Soil C, N and crop yields in Alabama's long-term `Old Rotation' cotton experiment

Abstract The Old Rotation cotton experiment at Auburn, Alabama, is the oldest, continuous cotton experiment in the world (cf. 1896). Long-term cropping systems provide a unique opportunity to observe the effects of 100 years of cropping on soil organic carbon (SOC). The objective of this paper was to summarize limited data on SOC and N cycling in this historic experiment. Soil organic C has been measured on the 13 plots (6 cropping systems) in 1988, 1992 and 1994. Long-term planting of winter legumes with no other source of N applied resulted in higher SOC (9.5 g C kg −1 ) in the plow layer (0–20 cm depth) compared to continuous cotton with no winter cover crops (4.2 g C kg −1 ). A 3-year rotation of cotton–winter legumes–corn–small grain–soybean resulted in 12.1 g C kg −1 . There was a significant ( P R 2 =0.54) to increasing SOC. Winter legume cover crops supplied between 90 and 170 kg N ha −1 . Where no N has been applied in fertilizer or from a legume crop, annual N removal in the cotton crop is around 13 kg ha −1 , about the same as that fixed in precipitation.