Severe drought has lasting effects on Amazon

Hannah Hoag in Nature: A study published today in the Proceedings of the National Academy of Sciences sheds light on the long-term effects of drought on the Amazon rainforest — giving clues about how the rainforest might be affected by global warming in the future. The researchers report that the severe drought that hit the rainforest in 2005 had lasting effects on the forest canopy, such that it remained damaged at least four years later.

The effects of the 2005 drought have been debated since 2007, when researchers reported in Science that photosynthesis within the canopy had increased, leading the Amazon basin to ‘green up’ during the dry period. But in 2010 another group reported that they were unable to reproduce the results using the same data3.

“The ‘green-up’ is a short-term response and a bit of a red herring,” says Oliver Phillips, a tropical ecologist at the University of Leeds, UK. But the latest study “transcends that debate”, he says. “The question of the underlying health of the forest is much deeper than the instantaneous response.”

A drawback of the method used in the earlier studies — which used satellite measurements to estimate forest greenness using reflected solar radiation — is that the data can be muddied by clouds and atmospheric aerosols. So for the latest study, Sassan Saatchi, a remote-sensing expert at the California Institute of Technology Jet Propulsion Laboratory in Pasadena, California, studied the forest’s microwave ‘silhouette’, showing its contours instead of its greenness. To look at canopy structure, he and his colleagues used microwave satellite data, which are unaffected by clouds, from a NASA probe. When it passed over lush canopy, the satellite sensor recorded a smooth signal. Bare branches, thinned leaves and missing trees showed more roughness.

The researchers found that more than 70 million hectares of rainforest in the western Amazon — an area nearly twice the size of California — were hit by the drought. And the canopy’s recovery dragged on long after the drought ended, with its biomass and fullness still below pre-drought levels in 2009 when the satellite suffered a mechanical failure. In 2010, an even stronger drought hit a larger swathe of the Amazon....

Aerial view of the Amazon, shot by lubasi, Wikimedia Commons via Flickr, under the Creative Commons Attribution-Share Alike 2.0 Generic license

Fertile soil doesn't fall from the sky: The contribution of bacterial remnants to soil fertility has been underestimated until now

Helmholtz Centre for Environmental Research: Remains of dead bacteria have far greater meaning for soils than previously assumed. Around 40 per cent of the microbial biomass is converted to organic soil components, write researchers from the Helmholtz Centre for Environmental Research (UFZ), the Technische Universität Dresden (Technical University of Dresden) , the University of Stockholm, the Max-Planck-Institut für Entwicklungsbiologie (Max Planck Institute for Developmental Biology) and the Leibniz-Universität Hannover (Leibniz University Hannover) in the professional journal Biogeochemistry.

Until now It was assumed that the organic components of the soil were comprised mostly of decomposed plant material which is directly converted to humic substances. In a laboratory experiment and in field testing the researchers have now refuted this thesis. Evidently the easily biologically degradable plant material is initially converted to microbial biomass which then provides the source material to soil organic matter.

Soil organic matter represent the largest fraction of terrestrially bound carbon in the biosphere. The compounds therefore play an important role not only for soil fertility and agricultural yields. They are also one of the key factors controlling the concentration of carbon dioxide in the atmosphere. Climatic change can therefore be slowed down or accelerated, according to the management of the soil resource.

In laboratory incubation experiment, the researchers initially labelled model bacteria with the stable isotope 13C and introduced the bacteria to soil deriving from the long-term cultivation experiment "Ewiger Roggenbau" in Halle/Saale. Following the incubation time of 224 days the fate of the carbon of bacterial origin was determined. "As a result we found fragments of bacterial cell walls in sizes of up to 500 x 500 nanometres throughout our soil samples. Such fragments have also been observed in other studies, but have never been identified or quantified", declares Professor Matthias Kästner of the UFZ.

...."This new approach explains many properties of organic soil components which were previously viewed as contradictory", says Matthias Kästner....

A canyon formed in the soft loess soil by a small stream that flows from the west into the Daxia He River, in the northeastern part of Linxia County (probably, Xihe Township), in western China. Shot by Vmenkov, Wikimedia Commons,  under the Creative Commons Attribution-Share Alike 3.0 Unported license