Litter decomposition and soil organisms within and outside of Camponotus punctulatus nests in sown pasture in Northeastern Argentina

Camponotus punctulatus builds big nests, up to 1.20 m high and 2 m in diameter, containing more organic matter and nutrients than the surrounding soil. The aim of this study was to relate litter decomposition in C. punctulatus nests with soil organisms. We expected a greater level of decomposition and more soil organisms within than outside the nests. The study site was a field of Setaria sphacelata, a common sown pasture in Northeastern Argentina, with 180 nests of C. punctulatus per hectare. To estimate decomposition rates we buried litterbags within and outside the nest (microsite type) at the beginning of each season and recovered those from the previous season. We used litterbags of different mesh size (7 mm, 2 mm, 100 μm and 1 μm) filled with 8 g of S. sphacelata litter. At the same time, we sampled the soil surrounding litterbags to estimate microbial dehydrogenase activity and the abundance of nematodes and mesofauna. Soil microbial activity was greater outside the nests, mesofauna were significantly more abundant inside the nests, and nematodes had similar abundance inside and outside the nests. Throughout all seasons, there was a greater proportion of Prostigmata and Mesostigmata in the nests, whereas Oribatida and Collembola were more abundant outside. Oribatid species composition differed between microsites. In the nests, there were two periods of higher decomposition (spring and summer) while outside the maximum occurred in spring, but only in litterbags of 7 and 2 mm mesh. The lack of macro and mesofauna (litterbags with 100-μm mesh) decreased organic matter decomposition in the nests in summer and induced phosphorus immobilization in winter. Ant activity and feeding preference, nest architecture and the plant community on C. punctulatus nests are suggested as plausible factors that modify soil organism abundance and decomposition.