Subsidy type and quality determine direction and strength of trophic cascades in arthropod food webs in agroecosystems

The subsidy hypothesis states that communities receiving nutrient subsidies will demonstrate top-down trophic cascades where predators indirectly increase plant biomass. This has been both confirmed and refuted, which might depend on whether the subsidy has mainly targeted the plant or the detrital food web compartment, and on the subsidy quality. This is particularly poorly understood for terrestrial communities such as heavily subsidized agroecosystems. Using cages covering 4 m(2) of ground in a long-term agricultural fertilization experiment, we tested whether subsidies targeting the detrital soil mesofauna compartment with organic fertilizers, or the plants with mineral fertilizer, impacted the direction and strength of trophic cascades in an arthropod-plant food web. We expected top-down controls of generalist arthropod predators (spiders, ground and rove beetles) on aphid densities to be stronger in organically fertilized plots due to enhanced alternative prey availability in the soil. Bottom-up control from barley quality on aphids was anticipated to be stronger in the mineral treatments. We examined how the quality (decomposability) of the organic subsidy governed the cascades by comparing treatments with labile (manure) and recalcitrant (hay) organic matter. Top-down forces dominated in food webs receiving organic subsidies, while bottom-up forces dominated under mineral fertilization. A high-quality, easily degradable organic subsidy propagated faster through the food chain, leading to a top-down trophic cascade with generalist predators having a positive effect on plant biomass in the labile but not in the recalcitrant organic treatment. Synthesis and applications. Management of agricultural soils that bolster the soil mesofauna, for example adding organic fertilizers, has potential to increase top-down biological control by naturally occurring generalist arthropod predators. Our research demonstrates how barley biomass can be enhanced in manure treatment in the presence of arthropod predators to a level comparable to that of mineral fertilizer.