Hydrogen evolution from white clover

Hydrogen (H 2 ) evolution from ‘Grasslands Huia’ white clover ( Trifolium repens L.) nodulated by Rhizobium trifolu TA1, was suppressed by 6% acetylene‐inair during acetylene reduction (AR) assays of nitrogen (N) fixation. Field measurements showed significant H 2 evolution and reduced nodule relative efficiency over summer in a laxly mown pasture, but virtually no H 2 was evolved from a closely grazed pasture. Plants transferred in winter from outdoors to a controlled cabinet with good growth conditions, showed increased AR rates and a decrease in nodule relative efficiency from 1 to 0.52 in 5 weeks. Defoliation of plants in a growth cabinet greatly reduced H 2 evolution, confirming that conditions of growth have large effects on H 2 evolution in white clover. No diurnal patterns of H 2 evolution or AR were observed in plants growing outdoors. The significance of H 2 evolution to AR assays of N fixation by white clover is discussed.