The climate benefits of high-sugar grassland may be compromised by ozone pollution

High sugar ryegrasses (HSG) have been developed to improve the uptake, digestion and nitrogen (N)-utilisation of grazing stock, with the potential to increase production yields and benefit climate by reducing methane (CH4) and nitrous oxide (N2O) emissions from livestock farming. In this study, the effects of tropospheric ozone pollution on the seasonal growth dynamics of HSG pasture mesocosms containing Lolium perenne cv. AberMagic and Trifolium repens cv. Crusader were investigated. Species-specific ozone (O3) dose-response relationships (seasonal means: 35, 41, 47, 51, 59 & 67ppb) based on the Phytotoxic Ozone Dose (PODy) were constructed for above and below ground biomass, injury, N-fixation and forage quality. The dynamics of effects of ozone exposure on HSG pasture changed over the course of a season, with the strongest responses occurring in the first 4-8weeks. Overall, strong negative responses to ozone flux were found for root biomass, root nodule mass and N-fixation rates, and ozone adversely impacted a range of forage quality parameters including total sugar content and relative and consumable food values. These results indicate that increasing ozone pollution could decrease the N-use efficiency and reduce the sugar content of managed pasture, and thereby partially detract from some of the suggested benefits of HSG.

N-fixation in legumes--An assessment of the potential threat posed by ozone pollution

The growth, development and functioning of legumes are often significantly affected by exposure to tropospheric ozone (O3) pollution. However, surprisingly little is known about how leguminous Nitrogen (N) fixation responds to ozone, with a scarcity of studies addressing this question in detail. In the last decade, ozone impacts on N-fixation in soybean, cowpea, mung bean, peanut and clover have been shown for concentrations which are now commonly recorded in ambient air or are likely to occur in the near future. We provide a synthesis of the existing literature addressing this issue, and also explore the effects that may occur on an agroecosystem scale by predicting reductions in Trifolium (clovers) root nodule biomass in United Kingdom (UK) pasture based on ozone concentration data for a "high" (2006) and "average" ozone year (2008). Median 8% and 5% reductions in clover root nodule biomass in pasture across the UK were predicted for 2006 and 2008 respectively. Seasonal exposure to elevated ozone, or short-term acute concentrations >100 ppb, are sufficient to reduce N-fixation and/or impact nodulation, in a range of globally-important legumes. However, an increasing global burden of CO2, the use of artificial fertiliser, and reactive N-pollution may partially mitigate impacts of ozone on N-fixation.

Highlighting the threat from current and near-future ozone pollution to clover in pasture

Globally, the legume-rhizobia symbiosis, contained within specialised organs called root nodules, is thought to add at least 30 Tg N annually to agricultural land. The growth and functioning of a modern white clover (Trifolium repens cv. Crusader) and red clover (T. pratense cv. Merviot) cultivar were investigated in current and future ozone scenarios in solardomes. Both cultivars developed leaf injury and had significant reductions in root biomass and root nodule number in response to ozone, with Crusader also displaying a reduced size and mass of nodules. In-situ measurements of N-fixation in Crusader by acetylene reduction assay revealed reduced N-fixation rates in a future scenario with an increased background and moderate peaks of ozone. The implications for the sustainability of temperate pasture are discussed.