Changes in soil C, N and δ15N along three forest-pasture chronosequences in New Zealand

Abstract

Changes in total soil carbon (C), nitrogen (N) and natural-abundance N isotopes (δ15N) were measured along three forest-to-pasture chronosequences on pumice soils in the Central North Island of New Zealand. On each of the three chronosequences, exotic pine forests had been converted to intensive dairy pastures 2-11 years before sampling and samples were also taken from remaining pine forests and long-term pastures (40-80 years old). The primary objective of the study was to test the hypothesis that surface-soil δ15N would increase over time following conversion of forest to pasture, due to greater N inputs and isotope-fractionating N losses (e.g. ammonia volatilisation) in pasture systems. Results supported our hypothesis, with linear regression revealing a significant (P<0.001) positive correlation between log-transformed pasture age (log10[pasture age+1]) and surface-soil δ15N. There was also a positive correlation (P<0.001) between pasture age and total soil C and N, and a negative correlation of pasture age with C:N ratio. Surface-soil δ15N was also positively correlated (P<0.001) with total soil N, and negatively correlated with C:N ratio when C:N was <13.6. These results suggested that as soils became more N-'saturated', isotope-fractionating N loss processes increased. Surface-soil δ15N in the pine forests was significantly less than subsoil δ15N, but there was no significant difference between the surface and subsoil in the long-term pastures, due to 15N enrichment of the surface soil. The difference in δ15N between the surface soil and subsoil may be a useful indicator of past land management, in addition to absolute δ15N values of surface soils. © CSIRO 2014.