Does Ammonia in Precipitation Inhibit Nitrate Removal by Cover Crops?.

It is well known that NH₄⁺ at low concentration can inhibit NO₃^- uptake and reduction (nitrate reductase or NRase) in plants. Atmospheric and precipitation NH₄⁺ has increased up to 60% in the Chesapeake Bay region in the past two decades, leading to the hypothesis that NH₄⁺ in precipitation can interfere with NO₃^- uptake and assimilation in cover crops. A series of experiments (hydroponic, perlite and soils) using rye and winter wheat was conducted. Perlite and hydroponic experiments were performed to determine the timing of inhibition of NRase. Hydroponic experiments were also used to evaluate inhibition of NO₃^- uptake. Experiments using winter wheat in three different types of soils were conducted as representative of three physiographic provinces: Hagerstown silt loam (Mountain), Glenelg silt loam (Piedmont) and Poplar sandy loam (Coastal Plain). In perlite experiments on both cover crops, we observed a 20% - 50% inhibition with exposure to 20µM NH₄⁺within 1 to 2 hours.  Hydroponic experiments showed a similar time course of NRase inhibition by NH₄⁺; however, parallel NO₃^‑ uptake studies showed nearly immediate inhibition. Winter wheat in Eastern Shore Poplar sandy loam showed NRase inhibition at 5µM, 10µM and 20µM NH₄⁺ within 6 days of treatment and NRase recovery at 2 weeks. Inhibition of NRase was also observed in Hagerstown silt loam at 10µM NH₄⁺ at 9 days post-treatment. No significant NRase inhibition was observed in Glenelg silt loam. Combined data suggest that NH₄­⁺ in precipitation at environmentally-realistic concentrations could interfere with NO₃^- absorption in cover crops and this inhibition is dependent upon NH₄⁺ concentration, cover crop and soil type. Rapid inhibition of NO₃^- uptake and subsequent inhibition of NRase on exposure to NH₄⁺ in precipitation may reduce the ability of cover crops to remove residual NO₃^-, leading to decreased effectiveness of cover crops under some circumstances.