The use of an unenclosed field fumigation system to determine the effects of elevated ozone on a grass-clover mixture.

A mixture of Trifolium repens L. (var. Grasslands Huia) and Lolium perenne L. (var. Melle) was established in an area with low ambient ozone concentrations and exposed to episodes of ozone using a simple, unenclosed field fumigation system. Control plots were exposed to ambient air. There were two cutting frequencies to simulate grazing and silage production respectively. This paper reports on the performance of the system and the effects on yield and species composition in the first year of fumigation. The system operated satisfactorily when the wind speed was between 1 and 5 m s . When the wind dropped suddenly below 1 m s there were short periods with high concentrations but these events were rare and did not last more than a few minutes. Above 5 m s there was lateral downwind spillage of ozone and at higher wind speeds it was not possible to maintain the target concentration of 50-70 nl 1 . The plots were fumigated only when the weather was suitable for ozone formation and when the wind speed was stable. Because the weather was poor, fumigation was restricted to 22 d between July and September. The mean 4-7 h concentrations were 50-70 nl l . Seven-hour mean ambient concentrations over the same period were 10-28 nl 1 . There were significant effects of ozone on yield and herbage composition. The effects varied between harvests, probably because of differences in the timing of cutting relative to the episodes, and to the differences in phenology between the species. Most of the loss of yield was due to effects on Trifolium, which was much more sensitive than Lolium. There was a persistent effect of ozone on the stolon density of Trifolium which lasted into the following year. It is suggested that this arose because the clover replaced damaged leaves very quickly, maintaining the canopy at the expense of the stolons. Persistent effects on stolons might lead to poorer winter survival and lower nitrogen fixation. Gaps in the sward as a result of loss of Trifolium might allow rapid weed invasion. There was no interaction between cutting frequency and ozone, which may have been due to the fact that the poor summer restricted the number of cuts and therefore decreased the difference between the two cutting frequencies.