Methane (CH₄) emission and environmental controls of CH₄-cycling microorganisms are unclear in inland floodplains. Here, we examined soil CH₄ emissions and the community composition of CH₄-cycling microorganisms under three vegetation types—mudflat (MF, no vegetation cover), Carex meadow (CM, mainly Carex brevicuspis), and reed land (RL, mainly Miscanthus sacchariflorus)—from water-adjacent areas to higher-elevation land in the Dongting Lake floodplain, China. The results showed that CH₄ emission is the highest in CM, while significant absorption was observed in the RL site. The abundance ratio of methanogen/methanotroph was the highest in CM, intermediate in MF, and lowest in RL. The Methanosarcinaceae family represented the dominant methanogens in the three sampling sites (41.32–75.25%). The genus Methylocystis (60.85%, type II methanotrophs) was dominant in CM, while Methylobacter and Methylosarcina (type I methanotrophs) were the dominant genera in MF (51.00%) and RL (50.24%), respectively. Structural equation model analysis showed that methanogen and methanotroph abundance were affected by water table depth, soil water content, and pH indirectly through soil organic content, total nitrogen, microbial biomass carbon, and microbial biomass nitrogen. These results indicated that the Dongting Lake floodplain may change from a CH₄ source to a CH₄ sink with vegetation succession with an increase in elevation, and the methanogen/methanotroph ratio can be used as a proxy for CH₄ emission in wetland soils. The continuous increase in reed area combined with the decrease in Carex meadow may mitigate CH₄ emission and enhance the CH₄ sink function during the non-flood season in the Dongting Lake floodplain.

Since a river system is a unidirectional network, the upstream influencing factors often interfere with those downstream. We quantify the effects of the TGD on the sediment exchange processes between the Yangtze River and Dongting Lake. Based on yearly sediment load data from 1981 to 2008, two multi-layer perceptron (MLP) models were constructed to predict the potential sedimentation in Dongting Lake without implementation of the TGD. The sediment discharge at Yichang station decreased from 622.5 to 61.1 Mt/year between 1981–1985 and 2003–2008, while the sedimentation in Dongting Lake reduced from 127.3 to 6.6 Mt/year. The MLP models indicate that only 27.9% of the decreased sediment load at Yichang station and 16.9% of that in Dongting Lake is caused by the TGD, while the rest is caused mostly by other upstream climate variations and anthropogenic impacts.

Climatic force might sharpen the latitudinal gradients of foliar nutrients directly (the Temperature–Plant Physiological hypothesis) or indirectly (either through soil nutrient, the Soil Substrate Age hypothesis, or plant functional type (e.g., herbs and trees) composition, the Species Composition hypothesis). However, the validities, effectiveness, and key drivers of these hypotheses have not been further examined globally. Here, we tested these hypotheses by synthesizing data from 2344 observations of leaf N, leaf P, and leaf K in terrestrial plants. The results indicated that leaves enriched nutrients towards the polar region. The validity of each hypothesis was confirmed, with the exception of the Soil Substrate Age hypothesis failing to predict leaf N, as the climatic influence on leaf N occurs through a mechanism opposite to what the hypothesis suggests. Additionally, among all hypotheses, the Species Composition hypothesis was the most effective model for leaf N, whereas the Substrate Age hypothesis was the most effective model for leaf P and leaf K. Soil, climate, and plant functional type collectively accounted for over half of the variations in leaf nutrients. Specifically, soil nutrient was the strongest determinant for leaf P and K, whereas plant functional type for leaf N. Taking into account changes in plant functional types and soil nutrients will improve the modeling of biogeochemical cycles under climate change. We expect further verification by global investigations of leaf stoichiometry using uniform methods.

Climate has shaped green leaf nitrogen (N) and phosphorus (P) patterns through its direct physiological effects (Temperature–Plant Physiology hypothesis), indirect pathways involving soil nutrients (Soil Substrate Age hypothesis), or vegetation composition (Species Composition hypothesis). However, the efficiencies of these hypotheses and the relative importance of the factors involved in predicting leaf litter N and P remain unresolved. We evaluated these hypotheses by analyzing 4657 global observations of leaf litter N and P concentrations and N/P ratios, demonstrating that litter stoichiometries diverged in plant functional types, and that litter N and N/P ratios declined with latitude, while P increased. The validity of each hypothesis in predicting latitudinal patterns of leaf litter P was confirmed, with the Species Composition hypothesis being the most effective model; however, all hypotheses failed to predict the litter N. Environmental and biological factors collectively explained over 40% of the variations in litter stoichiometries, with plant functional type, soil pH, and climatic factors being the most important drivers of litter N, P, and N/P ratio, respectively. The fundamentally different control mechanisms of litter stoichiometry patterns compared with those of green leaves challenge the idea that common hypotheses can predict biogeographic patterns across all leaf stages; thus, current litter element biogeochemical models and plant nutrition paradigms require revision.