Earthworm distribution and density across a climatic gradient within the Inland Pacific Northwest cereal production region

Despite their recognized importance in promoting crop yields and soil health, little is known about earthworm species composition and density within the cereal production region of the Inland Pacific Northwest (IPNW). The overall goal of this work was to understand the influence of climatic factors and soil properties on the presence/absence and density of earthworm species found in the region. The IPNW cereal production region includes a steep climatic gradient and can be divided into four agroecological classes (annual cropping, transition crop-fallow, crop-fallow and irrigated) (AECs) according to the dominant cropping systems present. Replicate sites were selected within the annual cropping, transition and crop-fallow AECs and earthworm density, diversity and soil properties were measured. Earthworm diversity was extremely low. Only two of the sampled sites had more than a single species, and these sites were both in the wettest AEC. Aporrectodea trapezoides was the predominant species sampled, and this species was present at every site where earthworms were found. Earthworms were present at all sites within the annual cropping and transition classes, while only 13.3% of sites within the crop-fallow AEC had earthworms. When only sites with earthworms were considered, earthworm metrics were greater in the driest crop-fallow AEC (density=151.2 individuals m−2) compared to 113.8 individuals m−2 in the transition and 86.4 individuals m−2 in annual cropping fields and a significant negative correlation was found with precipitation. The data suggest that while earthworms are largely absent from the driest sites, they can achieve relatively high density and biomass values when local conditions (soil properties and management) are favorable. A clear minimum threshold of mean annual precipitation (MAP) between 330 and 370mm was observed, with earthworms absent from all fields with MAP values below 330. This value is lower than those previous reported for A. trapezoides and demonstrates the species’ ability to deal with warm, dry conditions. Biomass values (mean of 94.3gm−2) for sites with earthworms were generally above those reported to be necessary to significantly influence crop yields, suggesting that further research focusing on quantification of the impact of earthworm on crop yields in this important agricultural region is warranted.