N:P ratios, δ¹⁵N fractionation and nutrient resorption along a nitrogen to phosphorus limitation gradient in an oligotrophic wetland complex

The vegetation N:P ratio is thought to be a diagnostic indicator of the nature of nutrient limitation in wetland vegetation. It should therefore be closely linked to other indicators of nutrient acquisition and conservation, such as nitrogen stable isotope fractionation (δ¹⁵N), nutrient resorption efficiency (RE) and resorption proficiency (RP). However, the interrelationships among these traits and the N:P ratio remain unclear. We compared tissue nutrient concentrations, N:P ratios, δ¹⁵N fractionation, RE, and RP along an N to P limitation gradient in an oligotrophic wetland valley in the South Island of New Zealand. Within the valley, the soil TN:TP ratio increased from 1.3 to 18.0 in three discrete wetlands along the gradient. In pooled data from all vegetation communities within each site, the mass-based vegetation N:P ratio correlated significantly (r²=0.35, P<0.01) to soil TN:TP ratios and increased from 10.2±2.7 to 13.5±3.6 along the N to P limitation gradient. This was accompanied by an increase in tissue δ¹⁵N enrichment from 2.05±1.12‰ to 6.27±1.70‰, consistent with more open N cycling and lower N demand. These trends held within all vegetation types, but were particularly strong in a Typha orientalis (C-strategist) community (soil TN:TP vs vegetation N:P correlation r²=0.78, P<0.001; δ¹⁵N increase from 1.81±0.44‰ to 7.73±1.79‰). The individual N and P concentrations and retention patterns were more species-specific and less responsive to the nutrient limitation gradient. T. orientalis maximised N resorption as N limitation increased (increasing NRE from 50.8±3.3% to 71.7±7.4%; reducing NRP from 0.70±0.12% to 0.36±0.13%) but did not alter PRE or PRP, whereas the S-strategist Schoenus pauciflorus maximised P resorption as P limitation increased (increasing PRE from 48.0±5.6% to 73.5±10.1%; reducing PRP from 0.053±0.008% to 0.015±0.004%) but did not alter NRE or NRP. These results show that the tissue N:P ratio and its associated δ¹⁵N enrichment are highly responsive indicators of the relative availability of N and P at the site and community level. However, they are not indicators of species-specific physiological requirements for N and P, or of likely responses of individual species to N or P enrichment, which are better interpreted from indicators such as RE and RP that describe nutrient retention behaviour.