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Freeze-drying Soil for Studying Reactive Nitrogen Gas Emissions

Freeze-dried soil mimics fresh soil most closely in terms of pH and reactive nitrogen gas emissions compared to air- and oven-dried counterparts.

Land-air gas exchanges exert a strong influence on the global climate. For instance, soil nitrogen cycling strongly affects Earth’s atmosphere through the release of reactive nitrogen (Nr) gases, including ammonia, nitrous acid, nitric oxide and nitrogen dioxide. Nitrous acid alone can contribute up to 80 percent of harmful hydroxide radicals. Therefore, quantifying soil Nr gas fluxes accurately is crucial towards a better understanding of atmospheric chemistry.

Traditionally, field soil samples are sieved and dried before conducting measurements in laboratory conditions. For long-term storage, soil samples are often air- or oven-dried, while freeze-drying has also been practised. However, few studies have compared the effects of different drying methods on resultant measurements.

Recently, a national team from various universities and institutions in China did just that. “We evaluated soil water content, pH, (in)organic N content, and Nr gas fluxes of air-dried, freeze-dried, oven-dried, and fresh soils from different land-use types,” said Dr. Wu Dianming, first author of the work published in Atmospheric and Oceanic Science Letters.

In general, all drying methods were found to increase the soil ammonium, nitrate and dissolved organic N contents compared with fresh soil. However, while nitrous acid flux, nitric oxide flux, and total emissions during a full wetting–drying cycle of fresh soil were also increased by air- and oven-drying, this did not occur for freeze-dried soil.

Furthermore, the soil pH of freeze-dried soil was closest to that of fresh soil, whereas air- and oven-dried samples registered significantly lower pH values. In other words, global soil Nr gas emissions might be overestimated if air- and oven-dried soil are used.

“The important implication of the finding is that we need to carefully evaluate the previous understanding of the mechanism of biogeochemical nitrogen cycling based on different drying methods,” concluded Dr. Wu. [APBN]