Nitrogen 'tie-up' can be positive

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Most no-till growers take into account the possible effects on a new crop of nitrogen 'tie-up', where soil micro-organisms go about their job of breaking down organic matter and mineralising organic nitrogen (which is not available to plants) into a soluble form plants can take up and use.

In some scenarios - such as when large quantities of vegetable matter are incorporated close to sowing - this can deprive plants of nitrogen needed for growth, and growers generally respond by applying extra nitrogen fertiliser to compensate for what is being tied up in the soil.

CSIRO Land and Water scientist Jeff Baldock says the tie-up process can also be a positive in a wellfunctioning farming system if it improves the soil's ability to hold nitrogen in the root zone, preventing it from leaching too far down for crop roots to reach.

"Work by my fellow soil scientists Dr David Roget and Dr Gupta Vadakattu suggests that soils with good levels of biological activity can hold up to 40 kilograms of nitrogen per hectare in the root zone for subsequent release and use by plants," Dr Baldock says. "This view is reinforced by deep soil measurements that revealed 250 to 1150kg/ha of nitrogen in the five metres of soil between one and six metres below the surface in paddocks in the Murray Mallee.

"There was virtually no nitrogen between one and six metres under nearby native scrub."

Dr Baldock says Dr Roget and Dr Vadakattu have commonly measured 'lost' nitrogen in the order of 200 to 600kg/ha under cropping paddocks, clearly showing the potential benefit of protecting nitrogen against leaching, particularly over summer, by holding it in the root zone where it can be accessed by plants.

Carbon is the main energy source for soil microorganisms that make nutrients, including nitrogen, available to plants and which also assist the nutrient uptake by plants.

Carbon also improves the soil's ability to hold moisture, and in modern stubble-retention no-till farming systems is most likely to enter the soil biological system in the form of roots and foliage.

Soil type influences soil carbon levels through its effect on plant biomass production. Different soils affect the extent to which tillage changes soil carbon densities. Poorer-structured soils have lower carbon levels, regardless of the intensity of tillage used.

In some instances this effect can lead to tillage systems with greater levels of soil disturbance, resulting in better plant growth and higher soil carbon inputs than lower-disturbance systems.

For example, soil under traditional tillage in a high-rainfall area with moderate temperatures might have very high carbon density, due to good crop yields resulting from the nutrient and agronomic management of crops and high biomass pastures between cropping phases.

However, in general terms reduced-tillage stubbleretention systems produce the highest potential carbon densities, and multiple cultivation cropping systems with heavy grazing or burning of stubbles the lowest.

GRDC Research Code CSO00030
More information: Dr Jeff Baldock, 08 8303 8537, Jeff.Baldock@csiro.au

[Photo (left): Reduced-tillage stubble-retention systems produce the highest potential carbon densities.]

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