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 d 13 C and d 15 N 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 d 13 C values and termed beta C . Similarly we calculated beta N values to test the relationship between vertical N decrease and d 15 N increase. Decreasing d 13 C values of litter with age suggests a physiological response of plants to decreased litter N concentrations. A decrease of litter d 15 N in the early succession stages and a second decline after 1300 years indicates reduced N 2 fixation. Beta C 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.Beta N 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 15 N provided by fungi. We found relations between beta C and beta N values suggesting that there might be shared processes shaping d 13 C and d 15 N vertical depth profiles, e.g. microbial cycling, transport or sorption.

Three decades following afforestation are sufficient to yield δ13C depth profiles

“…In addition, the accuracy of beta values as an approximation for decomposition at sites with low decomposition appeared to be problematic, since linear regressions at arable sites had low coefficients of determination and were predominantly non‐significant. The generally very good curve fitting of linear regressions to calculate beta values of previous and other studies ( Powers and Schlesinger , ; Garten Jr ., ; Brunn et al, ; Brunn et al, ) therefore suggest a limited applicability of beta values for sites with high soil disturbance and/or low decomposition rates. Investigation of soils in defined times after plowing or investigation of artificial soil columns in experimental setups may help to study the development of isotopic depth profiles at finer time scales and to better limit the application of our approach.…”

“…An Elemental Analyzer (Isotope Cube, Elementar, Hanau, Germany) with coupled isotope ratio mass spectrometry (IRMS) (Isoprime 100, Isoprime, Manchester, England) served to analyze C and N concentrations and C stable isotope ratios ( Brunn et al, ). Results are given in delta notation as δ 13 C‰ VPDB = [(R sample –R standard ) × R standard −1 ] × 1000 , where R is the 13 C/ 12 C ratio ( Coplen , ).…”

“…Relations between the vertical decrease of C in OM and the increase of δ 13 C values served to approximate decomposition. Therefore, linear regressions between log 10 -transformed carbon concentrations [log 10 (g C kg -1 )] (= x) and the respective stable isotope values [δ 13 C%] (= y) of the depth intervals (organic layers and of the mineral soil) were calculated (Garten Jr., 2006;Brunn et al, 2014;Brunn et al, 2016). Absolute values of regression slopes were termed beta, so that higher beta values approximate higher decomposition of OM and vice versa.…”

“…The vertical change of organic matter (OM) stable carbon (C) isotope ratios in topsoil is a widely reported phenomenon across varieties of ecosystems and environmental gradients, with OM being more enriched in the heavier isotope 13 C at greater depth (Nadelhoffer and Fry, 1988;Balesdent and Mariotti, 1996;Garten Jr., 2006). The analysis of these vertical δ 13 C changes have been proven to be a useful tool to investigate C processing, with linear regression slopes between logarithmized C concentrations and δ 13 C values in soil profiles serving to approximate decomposition of OM in soil (Garten Jr., 2006;Acton et al, 2013;Brunn et al, 2014;Brunn et al, 2016). However, debates on the temporal behavior of OM in soil (Amelung et al, 2008;Schmidt et al, 2011;Gregory et al, 2016) raise the question how fast δ 13 C depth profiles develop.…”

Effects of Depth and Land Cover on Soil Properties as Indicated by Carbon and Nitrogen-Stable Isotope Analysis

“…In contrast, when the soil depth profi le was increased, the value of δ 15 N became signifi cantly higher. According to [56,66,[74][75] It is noteworthy that this increase in δ…”