Earthworm abundance and species composition in organic forage production systems of northern Colorado receiving different soil amendments

Abstract

We evaluated the effects of soil amendments on earthworm communities in organic annual forage and perennial pasture systems in northern Colorado. In the annual forage study (1) an annual warm season grass teff (Eragrostis tef) and (2) bare fallow were main plot treatments and received one of three soil amendments: (1) composted dairy manure (CDM), (2) raw dairy manure (RDM), and (3) no amendment as control. For the perennial pasture study, CDM was topdressed onto a grass mixture consisting of orchardgrass, smooth and meadow bromegrass at rates ranging from 0 to 44.8Mgha−1. At both sites, earthworm and soil samples were collected in July 2009. The earthworms identified from both systems were composed of endogeic species Aporrectodea rosea (Savigny), A. tuberculata (Eisen), and A. turgida (Eisen), the first being found only in the perennial pasture. In the annual forage study, earthworm total abundance did not differ between teff and bare fallow treatments. However, within bare fallow treatment, earthworm total abundance was significantly affected by soil amendment, with CDM averaging approximately 1.4 and 5.4 times greater earthworm total abundance than RDM and the control, respectively. Earthworm total abundance was found to be positively correlated with soil Cu (R=0.51, P=0.03) and K (R=0.58, P=0.01). In the perennial pasture, earthworm total abundance tended to increase with an increase in the CDM rate to 33.6Mgha−1. However, no further increase was observed when the CDM rate was increased to 44.8Mgha−1. At this site, earthworm total abundance was negatively correlated with EC (R=−0.37, P=0.02). Our results suggest that high quality (low C/N ratio) dairy manure is important for maintaining a high earthworm population. Larger CDM application rates appear to discourage earthworm populations probably due to salinity stress. Further study is necessary to elucidate the exact effects of manure quality and quantity on earthworm populations in annual forages and perennial pastures.