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Estimated symbiotic N2 fixation by annual legume and lucerne pastures on 2 Vertosols with and without applied gypsum.

Brian Dear1,4, Mark Peoples2, Richard Hayes1,4, Anthony Swan2, and Steve Morris3

1Industry and Investment NSW, Agricultural Institute, PMB Pine Gully Rd, Wagga Wagga NSW 2650, Australia. brian.dear@industry.nsw.gov.au
2
CSIRO, Division of Plant Industry, GPO BOX 1600, Canberra ACT 2601, Australia. mark.peopples@csiro.au
3
Industry and Investment NSW, Agricultural Institute, Wollongbar, NSW 2477 Australia.
4
EH Graham Centre (Charles Sturt University and Industry and Investment NSW), Agricultural Institute, PMB Pine Gully Rd, Wagga Wagga NSW 2650, Australia

Key words

Subterranean clover, balansa clover, alfalfa, root, shoot

Abstract

The amount of symbiotic dinitrogen (N2) fixed by an annual legume pasture (subterranean clover + balansa clover) and a lucerne-annual legume pasture growing on vertosol soils was estimated at two sites (Grogan and Morangarell) over 3 years in southern NSW following treatment with 3 rates of gypsum. Based on shoot growth only, the annual legume pasture was estimated to fix between 91 and 146 kg N2/ha/yr at Grogan and 86-214 kgN2/ha/yr at the less sodic Morangarell site. When roots were also included these values increased to 156-254 kg N2/ha at Grogan and 148-370 kg N2/ha/yr at Morangarell. Estimates of N2 fixed by the lucerne-annual legume mixed sward at Grogan were similar to the annual legume sward, but at Morangarell estimates of N2 fixed by lucerne were both higher and lower depending on seasonal conditions. Gypsum increased estimated total N2 fixed by the annual and lucerne pastures by up to 16% and 37% respectively at Grogan, but had no effect at Morangarell.

Introduction

Legume-based pastures play an important role in the cropping zone of southern Australia by increasing soil nitrogen (N), acting as a disease break and providing income diversification. Placing a value on the N2 fixed by pastures has become increasingly important as farmers question the future role of pastures in the system given perceived potentially higher returns from cropping and the increased work load associated with running livestock. Most measures of N2 fixation by pasture legumes undertaken in Australia (e.g. see Peoples and Baldock 2001; Unkovich et al. 2010) and elsewhere in the world (Carlsson and Huss-Danell 2003) have generally indicated that the amounts of N2 fixed were related to legume dry matter (DM) production. However, many of these previous estimates of N2 fixation have focussed on measuring the N contained in the herbage (Peoples and Baldock 2001) and have ignored the below-ground contributions of N associated with root exudates (rhizodeposition; Wichern et al. 2008) and present in, or derived from, roots and nodules (Peoples et al. 2001). An improved understanding of the relationship between the N contained in the roots relative to top growth in both annual and perennial pasture legumes has allowed more realistic estimates of the total N2 fixed by legume pastures to be derived. The following study sought to quantify the likely N contribution of annual legume and lucerne pastures growing on heavy clay sodic Vertisols in southern New South Wales (NSW) over a three year period following amelioration with gypsum.

Materials and methods

Two pasture types (annual legume pasture and a lucerne (Medicago sativa L.) -annual legume mixture) were established on 2 sodic Vertosol soils at Grogan and Morangarell in southern NSW in autumn 1999. The annual pasture treatment was a mixture of 2 cultivars of subterranean clover (Trifolium subterraneaum L.) (cvs Riverina and Clare) and balansa clover (T. michelianum)(cv. Paradana). The lucerne treatment consisted of cv Aquarius, sown with the same annual legumes at half the rate as the annual only treatment. Three rates of gypsum were applied prior to sowing and incorporated by rotary hoeing, the rates varied to account for the different levels of sodicity at the two sites. The more sodic Grogan site received 0, 5, or 10 t gypsum/ha and the Morangarell site 0, 2.5 and 5 t/ha. The experimental design at both sites was a fully randomised split plot design with 4 replications, pasture type were main plots and gypsum rate sub plots within each main plot. Main plots were 4 m wide by 54 m long and subplots 4 m by 18 m.

Soil cores were taken at both sites to a depth of 1.5 m to characterise each site and analysed for pH, exchangeable cations and electrical conductivity. A detailed description of the analysis performed and the techniques used are given in Dear et al. (2010). The change in soil pH, exchangeable sodium and electrical conductivity down the profile at the two sites is given in Figure 1.

Figure 1. Change in soil pH, exchangeable sodium percentage (ESP) and electrical conductivity (EC) down the profile to 1.5 m at (a) Grogan and (b) Morangarell experimental sites.

Herbage dry matter (DM) yields were measured at intervals throughout the growing season and partitioned into annual legume and lucerne components. Direct measurement of symbiotic N2 fixation is problematic as available methods are not always reliable and can be expensive to apply (Unkovich et al. 2008). Although the amounts of N2 fixed by the lucerne and annual legume swards were not measured in the current experiment likely inputs of fixed N were approximated using relationships between legume herbage DM and N2 fixation derived from a large number of prior studies of subterranean clover and lucerne under Australian conditions (20 and 18 kgN/tDM for annual legume components and lucerne; respectively Unkovich et al. 2010), including studies previously conducted at the Grogan and Morangarell sites (Dear et al. 2003). Total amounts of N2 fixed (shoots + roots) were subsequently calculated by multiplying the estimated N2 fixed in shoots by a ‘root N factor’ (1.72 for subterranean clover and 2.0 for lucerne; Unkovich et al. 2010).

The data were statistically analysed by fitting linear mixed models using the Asreml package (Butler et al. 2007) in the R environment (R Development Core Team 2008). Least significant differences (l.s.ds) to determine statistical differences were calculated at P = 0.05.

Results

The application of gypsum increased herbage yields at the Grogan site on 10 of the 16 occasions pasture yield was measured over the 3 year period. Seasonal pasture responses to gypsum at the less sodic Morangarell site were few and statistically unimportant. A more detailed description of the seasonal and total productivity of the lucerne and annual swards is presented in Dear et al. (2010).

Estimates of shoot N2 fixation at the more sodic Grogan site varied with gypsum treatment, species and year (P<0.001). Based on shoot growth only, annual swards fixed on average between 91-146 kgN2/ha/yr and lucerne 82-138 kgN2/ha/yr (Table 1). At the Morangarell site there was a significant (P<0.001) species by year effect on estimated shoot N2 fixation, but it was unaffected by gypsum treatment with annual swards calculated to have fixed on average 86-214 kgN2/ha/yr, and lucerne pastures 109-147 kgN2/ha/yr and (Table 1).

Estimates of total N2 fixation (shoots + roots) at Grogan was influenced by species, gypsum treatment and year and ranged from 156-268 kgN2/ha/yr in annual swards and 157-267 kgN2/ha/yr in lucerne swards. Total N2 fixation estimates at Morangarell varied with species and year, but were unaffected by gypsum treatment. Annual pasture swards were estimated to fix up to 370 kgN2/ha/yr compared to 279 kgN2/ha/yr for lucerne.

Increases in herbage DM and amounts of N2 fixed due to gypsum application at the more sodic Grogan site ranged from 0-16% in annual swards and 0-37% in lucerne.

Table 1. Estimated biological N2 fixation (kgN2/ha/yr) in shoots and shoots+roots of annual pasture (AP) and lucerne/annual pasture (L) swards at (a) Grogan and (b) Morangarell in 1999, 2000 and 2001 in response to 3 rates of gypsum (0, 5, 10 t/ha at Grogan and 0, 2.5, 5 t/ha at Morangarell).

Year /plant part

Pasture type and gypsum rate (t/ha)

   
     

(a) Grogan

     

1999

AP0

AP5

AP10

Mean

L0

L5

L10

Mean

Shoots

91

89

92

91

88

80

77

82

Shoots+roots

157

155

158

156

167

155

149

157

2000

               

Shoots

133

152

154

146

115

145

155

138

Shoots+roots

231

264

268

254

220

281

301

267

2001

               

Shoots

93

102

102

99

97

107

110

104

Shoots+roots

161

176

175

170

183

205

213

200

l.s.d.(P = 0.05) between shoot N values=12, between total N values = 22

     

(b) Morangarell

     

1999

AP0

AP2.5

AP5

 

L0

L2.5

L5

 

Shoots

107

109

114

110

109

111

108

109

Shoots+roots

184

188

196

189

203

208

201

204

2000

               

Shoots

213

211

218

214

143

150

149

147

Shoots+roots

368

365

377

370

269

282

285

279

2001

               

Shoots

83

87

89

86

111

111

110

111

Shoots+roots

142

149

154

148

219

218

217

218

l.s.d. .(P = 0.05) between shoot N values = 10, between total N values = 19

Discussion

The contributions of N2 fixation for legume growth varies during the year (Dear et al. 1999) and with levels of available mineral N in the soil (Unkovich et al. 2010) although studies have shown that the proportion of legume N derived by biological fixation in the winter dominant rainfall zone of southern Australia is uniformly high (65-95%)(Peoples et al. 2001). The values used to estimate N2 fixed in the current study are based on average values derived from data collated from a large number of studies of lucerne and annual legumes in Australia (Unkovich et al. 2010). Given that previous measures of N2 fixation undertaken elsewhere in southern NSW by Peoples et al. (1998) and Dear et al. (1999) found that subterranean clover fixed between 19-34 kgN/t above-ground DM and lucerne 17-29 kgN/tDM and, the values of 20 and 18kgN/tDM used for subterranean clover and lucerne in the current study could therefore be considered to provide a conservative estimate of realistically achievable targets of total N2 fixation on these heavy clay sodic soils.

The estimates of yearly total shoot N2 fixed in the current study were 86-214 kgN/ha/yr for subterranean clover and 82-147 for lucerne/annual swards which were comparable to, or higher than, those observed by Peoples et al. (1998) for pastures in districts around Junee on red-brown earths, but were lower than values previously reported on a red earth at Wagga Wagga in a similar climatic zone (Dear et al. 1999). The values most likely reflect the lower yield potential of pastures on the vertosols compared to red earths and the effects of the drier seasonal conditions.

In the current study lucerne did not produce significantly greater N2 fixation than the annual pastures as has been reported previously (Peoples et al.1998; Dear et al. 1999), despite a potentially longer growing season. This is undoubtedly related to the generally dry conditions experienced over the summer periods that reduced the competitive advantage of a perennial species such as lucerne which has the capacity to utilise water outside the growing season of the annuals (Peoples and Baldock 2001; Sandral et al. 2006). Had more summer rain occurred generating additional lucerne growth, the resultant annual amounts of N2 fixed by lucerne would be expected to increase relative to the annual pastures.

Conclusion

Gypsum enhanced the productivity of the pastures at the sodic Grogan site which had high levels of surface sodicity (>10% ESP in top 10cm), and the herbage response was reflected in the increased estimates of N2 fixation in 2000 and 2001. Although the Morangarell site was sodic at depth, it had relatively low surface sodicity (<5% ESP in top 10cm) and hence herbage yield and N2 fixation did not respond to gypsum applications. The derived estimates of N2 fixation presented here were calculated using simple empirical relationships between shoot DM and symbiotic N2 fixation (Unkovich et al. 2010). This approach represents a means of assessing inputs of fixed N to farms where only pasture legume DM production is known. While it is acknowledged that the assumed size of below-ground pools of fixed N in the current study were approximations at best, we believe that errors associated with the inclusion of below-ground N in the calculations were far less than those that might be incurred by ignoring N contributed by the nodulated roots. It was clear from comparisons of the estimates of shoot N fixed with total (shoot + roots) inputs of fixed N that any conclusions about the relative role pastures play in building the organic fertility of soils in mixed farming systems could be substantially underestimated if determinations of N2 fixation were based solely on shoot data.

References

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