Waldir L. Roque

Bio: Waldir L. Roque is an academic researcher from Federal University of Paraíba. The author has contributed to research in topics: Tortuosity & Soil structure. The author has an hindex of 11, co-authored 27 publications receiving 345 citations. Previous affiliations of Waldir L. Roque include University of Rio Grande.

Soil structure changes induced by tillage systems

Abstract: Structure represents one of the main soil physical attributes indicators. The soil porous system (SPS) is directly linked to the soil structure. Water retention, movement, root development, gas diffusion and the conditions for all soil biota are related to the SPS. Studies about the influence of tillage systems in the soil structure are important to evaluate their impact in the soil quality. This paper deals with a detailed analysis of changes in the soil structure induced by conventional (CT) and no-tillage (NT) systems. Three different soil depths were studied (0–10, 10–20 and 20–30 cm). Data of the soil water retention curve (SWRC), micromorphologic (impregnated blocks) (2D) and microtomographic (μCT) (3D) analyses were utilized to characterize the SPS. Such analyses enabled the investigation of porous system attributes such as: porosity, pore number and shape, pore size distribution, tortuosity and connectivity. Results from this study show a tri-modal pore size distribution (PSD) at depths 0–10 and 10–20 cm for the soil under CT and a bi-modal PSD for the lower layer (20–30 cm). Regarding the soil under NT, tri-modal PSDs were found at the three depths analyzed. Results based on the micromorphologic analysis (2D) showed that the greatest contribution to areal porosity (AP) is given by pores of round (R) shape for CT (52%: 0–10 cm; 50%: 10–20 cm; 67%: 20–30 cm). Contrary to the results observed for CT, the soil under NT system gave the greatest contribution to AP, for the upper (0–10 cm) and intermediate (10–20 cm) layers, due to the large complex (C) pore types. For the μCT analysis, several types of pores were identified for each soil tillage system. Small differences in the macroporosity (MAP) were observed for the 0–10 and 20–30 cm between CT and NT. A better pore connectivity was found for the 0–10 cm layer under NT.

X-ray microtomography analysis of representative elementary volume (REV) of soil morphological and geometrical properties

Abstract: The representative elementary volume (REV) is related to the minimum size of a sample that is necessary for representative measurements of a property. The REV is frequently investigated in samples with characteristic sizes extending from millimeters to few centimeters. Most of the time, the obtained information in this kind of study does not reveal any important detail of the inner structure of a porous medium. Something that can only be attained using some high spatial resolution image analysis technique, as, for instance, the X-ray computed microtomography (μCT). In this research, 3D μCT images (60 μm of resolution) were used for investigating the REV for: soil macroporosity (MAP), number of macropores (NMAP), pore tortuosity (τ) and connectivity (Euler-Poincare characteristic, EPC) of a Rhodic Ferralsol under no-tillage (NT) and conventional (CT) tillage systems. Consecutive increasing volumes, centrally and randomly, with volumes ranging from ≈0.40 to ≈36,591 mm³, were selected inside the 3D images. Results showed that MAP and NMAP presented high variability for small investigated subvolumes and for the random selection scheme. The REV for MAP (under NT) was reached at volumes of ≈10,841 mm3 (centered scheme) and ≈17,216 mm3 (random scheme), respectively. For the CT, the REV for MAP occurred at volumes of ≈17,216 mm3 (random and centered schemes). The REV was not reached for NMAP, while the ratio between NMAP and sample subvolumes (here called specific NMAP, NMAPe) presented a REV of ≈17,216 mm3. The tortuosity and EPC were characterized by high variability for smaller subvolumes and random selection scheme. The REV for EPC was not found and for τ it was reached at volumes of ≈17,216 mm3 (both subvolume selection schemes and tillage systems). The results of our work present for the first time the REV definition for soil properties such as NMAPe, τ and EPC at the micrometer scale as well as the effect of tillage systems in the REV. The obtained results surprisingly demonstrated that the REV was not influenced by the tillage systems as well as it was the same for NMAPe, τ and EPC. In addition, the complexity of the soil porous system did not allow the REV determination for EPC, at least for the sample volume here analyzed.

X-ray computed tomography for assessing the effect of tillage systems on topsoil morphological attributes

Abstract: The sustainability and efficiency of crop production are sometimes dependent on the adopted tillage system. In Brazil, conservation systems such as no-tillage (NT) have been adopted as an alternative to conventional tillage (CT) method. Conservation tillage systems usually tend to minimize soil losses, water runoff, preserve soil moisture and reduce the disruption of the soil structure. Nevertheless, the influence of these managements in the soil porous system, in the micrometric point of view, is not presented in most of the published literature. Considering the importance of pores for the proper functioning of the soil, the main objective of this study was to analyze the effect of long-term NT and CT systems, in comparison to non-managed soil maintained under secondary forest (F) conditions on the pore system of a clay soil, using 3D X-ray computed tomography (μCT) images. To achieve this goal, 3D μCT images were obtained from samples of the surface soil (0.05-0.10 m and 0.10-0.15 m). Morphological attributes such as: macroporosity (MAP), number of macropores (NMAP), shape of pores, tortuosity (τ) and connectivity (C), estimated based on the Euler-Poincare characteristic (EPC), were employed to characterize the soil porous system. In addition, saturated hydraulic conductivity (Ks) and soil water retention curve (SWRC) were analyzed. The forest was characterized by the highest MAP. However, when comparing the tillage systems, the CT MAP was larger than that of NT. Ks showed the same trends, which indicates that Ks was directly influenced by MAP. However, SWRC analyses showed a greater volume fraction of mesopores and micropores under NT than under CT. The macropore size distribution reveals that large well-connected macropores (volume >1000 mm3) were responsible for the major contribution to MAP. However, the highest contribution to NMAP was given by macropores with volume up to 0.1 mm3. Tortuosity results indicate less sinuous pores for F, when compared to CT and NT. EPC results present better connectivity of pores in the shallow layer 0.05-0.10 m for F, while the opposite was observed for CT. The present research showed that from 3D μCT images, associated with results of hydraulic properties, it is possible to characterize the macroporous system of undisturbed samples. Research like this is important to infer about the impacts of management systems on soil.

3D analysis of the soil porous architecture under long term contrasting management systems by X-ray computed tomography

Abstract: The development of adequate soil structure is important for achieving good physical status, which influences the sustainability of agricultural areas. Different management systems lead to the development of a wide range of soil pore network characteristics. The objective of this research was to analyze the effect of three contrasting tillage systems (zero-tillage, ZT; reduced tillage, RT; conventional tillage, CT) in the soil porous system of an Oxisol. Samples were collected from the surface layer (0–10 cm). An area under secondary forest (F) was also assessed to provide an undisturbed reference. X-ray Computed Tomography (μCT) scanning of undisturbed soil samples and image analysis were employed for analysis of the pore network. The soil under ZT had the smallest porosity in comparison to the other management systems. The conventionally tilled soil had the largest porosity and the most connected pores. One large connected pore was responsible for around 90% of the porosity of the resolvable pores (>35 μm) studied for all the management systems. Pores of elongated shapes, which enhance water movement through the soil, were the most frequent pores in terms of shape.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Soil structure changes induced by tillage systems

Abstract: Structure represents one of the main soil physical attributes indicators. The soil porous system (SPS) is directly linked to the soil structure. Water retention, movement, root development, gas diffusion and the conditions for all soil biota are related to the SPS. Studies about the influence of tillage systems in the soil structure are important to evaluate their impact in the soil quality. This paper deals with a detailed analysis of changes in the soil structure induced by conventional (CT) and no-tillage (NT) systems. Three different soil depths were studied (0–10, 10–20 and 20–30 cm). Data of the soil water retention curve (SWRC), micromorphologic (impregnated blocks) (2D) and microtomographic (μCT) (3D) analyses were utilized to characterize the SPS. Such analyses enabled the investigation of porous system attributes such as: porosity, pore number and shape, pore size distribution, tortuosity and connectivity. Results from this study show a tri-modal pore size distribution (PSD) at depths 0–10 and 10–20 cm for the soil under CT and a bi-modal PSD for the lower layer (20–30 cm). Regarding the soil under NT, tri-modal PSDs were found at the three depths analyzed. Results based on the micromorphologic analysis (2D) showed that the greatest contribution to areal porosity (AP) is given by pores of round (R) shape for CT (52%: 0–10 cm; 50%: 10–20 cm; 67%: 20–30 cm). Contrary to the results observed for CT, the soil under NT system gave the greatest contribution to AP, for the upper (0–10 cm) and intermediate (10–20 cm) layers, due to the large complex (C) pore types. For the μCT analysis, several types of pores were identified for each soil tillage system. Small differences in the macroporosity (MAP) were observed for the 0–10 and 20–30 cm between CT and NT. A better pore connectivity was found for the 0–10 cm layer under NT.

Testing General Relativity with stellar orbits around the supermassive black hole in our Galactic center

Abstract: We demonstrate that short-period stars orbiting around the supermassive black hole in our Galactic center can successfully be used to probe the gravitational theory in a strong regime. We use 19 years of observations of the two best measured short-period stars orbiting our Galactic center to constrain a hypothetical fifth force that arises in various scenarios motivated by the development of a unification theory or in some models of dark matter and dark energy. No deviation from general relativity is reported and the fifth force strength is restricted to an upper 95% confidence limit of |α|<0.016 at a length scale of λ=150 astronomical units. We also derive a 95% confidence upper limit on a linear drift of the argument of periastron of the short-period star S0-2 of |ω_(S0-2)|<1.6×10^(-3) rad/yr, which can be used to constrain various gravitational and astrophysical theories. This analysis provides the first fully self-consistent test of the gravitational theory using orbital dynamic in a strong gravitational regime, that of a supermassive black hole. A sensitivity analysis for future measurements is also presented.