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Soil Fertility Specialist
Ontario Ministry of Agriculture
Food and Rural Affairs
581 Huron Street
Stratford, ON N5A 5T8
Canada
keith.reid{at}ontario.ca
Received for publication November 30, 2007.
Dear Editor,
In this paper, published in the Journal of Environmental Quality (2007), S.A. Khan and his co-authors have concluded that application of high rates of fertilizer nitrogen has caused the net loss of soil organic matter. This is a sensational claim, and it would be incredibly important if it was true, but the evidence presented in the paper does not unequivocally support such a conclusion. It is my belief that the authors have overlooked other, more plausible explanations for the changes in soil organic carbon that they have observed.
The main evidence presented by the authors is the decline in soil organic carbon (SOC) in the Morrow plots at the University of Illinois over the 51 yr period from 1955 to 2005. The greatest losses have occurred in the plots receiving the highest rates of NPK fertilizers, but the confounding factor, which has been ignored, is that the initial SOC contents in these plots were much higher than in the unamended or moderate fertilizer treatments. The graph of SOC concentrations over the past century clearly show that SOC was stable or increasing while livestock manure was being applied to these plots, and began to decline when this input of organic matter was replaced with mineral fertilizer. This is not unexpected, since SOC concentration is an equilibrium between carbon inputs and losses. Any changes to either side of the equation will result in a new equilibrium concentration being established. Other cases have been reported in the literature of a negative correlation between initial SOC concentration, and the change in SOC over time (VandenBygaart et al, 2002). It is much more likely that the decline in SOC is due to the change in the form of fertilizer than to the rate of fertilizer applied.
The authors also point to an extensive list of papers, which supposedly support their conclusion that nitrogen fertilizer is harmful to SOC. While this list does show a widespread decline in SOC concentrations, there is no evidence that the cause of this decline is nitrogen fertilizer. There is no consistent pattern of greater declines with higher nitrogen fertilizer rates, and the six studies (out of 25), which do include different nitrogen inputs show, no meaningful differences between the nitrogen treatments. The range in SOC change for the low N treatments was 0 to -21.9 Mg ha–1, while the range for the high N treatments was +0.5 to -20.5 Mg ha–1.
From the evidence presented in this paper, it would be fair to conclude that modern annual crop management systems are associated with declines in SOC concentrations, and that increased residue inputs from high nitrogen applications do not mitigate this decline as much as we might hope. It is not possible, however, to link the over-application of nitrogen fertilizers to the decline in SOC. It would be appropriate for the authors to restate their conclusions to reflect a more measured assessment of the evidence provided.
NOTES
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REFERENCES
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