Plants and earthworms control soil carbon and water quality trade-offs in turfgrass mesocosms.

Understanding how plants and earthworms regulate soil-based ecosystem services can guide design and management of built environments to improve environmental quality. We tested whether plant and earthworm activity results in trade-offs between soil carbon (C) retention and water quality. In a 2 × 2 factorial random block design, we introduced shrubs (Aronia melanocarpa) and earthworms (Lumbricus terrestris) to turfgrass (Lolium perenne) sandy loam mesocosms in a greenhouse. We measured soil respiration and soil microclimate every two weeks and leachate every two months. After 15 months, we assessed C and nitrogen (N) in bulk soil and aggregates (> 2000, 2000-250, 250-53 μm). Turfgrass mesocosms with earthworms retained less soil C (6.10 ± 0.20 kg/m), especially when warmer. Soils planted with shrubs were drier and had 7% lower mean respiration rates than soils without shrubs. Turfgrass mesocosms with both shrubs and earthworms retained more soil C (6.66 ± 0.25 kg/m), even when warmer, and held ~1.5 times more C in >2 mm aggregates than turfgrass-only mesocosms. Turfgrass mesocosms with shrubs and earthworms leached nitrate-N with increased respiration and retained phosphate-P and dissolved organic carbon (DOC) when wetter. In contrast, turfgrass mesocosms with only shrubs had the opposite response by leaching less nitrate-N with increased respiration, and more phosphate-P and DOC when wetter. Overall, shrub and earthworm activity in turfgrass mesocosms led to soil C-nutrient retention trade-offs. Our results reveal potential challenges in managing built environments to both retain soil C and improve water quality.