N.Z. soil report holds serious implications

Press, 30 June 1983, Page 19

N.Z. soil report holds serious implications

PA Hamilton Nitrogen-based fertilisers used with a number of New Zealand crops could be disappearing into thin air because of the unsuspected activities of bacteria in the soil.

The problem, which could have serious implications for nitrogenous fertiliser practices throughout the world, has been uncovered by a Waikato University research project funded by the Ministry of Agriculture.

A university scientist has described the findings as astonishing, and a Ruakura soils expert said the implications, unsuspected until now, were world-wide.

The study, financed by the Ministry after a university researcher’s suspicions were aroused two years ago, shows that vast quantities of essential nitrogen are probably being lost through the action of bacteria deliberately introduced to the soil because of their nitrogenfixing capability.

The., losses have been recorded consistently in laboratory trials, contradicting existing beliefs about the nitrogen cycle, and the

research team hopes to continue this year in an attempt to establish how widespread the problem is, and find a solution that could enhance efforts to correct the nitrogen deficiences of New Zealand soils. The main implication is that spreading nitrogenous fertiliser such s urea, or the natural decay or ploughingunder of nitrogen-fixing legumes, can result in its being uselesssly vented-off as quickly as it soaks into the ground in areas affected by the bacteria, known as rhizobia. The losses are believed to be taking place in areas where legumes other than clover are grown, particularly in hill-country farming districts. Clover areas are unaffected, for some as-yet unclear biochemical reason, but trials offer strong evidence that nitrogen loss is heavy where crops such as lucerne, lupin, lotus, and soy bean are grown. The researchers suspect that if theories linking nitrous oxide with damage to the ozone layer and increasing world temperatures are correct, the unexpected nitrogen losses are likely to

be contributing. A university biochemist, Dr Roy Daniel, said the findings would be published internationally later this year.

He said Ministry of Agriculture funding was being sought for continuation of the research, unparalleled anywhere in the world, to improve knowledge of how the losses happened and to find a way possibly through genetic engineering to counteract them. Legumes fix their own nitrogen, converting it into Usable nitrates or ammonia, through the actions of bacteria, known as rhizobia, in their root nodules. The presence of legumes leads to large colonies of rhizobia in the surrounding soil, and while they do not fix nitrogen it has not been suspected until now that they do any harm. Dr Daniel said “denitrification,” the venting-off of pure nitrogen or nitrous oxide, had been thought to take place among soil-living rhizobia only in the absence of oxygen, not in aerated soils.

But the study had shown that the process did take place in the presence of

oxygen, a result he described as quite unexpected. Except for those associated with clover, it had now been established for the first time that rhizobia were capable of aerobic denitrification. He said this cast doubt on the effectiveness of nitrogenous fertiliser, or ploughed-under legumes, in the areas affected. The study indicated that large quantities of the fertiliser, when broken down, were taken up by the rhizobia and expelled into the air before reaching the plants. Trials had shown that virtually all the nitrogen introduced to soil containing a variety of rhizobia disappeared within three or four days. Dr Daniel said results of the research so far could not be doubted. More than 80 strains of rhizobia had been tested and found capable of aerobic denitrifying, while 130 strains associated with clover had been tested at Ruakura and found to pose no problem.

It was essential now to continue the work and find out more about how rhizobia processed nitrogen, he said.