Optimum rates of surface‐applied coal char decreased soil ammonia volatilization loss

Fertilizer N losses from agricultural systems have economic and environmental implications. Soil amendment with high C materials, such as coal char, may mitigate N losses. Char, a coal combustion residue, obtained from a sugar factory in Scottsbluff, NE, contained 29% C by weight. A 30‐d laboratory study was conducted to evaluate the effects of char addition on N losses via nitrous oxide (N 2 O) emission, ammonia (NH 3 ) volatilization, and nitrate (NO 3 –N) leaching from fertilized loam and sandy loam soils. Char was applied at five different rates (0, 6.7, 10.1, 13.4, and 26.8 Mg C ha −1 ; char measured in C equivalent) to soils fertilized with urea ammonium nitrate (UAN) at 200 kg N ha −1 . In addition, there were two negative‐UAN control treatments: no char (no UAN) and char at 26.8 Mg C ha −1 (no UAN). Treatment applied at 6.7 and 10.1 Mg C ha −1 in fertilized sandy loam reduced NH 3 volatilization by 26–37% and at 6.7, 10.1, and 13.4 Mg C ha −1 in fertilized loam soils by 24% compared with no char application. Nitrous oxide emissions and NO 3 –N leaching losses were greater in fertilized compared with unfertilized soil, but there was no effect of char amendment on these losses. Because NO 3 –N leaching loss was greater in sandy loam than in loam, soil residual N was twofold higher in loam than in sandy loam. This study suggests that adding coal char at optimal rates may reduce agricultural reactive N to the atmosphere by decreasing NH 3 volatilization from fertilized soils.