Foliar and soil N and <i>δ</i>¹⁵N as restoration metrics at Pūtaringamotu Riccarton Bush, Christchurch city

Delta¹⁵N values and associated N concentrations may be useful nutrient cycling metrics for assessing restoration progress, and complement biodiversity assessment. We tested this hypothesis at Pūtaringamotu Riccarton Bush (‘Pūtaringamotu’), a low-rainfall urban forest fragment under restoration since 1974. Foliar and soil measurements came from four indigenous forests (Pūtaringamotu; Saltwater, high rainfall, logged until 2002; Ōkārito, high rainfall, never logged; Scotchman’s Creek, high rainfall, burrowing seabirds present). Despite different disturbance histories, Pūtaringamotu, Ōkārito and Saltwater results all fitted global <i>δ</i>¹⁵N regressions between rainfall and foliar N concentration. Enhanced foliar N concentrations accompanying low rainfall at Pūtaringamotu were similar to seabird-affected Scotchman’s Creek. Although Pūtaringamotu foliar <i>δ</i>¹⁵N values showed differences between taxa, they were less pronounced than for Ōkārito; Saltwater showed no effect. Further investigation of nutrient cycling restoration metrics at Pūtaringamotu should therefore emphasise progressive development of differences in foliar <i>δ</i>¹⁵N between plant taxa, pooled values being insufficiently sensitive.