Question to @aucropnutrition via twitter
What effect do high crop residues have on spread urea? Does some of the applied nitrogen (N) get used to break down the stubble and therefore become unavailable to the plant?
When urea dissolves on the surface of the residues in humid air or dew it gets converted to ammonium. Without soil contact (which buffers the alkalinity produced in the urea to ammonia conversion) some of the ammonium becomes ammonia gas, which can be lost to the atmosphere. Residues can contain high levels of urease enzyme which can promote ammonia production and increase nitrogen (N) losses compared to urea spread on bare ground.
Rainfall soon after broadcasting will dissolve the urea and help it move down into the soil and be less subject to volatilisation losses. If urea is “tucked” under the stubble N losses should also be reduced.
How crop residues affect the N that does get into the soil depends on the crop the residues come from, and how the residues are managed.
Residues from cereal crops have a low concentration of N and therefore a high carbon (C) to N ratio. Decomposition of residues requires N from another source. The possible sources of N are the soil or recently applied N fertiliser or manures. While this N is being used by microbes decomposing the residues it is less available for immediate use by growing plants. We say that the N being used in decomposition processes has been ‘immobilised’.
Immobilised N is only temporarily unavailable, not lost from the system. Once the residues are sufficiently decomposed, the N will be made available to plants as part of the soil organic matter mineralisation process. For example, a 3 t/ha wheat crop will immobilise approximately 21kg N/ha during a summer fallow in the northern grains region (Herridge 2011). The immobilisation period usually lasts from a few weeks to a few months.
Legume and oilseed residues
The residues of legume and oilseed crops contain more N and have a lower C/N ratio. There is already enough N in these residues to support the microbes involved in decomposition without immobilising soil or fertiliser N. These residues add additional available N to the soil as they decompose.
Management of residues affects the timing and degree of N immobilisation. Standing stubbles decompose much more slowly than those with greater soil contact. When residues are incorporated, their decomposition is likely to be more rapid and will immobilise the available N more quickly than if left on the surface or left standing.
Applying urea by broadcasting can lead to some loss of N as ammonia (gas) through volatilisation. Volatilisation losses are affected by many factors, but crop residues that are thick enough to prevent the broadcast urea granules from contact with soil can increase volatilisation losses.
Photo courtesy of the GRDC