Importance of Trifolium artense as a nitrogen fixer in
semi-arid tussock grasslands of New Zealand

Bill Lowther, Colin Boswell and Alison Rutherford
AgResearch, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand

1. Background The South Island tussock grasslands have a history of exploitive pastoral farming and the depletion of soil nitrogen is considered a key factor defining sustainability. The scarcity of indigenous nitrogen-fixing legumes has led to an assumption that minimal inputs of N occur in tussock grasslands that are not improved with fertiliser and seed This assumption neglects the potential importance of the widespread annual T. arvense (haresfoot clover).

2. Methods

• A 50 ha catchment was mapped into 6 land classes
• The % cover of T. arvense was determined
• Biomass and N-uptake of T. arvense (shoot+root) was measured over the spring/early summer growing period
• The proportion of N derived from symbiotic fixation was assessed by the 15N dilution technique on 3 land classes
• The amount of N fixed on the different land classes was then modelled
  

3. T. arvense cover T. arvense was present on 6 of the 7 land classes Cover of T. arvense varied over the land classes T. arvense was most widespread on the low-suuny face and the fan T. arvense was virtually absent from the low-shady face and gully
	% cover of T. arvense

4. Nitrogen

• N-uptake varied over the different landscapes
• A high % of N was derived from fixation
• N-uptake and N-fixed were related to cover of T. arvense
• When calculated on the area of the different land classes, N-fixed by T.arvense averaged 0.6 kg/ha over the whole catchment
• Rainfall was only 74 % of average and higher amounts of N could be expected in more favourable seasons
  

5. Conclusions

• T. arvense is often seen as an insignificant part of the landscape
• However, in spring/early summer it can be a major component
• Although the amount of N-fixed (˜ 0.6 kg/ha) is not high it makes a significant contribution in the context of the estimated low removal of N (˜ 1 - 3 kg/ha) under the existing grazing regime
• The potential importance of naturalised annual legumes should not be overlooked in developing N-budgets for unimproved semi-arid grasslands

Fan	Trifolium arvense

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