Researchers have provided a solution to one of the agricultural problems – soil nitrogen loss, which causes the emission of nitrous oxide, a greenhouse gas that contributes to global warming – by using biochar made from wheat straw.
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Plants need nitrogen to grow, an element that is involved in the formation of proteins and chlorophyll, but the nitrogen present in the soil may be subject to microbial transformations that lead to its loss, instead of remaining in the soil for the plant to benefit from.
Therefore, researchers from the Key Laboratory of Soil Ecology at the Agricultural Resources Research Center of the Institute of Genetics and Growth Biology of the Chinese Academy of Sciences wanted to test the possibility of directing microbial activity towards a pathway that helps retain nitrogen within the soil, while reducing nitrous oxide emissions.
For this purpose, the researchers used biochar, also known as "black gold," because it transforms agricultural waste into valuable materials. They manufactured it from wheat straw and then loaded it with manganese compounds to obtain a substance that they called, in the description of the study published in the journal "Biochar," "manganese-loaded biochar."
The idea that the researchers started from is not just adding biochar to the soil, but modifying its chemical properties so that it becomes more able to help in transferring electrons between the components involved in microbial reactions.
The researchers started by placing samples of rice soil in controlled laboratory conditions and tested the effect of adding manganese-loaded biochar compared to untreated soil, with the aim of determining the changes resulting from this addition.
The researchers then measured the ability of biochar to exchange electrons before and after it was loaded with manganese, and the results showed that this ability increased by about 80% in the modified material compared to unmodified biochar.
This is an important result, because it indicates that the substance has become more capable of participating in the oxidation-reduction reactions that microorganisms depend on.
In the next step, the researchers analyzed how nitrates are transformed by microorganisms, specifically whether they follow a pathway that leads to the formation of ammonium, or pathways that lead to the formation of gaseous nitrogen compounds.
They found that the modified material increased the proportion that goes down the nitrate-to-ammonium reduction pathway. The researchers then measured the cumulative amount of nitrous oxide emitted from soil samples during the experiment period, and compared the results between the treated and untreated soils, recording a significant decrease in emissions in the presence of manganese-loaded biochar.
Finally, they studied changes in the microbial community within the soil, monitored the expression of the NRFA gene, which is associated with the pathway for converting nitrate to ammonium, and analyzed the role of organic carbon in the soil in supplying microbes with the electrons needed for reactions.
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