Spatial variation of soil macrofauna and nutrients in tropical agricultural systems influenced by historical charcoal production in South Nandi, Kenya

The charcoal sector constitutes an important source of employment and revenue for many tropical agroecosystems. Better understanding of the effects of charcoal-making is thus warranted to guide actions aimed at minimising environmental externalities. Conversion of trees to charcoal eliminates canopy effects associated with the living trees while at the same time creates new conditions in and around spots where the charcoal is produced due to increased concentration of pyrogenic organic matter (PyOM). It is unclear, whether such unintentional PyOM additions play a role in the abundance and distribution patterns of soil macrofauna. A study was conducted in South Nandi (Kenya) to assess effects of PyOM on soil macrofauna, taking advantage of abandoned traditional earth-mound charcoal kilns, where Croton megalocarpus Hutch. and Zanthoxylum gilletii (De Wild.) P.G.Waterman trees were used in charcoal making. Soil and soil macrofauna samples were collected at increasing distances from the centre of the spots. Total C, non-pyrogenic C (non-PyC) and total N progressively increased with increasing distance from the centre of the spots, whereas soil pH, pyrogenic C (PyC), available P and exchangeable K decreased. The number of earthworms and centipedes in Z. gilletii spots (119 and 14 individuals m−2, respectively) was twice as high as in kilns where C. megalocarpus was used. Notably, while the number of earthworms in spots rich in Z. gilletii PyOM significantly increased with increasing distance from the centre of the spots, the opposite trend was observed for centipedes. In contrast, no significant differences in the spatial distribution of earthworms or centipedes were found in spots rich in C. megalocarpus PyOM. Furthermore, beetles, termites and crickets were significantly higher in C. megalocarpus than Z. gilletii spots, but sampling distance also had no significant influence. As hypothesised, source of PyOM played a major role in determining soil properties and macrofauna distribution patterns thus showing the value of abandoned charcoal-making spots in contributing to a mosaic of soil conditions that could ultimately affect soil productivity in tropical agricultural systems.