Magnitude of nitrate turbulent diffusion in contrasting marine environments

Difficulties to quantify ocean turbulence have limited our knowledge about the magnitude and variability of nitrate turbulent diffusion, which constitutes one of the main processes responsible for the supply of nitrogen to phytoplankton inhabiting the euphotic zone. We use an extensive dataset of microturbulence observations collected in contrasting oceanic regions, to build a model for nitrate diffusion into the euphotic zone, and obtain the first global map for the distribution of this process. A model including two predictors (surface temperature and nitrate vertical gradient) explained 50% of the variance in the nitrate diffusive flux. This model was applied to climatological data to predict nitrate diffusion in oligotrophic mid and low latitude regions. Mean nitrate diffusion (~ 20 Tmol N y−1) was comparable to nitrate entrainment due to seasonal mixed-layer deepening between 40°N–40ºS, and to the sum of global estimates of nitrogen fixation, fluvial fluxes and atmospheric deposition. These results indicate that nitrate diffusion represents one of the major sources of new nitrogen into the surface ocean in these regions.

Disentangling Turbulent Gas Diffusion from Non-diffusive Transport in the Boundary Layer

An analysis based on the law of linear momentum conservation demonstrates unequivocally that the mass fraction is the scalar whose gradient determines gas diffusion, both molecular and turbulent. It illustrates sizeable errors in previous micrometeorological definitions of the turbulent gas flux based on fluctuations in other scalars such as the mixing ratio or density. In deference to conservation law, we put forth a new definition for the turbulent gas flux. Net gas transport is then defined as the sum of this turbulent flux with systematic transport by the mean flow. This latter, non-diffusive flux is due to the net upward boundary-layer momentum, a Stefan flow forced by evaporation, which is the dominant surface gas exchange. A comparison with the traditional methodology shows exact agreement between the two methods regarding the net flux, but with the novelty of partitioning gas transport according to distinct physical mechanisms. The non-diffusive flux is seen to be non-negligible in general, and to dominate turbulent transport under certain conditions, with broad implications for boundary-layer meteorology.

Enhanced-Efficiency Phosphorous Fertilizer Impacts on Corn and Common Bean Crops and Soil Phosphorus Diffusion

Phosphorus (P) supply to crops is a major constraint on the quantity and quality of food production in tropical soils, which are often characterized by highly weathered soils having low phosphorus use efficiency. Increasing P fertilizer use efficiency is a good tool for increasing food production to feed an increasing world population. Enhanced efficiency P fertilizer is used to achieve this goal. The lack of information about soil P diffusive flux and corn and common bean yield response with increased efficiency P fertilizers justify studies to evaluate the performance of this type of fertilizer. The aims of this study were to evaluate P diffusive flux and corn and common bean crop response to P sources and rates. A laboratory trial was carried out to evaluate soil P diffusive flux in response to P sources (MAP and Policote coated MAP). Field trials were carried out to evaluate corn and common bean yields in response to P sources (MAP and Policote coated MAP) and rates. Policote, an additive based on water-soluble polymers, is an anionic copolymer with iron an aluminum affinity. Increasing contact time between P-fertilizers and soil reduced phosphorus diffusive flux. Policote coated P-fertilizer resulted in higher phosphorus diffusion than conventional phosphorus fertilizer. Phosphorus fertilization increased corn and common bean yields. Policote coated phosphorus fertilizer resulted in higher agronomic P efficiency use, corn, and common bean yields than conventional phosphorus fertilizer. Policote coated P fertilizer can be used as an enhanced efficiency fertilizer and is a more efficient way to deliver required phosphorous to plants. Reducing farm investment, increasing agricultural profits, preserve phosphatic rocks reserves, and avoid the overuse of phosphate fertilizer could be realized through the rational use of enhanced efficiency fertilizers and fertilizer rate use reduction.

Impacts of a deep reactive layer on sedimentary phosphorus dynamics in a boreal lake recovering from eutrophication

Using biogeochemical analyses of sediments and porewaters, we investigate the legacy of a brief, intense period of eutrophication on sedimentary phosphorus (P) cycling in a boreal lake (Enonselkä basin, Lake Vesijärvi, Finland). Point-source sewage inputs in the twentieth century caused deoxygenation of the lake and accelerated the focusing of iron (Fe) and manganese (Mn) oxides into deeper areas. Early diagenesis under Fe–Mn-rich conditions now favors rapid burial of P in these areas, likely as a combination of both oxide-bound P phases and authigenic manganous vivianite. A new P budget for Enonselkä basin shows that P burial causes an annual drawdown of 1.2% (± 0.2%) of the surface sediment P inventory, supporting a long-term trend towards recovery since the construction of a wastewater treatment plant in the mid-1970s. However, remineralization of organic matter and associated dissolution of Fe–Mn oxides continues to regenerate P from a deep reactive layer (20–60 cm depth) deposited at the height of past eutrophication, leading to an upwards diffusive flux of dissolved phosphate towards the surface sediments. The magnitude of this flux is similar to that of external P loading to the lake. The combined incoming fluxes of P are likely to retard the complete recovery from eutrophication by decades, despite ongoing restoration actions.

Crossover dynamics from superdiffusion to subdiffusion: Models and solutions

The Cattaneo or telegrapher’s equation describes the crossover from initial ballistic to normal diffusion. Here we study and survey time-fractional generalisations of this equation that are shown to produce the crossover of the mean squared displacement from superdiffusion to subdiffusion. Conditional solutions are derived in terms of Fox H-functions and the δth-order moments as well as the diffusive flux of the different models are derived. Moreover, the concept of the distribution-like is proposed as an alternative to the probability density function.