February 7, 2013
As the focus swings from 2012 to the 2013 cropping season, Hamilton district farmers will be contemplating their rotations, management and systems for the coming cropping season. Stubble management in high rainfall zones is often a challenge with farmers facing the ultimate question – to burn or not to burn. Research by DPI scientists has provided some insights into the impacts of different stubble managements and crops types on the levels of stored carbon. DPI Hamilton researcher Ms Penny Riffkin said soil carbon levels were expected to decline under cropping in the Dunkeld area where the starting soil organic carbon (SOC) was high regardless of the management regime. “The rate of decline will be greater where less stubble is retained,” she said. “However, the method of management will have a significant impact on maintaining or increasing SOC where starting SOC is low to medium,” she said. What is soil organic carbon? Soil organic carbon (SOC) is defined as the stored carbon within the soil and, together with other elements such as nitrogen, calcium, oxygen and hydrogen, forms soil organic matter (SOM). It is important for a variety of functions in the soil including; Biological functions – by providing energy and nutrients (particularly N, P, and S) for biological processes. Physical functions – by improving structural stability, water retention and altering the thermal properties of the soil. Chemical functions – by contributing to the cation exchange capacity and pH buffering in the soil. These functions are important for crop growth and yield, reducing erosion and for improving the ease with which cultural practices can be conducted by improving trafficability. The amount of soil organic carbon (C) depends on the balance between C inputs such as plant residues (stubble, roots and plant exudates) and outputs or losses as CO2 as a result of decomposition by microbes. The balance is strongly influenced by climate, soil type, topography, vegetation and management history. In cropping systems, about 80 per cent of C input is decomposed by microbes and lost within 1-2 years. This loss is accelerated under cultivation. The effects of stubble load on SOC At Dunkeld the research group modelled four different management methods on five different levels of stubble load. These included burning with 10 per cent of stubble retained Baling with 25 per cent of stubble retained Grazing with 75 per cent of stubble retained Mulching, standing or incorporating with 100 per cent of stubble retained. Results Under a burnt stubble regime, stubble loads of at least 10t/ha would need to be achieved in every year to maintain SOC levels from a starting base of three per cent. Stubble loads of around 5 t/ha would need to be achieved under grazing or mulching to maintain or increase SOC levels. Grain yields of at least 5.4 t/ha for wheat or 4.3 t/ha for canola would need be achieved in each year to maintain stubble residue loads of 10 t/ha. In summary • Stubble burning and baling reduces soil organic carbon (SOC) except where starting SOC is low ( less than two per cent). • Stubble mulching and grazing increases SOC except where the starting SOC is high (more than three per cent). • Losses of SOC can be as high as 0.9 t/ha per year from soils with a high starting SOC (five per cent) and where stubble is burnt. • Gains of 0.7 t/ha per year can be achieved from soils with a low starting SOC (1.5 per cent) and where stubble is mulched. Take home messages: Soil organic carbon (SOC) declines where stubble loads are small (< 3t/ha) regardless of how stubble is managed. • SOC is maintained or increases where stubble loads are high (>10t/ha) regardless of stubble management. • At medium stubble loads (5t/ha), burning and baling generally reduces SOC, whereas mulching and grazing generally increases SOC. • Losses of SOC can be as high as 0.9 t/ha per year from soils with a high starting SOC level (five per cent) and where stubble is burnt. • Gains of 0.7 t/ha per year can be achieved from soils with a low starting SOC level (1.5 per cent) and where stubble is mulched.