Carbon and nitrogen stable isotope ratios differ among invertebrates from field crops, forage crops, and non-cropped land uses1

Stable isotopes are an important tool for studying invertebrate food webs and movement of invertebrates in farmland. However, stable isotope values of farmland invertebrates have been reported for only a few crop types, and rarely for other land uses within farmland. We compared 13C and 15N of invertebrates captured in corn, soybean, hay, and hedgerows in eastern Ontario, Canada. 13C was significantly lower in invertebrates captured in hay and hedgerows than in invertebrates captured in soybean and corn, but was not different between invertebrates captured in corn and soybean. This suggests invertebrates may be moving between crop fields during the growing season, using alternative food sources within crop fields, or retaining 13C values from the previous year. When all invertebrates were examined together, 15N was significantly higher in invertebrates captured in manured corn than in those captured in soybeans, hedgerows, or manured or unmanured hay, but there was no difference between invertebrates captured in manured or unmanured corn. However, spiders from manured corn had significantly higher 15N than those from unmanured corn. Spiders had less variable 15N than other taxa in this study, because they occupy a single trophic level. This may make spiders more suitable for detecting changes in fertilization regimes. By demonstrating how invertebrate 13C and 15N vary with land use, this study will contribute to the understanding of agricultural food webs and of responses of invertebrates to land use change.