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Liming, water deficits and c2h2 reduction by subterranean clover

Allan G. Davey and R.J. Simpson

School of Agriculture and Forestry,The University of Melbourne,Parkville 3052

Acidification of soils beneath clover-based pastures is a serious problem in southern Australia (1,2,3). For example, nodulation of roots of subterranean clover (Trifolium subterraneum L.) can be restricted in very acid soil due to the poor ability of Rhizobium trifolii to colonize the soil (4). The experiments described in this paper examine whether plants with nodules confined to the uppermost layers of a limed and unlimed acid soil experience moisture stress and consequent reductions in nitrogen fixation as the soil dries in late summer.

Methods

Seedlings of cv. Mr Barker inoculated with R. trifolii (WU95) were planted individually into intact cores of an acid soil (5) which had been subjected to three treatments, namely, (i) uncultivated, (ii) cultivated to 12 cm and (iii) cultivated with incorporation of lime (5 t/ha). Pots were watered to field capacity during establishment of plants (12 weeks) after which half the pots in each treatment were watered from the base with 50% of the amount of water transpired by the control plants. Acetylene reduction activity (ARA) of nodulated roots was measured in a flow-through system (5). All treatments were replicated 5 times.

Results and discussion

As 47% of the number of root nodules were located in the top 4 cm of the soil profile, accounting for 48% of total nodule dry weight, most of the measurements presented in Table 1 have been taken from this region.

Table 1. Results of the pot trials. Data were collected 21 days after commencement of the water deficit. RWC=relative water content.

*0-4cm from the top of the soil profile. **day 0=commencement of water deficit. 'Tukey's Studentized range. ø 0.01M CaC12 1:5 w/v

Limed treatments showed no early advantage in their ARA, however, as well-watered plants grew older, ARA in unlimed soil declined more rapidly than in limed soil. Moisture stress, although having little effect on leaf water potential, significantly reduced ARA in all treatments.

1. Bromfield, S.M., Cumming, R.W., David, D.J. and Williams, C.H. 1983. Aust. J. Exp. Agric. Anim. Husb. 23 192-200.

2. Horsnell, L.J. 1983. Aust. J. Exp. Agric. 25 157-63.

3. Pinkerton, A. and Simpson, R.J. 1986. Aust. J. Exp. Agric. 26 681-9.

4. Richardson, A. and Simpson, R.J. (unpublished data).

5. Davey, A.G. and Simpson, R.J. 1986. AIAS Occasional Publication No. 25, pp. 33-4.

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