Earthworm Populations and Association with Soil Parameters in Organic and Conventional Ley Pastures

ABSTRACT

Earthworm density, age structure, species composition and biomass, and soil biological, chemical and physical parameters (n = 24) were compared between pairs (n = 6) of organic and conventional ley pastures on mixed cropping farms in New Zealand. No differences in population density or biomass of the earthworm species Aporrectodea caliginosa, A. trapezoides, Lumbricus rubellus or Octolasion cyaneum were detected between the paired fields. Of the soil parameters, only nitrate- N levels differed significantly between the pairs; this was higher in conventional fields. Nitrate—N levels in organic fields were significantly and negatively correlated with previous cropping intensity. The biomass of juvenile A. caliginosa was significantly correlated with some of the same parameters in the conventional and organic fields, these were anaerobically mineralizable N (AMN), bulk density and total porosity levels. The total earthworm biomass on conventional fields was positively and significantly correlated with C, N, cation exchange capacity (CEC) and macroporosity levels, and with hot water extractable C, Mg, AMN, CEC, and microbial biomass C levels, and pasture age for total earthworm density. For the organic fields, no significant association for total earthworm biomass or density with the above parameters were observed. Only the r values for bulk density and total earthworm density on conventional (-0.82) and organic (-0.80) fields were comparable. The different associations between earthworm population attributes and a number of soil biological and chemical parameters on conventional and organic fields, provides evidence for differing relationships between these soil parameters and earthworm populations in organic and conventional ley pastures.