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Annual rates of N2 fixation by pasture legumes on the central plateau of Tasmania

P.A. Lane

C/- Hopkins Pty. Ltd., 315 Main Road, Clenorchy, Tasmania, 7010

The success of any sown pasture depends largely on the use of a suitable legume as a source of nitrogen. An extensive field study has been undertaken to assess the annual rate of N2 fixation by two pasture legumes on the Central Plateau of Tasmania.

Method

An in situ acetylene reduction (AR) assay (1) and 15N dilution technique (2, 3) were used to estimate N2 fixation rates of two temperate pasture legumes, Caucasian clover (Trifolium ambiguum M. Bleb) and white clover (T. repens L.). growing in association with grasses at two locations on the Central Plateau of Tasmania.

The 15N dilution technique provided a means of calibrating the AR assay and the resulting ratios of C2H2 reduced to N9 fixed were used to convert monthly estimates of nitrogenase activity to actual rates of N2 fixation. By using this approach it has been possible to determine annual rates of N2 fixation.

Results and Discussion

The majority of N2 fixation was found to occur during periods of active growth from October through to January (Table 1). Factors such as low soil moisture in late summer and autumn and low temperatures (<5C) over the winter months. severely limited N2 fixation outside this period.

Table 1: Estimates of N2 fixation by T. ambiguum and T. repens at two Sites on the Central Plateau of Tasmania (kg N/ha).

Caucasian clover, which was only present at site 1 fixed considerably more N2 than white clover at this site, despite having an extended period of winter dormancy. More N2 was fixed by white clover at site 2 than at site 1, which was consistent with the slightly more favourable climatic conditions at site 2.

These estimates of N2 fixation compared favourably with other reported estimates for temperate pasture legumes growing in similar climatic regions (2, 4. 5). It was concluded that rates of N2 fixation in the range of 138 to 233 kg/N/ha/yr should be sufficient to maintain a productive permanent pasture not only on the Central Rlateau of Tasmnia but in most temperate regions of Australia where extensive methods of agriculture are practised.

1. Balandreau, J., and Dommergues. Y. 1973. Bull. Ecol. Res. Comm. (Stockholm) 17. 247-254.

2. Haystead. A.. and Lowe. A.C. 1977. J. Br. Crassld. Soc. 32. 57-63.

3. Edmeades. D.C.. and Coh, G.M. 1978. N.Z. J. Agric. Res. 21, 623-628.

4. Masterson, C.L.. and Murphy, P.M. 1976. in "Symbiotic Nitrogen Fixation in Plants" (P.S. Nutman Ed.) IBP7, 299-316. Cambridge University Rress, Cambridge.

5. Roglund. J.H., and Brock. J.L. 1978. N.Z. J. Agric. Res. 21, 73-82

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