Production, Consumption, and Transport of Greenhouse Gases in the Cropland Subsoil with a Shallow Groundwater Table.

Dissolved greenhouse gases (GHGs) in the percolating water under croplands are one of the potentials for the indirect emissions through various aquifers. Our previous study in the lysimeter field has revealed that annual emissions of dissolved nitrous oxide (N₂O) and carbon dioxide (CO₂) were quantitatively important as compared to the direct emissions. Moreover, there was a positive relationship between dissolved N₂O concentration and the aboveground efflux under upland conditions. We conducted one-year lysimeter field experiment to clarify the production, consumption, and transport of the three GHGs, including methane, in the soil. The lysimeter plots (3 by 3 by 1-m depth Fluvisol) with a 0.9-m depth of groundwater table were cultivated with different crops (paddy rice, soybean/wheat, and upland rice). Aboveground fluxes and concentrations in soil profile (gas or dissolved form) of the GHGs were monitored. The δ¹⁵N and δ¹⁸O of N₂O in aboveground efflux and dissolved in the percolating water under upland conditions in the summer were compared to identify the process and place for production.

Intensive methane production occurred in the flooded topsoil, and the concentration exponentially lowered with soil depth probably due to dilution and/or oxidation. The CO₂ concentration linearly rose with depth both under flooded paddy and upland conditions, indicating that CO₂ was produced in all the layers. The N₂O concentration rose with depth under upland conditions but fluctuated by the redox conditions in the flooded soil. Especially in the upland rice plots where crop biomass was unexpectedly quite low, much N₂O was produced in the subsoil as denitrification using leached surplus nitrate, where aboveground efflux was also high. The stable isotope analysis supported that N₂O produced in the subsoil of the upland rice plots was directly emitted into the atmosphere. Consequently, cropland subsoil with a shallow groundwater table is a hotspot for the GHG dynamics.