Impact of irrigation on the soil

It is likely to remain at the centre of debate this year as well.

All parties agree it is an important factor of soil quality; the arguments are about how to look after it.

Soil organic matter is increased or decreased by management. Because farmers and growers rarely alter one factor of management in isolation, the drivers of an effect on soil organic matter after a change in management can be difficult to identify.

Irrigation is an example and both decreases in soil organic matter and increases have been reported.

First principles can assist in teasing out what is happening. Irrigation is applied to overcome a soil moisture deficit and allow plants to keep photosynthesising and growing.

Roots will also keep growing, and plant litter will continue to arrive at the soil surface, providing food for soil organisms.

Maintaining soil moisture means that the soil organisms – including worms, insects, fungi and bacteria – will continue to live and respire.

One theory explaining a decrease in soil organic matter under irrigation is that water stimulates (microbial) soil respiration – the micro-organisms are mineralising the organic matter, releasing energy, carbon dioxide and nutrients such as nitrogen, phosphorus and sulphur, and so multiplying themselves at the expense of the organic matter.

Another theory is that the increase in plant growth is accompanied by increased harvest and removal of both carbon and nitrogen, resulting in progressive nitrogen limitation.

This, in turn, restricts carbon fixation (photosynthesis and hence growth is restricted) and so limits the capacity to sustain the soil organic matter which is constantly being respired by the soil organisms. Adding nitrogen to replace the extra that has been removed overcomes the restriction.

Professor Tony Parsons, now retired from Massey University, has been able to isolate the different drivers and show how the dynamic interaction between soil moisture and nitrogen affects both production and soil organic matter. He has also shown that irrigation reduces nitrogen loss to the environment, even when the nitrogen fertiliser is increased, because under irrigation the plants keep growing and taking up the nitrogen.

In the case of pasture plants, the pasture is harvested by the animals and removed in the product by tanker and stock truck.

Under irrigation and with fertiliser (such as on the Canterbury Plains, where increases in soil organic matter have been recorded after a change from sheep grazing to dairying), the soil moves to a new equilibrium in carbon capture from the atmosphere. As long as the greater rate of supply is sustained, there will be a larger pool of soil organic matter sequestered.

Each tonne of soil carbon will be associated with 80-100kg of nitrogen (as well as approximately 20kg of phosphorus and 14kg of sulphur), hence the importance of fertiliser addition.

Once a new irrigation and fertiliser regime has been in place for 10-20 years, it is likely that accumulation will reach equilibrium with mineralisation. But if the conditions that resulted in the increase in soil organic matter are changed, the equilibrium will again alter, and the ecosystem will move to a new dynamic equilibrium where inputs equal outputs.

This change could be restrictions in irrigation or in use of nitrogen fertiliser.

All parties, whether from an environmental or production perspective, agree that organic matter is an important component of soil quality. Increasing temperatures and drought mean that current soil organic matter is at risk of depletion – soil organisms continue to function after plants have stopped growing.

Irrigation, therefore, has an important part to play in soil quality in some parts of the country. The opportunity lies in managing New Zealand’s relatively abundant supplies of renewable freshwater through precision irrigation to benefit both the environment and food production. The challenge will continue to be achieving agreement on what constitutes ‘benefit’.

• Dr Jacqueline Rowarth, Adjunct Professor Lincoln University, has a PhD in soil science. She is also a former-elected director of DairyNZ and Ravensdown. The analysis and conclusions abover are her own. This email address is being protected from spambots. You need JavaScript enabled to view it.