Researchers from the Universidad Politecnica de Madrid and colleagues from the Chinese Academy of Science took samples of soil from a humid, subtropical area of southern China. In a laboratory, they simulated drought conditions on soils with and without added biochar and measured the resistance and resilience of the soil microbial populations.

“We know that fungal networks are supposed to be more resilient to perturbations than their bacterial counterparts, and based on previous studies with this soil we thought the soil microbial populations would shift to a more fungal-dominated one,” said Jorge Paz-Ferreiro, one of the authors of the study in Environmental Research Letters (ERL). “But the dominance of the microbial network did not shift in this way. Instead the whole community became more resistant and resilient to perturbation after the addition of biochar.”

To come up with their results, Paz-Ferreiro and colleagues measured enzyme activity and phospholipid-derived fatty acids (PLFAs), which are a marker for fungal activity. These measures are sensitive indicators of soil quality and respond rapidly to changes in soil management. The researchers used a resistance index, which relates to the value of a property before and after a perturbation, and a resilience index, which tallies how long the soil community takes to recover after a perturbation.

Adding biochar significantly increased the resistance to drought of the fungal community and bacterial communities, as well as the enzymes cellulose, b-glucosidase and phosphomonoesterase, the researchers found. Bacteria responded more intensely to drought than fungi, for both the control and the biochar-amended soil. But most strikingly, biochar seemed to offer an additional protection for both bacterial and fungal communities, compared with the control soil.

“The mechanism for this improvement is not clear,” said Paz-Ferreiro. “It could be because biochar is a porous material and therefore increases the habitats suitable for bacteria and fungi. Or it could be because biochar improves soil aggregation and water retention. Or perhaps it is due to the mineral content of the biochar and its high pH, which increased the pH of our soils by two points.”

While these results have universal relevance, Paz-Ferreiro is keen to point out that every biochar is different and that the biochar he and his colleagues used had a relatively low carbon/nitrogen ratio. It also had a significant amount of phosphorous and other nutrients, which are common in sludge or manure-derived biochars but absent in woody biochars.

“More work needs to be done to understand the mechanism of the interaction of biochar with the soil and to validate our results using biochars with contrasting characteristics,” he said.

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