Short-term casting activity of earthworm Pontoscolex corethrurus (Oligochaeta: Glossoscolecidae) after biochar additions

Abstract

Conversion of forests to cultivated farms through slash-and-burn or chop-and-char practices often results in rapid loss of soil organic matter (SOM) or conversion of inherent SOM into pyrogenic organic matter (PyOM). However, there is little knowledge about the short-term changes in soil macrofauna that may occur when large amount of biochar are added to the soil. A thirty-day microcosm study was conducted to assess effects of biochar derived from two trees, Croton megalocarpus Hutch. and Zanthoxylum gilletii (De Wild.) P.G.Waterman, on the activity of a geophagous earthworm, Pontoscolex corethrurus. A portion of the biochar was leached with either acetone or 2 M HCl, to remove easily mineralizable organic matter and ash contents, respectively. Each of the biochar types was mixed with soil at a rate equivalent to 5, 10 and 25 Mg ha−1. Casts were collected after 30 days and used as a measure of earthworms’ activity. Casts dry weight was affected more by amount than the type of biochar. The highest cast weight (188.1 g and 176.5 g) was recorded in microcosm that received 5 Mg ha−1 of C. megalocarpus and Z. gilletii biochar, respectively. Notably, the weight decreased with increasing biochar additions. Cast weight decreased by 4% in microcosms that received 10 Mg of C. megalocarpus biochar ha−1 and by 15% in microcosms that received the same biochar type at a rate of 25 Mg ha−1. Similarly, there was a 6% decline in cast weight in microcosms that received 10 Mg of Z. gilletii biochar ha−1 and an 8% decline in microcosms amended with 25 Mg ha−1 of the same biochar type. Easily mineralizable organic matter or nutrients were not responsible for the observed differences in cast production since leaching with acetone or HCl did not change the effects. The C and N content in casts and bulk soil were not significantly different, an indication that earthworms did not seek out biochar, but rather indiscriminately utilised the soil rich in biochar.