ABSTRACT Computer models have been used to assess soil organic carbon (SOC) stock change. Commonly, models require to determine soil bulk density (Db), a variable that is often lacking in soil data bases. To partly overcome this problem, pedotransfer functions (PTFs) are developed to estimate Db from other easily available soil properties. However, only a few studies have determined the accuracy of these functions and quantified their effects on the final quality of the spatial variability maps. In this context, the objectives of this study were: i) to develop one PTF to estimate Db in soils of the Brazilian Central Amazon region; ii) to compare the performance of PTFs generated with three other models generally used to estimate Db in soils of the Amazon region; and iii) to quantify the effect of applying these PTFs on the spatial variability maps of SOC stock. Using data from 96 soil profiles in the Urucu river basin in Brazil, a multiple linear regression model was generated to estimate Db using SOC, pH, sum of basic cations, aluminum (Al+3), and clay content. This model outperformed the three other PTFs published in the literature. The average estimation error of SOC stock using our model was 0.03 Mg C ha−1, which is markedly lower than the other PTFs (1.06 and 1.23 Mg C ha−1, or 15 % and 17 %, respectively). Thus, the application of a non-validated PTF to estimate Db can introduce an error that is large enough to skew the significant difference in soil carbon stock change.

O objetivo deste trabalho foi avaliar indicadores químicos, físicos e biológicos da degradação de pastagens em áreas contrastantes quanto à ocorrência de cupinzeiros. Foram avaliadas as áreas de pastagem com a cultivar Marandu de Urochloa brizantha (Syn. Brachiaria brizantha), quanto à ausência (pasto 1) ou à presença (pasto 2) de cupinzeiros, e área com vegetação nativa de Cerrado. Foram avaliados: granulometria e fertilidade do solo; atividade microbiana do solo; nitrogênio potencialmente mineralizável; produção da gramínea em oferta; liteira existente e depositada no período de 28 dias; e fração leve da matéria orgânica do solo. No pasto 2, os cupinzeiros foram contados e 20 ninhos foram sorteados para coleta e identificação de cupins. Cornitermes cumulans foi a única espécie coletada, com média de 128 ninhos por hectare, tendo ocupado 0,1% da área útil. Entre os indicadores avaliados, apenas a oferta e a liteira diferiram entre as áreas de pastagens. A maior densidade de cupinzeiros não pode ser relacionada à acidez do solo ou aos outros parâmetros avaliados. A presença de cupinzeiros não é indicador de degradação química e biológica da pastagem e não está associada à alteração na dinâmica de resíduos da parte aérea de capim-marandu.

The objective of this work was to evaluate chemical, physical and biological indicators of degradation of pastures in areas with contrasting occurrence of termite mounds. Pasture areas with the Marandu cultivar of Urochloa brizantha (Syn. Brachiaria brizantha) were evaluated for the absence (pasture 1) or presence (pasture 2) of termite mounds, and an area of native Cerrado vegetation. The following parameters were evaluated: soil texture and fertility; soil microbial biomass; potentially mineralizable nitrogen; forage on offer; existing litter and litter deposited on the soil surface in 28 days; and free light fraction of soil organic matter. In pasture 2, termite mounds were counted and 20 nests were randomly selected for collection and identification of termites. All active mounds were inhabited by the species Cornitermes cumulans, with a mean of 128 mounds per hectare, occupying 0.1% of the net area. Among the indicators evaluated, only forage on offer and litter deposited in 28 days differed between pasture areas. The greater density of termite mounds was not related with the higher acidity of the soil or with the other evaluated parameters. The presence of termite mounds is not an indicator of the chemical and biological degradation of the pasture and is associated with the dynamics of aerial residues of Marandu palisadegrass.