INTRODUCTION: Both reduced tillage methods and the addition of organic material have been seen to increase soil organic carbon (SOC) content in agricultural land as well as improve their fertility and soil health. The positive impacts and benefits seen from these methods have been recently published Stern report on the economics of climate change which promoted reduced tillage as a means of enhancing the storage of carbon in agricultural soils. Stern (2006) cited the example of the Chicago Climate Exchange, where a minimum four-year commitment to continuous zero-tillage on enrolled areas was valued at $1-2 per acre per year (equivalent to approximately £1.25-£2.50/ha). Following this, the UK Climate Change Programme set out to investigate the efficiency and effectiveness of a market based mechnaism which could enable the trading of GHG emissions from agriculture, forestry and other land management sectors which included looking at carbon storage in soils and forests as a potential option. 4.5 million ha of tillage land covers England and Wales, meaning that even a small increases in SOC storage per hectare of agricultural land could overall lead to a significant increases in carbon storage and sequestartion at a national level.
However, these changes in SOC are slow to occur and difficult to measure against the large background carbon content in arable soils in the UK. After a change in agricultural soil management practice such as the introduction of zero tillage or regular organic material additions, the SOC content will increase (or protentially decrease) towards an equilibrium (after 100 years or more) that is characteristic of the soil type, land use and climate. Annual rates of SOC accumulation (or depletion) therefore change over time and gradually decline as the new equilibrium is approached, when they become zero. Typically, c.50% of the SOC accumulation achieved after 100 years of introducing a management change, occurs within the first 20 years. Maintaining SOC at the new equilibrium level then becomes the main priority, which may be dependent on continuing or finding new management practices.
EFFECTS OF REDUCED TILLAGE ON SOC: ‘Reduced tillage’ is a term that is used to describe all non-plough based cultivation practices and has been suggested to increase SOC due to a reduction in soil disturbance and consequently the decomposition of organic matter (carbon). Approximately 50% of primary tillage practices in England and Wales in 2005 used mouldboard ploughing. Comparing the various arable planting methods used in the UK is difficult as only six tillage studies have been carried out. However, from this it can be estimate that the carbon storage potential of UK soils under zero tillage methods is is c.0.34% carbon content. This figure is then halved for reduced tillage methods. It must also be considered that these storage rates are only for the initial rate of increase, which is up to 20 years. After this annual rates of SOC storage will decrease until the equilibrium is reached.
As well as storing carbon in soils, reduced tillage has many benefits including the increase soil water infiltration rates and reduce water erosion, enhance soil water retention, and decrease production costs and fossil fuel (energy) consumption. However, zero tillage has also been shown to increase direct emissions of nitrous oxide (N2O) compared with conventional tillage, due to an increase in topsoil wetness and/or reduced aeration as a result of less soil disturbance. Nitrous oxide is a powerful greenhouse gas with 310 times the global warming potential of CO2, such that overall, increased N2O emissions may completely offset the balance of greenhouse gas emissions compared with the amount of C potentially stored through changing from conventional to reduced/zero tillage practices. However, the evidence is not clear and further work is required to determine the effect of contrasting tillage systems on N2O emissions, SOC storage and the overall balance of greenhouse gas emissions.
EFFECT OF ORGANIC MATTERIAL ADDITIONS ON SOC: The recycling of organic mattter (OM) to land provides a valuable source of nutrients, minerals and organic matter, and could potentially increase SOC levels. Currently, around 90 million tonnes of farm manures, 3-4 million tonnes of biosolids (treated sewage sludge) and 4 million tonnes of industrial ‘wastes’ are applied annually to agricultural land in the UK.
Muliutple experiments have examined the effects of OM additions on SOC. The results are based on average changes in SOC measured at 8 farm manure study sites, 10 biosolids study sites, 4 green compost study sites and 8 straw incorporation study sites. The potential SOC estimates increase when both OM additions and reduced tillag as seen in Table 1. These figure should be regarded as the initial (c.20 years) rate of SOC increase. It is debatable whether increases in SOC following the application of farm manures and soil incorporation of cereal straw can be considered genuine additional carbon storage, as nearly all of these materials are already applied to land. Similarly, only 1% of biosolids are presently landfilled. Only if the organic materials are diverted away from landfill, can the increased SOC be regarded as genuine additional carbon storage (against a recent/present day baseline). This is probably the case for compost and paper crumble applications, with c.480,000 tonnes of green compost and c.700,000 tonnes of paper crumble currently recycled to agricultural land. However, at current production and application rates they are only applied to relatively small areas of land (<50,000ha), although compost use on agricultural land is expected to increase at least 3 to 5-fold over the next decade.
The application of OM to agricultural soils can help to maintain (and enhance) existing SOC levels, as well as help to improve soil structure/ stability. This often results in an increase in soil water retention and water infiltration rates (thereby reducing the risks of soil erosion), improves plant nutrient uptake and acts as an organic fertiliser. This provides both cost and energy (fossil fuel) savings involved in manufacturing inorganic fertilisers. Moreover, the application of organic materials can lead to environmental pollution due to N and P losses to water courses, as well as gaseous emissions of N2O to the atmosphere. However, reductions in inorganic fertiliser N usage offset most of these losses following organic material additions.
FIGURE 2: CO2 stored/ saved from zero tillage/ reduced tillage and additional organic matterials
FINALLY: There is clearly scope for additional soil carbon storage/ accumulation from zero/reduced tillage practices and organic material applications, however, some changes may be limited or minimal. There are issues with SOC accumulation being finite and reversible, meaning SOC levels will only remain elevated if the practice is continued indefinitely. Only the application of biosolids (treated sewage sludge), compost and paper crumble appear to offer the same level of CO2-C ‘savings’ that have been predicted for land-use change options (e.g. reversion of arable land to permanent grassland, woodland or willow/poplar biomass production). With the probable exceptions of compost and paper crumble applications (which are largely a result of recent diversions away from landfill), almost all of the other organic materials considered were already applied to land, so it is questionable whether this can be regarded as genuine additional carbon storage (against a recent/present day baseline). Probably of more importance, is the maintenance of existing SOC levels and the avoidance of ploughing out permanent grasslands. The predominant justification for returning all organic materials to soil should therefore be for maintaining existing SOC levels, and completing natural nutrient and carbon cycles, not additional carbon storage for climate change mitigation per se. Similarly, should reduced tillage be encouraged, it should be for its protection of existing SOC levels and benefits to soil water retention and prevention of erosion, as well as reduced production costs and energy use.