Bulk Densities of Brazilian Amazon Soils Related to Other Soil Properties

Abstract

The results of this study provide the first baseline for predicting Db from soil properties for soils across the Amazon basin. Bulk density values are needed to convert nutrient content and organic carbon (OC) content to weight of nutrient and OC per unit area; unfortunately, common field methods to measure Db are limited with regard to reliable, complete, and uniform soil data. Much effort has been made in finding alternative solutions to predict Db from soil properties. We hypothesized that Db could be reliably estimated by multiple regression of OC, soil textural properties, and some chemical properties. Using the data of 323 soil horizons from the Brazilian Amazon basin, a stepwise multiple regression (SMR) procedure was developed to predict Db from other soil properties. Multiple regression relationships were obtained for all the data, which were also partitioned by layer and then by main soil order: Latossolos (Oxisols, 62 horizons) and Podzólicos (Alfisols and Ultisols, 212 horizons). The SMR on all the data showed that clay content is the best predictor of Db, accounting for 37% of the variation. Adding OC content increased the explained variance up to nearly 50%. Predictions of the models were improved when the data were partitioned by order and by horizon type. In the case of Latossolos (Oxisols), the use of OC and clay content as predictors increased the percentage of explained variation, reaching 71% using all layers and 79% for A horizons. The results of this study will provide a basis for estimating OC stocks in the Amazon basin.

Pedo-Transfer Functions for Estimating Soil Bulk Density in Central Amazonia

“…Using the equation of Bernoux et al (1998) to estimate soil density from the validation series of this study resulted in a R 2 = 0.56, which shows the degree of relationship between estimated density values for this model versus the values observed in the field. However, the graph of residuals versus estimated values showed clustering, suggesting that the residuals are not normally distributed, making the validity of this prediction questionable (Figure 4).…”

“…This study's best model for predicting bulk density was compared with three models from the literature that were generated from samples collected throughout the Amazon Basin (Bernoux et al, 1998;Tomasella and Hodnett, 1998;Benites et al, 2007). These equations were used to predict density from the data series used to validate our models.…”

“…Data are scarce because sampling undisturbed cores is laborious and data are only reliable if the methodological rigor is high. The result is that soil carbon stocks are commonly estimated from mean bulk density (Bd) values from the literature and from values for C concentration measured in the field (Bernoux et al, 1998).…”

“…This has stimulated predictions of soil density that exploit the relationship between this parameter and other variables commonly found in soil-related inventories in order to ensure the reliability of carbon stock assessments and to reduce evaluation costs (Federer et al, 1993;Bernoux et al, 1998Bernoux et al, , 2002Tomasella and Hodnett, 1998;Calhoun et al, 2001;Heuscher et al, 2005;Benites et al, 2007;Tranter et al, 2007;Steller et al, 2008;Gharahi-Ghehi et al, 2012;Chaudhari et al, 2013).…”

“…It should be noted that measuring the actual soil density values in the field is always more reliable than estimates based on variables that are equally or even more complex and with high spatial variability. Bernoux et al (1998) generated equations to estimate density from data series of the RADAMBRASIL inventories, published between 1973 and 1982 by the Ministry of Mines and Energy. Despite the vast amount of data on geology and geomorphology, vegetation, soils and land use, these surveys lack information on soil density.…”