ABSTRACT Stony soils have been increasingly used for agriculture production; however, little is known about their hydraulic properties due to problems, such as sample deformation and hydraulic continuity between samples and suction devices when the sampling and measurements are accomplished with traditional techniques. In this study, the traditional ring sampling technique was replaced by the sampling of undisturbed soil blocks coated with paraffin wax to preserve their structure. A saturated paste of fine-grained mineral particles was used to ensure contact and hydraulic continuity between samples and suction devices (sand table and ceramic plates). This allowed us to determine 30 water retention curves for three stony soils with coarse particle contents (> 2 mm) ranging from zero to 69 %. The van Genuchten model was fitted to the measured retention data and the root mean square errors were between 0.0034 and 0.0331 m3 m–3, with no outliers or odd behavior in the retention curves. These results showed that consistent water retention curves for stony soils can be determined with the technique proposed. Fine-grained minerals sandwiched between the surface of suctions sources and sampled blocks improve hydraulic continuity between them. These techniques can be applied to determine water retention properties in structured soil samples with coarse particles where it is unfeasible to collect structured soil samples with metal sampling rings.

ABSTRACT The relation between water content and water potential is a key soil hydraulic property. The presence of rock fragments in soils can affect this property in bulk soil. In this study, we focused on the relation property determined with a WP4 dew point potentiometer. The objective was to evaluate the hypothesis that breaking large gravel-size particles into smaller fragments is a suitable strategy for accurate WP4 measurements in coarse materials without affecting the retention properties. Ten initial samples of basalt rock fragments with different weathering degrees were collected from the A horizon of an Entisol (Neossolo). Rock fragments initially sized between 38 to 4 mm were successively fractured into smaller fragments (size classes of 4-2, 1-0.25, and <0.25 mm). For each size class, the retention properties (water content versus water potential) were determined with WP4 equipment. No significant effect of fragment size was detected on the determined retention properties. Using supplementary water retention data from a previous study, we found an overestimation of water retention at -1.5 MPa ranging from 0.08 to 0.12 g g-1 when rock fragments were removed, which is of the order of magnitude to the available water capacity of several soils. This study showed that fragments larger than the diameter of the WP4 sample cup could be broken to sizes smaller between 4 and 0.25 mm without affecting the retention properties. This is a suitable strategy to allow accurate WP4 measurements in coarse materials while considering the effect of such coarse material on retention properties.

ABSTRACT Finding the proper shaking setting to ensure total particle dispersion without altering the content of sand, silt and clay in soils composed of fragile particles such as saprolite fragments is still a research gap. This study aimed to identify the best shaking setting to optimize particle dispersion and minimize particle change in samples of Entisol (Neossolos). Samples were dispersed using an electric mixer at 12,000 rpm for 5, 15, 30 and 60 min, a horizontal shaker at 150 cycles min-1 for 1, 2, 4, 8 and 16 h with and without the use of nylon spheres, and an ultrasonic shaker at 70 and 110 J mL-1 for approximately 20 and 30 min, respectively. All the treatments affected particle distribution. Coarse sand reduced up to 14 % and clay content increased up to 18 % with the horizontal shaker with nylon spheres and the electric mixer. These changes were attributed to the breakdown of saprolite fragments, which represented around 1/3 of the coarse sand. Our results showed that the best setting was the horizontal shaker at 150 cycles min-1 for 1 to 2 h without using nylon spheres. This option allowed optimizing particle dispersion and minimizing changes in particle size distribution.

RESUMOPara o estudo do ciclo de nutrientes em uma floresta, é necessária a estimação precisa da quantidade de serapilheira depositada e de suas frações. Entre as formas reconhecidas para minimizar a variabilidade entre amostras está o uso do tamanho ótimo de unidades amostrais para a coleta da serapilheira. Entretanto, existem poucas recomendações nesse sentido, o que gera variação no tamanho da unidade amostral utilizada em experimentos similares. Assim, o objetivo deste trabalho foi determinar o tamanho e a forma ótimos da unidade amostral para quantificação de serapilheira em Floresta Estacional Subtropical. Para isso, foram alocadas, aleatoriamente, na área cinco parcelas de dimensões 3,0 x 2,0 m em cada uma das quatro estações do ano, subdivididas em 96 unidades básicas de 0,25 x 0,25 m. Em cada unidade básica foi coletada toda a matéria orgânica acima do solo, sendo esta separada nas frações folhas, galhos finos, resíduo. A estimativa do tamanho ótimo da unidade básica foi obtida por dois métodos distintos. Observou-se que o coeficiente de variação diminui à medida que aumenta o tamanho da unidade amostral; há diferenças na variabilidade da produção de serapilheira entre as parcelas na mesma estação de coleta; unidades amostrais retangulares são mais homogêneas entre si que as quadradas; recomenda-se a utilização de unidades amostrais de 0,40 m2 (1,5 m x 0,25 m) para a coleta de serapilheira em fragmento de Floresta Estacional Subtropical.

ABSTRACTTo study the nutrient cycle in a forest, a precise estimation of the amount of litter produced and its fractions is necessary. Among the recognized ways to minimize variability between samples is the use of optimal sampling size units to litter collection. However, there are a few recommendations found in another studies for this parameter, which generates variation in the size of the sampling unit used in similar experiments. The aim of this work is to determine the optimal size and shape of the sampling unit for quantification of litter in a Subtropical Seasonal Forest fragment. For this purpose, five plots of 3.0 x 2.0 meters in each season were randomly allocated in the area, subdivided into 96 basic units of 0.25 x 0.25 meters. In each basic unit all organic matter above ground was collected and separated in the fractions leaves, twigs and waste. The estimative of the basic units optimal size was obtained by two different methods. It was observed that the coefficient of variation decreases as the size of the sample unit is increased; there are differences in the production variability of litter between the plots in the same collection station; rectangular sampling units are more homogeneous than the square ones; the use of sample units 0,40m2 (1.5m x 0.25m) is recommend for litter quantification produced in Subtropical Seasonal Forest fragment.