Reply to Comments on “Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Production,” by R.L. Mulvaney, S.A. Khan, and T.R. Ellsworth in the Journal of Environmental Quality 2009 38: 2295–2314

We appreciate the opportunity to respond to the comments of Dr. Powlson and his colleagues (Powlson et al., 2010) and to further elaborate a critical but often overlooked aspect of synthetic N fertilizer usage in response to the world’s growing demands for food, fi ber, and energy. Th e work reported by Khan et al. (2007a) and Mulvaney et al. (2009) was undertaken to evaluate how synthetic N fertilization has aff ected soil storage of organic C and N, which is directly related to numerous physical, chemical, and biological processes and ecological functions that ultimately determine agricultural sustainability. Long-term cropping experiments provide the most reliable means for assessing these eff ects. To adequately investigate a topic with such important implications, the utmost care must be taken to ensure data integrity. Toward this end, an extensive eff ort was made in compiling baseline sets of soil C and N data from published cropping experiments. Only data sets involving synthetic N fertilization but not manuring were utilized because of their relevance to modern input-intensive agriculture, and these were documented in all cases with detailed records of crop and soil management. Th e Morrow Plots qualify in both respects, as the oldest experiment fi eld in the United States. Before the introduction of commercial fertilization, a substantial increase in soil organic C occurred despite moldboard plowing and the removal 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 (roughly equivalent to residue C removal) as an annual average per hectare. As documented by Fig. 2 of Khan et al. (2007a) and in a subsequent letter to the editor (Khan et al., 2008), this buildup reverted to decline following the shift to chemical-based management. Despite increasingly massive fertilizer (116–161 kg N ha−1 yr−1 on average) and residue (4.3–5.3 Mg C ha−1 yr−1 on average) inputs, there was very little evidence in 2005 of any soil C or N buildup or sequestration over time, relative to baseline data collected just before the introduction of synthetic N. Rather, the only signifi cant changes were net losses that were almost always more extensive for the subsurface than for the surface soil. Th ese fi ndings are substantiated by baseline data sets from Rothamsted and numerous other cropping experiments with ammoniacal fertilization and are also consistent with the temporal declines in soil C and N Reply to Comments on “Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Production,” by R.L. Mulvaney, S.A. Khan, and T.R. Ellsworth in the Journal of Environmental Quality 2009 38: 2295–2314