Impact of soybean cropping frequency on soil carbon storage in Mollisols and Vertisols

The high cropping frequency of soybean (Glycine max [L.] Merr.), mainly as a single annual crop, in the extensive agricultural systems of South America may adversely affect the soil organic carbon (SOC) storage, which may be different between soils depending on aggregation agents. The aim of this work was to evaluate the impact of the soybean cropping frequency on the SOC storage in different soil aggregate size classes in a Mollisol and in a Vertisol in the Northeastern Pampas of Argentina under no-tillage management. In each soil, the samples were collected at 0–5, 5–15 and 15–30 cm depths in eleven cropped and one uncropped fields. The number of months occupied with soybean in relation to the total number of months occupied with crops within crop sequences, over a 6-year period, was used to calculate the soybean cropping frequency. The SOC stocks in equivalent soil mass, the SOC concentration both in the whole sample and in different aggregate size classes, and the stratification ratio of the SOC stock and of the SOC concentration were determined. The increase in soybean cropping frequency reduced the SOC stock in both soils at 0–5 cm, and in the Vertisol at 5–15 and 0–30 cm but the change was evident only between the cropped and the uncropped situation. A decrease in soybean cropping frequency resulted in a higher amount of macroaggregates (> 250 μm), a higher SOC concentration and a higher stratification ratio in the Mollisol at 0–5 cm, whereas in the Vertisol the soybean cropping frequency did not affect the stratification ratio or the aggregate distribution in any size class. The increase in soybean cropping frequency reduced SOC storage only in macroaggregates (> 250 μm) in both soils at 0–5 cm, particularly in the largest macroaggregates (> 2000 μm), and more in the Mollisol than in the Vertisol. Our results show that a high soybean cropping frequency may severely affect the SOC storage in the Mollisol, and suggest that in the Vertisol this effect may lead to a reduction in the SOC storage in the long term.

► Impact of soybean cropping frequency was higher in Mollisol than in Vertisol.
► The soybean cropping frequency reduced the SOC stratification only in the Mollisol.
► In both soils the SOC was stored mainly in macroaggregates (> 250 μm).
► Carbon storage in macroaggregates was reduced by soybean cropping frequency.