Effects of agricultural practices on organic matter degradation in ditches

Abstract

Agricultural practices can result in differences in organic matter (OM) and agricultural chemical inputs in adjacent ditches, but its indirect effects on OM composition and its inherent consequences for ecosystem functioning remain uncertain. This study determined the effect of agricultural practices (dairy farm grasslands and hyacinth bulb fields) on OM degradation by microorganisms and invertebrates with a consumption and food preference experiment in the field and in the laboratory using natural OM collected from the field. Freshly cut grass and hyacinths were also offered to control for OM composition and large- and small mesh-sizes were used to distinguish microbial decomposition and invertebrate consumption. Results show that OM decomposition by microorganisms and consumption by invertebrates was similar throughout the study area, but that OM collected from ditches adjacent grasslands and freshly cut grass and hyacinths were preferred over OM collected from ditches adjacent to a hyacinth bulb field. In the case of OM collected from ditches adjacent hyacinth bulb fields, both microbial decomposition and invertebrate consumption were strongly retarded, likely resulting from sorption and accumulation of pesticides. This outcome illustrates that differences in agricultural practices can, in addition to direct detrimental effects on aquatic organisms, indirectly alter the functioning of adjacent aquatic ecosystems.

Partitioning the impact of environmental drivers and species interactions in dynamic aquatic communities

“…The importance of pesticides for explaining the abundances of taxa, that is, the composition of the community, is low, even in spring-at the peak of their importance. The low impact of pesticides can be regarded as surprising given the high and persistent pesticide loads in parts of the area due to the flower bulb industry (Hunting et al 2016, Barmentlo et al 2018; http://www.pesticidesatlas.nl), although it may also suggest that the community is welladapted to pesticides due to long-term exposure, so that very sensitive taxa may be missing from the area. The decrease in importance over the seasons up to early autumn is also exhibited in the other chemical predictors, namely pH, DO, and DOC.…”

“…We illustrate the applicability of this novel approach by analyzing a comprehensive dataset of aquatic ditch macrofaunal samples (Verdonschot et al 2011). This dataset describes a set of ditches that are exposed to different types of land uses that create a mosaic of different abiotic pressures affecting the adjacent aquatic system (Ieromina et al 2015, Hunting et al 2016, Musters et al 2019. Over the seasons, we expect to find in the macrofaunal community a shift in the relative importance of community processes, from those related to population growth toward an increased importance of interspecific interactions.…”

Spatial and temporal homogenisation of freshwater macrofaunal communities in ditches

“…Instead, knowing that community regulation is increased by increasing food limitation (Cadotte & Tucker, 2017;Musters et al, in press), the decreasing dissimilarity is likely to reflect an increase in food limitation or decrease in food quality over time, which is particularly evident in ditches adjacent to bulb fields where a myriad of agricultural chemicals are applied (Hunting et al, 2016Vijver et al, 2017). We consistently found a significant interaction effect between land use and year and a seasonal decrease in spatial β-diversity for the land-use types considered in this study.…”

“…Furthermore, based on a previous study of the quality of organic material from the ditches in the agricultural area for consumption by invertebrates, we know that ditches of the bulb-fields are under stronger human impact than ditches of the dairy farm grasslands (Hunting et al, 2016). So, for this study we divided the ditches into three categories according to the land use in the adjacent fields: dune ditches, bulb ditches, and grassland ditches (Appendix a: Figure S1).…”

“…These pressures include the continuous run-off of pesticides and nutrients, which may pose differential selection pressures that can directly (chemical toxicity) or indirectly (resource quality) affect the resident organisms (Allan, 2004;Herzon & Helenius, 2008;Hunting, Vonk, Musters, Kraak, & Vijver, 2016). As such, the study sites allow for assessing the effects of land-use practices on the spatiotemporal composition of these aquatic communities by considering the general null hypothesis that the macrofaunal samples collected in these ditches were derived from a highly connected, spatially and temporally uniform metacommunity, such that the relative species abundances at each location represents a random sample of the relative abundance of the species in the complete metacommunity.…”