Increasing soil pH reduces fertiliser derived N2O emissions in intensively managed temperate grassland

Abstract

Soil pH is generally considered a master variable, controlling a wide range of physical, chemical and biological properties, including a significant effect on microbial processes responsible for production and consumption of nitrous oxide (N2O), a potent greenhouse gas. Evidence of this pH impact on microbial denitrification mainly stems from observations in controlled laboratory experiments, while the results from field studies are mainly short-term, more variable and circumstantial. Soil pH is also one of the main factors controlling the availability of soil phosphorous (P), which has been also linked with N2O emissions. Here, we utilised an existing intensive grassland liming and P trial to investigate the effect of longer-term lime and P management and their interaction on N2O emissions and grassland productivity. The treatment plots were subject to different liming and P fertilisation strategies over 8 years and had a wide gradient of soil pH (5.1–6.9) and extractable P (2.3–8.3 mg kg−1). All plots received a total of 300 kg ha−1 of fertiliser nitrogen (N), applied in 8 splits across the growing season. N2O emissions, soil mineral N and grass yields were measured over 12-month period. We found a negative linear relationship between soil pH and cumulative N2O emissions, with a decrease in N2O emissions up to 39 % from limed plots compared to the unlimed control. The same effect was observed in relation to N2O emission factors and yield-scaled N2O emissions. Extractable soil P content had positive effect on yields, but no effect of P or P and pH interaction was observed in terms of direct N2O emissions or yield-scaled N2O emissions. We estimated that the increase in soil pH of grasslands in Ireland over the last 12 years potentially reduced national N2O emissions by 95 Gg CO2-eq yr−1, with potential for a further reduction by up to 254 Gg CO2-eq yr−1 if all the remaining acidic soils are brought up to optimal pH.