Abstract

Whether or not biosolids application facilitates long-term soil organic matter (SOM) stability in coastal dune forests remains unsolved. We conducted a long-term experiment in a coastal radiata pine (Pinus radiata D. Don) forest growing on sandy soils in New Zealand, where biosolids have been applied at moderate (3 t ha−1) and high (6 t ha−1) rates every three years for two decades. We determined total carbon (C) and nitrogen (N) contents, the amounts of iron and aluminum oxides, labile and recalcitrant C and N fractions and the chemical composition of SOM in the topsoil, and explore their relations to plant, microbial and geochemical controls. Biosolids application had increased total, labile and recalcitrant C by 6%–94%, 6%–56% and 0%–55%, respectively; increased total, labile and recalcitrant N by 14%–124%, 5%–214% and 0%–173%, respectively. Biosolids application had no significant effects on free and amorphous iron oxides, but significantly increased aluminum oxides with enhanced crystallization of amorphous aluminum. The metal/recalcitrant-C ratios, the recalcitrant/total-C ratios and the alkyl/O-alkyl-C ratios remained unchanged, indicating both the potential of mineral protection and the chemical recalcitrance of SOM were not reinforced. Decreased ratios of microbial lipid biomass to recalcitrant C further indicated biosolids application may limit microbial contribution to SOM stabilization. These limitations to confer higher stability to SOM under biosolids application is likely due to reduced belowground C allocation (root biomass and mycorrhizal associations) and microbial C limitation (higher C- versus nutrient-acquiring enzyme activities) and stress response (higher ratio of cyclopropyl-to-precursor fatty acids). Given unchanged mineral protection to SOM, decreased mycorrhizal associations coupled with increases in Gram-positive bacteria and microbial metabolic quotient may facilitate the decomposition of recalcitrant SOM. Hence, although biosolids application usually promoted immediate C accumulation, weak mineral protection and altered plant-microbial interactions may undermine its potential for long-term soil C sequestration in coastal dune forests.

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