Microbes use nitrogen (N) from the soil to decompose stubbles. When stubbles have a high carbon: nitrogen (C: N) ratio, soil mineral N is tied-up. Cereal stubbles have a high C: N ratio of around 80:1. Canola stubble C:N ratios are higher and legume stubbles are lower.
As stubbles break down soil mineral N is immobilised until crop residues decompose to have a C: N ratio of around 15:1. The question is if immobilisation of N will coincide with the early growth of the following crop.
If it does, extra N fertiliser may be needed to get the following crop off to a good start. Approximately 10 kg of additional N is required to break down one tonne of cereal stubble. A 6 t/ha wheat crop, leaving about 8t/ha of stubble may require up to an extra 80 kg N/ha at sowing to ensure the young crop can access enough N.
In time, the N becomes available as the stubble decomposes. Rootzones with good microbial activity can hold up to 40 kg/N/ha in the root zone for later release. Healthy root zones also reduce N leaching, keeping it where the plants can use it.
Burning stubbles makes some N available immediately, however up to 80% of the stubble N content is lost during the burn.
You may need to apply a similar amount of N fertiliser to compensate for immobilisation of N as to replace N lost through burning. Applying N to compensate for temporary immobilisation builds up paddock nitrogen stocks. Replacing N lost through burning maintains nitrogen levels.
Retaining stubbles also improves soil carbon levels and helps soil biological fertility.