Supporting data for "Global effects of soil invertebrates on litter decomposition and plant nitrogen uptake"

Plant decomposition is the principal process through which dead organic matter is transformed into inorganic forms, thereby replenishing the nutrient pool available for sustaining living vegetation. Nitrogen is recognized as the primary nutrient limiting net primary productivity in terrestrial ecosystems. The nitrogen mineralized from the decomposition is estimated to fulfill approximately 60% of plant nitrogen requirements. Invertebrates play a crucial role in the decomposition of dead organic matter, yet their contributions are frequently overlooked in ecosystem management, conceptual frameworks, and biogeochemical models. The influence of invertebrates on the processing of plant litter is often mediated by environmental variables and the distribution of decomposer invertebrates. This variation across regions may result in regional difference in the effects of invertebrates on litter decomposition and the subsequent plant nitrogen uptake, as well as their differential responses to global environmental changes. Therefore, this study aim to quantify the regional variations in the effects of invertebrates on global litter decomposition and the plant uptake of nitrogen mineralized during the decomposition process.

In chapter 1, we examined 476 case studies across 93 sites and performed a meta-analysis to estimate regional effects of invertebrates on forest litter decomposition. We then assessed how invertebrate diversity, climate and soil pH drive regional variations in invertebrate-mediated decomposition. We found that (1) invertebrate contributions to litter decomposition are 1.4 times higher in tropical and subtropical forests than in forests elsewhere, with an overall contribution of 31% to global forest litter decomposition; and (2) termite diversity, together with warm, humid and acidic environments in the tropics and subtropics are positively associated with forest litter decomposition by invertebrates. Our results demonstrate the significant difference in invertebrate effects on mediating forest litter decomposition among regions. We demonstrate, also, the significance of termites in driving litter mass loss in the tropics and subtropics. These results are particularly pertinent in the tropics and subtropics where climate change and human disturbance threaten invertebrate biodiversity, and the ecosystem services it provides.

In chapter 2, we performed a meta-analysis to quantify the effect of invertebrates on leaf litter decomposition in undisturbed (988 observations) and disturbed ecosystems (351 observations). We found that (1) globally, invertebrates are responsible for 33% of leaf litter decomposition in undisturbed ecosystems and 34% in disturbed ecosystems; (2) in undisturbed ecosystems, invertebrates contribute more to decomposition in the tropics and subtropics than temperate and boreal regions; and (3) in disturbed ecosystems, the contributions of invertebrates to decomposition are independent of tested climatic regions and environmental variables. These findings highlight the importance of invertebrates in maintaining nutrient cycling in both undisturbed and disturbed ecosystems. The preservation of invertebrate communities should be an important component of ecosystem management and restoration as they are vital for the continued delivery of essential ecosystem services, including decomposition.

In chapter 3, to assess the effects of invertebrates on plant uptake of nitrogen mineralized during the decomposition of plant litter, we conducted a 15-month experimental study in a subtropical secondary forest in Hong Kong (Tai Po Kau Nature Reserve). We employed the nitrogen stable isotope (15N) as a tracer to track nitrogen transfer from leaf litter to living plants and integrated metabarcoding to assess the impact of invertebrates on fungal functional guilds. Our results indicate that invertebrates significantly enhanced the amount of excess δ15N and litter-derived nitrogen in plant leaves by 59.63% and 72.10%, respectively. This increase corresponded with an 8.30% increase in nitrogen release from the litter and a 74.84% increase in the abundance of arbuscular mycorrhizal fungi. Our findings provide empirical evidence highlighting the importance of invertebrates in promoting nitrogen mineralization and the transport of mineralized nitrogen to plants. We demonstrate that the preservation of invertebrate communities should be acknowledged as an essential component of ecosystem restoration initiatives, given their crucial function in promoting the nitrogen use efficiency of plants.

In chapter 4, to assess the regional impact of invertebrates on plant nitrogen acquisition from decomposed litter. We employed a nitrogen stable isotope (15N) tracing approach alongside mesocosm experiments, conducted across two temperate and two tropical forests, to evaluate the influence of invertebrates on nitrogen mineralization and plant nitrogen uptake. We measured nitrogen isotope levels (δ15N) in plant tissues, soils, and termites. Our findings showed that in tropical forests, invertebrates significantly decreased excess δ15N in plant leaves by 93.65%, stems by 80.43%, roots by 62.77%, and soil by 67.67%, whereas in temperate forests, their influence was not observed. The considerable reduction in plant uptake of litter nitrogen in tropical forests may be attributed to termites removing enriched litter and sequestering N in their mounds, which potentially benefits surrounding vegetation. Over the long term, the redistribution of nitrogen by invertebrates suggests their role as key nitrogen reservoirs, retaining nitrogen within the biosphere and mitigating leaching losses, particularly in tropical rainforests where soil nutrients are susceptible to heavy rainfall.