Abstract
Land-use change in the Amazon basin has occurred at an accelerated pace during the last decade, and it is important that the effects induced by these changes on soil properties are better understood. This study investigated the chemical, physical, and biological properties of soil in a field under cultivation of soy and rice, and at an adjacent primary rain forest. Increases in soil bulk density, exchangeable cations and pH were observed in the soy field soil. In the primary forest, soil microbial biomass and basal respiration rates were higher, and the microbial community was metabolically more efficient. The sum of basal respiration across the A, AB and BA horizons on a mass per area basis ranged from 7.31 to 10.05 Mg CO2-C ha-1yr-1, thus yielding estimates for total soil respiration between 9.6 and 15.5 Mg CO2-C ha-1yr-1 across sites and seasons. These estimates are in good agreement with literature values for Amazonian ecosystems. The estimates of heterotrophic respiration made in this study help to further constrain the estimates of autotrophic soil respiration and will be useful for monitoring the effects of future land-use in Amazonian ecosystems.
Highlights
During the last decade the agricultural frontier in the Amazon has progressively migrated northward away from the crescent of deforestation that arches across the south and southeastern portion of the Amazon basin (Nepstad et al 2006; 2008)
Compared to the Tapajós National Forest (TNF), the soy field shows large differences in pH, exchangeable cations, and P, differences which are most likely related to liming and fertilization
The authors suggest that dissolved organic matter (DOM) inputs may drive higher rates of soil respiration by stimulating an opportunistic subset of the soil bacterial community, and the types of microorganisms vary through time in response to different rates and types of C inputs, suggesting that temporal shifts in community structure may directly affect soil respiration rates following C inputs
Summary
During the last decade the agricultural frontier in the Amazon has progressively migrated northward away from the crescent of deforestation that arches across the south and southeastern portion of the Amazon basin (Nepstad et al 2006; 2008). The majority of land-use change in Amazonia has been deforestation followed by cattle pasture installation. During recent years the expansion of large-scale industrialized agriculture has become a major land-use change (Nepstad et al, 2006). The area under intensive mechanized agriculture in the Brazilian Legal Amazon increased by > 3.6 million ha during 2001-2004, with 87% of this growth in the State of Mato Grosso (Morton et al, 2006). Soybeans occupy a larger area than any other crop in Brazil, and during the last 10 years, Amazonia became the region of Brazil with the largest increase in area under soy cultivation (Costa et al, 2007). The 2006 Censo Agropecuário conducted by the Brazilian Institute of Geography and Statistics (IBGE, 2006) showed that, of the regions of Brazil, the North (Amazonia) had the largest increase in area under soy cultivation: 275.5%. There are indications that there is considerable potential for expansion of the area under soybean cultivation in the Amazon. Vera-Diaz et al (2007), using a regression model of soybean yield that integrates the major climatic, edaphic, and economic determinants in the Amazon Basin, estimated that 20% of the Legal Amazon could yield 2 Mg.ha-1 of soybeans, a lower-threshold value believed to make soybean cultivation economically viable
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