State and trend of surface soil organic carbon stocks and associated nutrients in Tongatapu, Kingdom of Tonga

U. Stockmann, M. Farrell, T. Carter, S. Krawitz, V. N.L. Wong, V. Minoneti, D. Finau, A. Sharma, J. Barringer, V. T. Manu, S. Karunaratne, B. C.T. Macdonald

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Context. The status and temporal trend in soil organic carbon (SOC) stocks are unknown in Tongatapu, the main island and agriculture hub of the Kingdom of Tonga. This is a critical shortcoming in relation to agricultural sustainability and food security. 

Aim. Evaluate the soil organic carbon stocks within Tongatapu’s main soil series relative to historic data. 

Methods. Historical studies were used to establish a SOC baseline for different soil series on Tongatapu. These sites were revisited and sampled in 2018 and the SOC content of the soil samples was determined using a dry combustion analytical method. Changes in the soil nitrogen (N), phosphorus (P), and sulfur (S) nutrient pools were estimated using stoichiometry. 

Key results. There was a decline in SOC stocks over the last 20–30 years. The mineralisation of soil organic matter, as represented by the decline in SOC stocks, has likely also reduced the pool of N, P, and S. Soils with the greatest SOC stocks (>50 t C ha−1) declined at the highest rates relative to soils with SOC stocks of 20–30 t C ha−1

Conclusion. Agricultural practices that are employed on Tongatapu have caused the decline of SOC stocks and are threatening sustainable agriculture production. 

Implications. This study shows that new farming and soil management practices should be employed that build SOC and broader soil functions.

Original languageEnglish
Article numberSR23071
Number of pages8
JournalSoil Research
Volume62
Issue number7
DOIs
Publication statusPublished - 26 Sept 2024

Keywords

  • atolls
  • farming systems
  • Pacific soils
  • root cropping
  • soil monitoring
  • soil organic carbon
  • sustainability
  • volcanic islands

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