Raising the pH solves the problem of N2O greenhouse gas emissions

by Pawel Lycus

For the last decades, our population multiplied more rapidly than ever before, what have resulted in the growth in food demand. Modern agronomic practices have greatly improved crop production per unit land entailing excessive application of nitrogen (N) fertilizers to soils over the past century. Providing more nitrogen, than can be utilized by plants results in reactive N leaching to environment, thus causing its pollution.

One feet under – respiration in the absence of oxygen

Soil is a peculiar ecosystem, where the oxygen concentrations change rapidly and nutrient availability is generally low, still, soil is densely colonized by microorganisms, who, as all living creatures have to respire in order to provide energy needed for life processes and microbes prefer aerobic respiration (utilizing oxygen as an electron acceptor). However, when experiencing oxygen limitations, microbes are able to gain energy by exploiting other electron acceptors, and nitrate is the most favorable one. The step-wise reduction of soluble nitrate and nitrite (NO3 and NO2) through gaseous intermediates nitric oxide and nitrous oxide (NO and N2O) to dinitrogen gas (N2), called denitrification (Fig.1) is an alternative respiratory pathway in microorganisms and drives the nitrogen cycle in soil.

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Figure 1. Denitrification pathway. NAR, NIR, NOR, N2OR are the enzymes catalyzing following steps of nitrate reduction to dinitrogen gas. NO, N2O and N2 are gases, and therefore can escape the soil and reach atmosphere.

Backstage greenhouse gas nitrous oxide

Nitrous oxide gas, the intermediate product of denitrification has a great global warming potential, 300 times greater than CO2 and together with methane, these three are the most dangerous greenhouse gases. Although its threatening nature and quick rise of emission (Fig.2), the N2O has been paid not much attention until recently. The total N2O emission from agricultural soils is assigned to 50% of the estimated global N2O emission. Soil denitrification, furthermore, is the main source of N2O that affects the stratospheric ozone layer, thus contributing to its depletion. In order to develop mitigation strategies for N2O emission from anthropogenic ecosystems we need to improve our understanding of processes involved in its production and neutralization.

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Figure 2. N2O greenhous gas emission through the last decades. (Earth System Research Lab.)

Soil pH drives the N2O emissions

Due to intensive agronomics and overuse of fertilizers, many of soils are undergoing progressive acidification, and those are yet understudied, although available empirical data assert that these soils are the main source of N2O emissions. Soil microbial emissions of N2O depend on several environmental factors, like: soil N, carbon (C), pH, temperature, oxygen supply and water content, as well as the structure of microbial community (who is there), however, the overall mechanism has not yet been comprehended. Soil pH among others is the key driving factor that affects N2O:N2 product ratio of denitrification process, the lower the pH the more N2O is being released to the atmosphere, without being transformed to N2 (Fig.3). Intra-scales examinations of model organisms, indigenous bacterial communities and soil ecosystems indicate that the phenomenon is more general and pH steers the overall emission of N2O.

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Figure 3.The N2O/(N2O+N2) product ratio of denitrification and its dependence on the pH of soil. (Collaborative work of NMBU Nitrogen Group)

Problem of EU concern

The arising problem concerning antropogenic N2O emission attracted the attention of EU authorities, and the Marie Curie ITN Project NORA (Nitrous Oxide Research Alliance) has been founded in the year 2013. The joined forces of top class scientists involved in the nitrogen cycle research in Europe are working together trying to find explanations and solutions for N2O greenhouse gas emissions.

N2O reductase enzyme, the only thing we have

The N2O is a chemically inert gas and the only available weapon that can help us fighting the anthropogenic N2O emissions is the N2O reductase enzyme (NosZ) carried by several microorganisms. The enzyme does perform N2O reduction at the physiological level above 6 pH units, however. In the face of progressing soil acidification the enzyme tends to fail, thus leading to increasing emissions of N2O from these soils. The experimental approach and data provided by NMBU Nitrogen Group has paved the road for the basic understanding of the phenomenon. Researchers from NMBU-NG have been speculating that low pH affects the machinery for correct assembly of the NosZ, thus making the enzyme unable to perform N2O reduction. I joined the Nitrogen Group in 2013 as one of the Marie Curie ITN NORA fellows. My research focuses on the ecophysiology of denitrifying organisms and their roles in N2O emission. Unravelling the enigmas of impaired N2O reductase caused by low pH I work with the model organism in denitrification – Paracoccus denitrificans. My “pet” has been well studied and characterized, and this available knowledge helped me and my advisors to develop a novel approach for resolving “the pH question” concerning N2O reduction at the molecular level, which has a global effect, however. The solution is just around the corner!

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