Changes in N cycling in the rhizosphere of canola lead to decreased N requirements in a following wheat crop.

Cathryn A O’Sullivan1, Elliott G Duncan2, Kelley Whisson1, Karen Treble1, Margaret M Roper1, Mark B Peoples3.

1 CSIRO Agriculture, Private Bag 5, Wembley, WA 6913, Australia.

2 Future Industries Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, SA 5095, Australia.

3 CSIRO Agriculture, Clunies Ross St, Black Mountain, ACT 2601, Australia.


Growing canola in rotations is known to increase the yield of following cereal crops. In addition to providing a disease break and better weed management, it has been suggested that following crops have lower N fertiliser requirements. The aim of this study was to confirm the lower N requirement of wheat following canola and to postulate possible mechanisms to explain this effect.

In a field study, wheat was grown under several N fertiliser levels in paddocks where either canola or wheat/pasture were grown in the previous year. In parallel, laboratory and glasshouse studies were used to examine the impact of canola  on nitrification, N mineralisation and N immobilisation rates.

Two seasons of field data in Wongan Hills, WA, and one season in Merredin, WA, showed that wheat had a lower N fertiliser requirement following canola than a wheat or pasture rotation.  In the laboratory, nitrification rates were significantly lower in the rhizosphere of canola cv. Hyola 404RR than wheat cv. Janz, while N immobilisation and remobilisation rates were significantly higher.

To explain this we hypothesise that decreased nitrification rates conserve N as NH4+ during the canola season leading to increased N immobilisation rates and an elevated organic N pool that is likely to be stored over hot and dry summers in the WA region.  this organic N pool can be remobilised providing an alternative N source for the following crop. Further study is needed to fully explore this concept.