Vertical distribution of carbon and nitrogen stable isotope ratios in topsoils across a temperate rainforest dune chronosequence in New Zealand

Abstract

Chronosequences can provide valuable insights into carbon (C) and nitrogen (N) dynamics across natural gradients with C and N stable isotopes serving as powerful tool investigating these dynamics. We studied changes in δ13C and δ15N values in litter, organic layer and mineral soil on dunes across the Haast chronosequence (New Zealand), which spans 120 to 2870 years of pedogenesis beneath a temperate rainforest. Decomposition was approximated from linear regression slopes between C concentrations and δ13C values and termed betaC. Similarly we calculated betaN values to test the relationship between vertical N decrease and δ15N increase. Decreasing δ13C values of litter with age suggests a physiological response of plants to decreased litter N concentrations. A decrease of litter δ15N in the early succession stages and a second decline after 1300 years indicates reduced N2 fixation. BetaC values increased during early ecosystem development and at old sites, and were lowest at the intermediate stages (1500 years), which suggests decomposition did not decrease constantly with time. BetaN values were lowest at the youngest site and increased within the first 200 years, likely because litter as the uppermost part of the vertical depth profile reflected an increased supply of N depleted in 15N provided by fungi. We found relations between betaC and betaN values suggesting that there might be shared processes shaping δ13C and δ15N vertical depth profiles, e.g. microbial cycling, transport or sorption.