use of FeEDDHA fertilizer. Upon interaction with soils, racemic and meso o,o-FeEDDHA largely remained in solution, whereas o,p-FeEDDHA and rest-FeEDDHA were largely removed. In plant experiments it was found that, on soils in which plants from the blank treatment became chlorotic, the iron concentration in soil solution, governed by racemic and meso o,o-FeEDDHA, determined iron uptake by plants. When introduced into soil-plant systems, the concentration of racemic and meso o,o-FeEDDHA underwent an initial concentration drop due to adsorption, and gradually declined further. In particular for meso o,o-FeEDDHA, this gradual decline was not related to iron uptake by plants. The gradual decline could not be explained by biodegradation, which did not significantly affect the concentration of FeEDDHA components. From mechanistic multi surface modeling and batch interaction experiments it was concluded that there is a basis for assuming that under soil conditions iron can be displaced from o,o-FeEDDHA components by copper. This displacement reaction was closer examined in goethite suspensions, and it was found to predominantly take place on reactive surfaces rather than in solution. Factors enhancing adsorption also enhanced the rate of the displacement reactions. The observed concentration behaviour of meso o,o-FeEDDHA in soil-plant systems could be explained from the displacement reaction. Furthermore, the effectiveness of EDDHA ligands in chelating and mobilizing iron from soil, after delivering iron at the plant root (the so-called “shuttle mechanism”) was examined. Plant experiments provided experimental support for a re-chelation mechanism, but results from batch interaction experiments indicated that the efficiency in chelating iron from soil is probably low as a result of complexation of other cations, in particular copper. In conclusion a conceptual model for the behaviour of FeEDDHA components in soil-plant systems was composed. The essence of the model consists of three processes: 1) FeEDDHA adsorption, 2) iron displacement from FeEDDHA by copper on a soil reactive surface followed by release of CuEDDHA into soil solution, and 3) re-adsorption of CuEDDHA. Clay content, iron(hydr)oxide content and copper content were identified as soil characteristics substantially compromising the effectiveness of FeEDDHA components

Grazing in a megagrazer-dominated savanna does not reduce soil carbon stocks, even at high intensities

Recent studies suggest that wild animals can promote ecosystem carbon sinks through their impacts on vegetation and soils. However, livestock studies show that intense levels of grazing reduce soil organic carbon (SOC), leading to concerns that rewilding with large grazers may compromise ecosystem carbon storage. Furthermore, wild grazers can both limit and promote woody plant recruitment and survival on savanna grasslands, with both positive and negative impacts on SOC, depending on the rainfall and soil texture contexts. We used grazing lawns in one of the few African protected savannas that are still dominated by megagrazers (> 1000 kg), namely white rhinoceros Ceratotherium simum, as a model to study the impact of prolonged and intense wild grazing on SOC stocks. We contrasted SOC stocks between patches of varying grazing intensity and woody plant encroachment in sites across different rhino habitat types. We found no differences in SOC stocks between the most- and least grazed plots in any of the habitats. Intermediately grazed plots, however, had higher SOC stocks in the top 5 cm compared to most and least grazed plots, but only in the closed-canopy woodland habitat and not in the open habitats. Importantly, we found no evidence to support the hypothesis that wild grazing reduces SOC, even at high grazing intensities by the world's largest megagrazer. Compared to the non-encroached reference plots, woody encroached plots had higher SOC stocks in soils with low clay content and lower SOC stocks in soils with high clay content, although only in the top 5 cm. Accordingly, our study highlights that wild grazers may influence SOC indirectly through their impact on tree-grass ratios in grassy ecosystems. Our study thus provides important insights for future natural climate solutions that focus on wild grazer conservation and restoration.Keywords: fire, grazing impact, rewilding, soil carbon, white rhinoceros, woody encroachmen