Nematode and Protist Community Responses in Soil to Land Use Conversion from Conventional to Organic Farming

Abstract

Soil organisms at higher trophic position in the food web, such as nematodes or protists, play a key role in shaping the composition and functioning of soil communities. However, intensive agriculture can put these organisms under pressure and restoring communities of soil organisms will be vital for sustainable agriculture. Here, we examine how changing land use from conventional to organic farming influences the composition and diversity of belowground nematodes and protists. We collected soil samples from 70 organic and conventional farmers’ fields on clay and sandy soils, and used 18S sequencing to determine community composition and diversity. In order to test effects of time since conversion from conventional to organic management, we used organically farmed soils that had been converted at time periods varying from 1-25 years ago, each being paired with a nearby conventional field in order to account for local environmental differences in climate and soil conditions.The ASV richness and Shannon diversity of protists was lowest in organic fields and protist community composition differed between organic and conventional fields in clay soils. Similarly, Shannon diversity of nematodes was lower in organic fields in clay, but there was no difference in community composition and species richness between conventional and organic fields. Time since conversion to organic management impacted community composition for both nematodes and protists in clay soils and increased protist richness and diversity in both soils. However, we found a similar trend for the conventional fields if we assigned them the age of the paired organic field. This suggests that another factor than time was underlying our gradient. Although pH, soil organic matter content, and microbial biomass were not related to time since conversion, they could explain functional and taxonomic community composition of both nematodes and protists. Shifts in the relative abundance of feeding/functional groups of protists and nematodes were variable and depended on soil and management type.We conclude that changing land use from conventional to organic farming may lead to changes in community composition and diversity in higher trophic levels of the soil food web. Although these changes in the soil food web were partly explained by soil (a)biotic properties, we were not able to untangle how those influences developed over time.

Keywords: nematodes, protists, sustainable agriculture, chronosequence