Tillage in Ireland has become increasingly acknowledged as a low-emissions sector. Carbon loss from predominantly worn tillage ground is regarded as being low at around 1t/ha compared with over 8t/ha from intensive dairy operations.
Loss of nitrous oxide and ammonia emissions to the atmosphere are also very low.
So the sector has a relatively clean bill of health in terms of gaseous emissions, but we possibly score less well on biodiversity and nitrate loss to water.
Some tillage areas on light, worn soils are associated with nitrogen loss and high nitrate levels in water. This is mainly associated with nutrient loss in autumn and winter. So, now, there is increased focus on land use to reduce the opportunity for potential leakage.
There is also concern about the overall level of biodiversity on intensive tillage farms.
There can be little doubt but that the need to protect crops against substantial insect and disease attack and from weed infestations that damage yield potential and increase harvest difficulties reduces the overall diversity that can be found in working tillage crops.
Tillage and the presence of stubbles over winter is critical to the survival of some bird species
That said, the presence of a range of different crops sown in rotations and harvested at different times of the year add biodiversity in themselves.
Tillage and the presence of stubbles over winter is critical to the survival of some bird species and so tillage is an essential part of a biodiverse agricultural system.
Let us begin with emissions, where we are generally regarded as best in class. But that is largely because we have already lost a lot of carbon from worn tillage soils.
These can still function as a potential reservoir for new organic matter and carbon storage and many farmers are now acting to address these issues.
While cover crops were originally spurred on by GLAS payments, many growers are now beginning to see the benefits from this practice and they want to do things better (sowing earlier) to get even greater benefits.
There is no single figure for the total area sown to catch crops, but it is likely to be around 40,000ha. These crops sequester carbon, reduce nutrient loss and increase soil organic matter.
The straw incorporation scheme is also a great incentive to act for soil organic matter. Incorporated straw soaks up some nitrogen, increases earthworm numbers and helps to open soils up through improved structure.
While it takes extra diesel to chop and incorporate the straw, much less fuel will be required for crop establishment in the longer term.
Together, these actions are helping to sequester and store more carbon in tillage soils while also improving water percolation and water filtration. So, things are improving.
Tillage incentive and grass
The tillage incentive measure has seen an amount of grassland come into tillage this year. This is both temporary and permanent pasture where soil carbon levels are regarded as being higher.
The ploughing of grassland, particularly permanent pasture, can result in a significant loss of soil organic matter and soil carbon reserves within a few years where cultivation is continued over time. This is described graphically in Figure 1.
In the first two to three years of ploughing old permanent pasture, soil organic matter (OM) levels hold relatively stable but after that there can be a significant reduction resulting in a loss of carbon to the atmosphere.
This is partly because the very high level of biological activity in permanent pasture is accelerated by the sudden availability of oxygen from loosened soil and less organic biomass is being produced by the tillage crop.
Soil organisms that recycle OM in the soil will feed on the decaying grass sward for the first few years. But when that food source is used up, they begin to attack organic matter that is loosely bound within soil structure.
At that point the loss of OM is greater than the addition of OM by the tillage crop. This balance survives for a while, but then bugs will die of hunger thus adding to the collapse.
Two elements of tillage are largely responsible for reducing soil OM and carbon:
So reduced and shallower cultivation could be potentially beneficial on higher OM soils.
Land coming into tillage from grass might best be managed by non-inversion systems in some or all earlier years to help delay the inevitable loss of organic matter.
As the loss of soil organic matter forces the army of soil microorganisms to look for other foods, one must ask if the addition of organic matter to these soils in the early years post grass would provide an alternative food source to slow down soil OM decline?
If we could do both of these, could we reduce the carbon and OM loss associated with cropping permanent pasture?
Moving land use from grassland back into tillage crops would certainly add to the overall level of biodiversity in parts of the country.