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Mineralisation of soil organic nitrogen under long term rotations

D.P. Heenan and K.Y. Chan

NSW Agriculture, Agricultural Research Institute Wagga Wagga NSW 2650

Wheat farmers in Australia have traditionally relied on soil organic matter as the main source of nitrogen (N) for crop production. Following cereal crops the supply of soil organic nitrogen has been rejuvenated by a subterranean (sub) clover (Trifolium subterraneum) pasture phase. When cropping becomes more economically viable and the pasture phase has declined, crop yields and grain protein have suffered. A long-term experiment commenced in 1979 at Wagga Agricultural Research Institute to examine in part the ability of lupins (Lupinus angustifolius) to maintain soil nitrogen supply and crop production in a cropping rotation.


Rotations studied were lupins/wheat, sub clover/wheat, and wheat/wheat with legumes being 50% of the rotation in appropriate treatments. Soil mineral N supply, mineralisation rate, and loss were estimated in the surface 15 cm throughout the season, by sequential sampling, using covered and uncovered PVC (5.0 cm ID) tubes pushed into the soil and left for 2-6 weeks (1). Measurements were taken over two seasons between 27 April 1989-11 December 1989 and between 26 April-4 December 1990 and were only taken during the wheat phase of the rotation.

Results and discussion

Amount of soil N mineralised in legume/wheat rotations was greater in 1990 than in 1989, probably due to differences in climate and/or growth of the previous legume crop (Table 1).

Table 1. Soil N mineralised (kg N/ha), and lost (kg N/ha) from surface 15 cm and the percentage of mineralised N taken up by the wheat crop.

Higher losses in 1990 were most likely related to differences in rainfall and amounts of soil N mineralised. In both seasons highest levels of soil organic N mineralised occurred in sub clover/ wheat followed by lupins/wheat and wheat/wheat. Loss of mineral N from the surface 15 cm can easily be related to the amount mineralised, being greatest for sub clover/wheat and least for wheat/wheat. There was no difference between seasons in percentage of mineralised N taken up by the wheat crop. However, differences between rotations were evident, with lowest percentages estimated for sub clover/wheat, suggesting that this system was producing in excess of requirements by the crop. This was also indicated by the low 100 grain weight in both seasons and symptoms of 'haying off' in 1990 in sub clover/wheat.


1. Raison, R.I., Connel, M.J. and Khanna, P.K. 1987. Soil Biol. Biochem. 19, 521-530.

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