Pasture degradation in the central Amazon: linking changes in carbon and nutrient cycling with remote sensing

Abstract

The majority of deforested land in the Amazon Basin has become cattle pasture, making forest-to-pasture conversion an important contributor to the carbon (C) and climate dynamics of the region. However, our understanding of biogeochemical dynamics in pasturelands remains poor, especially when attempting to scale up predictions of C cycle changes. A wide range of pasture ages, soil types, management strategies, and climates make remote sensing the only realistic means to regionalize our understanding of pasture biogeochemistry and C cycling over such an enormous geographic area. However, the use of remote sensing has been impeded by a lack of effective links between variables that can be observed from satellites (e.g. live and senescent biomass) and variables that cannot be observed, but which may drive key changes in C storage and trace gas fluxes (e.g. soil nutrient status). We studied patterns in canopy biophysicalbiochemical properties and soil biogeochemical processes along pasture age gradients on two important soil types in the central Amazon. Our goals were to (1) improve our understanding of the plot-scale biogeochemical dynamics of this land-use change, (2) evaluate the effects of pasture development on two contrasting soil types (clayey Oxisols and sandy Entisols), and (3) attempt to use remotely sensed variables to scale up the sitespecific variability in biogeochemical conditions of pasturelands.The biogeochemical analyses showed that (1) aboveground and soil C stocks decreased with pasture age on both clayey and sandy soils, (2) declines in plant biomass were well correlated with declines in soil C and with available phosphorus (P) and calcium (Ca), and (3) despite low initial values for total and available soil P, ecosystem P stocks declined further with pasture age, as did a number of other nutrients. Spectral mixture analysis of Landsat imagery provided estimates of photosynthetic vegetation (PV) and non-photosynthetic vegetation (NPV) that were highly correlated with field measurements of these variables and plant biomass. In turn, the remotely sensed sum PV 1 NPV was well correlated with the changes in soil organic carbon and nitrogen, and available P and Ca. These results suggest that remote sensing can be an excellent indicator of not only pasture area, but of pasture condition and C storage, thereby greatly improving regional estimates of the environmental consequences of such land-use change.

Nitrous oxide fluxes and nitrogen cycling along a pasture chronosequence in Central Amazonia, Brazil

“…Furthermore, the variation in soil mineralogy on P availability likely contributed to the differences in pasture productivity and N cycling across the Amazon basin (see Davidson et al, 2004). The phosphorus status of pastures on Ultisols in Rondônia is better (see Garcia-Montiel et al, 2000) than the pastures in this study (Table 1) or a nearby study on forest-to pasture chronosequences (Asner et al, 2004), and in the Paragominas region, eastern Amazon (see Verchot et al, 1999). The better P status at the Rondônian sites may have led to higher pasture productivity and hence, the potential of increased levels of SOM and N-mineralization.…”

Role of soil carbon in the landscape functioning of the Alto São Bartolomeu watershed in the Cerrado region, Brazil

“…Some of these vegetation indices are considerably good indicators of biomass and, when combined, they can be correlated with the carbon in the vegetation (Aragão & Shimabukuro, 2004). Some studies have correlated these indices with SOC and can be used to estimate functioning patterns of ecosystems and also to inform soil-vegetation management strategies (Asner et al, 2004). Experiments have shown high correlations between vegetation indices and carbon from plant biomass, as observed by Watzlawick et al (2009) in an Araucaria forest in the state of Rio Grande do Sul and by Bolfe et al (2012) in agroforestry systems in the Amazon, both in Brazil.…”

Solar Radiation Utilization by Tropical Forage Grasses: Light Interception and Use Efficiency

“…(Boddey et al, 2004, Meirelles et al, 2011. At the same time, the majority of deforested land in the Amazon Basin has become cattle pasture, making forest-to-pasture conversion an important contributor to the carbon and climate dynamics of the region (Asner et al, 2004).The terminology here follows the International Forage and Grazing Terminology Committee (Allen et al, 2011):"the term grassland bridges pastureland and rangeland and may be either a natural or an imposed ecosystem. Grassland has evolved to imply broad interpretation for lands committed to a forage use".…”