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The use of ley pastures to arrest soil fertility decline in central Queensland.

Michael Braunack1 2 and Maurice Conway1

1 QDPI&F, LMB 6, Emerald, Qld 4720, email Maurice.Conway@dpi.qld.gov.au
2
Now CSIRO, Locked bag 59, Narrabri, NSW 2390 email michael.braunack@csiro.au

Abstract

With the increasing cost of nitrogenous fertiliser growers are looking for alternative strategies to maintain or increase soil nitrogen levels. An opportunity was taken to sample butterfly pea (Clitoria ternatea) pastures of various ages to compare the total nitrogen contribution with that of adjacent native grassland (Queensland bluegrass, Dichanthium sericeum). Butterfly pea is a productive, perennial legume adapted to tropical conditions. Further assessment of butterfly pea pastures was made by comparing total nitrogen and carbon levels (0-10 & 10-20 cm) in native vegetation and cropping soils with that of the ley pasture across a range of sites.

Total nitrogen level under butterfly pea and grass pasture and that under native grassland was similar where biomass was greater than 2000 kg/ha, but was lower than native grassland where biomass under butterfly pea pasture was less than approximately 2000 kg/ha for both depths under drought conditions. Across a range of sites it was found that total soil nitrogen was greatest under native vegetation compared with cropping and butterfly pea soils, with levels being slightly higher in cropping than in soils with butterfly pea. A similar result was observed with total soil carbon levels.

Butterfly pea ley pastures can build soil total nitrogen to a level similar to that under native vegetation, depending on the biomass produced. In the short term butterfly pea ley pasture will maintain total soil nitrogen at a level similar to a cropped soil with nil or a budget application rate of nitrogen fertiliser.

Key Words

Total soil carbon, total soil nitrogen, native vegetation

Introduction

A fundamental requirement of any sustainable cropping system is the provision of adequate nutrients to optimise crop performance, either by applying fertilisers or by exploiting inherent soil fertility. There are two main soil types that are cropped in the region, known locally as ‘Open Downs’ soils (shallow black vertosols, often overlying decomposing basalt and supporting open bluegrass grasslands in their native state) and ‘Brigalow scrub’ soils (black, brown and grey vertosols and sodosols, originally supporting Brigalow (Acacia harpophylla) dominant woodlands (Spackman and Garside, 1995). A number of factors combine to make the use of ley legumes attractive as a low cost method of restoring soil N fertility in central Queensland cropping systems, including: the relatively high cost of N fertiliser, high variability of seasonal rainfall, making prediction of optimal N fertiliser application rates problematic, and the fact that the majority of central Queensland grain growers also support a beef cattle enterprise.

This paper presents the results from sampling butterfly pea (Clitoria ternatea) pastures of various ages to compare the total nitrogen contribution with that of adjacent native grassland. Further assessment of butterfly pea pastures was made by comparing total nitrogen and carbon levels in native vegetation and cropping soils with that of the ley pasture across a range of sites.

Methods

Soil samples were collected from two depths (0-10 and 10-20 cm, vertosol soil) under butterfly pea and butterfly pea plus grass pastures in 1999, which corresponded to 6, 4, 3, 2, 1 and 0 years after pasture establishment. These samples were analysed for total nitrogen (%) (Bruce and Rayment, 1982). Corresponding plant biomass samples were collected from 1 m2 quadrats at the same time. A wider survey was conducted to compare total soil nitrogen and total soil carbon (Walkley and Black) (0-10 and 10-20 cm) under cropping, butterfly pea and undisturbed soils in close proximity at a range of sites covering the two major cropping soils of central Queensland. This was a one off sampling to determine level of total soil nitrogen and carbon at that point in time. Three cores were collected from the 0-10 and 10-20 cm depths from cropping areas, butterfly pea ley pastures and from adjacent native vegetation at 4 sites and individual sites and depths bulked for analysis. In this second survey, butterfly pea pastures had been established for five years prior to sampling. Since the samples were bulked to determine total soil nitrogen and carbon no statistical analysis was possible. The data represent values of total soil nitrogen and carbon at one point in time

Results

Total nitrogen level under butterfly pea plus grass pasture and that under native grassland was similar where biomass was greater than 2000 kg/ha, but was lower than native grassland where biomass under butterfly pea pasture alone was less than approximately 2000 kg/ha for both depths under drought conditions (1996/97) (Figure 1). This suggests that the total biomass produced is more important than the length of time the pasture has grown in contributing to total soil nitrogen. The biomass represented in Figure 1 was that present at sampling in 1999.

Figure 1. Total nitrogen (%) for 0-10 & 10-20 cm depths and plant biomass (kg/ha) for pastures of different ages. (Native = ungrazed stock route, BFP_93 = Butterfly pea pasture planted 1993, BFP+G_93 = Butterfly pea plus grass pasture planted 1993, _96, _97, _98, _99 correspond to the year of planting of both Butterfly pea & Butterfly pea plus grass. Samples collected 1999)

The results from the wider survey showed that the native vegetation had the greatest levels of total carbon (Figure 2) and nitrogen, (Figure 3) with the cropping and the butterfly pea ley pasture having similar levels.

There is variation in total carbon and total nitrogen levels between sites, which reflect environmental and climatic differences. The total levels are lower in the 10-20 cm depth compared with the 0-10 cm depth (Figure 4, 5). This may be expected since leaf litter and crop residues fall on the soil surface and then need to be incorporated by soil macro fauna before breakdown can occur. We cannot explain why the levels of nitrogen and carbon are so high for the native vegetation site at Capella. Similar soil carbon levels have been measured in a recent survey for soil health workshops of a wide range of crop/pasture combinations across central Queensland (Conway, pers. comm. 2008).

At this early stage it appears that butterfly pea has not restored soil fertility to a level greater than cropped areas. This situation may change as the time under ley pasture increases. It should be remembered that the data represents only one sample from each site and the ley pasture has only been in place for a relatively short period. The soil carbon levels reported are approximately equivalent to 20 years cultivation, with the exception of the Capella site which is about 10 years cultivation (Dalal and Mayer, 1986). This suggests that most sites would be responsive to nitrogen application. The butterfly pea pasture appears to be able to maintain soil fertility at a similar level to the cropped soil.

Conclusion

Butterfly pea ley pastures can build soil total nitrogen to a level similar to that under native vegetation, if the biomass produced is greater than 2000 kg/ha except in drought years.

In the short term butterfly pea ley pasture will maintain total soil nitrogen at a level similar to a cropped soil with nil or a budget application rate of nitrogen fertiliser.

Figure 2. Total soil carbon (%) for cropping, BFP and native pasture sites (K=Kilcummin, O=Orion, C=Capella, B=Baralaba)

Figure 3. Total soil nitrogen (%) for cropping, BFP and native pasture sites (sites as in Figure 2)

Figure 4. Average total soil nitrogen (%) for crop, BFP and native vegetation (4 samples for each column)

Figure 5. Average total soil carbon (%) for crop, BFP and native vegetation

References

Dalal RC and Mayer RJ (1986). Long-term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. II Total organic carbon and its rate of loss from the soil profile. Australian Journal of Soil Research 24: 281-292.

Rayment GE and Higginson FR (1992). Australian laboratory handbook of soil and water chemical methods. Inkata press, Melbourne.

Spackman GB and Garside AL (1995). Major factors affecting grain production in central Queensland and their implications for sustainable grain farming systems. A review prepared for the Grains Research and Development Corporation, July, 1995, 77p.

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