Carbon and nitrogen dynamics in ageing earthworm casts in grasslands of the eastern plains of Colombia

The effects of a large species of anecic earthworm, Martiodrilus carimaguen sis Jimenez and Moreno, on soil C and N dynamics were investigated in a nat ive savanna and a man-made pasture of the eastern plains of Colombia. We co mpared, across time (11 months), the total C, total N, NH4+ and NO3- conten ts in the earthworm casts, the underlying soil and the adjacent soil. Addit ional sampling of root biomass and macrofauna was performed. In the two man agement systems, the total C and N contents were higher in casts (4.33-7.50 %) than in the bulk soil (2.81-4.08%), showing that the earthworms selected food substrates with high organic contents. In general, C contents signifi cantly increased during cast ageing (+100%), possibly because of CO2 fixati on processes, dead root accumulation and/or macrofaunal activities in casts . In fresh casts, NH4+ levels were very high (294.20-233.98 mu g g(-1) dry cast) when compared to the soil (26.96-73.95 mu g g(-1) dry soil), due to t he intense mineralisation processes that occurred during the transit of soi l and organic matter through the earthworm gut. During the first week of ca st ageing, NH4+ levels sharply decreased, while NH3- levels showed successi ve peaks in the casts, the underlying soil and the adjacent soil. These res ults suggested the rapid production of NO3- by nitrification processes in t he fresh casts, followed by diffusion to the nearby soil, first vertically, then horizontally. After 2 weeks of cast ageing, NH4+ and NO3- levels only showed slight variations, likely because of organic matter protection in s table dry casts. The root biomass was higher (1.6-4.7 times) below the old earthworm casts. The ecological significance of these results is discussed.