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      Changes in natural ¹⁵n abundance in pastoral soils receiving differing amounts of superphosphate fertilizer and irrigation for 50 years

      Mudge, Paul Lawrence; Schipper, Louis A.; Ghani, A.; Upsdell, M.; Baisden, W. Troy
      DOI
       10.2136/sssaj2012.0333
      Link
       www.soils.org
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      Mudge, P. L., Shipper, L. A., Ghani, A., Upsdell, M. & Baisden, W. T. (2013). Changes in natural ¹⁵n abundance in pastoral soils receiving differing amounts of superphosphate fertilizer and irrigation for 50 years. Soil Biology & Biochemistry, 77(3), 830-841
      Permanent Research Commons link: https://hdl.handle.net/10289/7683
      Abstract
      Intensification of pastoral agriculture has led to increased N losses to the wider environment. This has led to an emphasis on improving our understanding of soil N dynamics, and development of management practices which mitigate N losses. The natural abundance of the stable isotope δ¹⁵N relative to ¹⁴N (δ¹⁵N) in soils can provide an integrated measure of past N cycle processes, and in particular soil δ15N can reflect past N losses, because during most N transformation processes 14N is preferentially lost. We therefore hypothesized that pastoral soils under intensive management regimes (with high N inputs, cycling, and loss) would become progressively enriched with ¹⁵N relative to soils under less intensive management. To test this hypothesis we analyzed archived surface soils from two long-term grazed field trials in New Zealand, where different rates of irrigation and superphosphate fertilizer had been applied for ∼50 yr. In all treatments except one (a control treatment receiving no fertilizer), δ¹⁵N increased with time and the increase was greater in treatments receiving more superphosphate or irrigation (average increases ranged from ∼0.015–0.034 ‰ yr–1). Pasture production and grazing intensity also increased with increasing fertilizer rate and irrigation frequency, and we found positive correlations between the average rate of change in soil δ¹⁵N, and total pasture production (r² = 0.77, p = 0.02), clover production (r² = 0.95, p < 0.001), and calculated N losses (r² = 0.98, p < 0.001) over ∼50 yr. We suggest that most of the difference in δ¹⁵N observed between treatments was due to the influence fertilizer and irrigation had on pasture production, and the resulting effects this had on isotope fractionating N loss processes. Soil δ¹⁵N could therefore be a useful indicator of past management intensity and N cycling and loss from pastoral systems.
      Date
      2013
      Type
      Journal Article
      Publisher
      Soil Science Society of American
      Collections
      • Science and Engineering Papers [3019]
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