The nitrate scavenging ability of phalaris and lucerne in subterranean swards

B.S. Dear^1,2, P.S. Cocks², M.B. Peoples³ and A.D. Swan^1,3

¹NSW Agriculture, Wagga Wagga, NSW.²University of Western Australia and CRC for Legumes in Mediterranean Agriculture, Nedlands, WA. ³CSIRO, Canberra, ACT.

Abstract

The uptake of mineral nitrogen by perennial species (N) was compared in subterranean clover (Trifolium subterraneum L.) swards containing lucerne (Medicago sativa L.) or phalaris (Phalaris aquatica L.) at densities ranging from 5-40 plants/m² over a two-year period. The annual amounts of soil mineral N accumulated in lucerne shoots ranged from 60-170 kg/N/ha and in both years N uptake increased with lucerne density. The assimilation of mineral N by phalaris varied from 110-240 kg/N/ha/annum, but in contrast to lucerne, shoot uptake decreased with increasing phalaris density. The greater effectiveness of phalaris relative to lucerne at taking up mineral N at low plant densities (5-10 plants/m²) was attributed to the superior ability of phalaris to compensate for low plant density by increasing individual plant size. Lucerne was found to be relatively effective at scavenging for mineral N, biologically fixed N providing only 39-59% of lucerne’s N requirements for shoot growth over the duration of the experiment. Available soil mineral N in the soil surface in winter varied with perennial species and reflected the differential scavenging ability of the two perennials.

Key words

Lucerne, phalaris, subterranean clover, nitrate, density, nitrogen.

Introduction

Soil acidification is linked in part to the inefficient utilisation of nitrate mineralised from plant residues and its subsequent leaching to depth down the soil profile (4, 9). The root system of annual species germinating in autumn is poorly developed compared to perennial species and appears unable to effectively utilise nitrate moving down with the wetting front, resulting in a bulge of nitrogen (N) accumulating at depth. The ability of perennials to trap nitrate could be affected by their dormancy/activity in autumn, their density and the area exploited by their root systems. In the case of perennial legumes such as lucerne, it might also depend on the proportion of plant N sourced from biological fixation compared to uptake of available mineral N.

The following paper describes the total amount of mineral N taken up by phalaris (Phalaris aquatica L.) and lucerne (Medicago sativa L.) over two growing seasons when grown at different densities in association with subterranean clover (Trifolium subterraneum L.).

Materials and methods

The experimental site was located at the Agricultural Research Institute, Wagga Wagga, New South Wales, (147⁰21'E, 35⁰03'S; 219 m above sea level) on a red earth soil (Gn2.11)(9). Subterranean clover was grown as a pure sward and as a mixture with either lucerne or phalaris sown at 5, 10, 20 or 40 plants/m² in a randomised complete block design with 4 replications. The subterranean clover component consisted of a 1:1:1 mixture of three cultivars of subterranean clover of increasing maturity: Dalkeith, Seaton Park and Goulburn. Details of the establishment of the experiment are provided in previous publications (2,3).

Herbage yields were assessed at intervals of about 6 weeks during the growing season by cutting a 45 x 75 cm quadrat located at the centre of each plot to a height of 4 cm. The sample cut from within the quadrat was separated into clover and perennial fractions to determine the botanical composition, dried at 70⁰C for 48 h and weighed.

After sampling, plots were grazed and then mown to ensure an even sward height and the cut material removed from the plot. Total herbage production for each year was calculated.

At each sampling, samples of phalaris, lucerne and subterranean clover were taken, dried at 60⁰C in a forced-draught oven for 48 h and then ground in a Wiley mill and a subsample finely ground with a ring grinder. Total N and the ¹⁵N composition were determined by combustion in an automatic N and carbon analyser (ANCA-SL) interfaced to a 20-20 stable isotope mass spectrometer (Europa Scientific, Crewe, UK). The ¹⁵N natural abundance technique (7) was used to estimate N₂ fixation by comparing the ¹⁵N concentrations of the legume against the non-N₂ fixing reference plant (phalaris) (5, 7, 8). The amount of mineral N taken up by the lucerne was determined by deducting the amount of fixed N from the total N accumulated in the herbage.

Results

Total soil N taken up by the two year old phalaris sward ranged from 110 to 210 kg/ha with uptake decreasing with increasing phalaris density (Figure 1). Soil N uptake by lucerne swards over the same period ranged from 60 to 100 kg/ha but in contrast to phalaris the amount increased with increasing lucerne density. In the following year, uptake ranged from 150 to 240 kg N/ha in phalaris and 50 to 170 kg N in lucerne. Perennial density had the same effect on N uptake as in the previous year. Available mineral nitrate levels in the surface profile in autumn-winter were higher in the lucerne swards than in phalaris in both years (Figure 2). Within each species, available nitrate levels declined with increasing perennial density.

Discussion

The results indicate that both phalaris and lucerne are effective at scavenging mineral N but these perennials respond differently as their density changes. Lucerne sourced a significant proportion of its N needs (40-60%) (2) from soil N. Phalaris was effective at scavenging for available mineral N even at relatively low plant densities of 5 plants/m². The decline in N uptake with increasing phalaris density was not due to any decrease in the N content of phalaris tissue which was relatively independent of phalaris density (2) nor was it due to decreasing subterranean clover content as measurements indicated that N fixed by subterranean clover was similar across all phalaris densities (2). It was, however, associated with a reduction in the size and vigour of individual phalaris plants with increasing plant density.

Lucerne was also relatively effective at scavenging for mineral nitrogen and relied on a relatively low proportion of biologically fixed nitrogen (%) over the two year period. In contrast to phalaris, lucerne mineral N uptake increased with increasing lucerne density. As with phalaris the N content of the lucerne remained constant with density but unlike phalaris, lucerne yields increased with increasing density and accounted for much of the increase in N uptake.

Including either phalaris or lucerne in a sward will potentially reduce the amount of mineral N leached to depth but lucerne densities of 20 or more plants/m² are required to be as effective as low densities (5 plants/m²) of phalaris. One reason for the decreased N scavenging ability of lucerne at low densities is the inability of lucerne plants to increase plant size at low densities. In contrast phalaris responds to low densities by greatly increasing plant size and basal area (3). This would allow phalaris to exploit a larger surface soil volume compared to lucerne.

The levels of available mineral N present in the surface soil were consistently lower under swards containing phalaris. These lower mineral N levels in phalaris pastures could be expected to reduce the vigour of broadleaf and annual grass weeds and is this is consistent with field observations.

	Figure 1: Mineral N uptake by lucerne and phalaris tops at different densities over 2 seasons.
	Figure 2. Available nitrate (ug N/g soil) in surface 10 cm soil during winter in swards of pure subterranean clover (nil perennial) or subterranean clover in mixtures with different densities (nil-40 plants/m²) of lucerne and phalaris in successive seasons.

References

1. Dear, B.S. 1998. The ecology of subterranean clover growing in association with perennial species. Ph.D Thesis, University of Western Australia, Perth.

2. Dear, B.S., Cocks, P.S., Peoples, M.B., Swan,_,A.D. and Smith, A. 1999. Aust. J. Agric. Res. 50, 1047-1058.

3. Dear, B.S., Cocks, P.S., Swan,A.D., Wolfe, E.C. and Ayre, L.M.2000. Aust. J. Agric. Res. 51, 267-278

4. Helyar, K.R. and Porter, W.A. 1989. In ‘Soil Acidity and Plant Growth’. (Ed. A.D. Robson) (Academic Press: Sydney.) pp. 61-101.

5. Ledgard, S.F., Simpson, J.R., Freney, J.R. and Bergersen, F.J. 1985. Aust. J. Agric. Res. 36, 663-676.

6. Northcote, K.H. 1979. 'A Factual Key for the Recognition of Australian Soils', 4th Ed (Rellim Technical Publications: Glenside, S. Aust.).

7. Peoples, M.B., Turner, G.L., Shah, Z., Shah, S.H., Aslam, M., Ali, S., Maskey, S.L., Bhattarai, S., Afandi, F., Schwenke, G.D. and Herridge, D.F. 1997. Proceedings International Workshop on Managing Nitrogen Fixation in the Cropping Systems of Asia. (ICRISTAT, Pradesh, India.) p.57-75.

8. Peoples, M.B., Gault, R.R., Scammell, G.J., Dear, B.S., Virgona, J.M., Sandral, G.A., Paul, J., Wolfe, E.C. and Angus, J.F. 1998. Aust. J. Agric. Res. 49, 459-474.

9. Ridley, A.M., Helyar, K.R., and Slattery, W.J. 1990. Aust. J. Exp. Agric. 30, 195-201.

Introduction

Materials and methods

Results

Discussion

References

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