Controls on organic and inorganic soil carbon in poorly drained agricultural soils with subsurface drainage

Many productive agricultural soils have naturally poor drainage characteristics and may intermittently pond water even where artificial drainage infrastructure is present, especially in topographic depressions. Soil organic carbon (SOC) is often higher in depressions than uplands, but whether temporary ponding increases SOC by suppressing decomposition remains an important knowledge gap. We measured SOC and inorganic C (carbonate) along topographic gradients from tile-drained depressions to adjacent uplands and tested their relationships with hydrological and biogeochemical properties in corn/soybean fields in Iowa, USA, and examined soil respiration and its stable C isotopes (delta C-13) by lab incubation. The 0-30 cm SOC concentration was greatest at depression bottoms, as expected, while carbonate C was highest at boundaries between depressions and uplands. However, only carbonate C, not SOC, increased in depressions with increasingly poor drainage (greater ponding duration). Silt + clay content was the strongest positive predictor of SOC, while ponding duration and oxalate-extractable iron were negatively related to SOC in a statistical model (R-2 = 0.83). These negative relationships are consistent with suppression of crop biomass production and iron-mediated decomposition in periodically anoxic soil. Soil C/N ratios were similar in depressions and uplands, indicating that plant detritus did not accumulate with ponding. Stable C isotopes of respiration from incubated soils indicated a similar C-3/C-4 plant mixture in depressions and uplands, consistent with decomposing soybean and corn residues. In contrast, depression soil organic matter had lower delta C-13 and delta N-15 values than uplands, more consistent with pre-agricultural prairie plants than crop residues. Accumulation of SOC in these agricultural depressions is more likely explained by erosion than by suppression of decomposition due to temporary ponding. Gaining additional SOC may require fundamental changes in management, or wetland restoration.