The nitrogen handling characteristics of white clover (Trifolium repens L.) cultivars and a perennial ryegrass (Lolium perenne L.) cultivar

Ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) have co ntrasting responses to soil mineral N availability and clover has the abili ty to fix atmospheric N-2 symbiotically. It has been hypothesized that thes e differences are the key to understanding grass-clover coexistence and veg etative dynamics in pastures. However, the whole plant response of clover a nd ryegrass to mineral N availability has not been fully characterized and inter-cultivar variability in the N-handling dynamics of clover has not bee n assessed. A detailed experimental study to address these issues was under taken. For all clover cultivars and ryegrass, mass specific mineral N uptak e rates (of whole plants) were similar saturating functions of mineral N av ailability. For all clover cultivars total N assimilation rates, whole plan t C : N ratios and root : shoot ratios were independent of mineral N availa bility. Clover growth rates were also independent of mineral N availability except for a slight (<10%) reduction at very low N availability levels. Sp ecific N-2 fixation rate (whole plant) was precisely controlled to ensure f ixation balanced the deficit between mineral N uptake and the total N assim ilation required to maintain constant whole plant C : N ratio. There was al ways a deficit between N uptake and the total N assimilation required to ma intain C : N ratio. Consequently, some NP fixation remained engaged even at high mineral N availability levels. All inter-cultivar variation in N-2 fi xation dynamics could be attributed to variations in growth rate. Clover ma ss specific growth rate declined as plant size increased. Ryegrass specific growth rate, whole plant C : N ratio and root : shoot ratio were dependent on N availability. Increased N availability led to increased growth rate a nd decreased C : N and root : shoot ratios. Specific growth rate was also d ependent on plant size, growth rate declining as plant size increased. It i s concluded that clover inter-cultivar variation in field performance is un likely to be a consequence of variation in N-handling characteristics. Inte r-cultivar differences in growth rate are likely to be a much more importan t source of variation.