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Abstract. Tropical ecosystems contribute significantly to global emissionsof methane (CH4), and landscape topography influences the rate ofCH4 emissions from wet tropical forest soils. However, extreme eventssuch as drought can alter normal topographic patterns of emissions. Here weexplain the dynamics of CH4 emissions during normal and droughtconditions across a catena in the Luquillo Experimental Forest, Puerto Rico.Valley soils served as the major source of CH4 emissions in a normalprecipitation year (2016), but drought recovery in 2015 resulted in dramaticpulses in CH4 emissions from all topographic positions. Geochemicalparameters including (i) dissolved organic carbon (C), acetate, and soil pH and (ii) hydrological parameters like soil moisture and oxygen (O2)concentrations varied across the catena. During the drought, soil moisturedecreased in the slope and ridge, and O2 concentrations increased in thevalley. We simulated the dynamics of CH4 emissions with theMicrobial Model for Methane Dynamics-Dual Arrhenius and Michaelis–Menten (M3D-DAMM), which couples a microbialfunctional group CH4 model with a diffusivity module for solute and gastransport within soil microsites. Contrasting patterns of soil moisture,O2, acetate, and associated changes in soil pH with topographyregulated simulated CH4 emissions, but emissions were also altered byrate-limited diffusion in soil microsites. Changes in simulated availablesubstrate for CH4 production (acetate, CO2, and H2) andoxidation (O2 and CH4) increased the predicted biomass ofmethanotrophs during the drought event and methanogens during droughtrecovery, which in turn affected net emissions of CH4. A variance-basedsensitivity analysis suggested that parameters related to aceticlasticmethanogenesis and methanotrophy were most critical to simulate net CH4emissions. This study enhanced the predictive capability for CH4emissions associated with complex topography and drought in wet tropicalforest soils. 

Dissolved organic carbon (DOC) and nitrogen (DON) are important energy and nutrient sources for aquatic ecosystems. In many northern temperate, freshwater systems DOC has increased in the past 50 years. Less is known about how changes in DOC may vary across latitudes, and whether changes in DON track those of DOC. Here, we present long‐term DOC and DON data from 74 streams distributed across seven sites in biomes ranging from the tropics to northern boreal forests with varying histories of atmospheric acid deposition. For each stream, we examined the temporal trends of DOC and DON concentrations and DOC:DON molar ratios. While some sites displayed consistent positive or negative trends in stream DOC and DON concentrations, changes in direction or magnitude were inconsistent at regional or local scales. DON trends did not always track those of DOC, though DOC:DON ratios increased over time for ~30% of streams. Our results indicate that the dissolved organic matter (DOM) pool is experiencing fundamental changes due to the recovery from atmospheric acid deposition. Changes in DOC:DON stoichiometry point to a shifting energy‐nutrient balance in many aquatic ecosystems. Sustained changes in the character of DOM can have major implications for stream metabolism, biogeochemical processes, food webs, and drinking water quality (including disinfection by‐products). Understanding regional and global variation in DOC and DON concentrations is important for developing realistic models and watershed management protocols to effectively target mitigation efforts aimed at bringing DOM flux and nutrient enrichment under control.

 

A comprehensive cross‐biome assessment of major nitrogen (N) species that includes dissolved organic N (DON) is central to understanding interactions between inorganic nutrients and organic matter in running waters. Here, we synthesize stream water N chemistry across biomes and find that the composition of the dissolved N pool shifts from highly heterogeneous to primarily comprised of inorganic N, in tandem with dissolved organic matter (DOM) becoming more N‐rich, in response to nutrient enrichment from human disturbances. We identify two critical thresholds of total dissolved N (TDN) concentrations where the proportions of organic and inorganic N shift. With low TDN concentrations (0–1.3 mg/L N), the dominant form of N is highly variable, and DON ranges from 0% to 100% of TDN. At TDN concentrations above 2.8 mg/L, inorganic N dominates the N pool and DON rarely exceeds 25% of TDN. This transition to inorganic N dominance coincides with a shift in the stoichiometry of the DOM pool, where DOM becomes progressively enriched in N and DON concentrations are less tightly associated with concentrations of dissolved organic carbon (DOC). This shift in DOM stoichiometry (defined as DOC:DON ratios) suggests that fundamental changes in the biogeochemical cycles of C and N in freshwater ecosystems are occurring across the globe as human activity alters inorganic N and DOM sources and availability. Alterations to DOM stoichiometry are likely to have important implications for both the fate of DOM and its role as a source of N as it is transported downstream to the coastal ocean.