Insights on agricultural nitrate leaching from soil block mesocosms

Quantifying nitrate leaching in agricultural fields is often complicated by inability to capture all water draining through a specific area. We designed and tested undisturbed soil monoliths (termed “soil block mesocosms”) to achieve complete collection of drainage. Each mesocosm measures 1.5 m × 1.5 m × 1.2 m and is enclosed by steel on the sides and bottom with a single outlet to collect drainage. We compared measurements from replicate mesocosms planted to corn (Zea mays L.) with a nearby field experiment with tile‐drained plots (“drainage plots”), and with drainage from nearby watersheds from 2020 through 2022 under drought conditions. Annual mesocosm drainage volumes were 6.5–24.6 cm greater than from the drainage plots, likely because the mesocosms were isolated from the subsoil and could not store groundwater below the drain depth, whereas the drainage plots accumulated infiltration as groundwater. Thus, we obtained consistent nitrate leaching measurements from the mesocosms even when some drainage plots yielded no water. Despite drainage volume differences, mean flow‐weighted nitrate concentrations were similar between mesocosms and drainage plots in 2 of 3 years. Mesocosm annual drainage volume was 8.7 cm lower to 16.7 cm higher than watershed drainage, likely due to lagged influences of groundwater. Corn yields were lower in mesocosms than drainage plots in 2020, but with irrigation, yields were similar in subsequent years. Mean 2020 surface soil moisture and temperature were similar between the mesocosms and nearby fields. Based on these comparisons, the mesocosms provide a robust method to measure nitrate leaching with lower variability than field plots. Core Ideas We tested a new field study design to measure drainage from undisturbed soil mesocosms planted to corn. Annual mesocosm drainage was higher than drainage plots but lower or higher than nearby watersheds during drought. Drainage and nitrate differences among mesocosms and drainage plots were likely due to groundwater storage. Corn yields from mesocosms and drainage plots were equivalent when irrigation was applied. The mesocosm design reduced variability in drainage and nitrate leaching measurements relative to drainage plots. Plain Language Summary Nitrate leaching is a major cause of water pollution, but it is challenging to measure. We tested a method to measure nitrate leaching by enclosing blocks of soil within a field in steel boxes, each with a single drain for water quality mo