Benefits in plant N uptake via the mycorrhizal pathway in ample soil moisture persist under severe drought

Under global change, drought is a threat of increasing importance to plant growth and nutrition. We employed isotopic labeling on potted tomatoes (Solanum lycopersicum, cv. 'Micro Tom') to investigate the role of an arbuscular mycorrhizal fungus (AMF – Rhizophagus irregularis) in plant uptake of nitrogen (N) from either inorganic (15NH4Cl) or organic source (15N-labeled clover biomass) along a soil moisture gradient. 15N was provided into root-excluding mesh bags inserted in pots with mycorrhizal (M) and nonmycorrhizal (NM) plants. After labeling, a substrate moisture gradient ranging from ample moisture to the wilting point was maintained for the terminal 4 weeks. M plants acquired strikingly more 15N derived from either the inorganic or organic source than their NM counterparts. The advantage of M plants in 15N uptake was manifested across the entire moisture range, although a declining trend was observed toward the drought extreme. M plants exploited up to threefold more 15N if it was provided in the inorganic form than in the organic form, probably due to faster availability of ammonium dissolved from 15NH4Cl and adsorbed on soil particles as compared to ammonium ions derived from clover biomass via mineralization. Interestingly, NM plants received more 15N from the organic than from the inorganic source. Mycorrhiza caused a fourfold increase in N acquisition from mineral sources but only by twofold from organic sources. We speculate that this was partly caused by higher rates of N transformation of ammonium ions released by mineralization to more mobile forms (nitrification) that happened inside mesh bags with the clover biomass, and we discuss possible mechanisms responsible for such increased mobility.