Farm Table says:
What is the problem?
The majority of dryland croppers in Australia retain stubble to retain moisture, protect soil and maintain fertility. However, there is focus now on flexible stubble management in order to avoid unprofitable or unproductive situations, including the N inefficiency leading to reduced yield.
What did the research involve?
This paper reviewed the factors leading to N tie up or immobilisation. The results are from a number of short and long-term experiments in southern NSW.
What were the key findings?
Harden long term site
- Average wheat yield has been reduced by 0.3t/ha in stubble retained versus stubble burnt treatments, but the negative impacts of stubble were greater in wetter seasons
- Rarely found significant differences in the starting soil mineral N pre-sowing.
- Nine-year experiment managed using no-till, controlled traffic, inter-row sowing (spear-point/press-wheels on 305mm spacing) in a canola-wheat-wheat system.
- Un-grazed treatments, retaining stubble, rather than burning had no impact on the yield of canola or the first wheat crop over the nine years, but consistently reduced the yield of the second wheat crop by an average on 0.5t/ha.
- Grazing stubbles never reduced the yield of any crop at the site, but increased the yield of the second wheat crop.
The evidence emerging from these studies suggests that even where cereal crop residues are retained on the soil surface (either standing or partially standing) and not incorporated, significant N immobilisation can be detected pre-sowing in some seasons.
The paper presents the following take home messages:
- Cereal stubble should be thought of as a source of carbon (C) for microbes, not as a source of nitrogen (N) for crops. In no-till systems, only approximately 6% of the N requirement of crops is derived from the stubble.
- Nitrogen tie-up by cereal residue is not just a problem following incorporation — it occurs in surface-retained and standing-stubble systems and can reduce wheat yields by 0.3t/ha to 0.4t/ha.
- Management is reasonably straightforward — supply more N (5kg N for each t/ha of cereal residue) and supply it early to avoid impacts of N tie-up on crop yield and protein.
- Deep-banding N can improve the N uptake, yield and protein of crops, especially those in stubble-retained systems.