MAGGE Mitigating Agricultural Greenhouse Gas Emissions by improved pH management of soils

Direct emissions of nitrous oxide (N2O) from farmed soils account for the largest share of plant production?s climate forcing. The FACCE ERA-GAS project ?Mitigating Agricultural Greenhouse Gas Emissions by improved pH management of soils (MAGGE-pH)? explored possibilities to reduce N2O emissions by liming soils beyond the minimum needed for crop growth. The idea was based on the knowledge that the reduction N2O to harmless N2 by denitrifying bacteria is hampered at low pH. To test whether this pH effect can be exploited to curb N2O emissions from plant production, N2O emissions were measured throughout two years in various liming experiments in Europe and New Zealand. Research further included manipulative laboratory studies and molecular assessments of functional microbial communities in long-term and more recently limed soils. While process studies confirmed that liming acid soils reduces N2O emissions under denitrification conditions when oxygen availability is limited, we also found that liming increases N2O production by nitrification, a process that oxidizes ammonium to nitrate in the presence of oxygen. This was explained by a pH-driven change in the ratio of NH3 oxidizing bacteria and archaea. The net effect of liming therefore depends on which of the N2O forming processes prevails. This was also found in the field experiments. MAGGE-pH provides data that can be used to estimate the mitigation potential of liming depending on local soil conditions. The Achilles heel of N2O mitigation by liming could be the emission of carbonate-CO2, which could more than negate the effect of reduced N2O emission on climate forcing. However, the conventional view adopted by IPCC that 100% of the carbonate CO2 evades is wrong, and we argue that the effect of the extra lime needed to take pH above the minimum required for good crop growth is more likely to be a CO2 sink. Recent soil monitoring projects in Germany, Ireland and UK demonstrate an ongoing gradual acidification of cultivated surface soils in Central Europe. If not halted and reversed, this acidification will plausibly increase N2O emissions and decrease yields, eventually resulting in an increased GHG footprint per unit of yield. MAGGE-pH addresses this problem by exploring limitations to proper pH management for selected regions by means of socio-economic modelling.

We tested the hypothesis that liming acidic soils decreases N2O emissions. On the field level, N2O mitigation by liming depended on weather (drought, freeze-thaw) and time of liming. Rapid pH rise transiently increased N2O emissions, likely by changing the nitrifier community. Own incubation experiments and mesocosm experiments with Norwegian soil conducted at Thünen Institute using advanced 15N2 approaches revealed that the pH effect on N2O emissions strongly depended on the extent of denitrification induced by soil management. Hence, the liming effect should be seen dynamically in conjunction with, e.g., ammonium fertilization and crop residue incorporation, which first stimulate nitrification before inducing denitrification due to oxygen consumption. MAGGE-pH has risen the awareness of practitioners and extension service about the advantages of proper pH management in Norway, and has feed into follow up projects such as EarthResQue and GREENMOVE.

Climate forcing by crop production is dominated by N2O emissions. Although emissions can be marginally reduced by "good agronomic practice", we need novel approaches to make substantial progress. This is what MAGGE-pH is about, by deliberately targeting the microbial processes responsible for production and consumption of N2O in soils. Our proposed research is based on the emerging understanding of how soil pH controls the N2O/N2 product ratio of denitrification. This research has shown unequivocally that the ratio declines with increasing pH within the agronomically permissible range of 5.5 -7.0. We therefore see a possibility to mitigate N2O emissions by increasing the pH of moderately acidic soils beyond the threshold needed to secure adequate crop growth. This necessitates policy instruments to secure effective implementations, and MAGGE-pH will deliver socio-economic analyses to identify such instruments. The evidence for the pH effect on N2O emissions stems from laboratory experiments and a few field experiments. Now we need stringent testing of different liming strategies under realistic field conditions. This will be the core activity in MAGGE-pH and generate emission factors for a range of N fertilizers/manure/urine applications by pH. We will also draw on ongoing research which tests non-calcareous rock powder as a replacement for traditional lime (carbonates). This is highly relevant, because CO2 emissions from carbonates can negate their potential N2O reducing effect. MAGGE-pH will also explore a series of novel approaches to manage soil pH via carbonate amended fertilizers and not the least via manure, which hold the potential to reduce anthropogenic N2O emissions even further by minimizing NH3 volatilization. Thus, the project adds "blue sky? research to the core objectives, and puts it to a critically evaluation by a broad European consortium consisting of researchers, fertilizer companies and practitioners.