Leaf and root litter profoundly impact soil carbon sequestration and nutrient cycling in terrestrial ecosystems. Recent evidence indicates that within single-species contexts resource traits are coordinated between leaves and roots driving parallel decomposition dynamics of leaf and root litters, yet it remains unclear whether this coordination also underlies parallel mixing effects in leaf and root litter mixture decomposition. In a 501-day field experiment in a temperate steppe, we incubated leaf and fine root litters from six species alone and in all pairwise mixtures. We assessed the relationship between leaf and fine root litter decomposition responses to litter mixing, and examined how trait dissimilarity between component species and decomposition responses of four carbon fractions (soluble compounds, hemicellulose, cellulose and lignin) shape this relationship. We found litter trait dissimilarities drove contrasting fraction-level responses to litter mixing. Most leaf and fine root litter mixtures exhibited non-additive effects in soluble-compound and cellulose decomposition, with soluble compounds contributing most to the overall non-additive effects of mixed leaf and fine root litters. Coordinated dissimilarity in leaf and root traits led to parallel decomposition responses of leaf and root soluble compounds to litter mixing, but to negative correlations for hemicellulose and cellulose and no correlation for lignin. These divergent fraction-level relationships blurred overall coordination of decomposition response between leaf and fine root litters to litter mixing, causing uncoordinated bulk-litter mixing effects. Our results demonstrate that resolving fraction-level processes is critical for understanding mixed-litter decomposition and for predicting ecosystem carbon and nutrient fluxes under changing plant communities.

Ecosystem temporal stability (TS) determines its ability to maintain structure, function and services under external disturbances, playing a critical role in the global carbon cycle and climate regulation. However, the capability of numerical models to simulate the TS of ecosystem carbon uptake remains insufficiently assessed. This study evaluated the performance of nine terrestrial ecosystem models in simulating gross primary productivity (GPP) and its TS and employed Random Forest (RF) models with Shapley Additive Explanations (SHAP) to identify key factors contributing to model biases. Site-scale analysis based on flux tower observations indicated that most models underestimated GPP while overestimating its TS, with the most pronounced biases occurring at the interannual scale. These discrepancies primarily stemmed from errors in simulating vegetation phenology, specifically the carbon uptake period and physiological traits, particularly peak GPP within a year. At the global scale, regions with higher carbon uptake tended to exhibit greater TS, yet significant discrepancies existed among models. Notably, RF and SHAP analyses indicated that leaf area index was more important than climate and geographical factors in explaining model divergence for simulating GPP and its TS. The study revealed systematic biases in the current models’ representation of TS, highlighting the potential vulnerability of ecosystems. These uncertainties among models may lead to an overestimation of ecosystem resilience, introducing uncertainties in global carbon budget estimates and potentially misguiding scientific assessments and policy decisions regarding future climate change responses. Therefore, improving carbon cycle simulation mechanisms is essential for enhancing model predictive capabilities.

Nitrogen (N) availability critically limits plant productivity in nutrient-depleted coral island ecosystems, necessitating substantial inputs of exogenous N fertilizer. However, excessive or unbalanced fertilization poses risks to environmental sustainability. In this study, we assessed how three N fertilizer forms, ammonium (NH₄⁺-N), nitrate (NO₃⁻-N) and amide nitrogen (NH₂-N), affect soil properties and plant performance in coral sand environments. A 15N-labeled greenhouse experiment was conducted using two island-adapted species, Ficus microcarpa and Terminalia catappa. Results showed that NO₃⁻-N markedly enhanced nitrogen retention, microbial biomass nitrogen and overall plant growth, while NH₄⁺-N promoted microbial biomass carbon. Ficus microcarpa and T. catappa both exhibited superior growth under NO₃⁻-N, although T. catappa achieved higher leaf nutrient concentrations with NH₂-N, reflecting differences in nutrient uptake preferences. Isotopic tracing revealed greater nitrogen retention in soils than in plant tissues, with NO₃⁻-N fertilization yielding the highest nitrogen recovery efficiency. These findings highlight the importance of nitrogen form in shaping soil–plant interactions in sandy, alkaline soils and offer mechanistic insights for designing targeted, sustainable fertilization strategies for coral island ecosystems.