Pasture measurements and bio-economic analyses to assess effects of climate, grazing pressure and pasture rundown on soil carbon and returns from legume-based sown pastures in the Condamine region of Southern Queensland. Final Report on project AOTGR1-137 'Increasing soil carbon in degraded cropping and grazing land'

Pasture measurements and bio-economic analyses to assess effects of climate, grazing pressure and pasture rundown on soil carbon and returns from legume-based sown pastures in the Condamine region of Southern Queensland. Final Report on project AOTGR1-137 'Increasing soil carbon in degraded cropping and grazing land'

The Condamine catchment has been identified as a key area in Australia where there is potential to build soil carbon. There are approximately 1 million hectares of degraded crop and grazing lands in the region that could be potentially improved through establishment of sown pastures, particularly legume-based pastures that have capacity to add nitrogen, lift productivity and build soil carbon.

This document provides a final report on several sub-components for the Condamine Alliance project “Increasing soil carbon in degraded cropping and grazing Land (AOTGR1-137)”. The overall objectives of this project were to measure, assess and communicate the impact and feasibility of practices for increasing soil carbon sequestration in cropping and grazing land in the Condamine catchment. Field studies from July 2012 to March 2015 at nine trial sites tested the value of returning crop land to pasture, renovation of pastures and the use of manure and inorganic fertilizer.

As part of the overall project, the objectives addressed in this report focus on bio-economic analyses to assess the value of sown pastures in the Condamine region in terms of their capacity to build soil carbon and provide significant economic benefits to industry. The influence of seasonal conditions, land type and management were considered. This included the effects of grazing pressure, manure, fertilizer and importantly the use of legumes. A sown pastures version of the GRASP grazing systems model was used to estimate the effects of season and management on pasture growth and condition, beef production, economic returns, soil carbon sequestration and green-house gas emissions. Nitrogen available for pasture growth was a key component of the model concerning effects of pasture rundown and the influence of legumes. Potential effects of other nutrient limitations particularly phosphorous were recognised but not included in simulation analyses. Soil tests and pasture measurements of net primary production from exclosures and pasture yield/composition from grazed paddocks at the trial sites were used to calibrate and test the modelling process.

The sub-tropical sub-humid inland climate of the Condamine region is favourable for growing sub-tropical grass-legume sown pastures in most years. The long-term average rainfall for the region is 672 mm. Weather conditions during the trial period were variable with several extended periods of hot and dry drought conditions; particularly in 2013 and 2014. The modelling and simulation studies were an important tool for overcoming the influences of climate variability on results, as well as reasonable inferences due to the short term duration of the project.

The mean observed value of soil carbon (0-30 cm) across all trial sites was 1.13%. Values ranged from 0.63 to 0.88 % carbon (31 to 35 t/ha) on light sandy soils of the Brigalow and Alluvial plains to an average of 1.68 % carbon (52 t/ha) for the black cracking clays of the Basalt uplands with some values up to 2.16%.

The mean annual growth of pastures across all sites, pastures, grazing pressures and climatic conditions was estimated to be 3076 kg/ha. This mean was substantially higher on the more fertile clay soils of the region’s Basalt Uplands (3898 kg/ha) but lower on less fertile loam soils of Alluvial Plains (2648 kg/ha) and the sandstone derived soils of the Brigalow Uplands (2708 kg/ha).

The optimum commercial grazing pressure in terms of maximum economic return per hectare was estimated to be 25 to 30% utilisation of pasture growth and this was consistent across all trial sites and land types. Lower utilisation levels were estimated to increase live weight gains per head but this reduced overall economic returns. Higher utilisation levels increased short-term economic gain but were likely to cause detrimental effects on pasture condition if persistently used, and also reduce live weight gains per head. This led to lower gross margins and would ultimately lead to reduced soil carbon.

Use of legume-based sown pastures in the GRASP simulation experiments was estimated to maximise economic returns at all sites. At the optimum grazing pressure the mean gross margin across all sites of legume-based sown pasture was $78.60/ha compared to $44.50/ha and $40.50/ha respectively for sown grass pastures and native pasture.

Soil carbon sequestration rates were estimated from simulation modelling to be much higher in the first decade after planting a sown pasture than in later decades. This reflected changes in pasture productivity associated with pasture rundown and the progress of soil carbon levels towards new equilibrium conditions. In 50 year simulations (repeated for four different time periods at each site), the mean carbon sequestration of sown grass pastures for the first decade was 459 kg/ha/year compared with 10, 15 and -36 kg/ha/year over the last three decades. Grass-legume pastures had the highest carbon sequestration rates. They were estimated to sequester an average of 595 kg/ha/year for the first decade after sowing and 113 for the second decade. This was followed by an average of 32 kg/ha/year over the last three decades.

Cattle methane emissions were calculated on the basis of dry matter intake. Estimates from GRASP simulations showed that sown pastures should help to reduce green-house gas emissions because:

• the additional mean annual soil carbon sequestered by sown grass pastures (relative to native pasture) was 412 kg CO2e /ha and this was substantially more than the corresponding increase in methane emissions (58 kg CO2e/ha) due to higher stocking rates
• the difference was greater for sown grass-legume pastures. Legume-based sown pastures increased carbon sequestration relative to native pasture by 1411 kg CO2e /ha compared with the much lower increase in methane emissions of 127 kg CO2e /ha.

Nitrous oxide emissions as CO2e were estimated from legumes (due to nitrogen fixation) and from livestock urine and faeces due to increased stocking rate on sown pastures. These estimates were low in comparison to carbon sequestration and were lower than methane emissions.

Key messages communicated to producers at a series of field days in March 2015 were:

• Degraded crop and grazing lands are improved through establishment of legume-based pastures with bonus payoffs in production, carrying capacity, economic returns, and GHG emissions and sequestration rates.
• Sown pastures are usually most productive in the first few years after planting and then gradually decline in productivity (known as 'pasture rundown' ) in the following years because of nutrient limitations mainly nitrogen
• Maintaining legumes in pastures increases soil nitrogen, pasture growth and cattle production.
• Legumes can help to offset pasture rundown.
• While droughts cause significant losses in some years, nitrogen is limiting in most years.
• Stocking rates should aim to utilise 25 to 30 % of pasture growth.
• Increased pasture production builds soil carbon which improves soil health.

The study has highlighted several concepts that include the following:

• Pasture rundown is a consistent feature of sown pastures in grazing systems and therefore needs to be taken into account in farm management planning processes and carbon sequestration rate calculations
• Legume contributions of nitrogen to foster additional grass growth were important at all sites to either offset or overcome the effects of pasture rundown. They enable production to plateau at a higher level than grass only pastures, which emphasises the importance of legumes in sown pastures.
• Legumes are a relatively minor cost when establishing a sown pasture but they contribute greatly to the profitability of sown pastures. This highlights the value of developing technologies to improve the reliability and resilience of agricultural practices to successfully establish and maintain palatable legume-based sown pastures.

Lucerne was demonstrated to be a highly successful legume for pastures on the heavier clay soils of the Condamine region, however, more summer active legumes might give additional benefits in the regions summer dominant rainfall climate. The summer growing legumes, shrubby stylos, particularly Caatinga stylo, desmanthus, Wynn cassia, fine-stem stylo and leucaena are options that should be considered.

The sown pastures version of the GRASP model has proved to be a useful tool in several ways. Firstly, it provided a mechanism to integrate information from the trial sites and elsewhere which was then used to interpolate and extrapolate data across sites, time periods and levels of output relevant to industry. Secondly, the simulation results are providing industry with information for discussion that would be otherwise not available, and thirdly GRASP and the sown pastures version of GRASP are providing a platform that may well be useful to other projects.

The Condamine Catchment is a highly productive agricultural region and this study has shown that it has the potential to significantly increase soil carbon over a large area with legume-based pastures. The region stands out as an area in Australia to continue work to demonstrate, test the value and seek adoption of sown pastures. Therefore it is recommended that further work be conducted in this field to work with the farming community in planned extension programs to demonstrate the advantages of legume-based pastures for increasing productivity, building soil carbon and improving economic returns. This work should integrate field studies, analyses and communications that emphasise farming practices that help to overcome risks including the risks that are linked with agronomy, grazing management, financial issues and climate variability.

The need for this work to continue is accentuated by the continuing rise of carbon dioxide levels in the atmosphere and resulting effects on rising global temperatures and increased climate variability, and also to clarify issues concerning the storage of carbon in soils as a greenhouse gas abatement strategy.