Molecular Composition and Stability of Soil Organic Matter in Tropical Soils- A Belowground View to Afforestation Through Payment for Ecosystem Services in Costa Rica

Abstract:

Shifts in the molecular composition of organic matter has the potential to regulate the persistence and stability of carbon (C) in soils. Soils, which act as the largest terrestrial sink of C, are not well recognized in atmospheric C mitigation strategies in the tropics; this is especially valid in Payment for Ecosystem Service (PES) programs that focus on C sequestration in aboveground biomass. The objective of this research was to identify the potential of soils to store persistent C under PES and non-PES land uses in Northeastern Costa Rica, a country that leads the world in managing for carbon neutrality. We used a space-for-time approach to characterize the quality (i.e., FT-ICR-MS molecular composition) and quantity of soil organic matter under PES plantations, PES-contracted conservation forests, and at adjacent non-PES agricultural plot that represented pre-plantation land uses at several different sites (i.e., land owners). Our results show that a principal components analysis of the molecular composition of organic matter differed among PES land cover, but not between PES and non-PES land uses (PCA axis 1; P = 0.0007, N = 72). Soil depth (0-5 vs. 5-10 cm) also influenced the variation seen in the molecular composition of soil organic matter (PCA axis 1; P = 0.013). More specifically, PES-conservation forests were relatively enriched in protein- and unsaturated hydrocarbon-like compounds, and relatively depleted in lignin-like compounds compared to PES-plantation forests. We also observed an effect of PES-plantation tree species on the relative abundance of condensed hydrocarbon-like compounds, highlighting that more PES forest type is important in shaping soil C composition. Furthermore, aromatic compounds were highest in 0-5 cm soils from PES-conservation forests compared to PES plantations and non-PES agricultural land uses, suggesting that the increase in aromatic compounds accumulate, as more labile carbon is preferentially degraded. This was also reflected in the lower active-to-total C ratio seen in PES-conservation forests relative to non-PES agricultural land uses. The quantity of soil C followed a similar trajectory, emphasizing that PES-conservation forests have more persistent (i.e., molecularly complex) organic matter compared to PES-plantations or non-PES agricultural land uses.