Land use and carbon substrate identity drive differences in denitrification and N2O reduction in agricultural soils

Andrew Curtright
Plant, Soil and Microbial Sciences

Presented at the All Scientist Meeting and Investigators Field Tour (2021-09-23 to 2021-09-23 )

Soil-borne denitrifiers convert nitrate and nitrite to either nitrous oxide (N2O) or inert N2 gas. N2O is a potent greenhouse gas and ozone depletant, and it is therefore crucial to understand the drivers of N2O emissions from soils. Although the genetic basis of denitrification has been described, the biological drivers of N2O production and reduction are difficult to predict and therefore manage. The carbon © preference of soil microorganisms is a relatively unexplored driver of community functioning. Of the diversity of microbes present in the soil, those organisms that are active—and therefore contributing to community function—are those able to utilize the types of C that are immediately available to the community. Different land uses can drastically vary the type and quantity of soil C inputs, thereby exposing the soil microbial community to different C substrates and potentially altering microbial C preference. We hypothesized that, by selectively stimulating distinct subsets of microorganisms, the identity of available C could drive different rates of denitrification and N2O reduction within the same soil. Moreover, we expected that a history of differing C inputs would affect the C-utilization profiles of the denitrifier communities across soils from distinct land uses. To address these questions, we determined potential rates of N2O production and N2O reduction under anaerobic conditions for soils from three distinct land uses supplied with 12 different C substrates. Both N2O production and N2O reduction differed widely between C substrate additions. Glucose, amino acids, and organic acids stimulated the greatest production of N2O. On the other hand, N2O reduction was highest when soils were amended with amino acids, proteins, and organic acids. Land use also impacted rates of denitrification and N2O reduction. Across all substrates, soils under conventional agriculture produced less N2O and had lower rates of N2O reduction. Differences between soils from reduced input agriculture, which utilizes cover crops, and perennial switchgrass were more subtle. These results indicate the importance of the identity of available C substrates in the soil for driving microbial activity. Moreover, we show that land management affects how soil denitrifiers utilize available C, potentially by altering the legacy of soil C inputs.

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