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

doi:10.1071/SR23071

RESEARCH ARTICLE (Open Access)

Next Contents Vol 62(7)

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

U. Stockmann ^A , M. Farrell

^B , T. Carter ^B , S. Krawitz ^C , V. N. L. Wong

^C , V. Minoneti ^D , D. Finau ^D , A. Sharma ^E , J. Barringer ^F , V. T. Manu ^D , S. Karunaratne

^G and B. C. T. Macdonald

^A ^*

+ Author Affiliations

- Author Affiliations

^A CSIRO Agriculture and Food, Soils and Landscapes Group, Canberra, ACT, Australia.

^B CSIRO Agriculture and Food, Footprints and Credentials Group, Adelaide, SA, Australia.

^C School of Earth, Atmosphere and Environment, Monash University, Melbourne, Vic, Australia.

^D Ministry of Agriculture, Food and Forests, Kingdom of Tonga, Nuku‘alofa, Tonga.

^E Fijian Ministry of Agriculture and Waterways, Suva, Fiji.

^F Manaaki Whenua Landcare Research, Lincoln, New Zealand.

^G CSIRO Agriculture and Food, Footprints and Credentials Group, Canberra, ACT, Australia.

^* Correspondence to: ben.macdonald@csiro.au

Handling Editor: Thomas Bishop

Soil Research 62, SR23071 https://doi.org/10.1071/SR23071

Submitted: 19 April 2023 Accepted: 1 September 2024 Published: 26 September 2024

© 2024 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)

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.

Keywords: atolls, farming systems, Pacific soils, root cropping, soil monitoring, soil organic carbon, sustainability, volcanic islands.

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