Assessment of methylammonium as an analog for ammonium in plant uptake from soil

Abstract

We tested radioactive methylammonium (14CH3NH inf3 sup+ ) as a tracer for ammonium (NH4 +) in root uptake measurements from soil. Tomato (Lycopersicon esculentum Moll. cv T5) in 3 L pots filled with loamy sand soil received 40, 200, or 600 μmol 14CH3NH3 + or 15NH4 +. During a 4 h period, the plants absorbed 14CH3NH3 + at slower rates than 15NH4 +. Estimates of NH4 + absorption based on 15NH4 + absorption were 0.9–7.9 μmol NH4 + g−1 plant dry weight h−1, whereas those based on 14CH3NH3 + absorption were 0.2–1.0 μmol NH4 + g−1 plant dry weight h−1. After 4 h, approximately one-half of the applied 15N was not recovered in the plants or soil KCl extracts; apparently, this 15N was either immobilized or nitrified and denitrified by soil biota. By contrast, almost all the 14CH3NH3 + remained in the soil solution after 4 h, but after a 10 d incubation, approximately 20% had been released as 14CO2. These differences in plant absorption rates and movement through soil pools indicate that CH3NH3 + cannot be used reliably as an NH4 + analog in soil.