Changes in δ¹⁵N in pastoral soils under varying management intensity
type: CreativeWork
description: In New Zealand, nitrogen (N) inputs and losses have increased dramatically following establishment (and subsequent intensification) of European style pastoral agriculture from the 1800s. An indicator that could identify soils which are most vulnerable to N loss would be useful to help target management practices aimed at reducing unwanted N losses. The natural abundance of ¹⁵N relative to ¹⁴N (δ¹⁵N) in soils is one potential indicator of N loss, because during most N transformations in soils, ¹⁴N is preferentially processed and lost (e.g. via ammonia volatilisation, denitrification or nitrate leaching). Therefore the overarching hypothesis for this thesis was that pastoral soils under intensive management regimes (with high N inputs, cycling and losses) would become progressively enriched with ¹⁵N relative to soils under less intensive management. This hypothesis was tested by measur ing δ¹⁵N in soils from four forest-to-pasture chronosequences, and in archived soils from six long-term (4–57 year) grazed field trials with different fertiliser or irrigation regimes. Three of the forest-to-pasture chronosequences were on pumice soils where pine forests had been converted to dairy pastures. The fourth chronosequence was on a podzol soil in Northland, where native scrub had been converted to sheep grazed pastures. Surface soil δ¹⁵N on the pumice soil increased significantly from pine forests (2 ‰) to long-term pastures (4.1 ‰). In contrast, there was no clear relationship between pasture age and soil δ¹⁵N for the chronosequence on the podzol soil. The Northland soil displayed extreme podzolization and weathering, and had been previously disturbed by gum diggers, implying results could be relatively unique to this soil. The two longest field trials were at Winchmore on the Canterbury plains, where different rates of superphosphate and irrigation had been applied for ~50 years. Soil δ¹⁵N increased more in treatments receiving higher rates of superphosphate or more frequen…
abstract: In New Zealand, nitrogen (N) inputs and losses have increased dramatically following establishment (and subsequent intensification) of European style pastoral agriculture from the 1800s. An indicator that could identify soils which are most vulnerable to N loss would be useful to help target management practices aimed at reducing unwanted N losses. The natural abundance of ¹⁵N relative to ¹⁴N (δ¹⁵N) in soils is one potential indicator of N loss, because during most N transformations in soils, ¹⁴N is preferentially processed and lost (e.g. via ammonia volatilisation, denitrification or nitrate leaching). Therefore the overarching hypothesis for this thesis was that pastoral soils under intensive management regimes (with high N inputs, cycling and losses) would become progressively enriched with ¹⁵N relative to soils under less intensive management. This hypothesis was tested by measur ing δ¹⁵N in soils from four forest-to-pasture chronosequences, and in archived soils from six long-term (4–57 year) grazed field trials with different fertiliser or irrigation regimes. Three of the forest-to-pasture chronosequences were on pumice soils where pine forests had been converted to dairy pastures. The fourth chronosequence was on a podzol soil in Northland, where native scrub had been converted to sheep grazed pastures. Surface soil δ¹⁵N on the pumice soil increased significantly from pine forests (2 ‰) to long-term pastures (4.1 ‰). In contrast, there was no clear relationship between pasture age and soil δ¹⁵N for the chronosequence on the podzol soil. The Northland soil displayed extreme podzolization and weathering, and had been previously disturbed by gum diggers, implying results could be relatively unique to this soil. The two longest field trials were at Winchmore on the Canterbury plains, where different rates of superphosphate and irrigation had been applied for ~50 years. Soil δ¹⁵N increased more in treatments receiving higher rates of superphosphate or more frequen…
author: Mudge, Paul Lawrence
keywords: natural abundance, nitrogen isotopes, pasture, grazing, land use change, grassland
license: https://researchcommons.waikato.ac.nz/bitstreams/ede4282c-58a9-48fe-81f1-1a1fbd193132/download
date published: 2012
date modified: 2026-07-24T05:57:15Z
provider: University Waikato
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Waikato