Effect of fertilizer, legumes and cropping frequency on soil organic matter in a long-term rotation – changes after 6 years of zero tillage
Society is interested in methods of increasing C storage in soil, so as to reduce CO, concentration in the atmosphere, which may contribute to Global warming. Scientists have demonstrated that the adoption of better management practices, such as proper use of fertilizers, including legumes in rotations, cropping more frequently or employing less summerfallow, may increase C storage in soils. However, the question has been raised regarding the accuracy of the amounts of C believed to be gained, because most of the past calculations were made on a volume basis rather than the more accurate equivalent mass basis. A long-term crop rotation study being conducted on a thin Black Chemozemic fine-textured soil at Indian Head was sampled in May 1987 and in September 1996 and total organic C and N determined using Carlo Erba combustion technique. The experiment was managed with conventional tillage from 1958 to 1989 and changed to no-tillage in 1990. We calculated C and N in the 0-7.5 and 7.5- 15 cm depth using the volumetric and equivalent mass approaches, but our conclusions are based on the 0-7.5 cm depth which was the only one showing significant effects. Both methods of calculation confirmed the efficacy of adopting BMPs for increasing C storage in soil, and showed that the latter was directly associated with the amount of crop residues returned to the soil. However, the results also showed that where bulk density is substantially different between sampling times, the volume basis of analysis can lead to erroneous conclusions. The equivalent mass analysis showed that in this experiment, 6 years of no-tillage has not increased soil organic matter; instead, systems that are not being fertilized are continuing to lose organic matter, while those being fertilized, or which have high fertility due to inclusion of legumes, are maintaining the organic matter at a constant level. We suggest that scientists who have appropriate data should consider reanalyzing them using both techniques to determine if their previous conclusions are still valid.
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