Managing soil carbon and nitrogen in kikuyu pastures in the Fitzgerald Biosphere

Timeframe: June 2016 to February 2018

Funding source: Action on the Ground: Australian Government Department of Agriculture

Project Coordinator: Karryn Dorrell.

Partners (providing technical support): Department of Agriculture and Food WA, University of Western Australia Centre for Excellence in Natural Resource Management, University of Queensland.

Additional support: South Coast NRM

Summary

The objective of this four-part project was to investigate ways of reducing nitrous oxide emissions whilst increasing soil carbon stores in South Coast farming systems.

The aim of the first sub-project was to compare nitrous oxide emissions from different companion legumes grown into established kikuyu pasture to identify the best options for reducing reliance on nitrogen-based fertilisers. This sub-project ended early when poor seasonal conditions resulted in insufficient establishment of the legume varieties that were to be compared.

The aim of the second sub-project was to monitor nitrous oxide emissions when synthetic nitrogen was applied to clover and kikuyu pastures. Results were variable due to a number of constraints, but showed an increase in soil carbon in the sites that were fertilised with synthetic nitrogen.

The aim of Sub-project three was to compare carbon sequestration capabilities of soils under three different growing regimes: established perennial pastures, annual pasture and cropping. Results showed there was little difference in carbon fixation between them.

The objective of the final sub-project was to establish eight baseline carbon sequestration sites on newly sown perennial pastures. More than 700 ha of perennials were sown on eight properties across the region. Baseline soil carbon measurements were taken, with the long-term view of using the data in future carbon monitoring projects.

The short (17 month) experiment timeframe was one of the main limitations of the project.

Project details and outcomes

Sub-project 1 Establishment of a large-scale key demonstration site that would trial a broad range of legume options and identify the better performing legumes in kikuyu-based pastures.

Trial location: Gairdner, WA (65 km from Jerramungup)

It was intended that once established, the persistence and productivity of the different legume varieties would be compared, and the impact the presence of legumes had on sward production in the kikuyu would be measured. The legumes sown, as both pod and seed, and at varying densities, were: Avilla, Bladder Clover, Margarita and Charano.

The decision was made to end this trial early when the legumes failed to establish in the first half of 2014 due to insufficient rain.

Sub-project 2: Monitoring of nitrous oxide emissions when synthetic nitrogen is applied to clover and kikuyu pastures.

Trial location: Gairdner, WA (approximately 50 km from Jerramungup)

The objective of this sub-project was to set up small trial plots in existing clover and kikuyu pastures and compare pasture productivity and nitrous oxide-nitrogen gas flux emissions after different synthetic nitrogen treatments were applied. The nitrous emission monitoring started in August 2013 and was conducted seven times, coinciding with rainfall events of above 10 mm.

Biomass estimates were highly variable and there were no significant differences across the treatments. Nevertheless, trends in mean data suggested clover was more productive than kikuyu and the nitrogen treatments were more productive than the controls.

There was high variability in the results, attributable to several factors, the most important being small field plot size, the strong non-wetting nature of the soil type, the sporadic nature of the denitrification process and the inherently high spatial variability of soil processes in undisturbed pasture soils.

The greatest constraint was the short duration (17 months) of the pasture treatments. Unfortunately, this short time period meant no contingency options for analysis delays and other unforeseen issues that were encountered.

Sub-project 3: Determination of carbon sequestration capabilities of soils with established perennial pastures (compared with annual pasture and cropped soils).

Four farming families, whose properties represent different rainfall zones, geographical locations and soil types, participated in this trial. Carbon levels in the soil and pasture growth were measured and compared with adjacent paddocks that had annual pasture or were being cropped.

Total organic carbon content in the soils tested were at moderate to good levels. Of note, none of the participants strained their perennial systems or over grazed their annual pastures, particularly during the summer months when all soil types in the southern region of Western Australia are susceptible to topsoil loss through wind erosion.

The comparison study was the most interesting component of this investigation, with the two cropping versus perennial rotations returning the closest difference in carbon sequestration data to the perennial pasture within the top ten centimetres. Carbon content from deeper within the soil (0-30cm) consistently showed that it was slightly better from both perennial pasture sites.

This was attributed to the practice of retaining crop stubble and maintaining sufficient ground surface cover, together with improved stocking and grazing management strategies over the summer period to prevent over grazing and wind erosion, and the use of minimum tillage to retain soil moisture and maintain soil structure.

The most unexpected result came from the comparison between the perennial pasture system and the permanent annual pasture system. Although the carbon content results show an unexpected high content value of total organic carbon, it is important to note that the permanent annual pasture system was more susceptible to breakdown.

The farmers involved commented that the perennial system, particularly the kikuyu, had visibly healthier soil than the other pasture systems on their farms. However, the data did not find a strong correlation between perennial farming systems having an advantage over annual farming systems – this likely because of the short duration of the project.

Sub-project 4: Establishment of eight satellite baseline carbon sequestration sites on newly sown perennial pastures.

Baseline soil carbon data was obtained from paddocks that were newly sown to perennial pasture, with the long-term view of using the data in future carbon monitoring projects. More than 700 ha of perennials, predominantly Lucerne, gatton panic, saltbush and kikuyu, were sown in eight properties across the region. The sites were selected so that they were evenly dispersed across rainfall and soil types within the shire. This sub-project served to increase awareness of the benefits of perennial pastures.