Summary
Agriculture is a major contributor of greenhouse gases, including carbon dioxide. Australia, like most other developed countries of the world, is committed to understanding and controlling emission of greenhouse gases, and there is a need to use farm management practices that minimise emissions. The global soil carbon (C) pool is large in comparison with the atmospheric C pool, so small changes in the size of the soil C pool may have a considerable effect on atmospheric carbon dioxide levels. Apart from its potential to influence atmospheric carbon dioxide, soil organic C is of critical importance in the maintenance of soil quality in natural and agricultural systems. Soil C sequestration (accumulation) represents the net balance between C inputs to and C losses from the soil.
The main process responsible for input of C to the soil is below-ground allocation of photosynthetically fixed C through plant root growth. C loss occurs mainly through emission of carbon dioxide during microbial respiration (organic matter decomposition) and root respiration. The capacity of a soil to sequester C depends on a range of soil, management and climatic factors. Australia has identified an opportunity to increase C sequestration through improved management of agricultural soils. In soils under pasture, increases in C sequestration have been demonstrated, but the effects of management practices are still unclear. The existence in Hamilton of a long-term (25 year) pasture experiment comparing a range of superphosphate fertiliser application rates and sheep stocking rates presented an opportunity to assess the influence of these management factors on soil C sequestration in the high-rainfall zone of Victoria.
This study documents measured pasture and animal productivity, measured organic C in various soil pools, and modelled projections of future trends in soil C using the Roth-C model. The objectives of the study were (1) to determine the effects of superphosphate application and sheep stocking rate on soil C sequestration under pasture in the high-rainfall zone of western Victoria, (2) to compare potential long-term trends in soil C in the different pasture systems and (3) to identify management practices that landholders may use to promote soil C sequestration. Pasture production was strongly increased by phosphorus (P) application, ranging from 4 t dry matter/ha at nil fertiliser up to 16 t dry matter/ha at the highest P application level of 33 kg P/ha annually. This increase in pasture production allowed a three-fold increase in stocking rate from 9 to 28.5 dry sheep equivalents/ha, and was accompanied by an increase in wool production from 55 to 133 kg wool/ha. Soil C sequestration was not significantly affected by either P application rate or stocking rate, even after 25 years of treatment. However, increasing rates of P application produced a trend of slowly increasing C sequestration, that would only be detectable by soil analysis if the higher application rates were continued for periods in excess of 30 years. The RothC model gave good prediction of soil C changes that were measured over the previous 10 years of the experiment.
Long-term modelling scenarios indicated that management resulting in low pasture production would lead to slow C loss, particularly in soils of initially high C content, that would be difficult to measure over periods of less than 50 years. Management resulting in high pasture production, on the other hand, would lead to long-term C gains, though they would not be detectable for 20-30 years. Under high productivity, C sequestration would increase by 0.4-0.5 t C/ha.year during the first 50 years, eventually leading to an increase of 18-63% in the top 30 cm, depending on the starting level of soil C. Such potential increases are significant when compared with C sequestration achieved by various management improvements to grasslands across the world.
Thus, increased plant and animal production through P fertiliser use is generally consistent with gradually increasing soil C sequestration, but the long time-scales involved make it difficult for individual farmers to influence soil C levels on their farms. Nevertheless, by maximising plant production through appropriately matched P application and grazing regimes, sheep producers can have confidence that these grazing systems are not likely to be detrimental to soil C sequestration. The maintenance, and eventual improvement, of soil C levels will provide direct benefits to farmers in terms of soil structure and fertility, water retention, reduced erosion, and improved sustainability of the farming enterprise. This work contributes new information to the discussion of C sequestration issues relating to sheep grazing systems.
