Dissolved inorganic and organic carbon export from tile-drained midwestern agricultural systems

While carbon is a critically important natural element cycling through the soil profile of agricultural systems, few stud-ies have examined the flux of dissolved organic carbon (OC) and inorganic carbon (IC) through artificially-drained cropped fields. In this study, we monitored eight tile outlets, nine groundwater wells and the receiving stream during a March to November period in 2018 to quantify subsurface IC and OC flux from tiles and groundwater to a perennial stream from a single cropped field in north-central Iowa. Results showed that carbon export from the field was dom-inated by IC losses through subsurface drainage tiles that were 20x higher than dissolved OC concentration in tiles, groundwater and in Hardin Creek. IC loads from tiles comprised approximately 96 % of the total carbon export. De-tailed soil sampling within the field quantified TC stocks to a 1.2 m depth (246,514 kg/ha), and based on the maximum annual rate of IC loss from the field (553 kg/ha per year), we estimated that approximately 0.23 % of the TC content (0.32 % of the TOC content and 0.70 % of the TIC content) of the shallow soils was lost in a single year. Loss of dis-solved carbon from the field is likely offset by reduced tillage and additions of lime. Study results suggest that attention should be given to improved monitoring of aqueous total carbon export from fields for accurate accounting of carbon sequestration performance.

Automated summary

This study examined the carbon flux from artificially-drained cropped fields. The results showed that the dominant carbon export was from inorganic carbon (IC) losses through subsurface drainage tiles. Detailed soil sampling revealed that approximately 0.23% of the total carbon content in shallow soils was lost in a single year. The study suggests the need for improved monitoring of total carbon export from fields for accurate assessment of carbon sequestration.