Quantification of soil nutrient stocks and stoichiometric ratios in Eucalyptus pellita biomass plantation chronosequence in Papua New Guinea

Eucalyptus pellita biomass tree plantations have been widely established in the native grasslands of Markham Valley in Papua New Guinea for bioenergy production and carbon capture. However, the impacts of converting grasslands to energy plantations, and the ensuing soil transformations across the plantation chronosequence, remain largely unexplored. Such land use transitions are associated with depletion of C sinks, increased C emissions, and potential implications for climate change. This study aimed to evaluate dynamic changes in key soil physico-chemical properties, as well as nutrient stocks of soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP), alongwith their stoichiometric ratios. A chronosequence of E. pellita plantations aged 2-, 4-, 7- and 10-years along with native grassland sites, was examined across two sampling seasons (January and June). The results indicated significant (p < 0.05) impacts of tree stand age on SOC and TP concentration and stocks, while TN remained unaffected. Soil NH₄⁺-N concentration declined with the plantation age, contrasting with relatively steady NO₃^–-N levels across the chronosequence. SOC and TP stocks within the top 45 cm of soil depleted at rates of 0.19 Mg (Mega Gram) ha^-1y^-1 and 0.011 Mg ha⁻¹ y^-1, respectively. The broader stoichiometric ratio of C:N (> 25:1) observed in soils of 4- and 7-year-old plantations suggest potential nitrogen immobilization and mineral N deficiency to tree nutrition. Conversely, the C:P ratios across the chronosequence were < 200, favoring the net mineralization of organic P compounds. Overall, afforestation of grasslands with E. pellita for biomass production does not appear ecologically beneficial in the short-term (<10 years), when considering only the belowground SOC sequestration potential.