COS 211-3 - Stocks and biogeochemical cycling of soil-derived nutrients in an ultramafic rain forest in New Caledonia

Background/Question/Methods

The biogeochemical cycling of nutrients comprises a broad array of direct and indirect interactions and processes whose deciphering lies in estimating quantities of nutrients in the various pools and the prevalence of interconnecting flows. In rain forest ecosystems, the partitioning of nutrient estimations are complex as they occur over a broad range of substrates with sometimes peculiar characteristics. This is particularly true for New Caledonian rain forests which despite being renowned for their unique biodiversity (i.e., ~2100 vascular plant species thriving on a ~5,500 km² surface) and exceptional endemicity ( >85%), evolved on some of the most nutrient impoverished soils globally. This study aimed to determine nutrient stocks and elucidate the biogeochemical cycling of nutrients in a remnant lowland rain forest that developed over millennia on exceptionally nutrient-poor ultramafic soils in southern New Caledonia. Based on an inventory of a 1-ha permanent plot, exhaustive plant tissue sampling (leaves, bark, twigs, wood, sap and latex) was undertaken of all large trees (diameter at breast height ≥ 15 cm) in a 0.25-ha subset in tandem with collecting 100 soil samples and litter fall over one year.

Results/Conclusions

All samples were analysed for major nutrient concentrations, and the results show that most of the magnesium was contained in the soil (96.9%), whereas a large fraction of calcium (46.5%) and phosphorus (16.0%), and most of the potassium (81.5%) were contained in the standing biomass. Soil concentration gradient indicated that potassium predominantly originated from atmospheric deposition, as might be expected for an extremely weathered soil, while the absence of a phosphorus soil gradient in the plot indicated fast recycling. Additionally, it was demonstrated that calcium was retained in the topsoil horizon, and magnesium leached down the soil profile because of different degrees of uptake. Based on these results, the removal of biomass through logging would empty most stocks of nutrients essential for the existence forest systems. Globally, this study has revealed how tightly these soil-derived nutrients are cycling in this system and deliver essential information for efforts to restore rain forests in New Caledonia. Future research should focus on more detailed measurements collected over a longer period focusing on roots but also tree growth rates and nutrient inputs and outputs to advance a fuller mechanistic understanding of nutrient fluxes in such complex ecosystems.