Published online 1 May 2008
Published in J Environ Qual 37:739-740 (2008)
DOI: 10.2134/jeq2008.0001re
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
LETTERS TO THE EDITOR
Reply: Comment on "The Myth of Nitrogen Fertilization for Soil Carbon Sequestration", by S.A. Khan et al. in the Journal of Environmental Quality 36:1821–1832
Reply
We appreciate Mr. Reid's interest in our paper, and welcome the opportunity to further explain our findings that intensive use of N fertilizers in modern agriculture has not promoted soil C sequestration. These findings are neither sensational nor novel, but rest entirely on long-term data from the Morrow Plots and many other field studies throughout the world.
While acknowledging that "modern annual crop management systems" lead to a decline in soil organic C (SOC), Mr. Reid also maintains that heavy use of synthetic N fertilizers cannot be linked to this decline, despite the fact that these fertilizers are an integral part of modern cropping practices. Such a view is at odds with a century of SOC changes reported for the Morrow Plots in Fig. 2 of Khan et al. (2007). Before the introduction of commercial NPK fertilization, SOC increased substantially with moldboard plowing, regardless of the initial level and without the return of aboveground residues, when corn was grown following oats or alfalfa, with a modest application of dairy manure that supplied approximately 20 to 30 kg N and 2 Mg C as an annual average per hectare. With modern management that began for these two subplots in 1967, the annual input of synthetic N averaged 116 to 161 kg ha–1, as compared with 4.3 to 5.3 Mg C ha–1 supplied by the return of above- as well as below-ground residues. According to the equilibrium concept mentioned by Mr. Reid, the increased C input should have caused an upward shift in SOC, rather than the dramatic decline that was actually observed. Clearly, C losses exceeded inputs, and this is most logically attributed to a massive increase in the input of N.
Besides data from the Morrow Plots, our paper utilizes a substantial collection of published literature from field trials around the world involving synthetic N fertilization, with emphasis in Table 3 on studies with initial (baseline) soil sampling. A decline usually occurred in SOC over time, despite the considerable range that existed in soil type and sampling depth, duration of the study period, quantity and quality of residue C inputs, tillage, and climatic regime. Given the confounding effects of such variability, a meaningful relationship would not be expected in quantifying the magnitude of SOC change on the basis of fertilizer N rate alone.
If agriculture is to play a role in reducing greenhouse gas emissions, not only is there a need to increase SOC storage, but this must be accomplished with a full accounting of C input costs. These costs are by no means negligible for the energy-intensive "modern annual crop management systems" referred to by Mr. Reid. As noted in our paper with reference to Schlesinger (2000), there is a C cost associated with the manufacture, transport, and application of N fertilizers, which necessarily increases with the N rate applied, as does the cost associated with N2O emission (De Klein et al., 2006; Crutzen et al., 2008). A comprehensive accounting would include C costs associated with planting, tillage, harvesting, grain transport, pesticide synthesis and application, P and K fertilization, etc. Table 1
incorporates both of the latter costs in estimating net greenhouse gas emissions with and without the lower N rate applied to Morrow Plots that had been unfertilized before 1955. According to these estimates, not only did NPK fertilization fail to reverse the SOC decline, the fertilizer inputs had a considerable atmospheric impact, especially with regard to continuous corn or the corn–soybean rotation that currently dominates the USA Corn Belt. Under these circumstances, it would be appropriate to conclude that N fertilization is of no benefit for soil C sequestration but does contribute to CO2 enrichment of air.
NOTES
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REFERENCES
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