Impacts of ozone pollution on productivity and forage quality of grass/clover swards

This study explored the influence of ozone and interactions with soil nutrient regime and composition on the productivity and nutritive quality of ryegrass/clover swards. Established mesocosms containing ryegrass and/or white clover were grown at two levels of soil nutrition. Mesocosms were exposed in three (replicate) open-top chambers per treatment to accumulated ozone concentrations above 40 ppb during daylight hours (AOT40) of 60, 3900, 9450 and 17,160 ppb h. Effects on productivity were determined at 28-day intervals over the course of the experiment, and nutritive quality was determined for late-season forage. Ozone exposure was inversely related to clover productivity and nutritive quality, and this was reflected in a similar relationship for consumable food value (CFV). There was no effect of ozone on the productivity of ryegrass and a marginal effect on nutritive quality. In ryegrass/clover mixtures, the clover component was diminished by ozone, and the gaps created were populated by ryegrass with the result that there was no significant effect of ozone on productivity or nutritive quality of ryegrass/clover mixtures. Interestingly, adverse effects of ozone on clover productivity were mitigated in mixtures compared with monocultures. Accumulated stomatal ozone uptake above a threshold of 8 nmol m−2 s−1 was found to out-perform AOT40 as a predictor of the effects of ozone on the CFV.