Organic rice cultivation enhances the diversity of above-ground arthropods but not below-ground soil eukaryotes

Organic farming generally has greater biodiversity compared to conventional farming. However, few studies have examined both above- and below-ground biodiversity in organic farming of the rice field, and little is known about whether and how biodiversity is affected by the duration of organic farming. In this study, we conducted a two-year investigation on one conventional farm, as well as two organic farms with different management durations (1 and 5 years before sampling). Compared to conventional farming, species richness increased by 40% for arthropods in organic farming, with 75% for pests and 35% for natural enemies, but there were no differences for soil microbial eukaryotes. No significant difference in species diversity of arthropods and soil microbial eukaryotes was found between two organic farms, which may indicate that five years is not yet long enough to achieve significant changes in arthropod and eukaryotic communities. We used a compatibility index (C_c-o) to evaluate the trade-off between biodiversity, rice yield, and economic benefits of conversion from conventional to organic farming. Results showed that the yield-based value of C_c-o was close to 0 for above-ground arthropods and negative for soil eukaryotes. However, C_c-o values based on economic benefits were positive for all taxa. Our study indicates that both one year and five years durations of organic farming could improve the diversity of above-ground arthropods, but not that of below-ground soil eukaryotes in the rice field. There was an equivalent gain in biodiversity and loss in yield for above-ground arthropods under organic farming, but win-win scenarios may occur when the economic benefits are considered.