Paired 15N-labeling evidence for efficient nitrate utilization by mature temperate broadleaved trees

Different nitrogen (N) forms might be used by various plant functional groups to reduce competition and promote coexistence. Broadleaved trees are generally considered prefer to take up nitrate, whereas conifers prefer to ammonium. However, this belief lacks in situ evidence from mature trees under field conditions. We conducted in situ paired 15N labeling on mature trees of six common species (three broadleaved, three conifers) in northeast China to assess their 15N-ammonium and 15N-nitrate uptake and reveal mechanisms underlying interspecific variation in N form utilization. All six species had higher uptake rates for 15N-nitrate than for 15N-ammonium, with 15N-nitrate rates 1.5- to 4.8-fold higher than those of 15N-ammonium. On average, the studied broadleaved species took up 15N-nitrate 3.4 times faster than 15N-ammonium (2.6 ± 0.4 vs.0.8 ± 0.1 μg 15N g-1 d-1). 15N-nitrate contributed 75% to total N uptake of broadleaved trees and 66% to that of conifers. Whole plant 15N-nitrate recovery was 2.7-fold higher than 15N-ammonium in the broadleaved species and was substantially higher than that in conifers (15.0 ± 1.8% vs. 2.8 ± 0.4%). Soil pH greatly affected 15N-nitrate uptake, possibly through soil pH modulation on soil nitrification and nitrate availability. Leaf nitrate reductase activity (NRA) and mycorrhizal colonization were also partially correlated with interspecific variation in nitrate acquisition. Our field experiments provided empirical evidence that common broadleaved species and conifers in northeast China predominantly take up nitrate over ammonium, particularly the studied broadleaved species, which may facilitate their growth and dominance in the future under changing climate.