Heavy use of synthetic nitrogen fertiliser not only burns away soil carbon, but reduces organic nitrogen levels in the soil and thus increases farmers' reliance on bought-in fertiliser, according to two related reports out of the United States.
A team from the University of Illinois used the Morrow Plots, the oldest continuously cropped experimental fields in the US, to study how fertilisation regimes begun in 1955 affected soil carbon and organic nitrogen levels.
The researchers noted that when NPK fertilisers began to be applied on the plots in the mid-1950s, yields leaped by 140 per cent.
At the same time, there was an expectation that fertilisation would help the soil build a large reservoir of organic nitrogen, and carbon levels would rise because of massively increased residue incorporation.
Neither of these forecasts proved true, the researchers say.
They instead found that after half a century of synthetic fertilization that exceeded grain nitrogen removal by 60 to 190 per cent, soil carbon levels across the Morrow Plots increased modestly under some moderate fertiliser regimes but declined substantially under heavy fertiliser applications.
Soil nitrogen levels declined in every plot but one in testing across corn-corn, corn-soybean and corn-lucerne cropping rotations.
Under a corn-soybean rotation with no fertiliser inputs, for instance, the difference between organic matter levels recorded in 1955 and the average of the subsequent 51 years was -0.9 g/kg at 0-15 cm.
The same rotation under a high NPK fertiliser regime showed a difference of -4.8 g/kg.
The differences in soil nitrogen concentrations for this trial ranged from -0.075 g/kg for an unfertilised plot to -0.441 g/kg in the high NPK plot.
Soil nitrogen depletion increased at depth – measurements were taken at three levels down to 46 cm – because, the researchers suggested, activity around dense plant roots had helped slow the decline in upper levels.
"There is an obvious implication that soil fertility and organic matter evaluation should not be confined to the plow layer, an expedient practice that has traditionally been followed in production agriculture and is often a major limitation in assessing the long-term impact of production practices," the researchers wrote.
In two separate papers published in the Journal of Environmental Quality, the team argued the effect of large nitrogen applications was to artificially boost microbial populations which subsequently consumed carbon contained in crop residue and soil as part of a rebalancing of the carbon:nitrogen ratio.
"Given the fundamental coupling of microbial C and N cycling, the dominant occurrence of both elements in soil organic forms, and the close correlation between soil C and N mineralization, the loss of soil organic carbon has serious implications for the storage of soil N," the researchers wrote.
"The loss of organic N decreases soil productivity and the agronomic efficiency of fertilizer N and has been implicated in widespread reports of yield stagnation or even decline for grain production in Asia."
"If the potential benefits of N fertilizers are to be fully realised, applications must be adequate but not excessive, so as to maximise the economic profitability of crop production while minimising microbial oxidation of residue C and native soil organic matter."
* The papers 'The Myth of Nitrogen Fertilization for Soil Carbon Sequestration' (Khan, 2007) and 'Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Production' (Mulvaney, 2009) can be downloaded for free from www.jeq.scijournals.org.
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