Ion exchange membranes are sensitive indicators of ammonium and nitrate released from green manures with low C/N ratios

• Ion exchange membranes are a sensitive indicator of NH4-N and NO3-N dynamics.
• Clay content slows decomposition, but not mineralization, of green manure residues.
• Greater mineral N released from green manure residues with C/N ratio of 8 than C/N = 12.

Green manure is a valuable nitrogen (N) source, but must undergo decomposition, mineralization and nitrification to release mineral N (NH4-N and NO3-N). The pattern of soil mineral N released from green manure is related to its biochemical composition (e.g., C/N ratio) and soil texture. The objective of this study was to investigate the use of ion exchange membranes (IEMs) as a sensitive indicator of N mineralization and nitrification from green manure (mixture of pea and oat residues) having C/N = 8 and C/N = 12, mixed into sandy clay loam and sandy loam soils. The green manures decomposed rapidly and released mineral N that was captured by IEMs inserted in the soil during a 6-wk greenhouse incubation. Net NH4 and NO3 produced after 6-wk was greater in soil mixed with green manure having C/N = 8, and in the sandy clay loam mixed with green manure having C/N = 12, than unamended control soil. The IEM- NH4-N concentration peaked one week after the green manure incorporation, with 0.13 μg IEM- NH4-N cm−2 wk−1 in the sandy clay loam and 0.19 μg IEM- NH4-N cm−2 wk−1 in the sandy loam soil, and declined thereafter. While the IEM- NO3-N increased steadily in the sandy clay loam during the 6-wk incubation, there was a plateau (green manure C/N = 8) or decline (green manure C/N = 12) in IEM- NO3-N concentration in the sandy loam soil. Net N mineralization and nitrification rates corresponded to the temporal fluctuations in mineral N detected with IEMs, confirming that this tool is a sensitive indicator of NH4-N and NO3-N dynamics in soils amended with green manures having low C/N ratios.