Relationships between leaf δ¹⁵N and leaf metallic nutrients

Abstract

Rationale Nitrogen (N) isotopic ratios (δ15N values) of plants are primarily determined by the δ15N values of their N sources. Metallic nutrients affect plant N uptake. However, there is little knowledge of the relationships between leaf δ15N values and leaf metallic nutrients. The δ15N values are often lower in soil nitrate (NO3−) than in ammonium (NH4+) due to large isotopic fractionation during nitrification. Plants acquire more NO3− than NH4+ when accumulating high potassium (K), calcium (Ca) and magnesium (Mg) to maintain charge balance. In addition, plants that absorb more NO3− than NH4+ increase the soil pH and decrease the availability of iron (Fe), manganese (Mn) and zinc (Zn). We therefore hypothesized that leaf δ15N values correlate negatively with K, Ca and Mg contents, while positively with Fe, Mn and Zn contents. Methods Leaves of non‐N‐fixing plants were sampled across an approx. 6000 km transect in China and their δ15N values and metallic nutrient content were determined using elemental analyzer/isotope ratio mass spectrometry. Results Inconsistent with the hypothesis, leaf δ15N values correlated positively with leaf K, Ca and Mg, indicating higher δ15N values of soil NO3− than NH4+. Higher δ15N values of soil NO3− revealed stronger denitrification than nitrification in the study regions because isotopic fractionation occurs during both processes. Leaf δ15N values correlated negatively with Fe, relating to decreases in soil Fe availability, which might be attributed to oxidation of Fe2+ to Fe3+ supplying electrons for denitrification, while greater uptake of NO3− than NH4+ of plants increases soil pH. Leaf δ15N values correlated positively with Zn and did not correlate with Mn. These observed relationships between leaf δ15N values and metallic nutrients, except Mn, were independent of vegetation or soil types. Conclusions This study has enriched our knowledge of associations between metallic nutrients and N cycling in plant–soil systems, especially for the roles of Fe in soil N transformations and K, Ca and Mg in plant N uptake.

Effects of high Ca and Mg stress on plants water use efficiency in a Karst ecosystem

“…However, the limitation of plant nutrients does not rule out the dilution effects due to the specific Karst environment. For example, K, Ca, and Mg contents can influence the NO 3 − and NH 4 + uptake of plants ( Chen et al, 2021 ) and the antagonistic interaction between K and Ca ( Haghshenas, Arshad & Nazarideljou, 2018 ). Further study should consider using the isotope tracing method to reveal the specific sources of elements and determine the influence of elements on each other.…”