Tile drainage history drives divergent soil carbon responses in moderately well‐drained and poorly drained soils

Abstract

Subsurface tile drainage is used globally in agroecosystems to lower water tables and improve soil conditions for crop production. Although drainage impacts on nutrient cycling and water quality are well documented, its influence on soil carbon (C) and nitrogen (N) stocks in mineral soils remains poorly understood. We evaluated C and N storage in grass seed agroecosystems under a Mediterranean climate across three drainage histories: <5 years since tile installation (New), >10 years since installation (Old), and Untiled fields in moderately well‐drained (MWD) and poorly drained (PD) soils. Cumulative C and N stocks were calculated using equivalent soil mass and compared throughout the soil profile using generalized additive mixed models. MWD soils stored significantly more C and N than PD soils regardless of drainage history, highlighting the importance of inherent soil properties in shaping soil C and N storage. In PD soils, drainage history had no significant effect on C or N stocks. In contrast, MWD soils with New tile systems contained 12.6 Mg C ha ^{− 1} less C and 1.2 Mg N ha ^{− 1} less N than Untiled systems at 3500 Mg soil ha ^{− 1} (∼35‐cm depth) and 6000 Mg soil ha ^{− 1} (∼57‐cm depth), respectively. Differences between Old and Untiled systems were small and largely confined to the upper 10–20 cm. These results suggest that drainage impacts on soil C and N depend on both natural drainage class and drainage history. Because these patterns were inferred using a space‐for‐time comparison, whether drainage effects diminish over time requires confirmation through long‐term monitoring.