Sodium limits litter decomposition rates in a subtropical forest: Additional tests of the sodium ecosystem respiration hypothesis

Abstract Sodium (Na) is unimportant to plants but critical for detritivore decomposers in brown food webs. Na’s limiting impact on detritivores and decomposition in inland tropical forest has already been demonstrated, but its role in regulating microbial decomposers remains unexplored. This study filled in this gap by assessing the response of detritivores and microorganisms to three levels of Na subsidies (0.005%, 0.05% and 0.5% water solution of NaCl) during the decomposition of broad-leaf and needle litter in a subtropical forest (Nanjing, China, 300 km inland from Pacific ocean). The relative contributions of detritivores vs microbes to litter decomposition were tested using litterbags with 5 mm and 0.2 mm mesh. Litter mass loss, soil microbial respiration, and soil enzyme activities were investigated in an eight-month field experiment. Our results showed that both 0.05 and 0.5% Na subsidies increased invertebrate detritivore decomposition of litter at least 1.5 fold, while it decreased the microbial contribution to litter decomposition by ≥7.2%. However, low-level Na subsidies (0.005%) increased microbial, but not detritivore, activity in recalcitrant litter. Despite enhanced litter mass loss from invertebrate detritivores, we found decreased microbial enzyme activities involved in carbon (C), nitrogen (N) and phosphorus (P) cycling under high (0.5%) Na subsidies. This suggests that at 0.5% of Na, microbes experience Na-toxicity and mineralization of C, N, and P nutrients likely decreased in forest ecosystems. These results suggest that neither Na shortage nor Na excess would be unfavorable for carbon-release. As increased with anthropogenic input or climate change in the future, sodium may have multiple effects on soil organic matter dynamics, ecosystem biogeochemical cycling, and plant nutrition through regulating microbial processes in forests.