River damming has been practised for millennia, with the first dams built before 2000 BCE in the Egyptian empire 1. The number of dams increased steadily prior to the Second World War, but expanded rapidly thereafter, peaking in the 1960s and 1970s, with most construction in North America and Western Europe 2. A second surge in dam construction began in the early 2000s, with over 3,700 hydroelectric dams either planned or under construction worldwide during this construction boom 3 , each with a generating capacity of >1 megawatt (MW). Many of the new dams are being constructed in South America, Asia and the Balkans, largely driven by the need to expand energy production in growing economies 3,4. Indeed, by 2015, dammed reservoirs supplied around 30-40% of irrigation water globally 5,6 , and 16.6% of the world's electricity was generated by hydropower 7. Almost two-thirds of the world's long rivers (that is, those >1,000 km) are no longer free-flowing 8 and the current surge in dam construction-motivated by the 2016 Paris Agreement and the need for greater renewable energy generation-is expected to double river fragmentation by 2030 (ref. 9). Accordingly, freshwater ecosystems have been referred to as the 'biggest losers' of the Paris Agreement 10. Nutrients, such as carbon (C), nitrogen (N), phosphorus (P) and silicon (Si), are transported and transformed along the land-ocean aquatic continuum (LOAC), forming the basis for freshwater and, ultimately, marine food webs.

Soil erosion by water impacts soil organic carbon stocks and alters CO 2 fluxes exchanged with the atmosphere. The role of erosion as a net sink or source of atmospheric CO 2 remains highly debated, and little information is available at scales larger than small catchments or regions. This study attempts to quantify the lateral transport of soil carbon and consequent land−atmosphere CO 2 fluxes at the scale of China, where severe erosion has occurred for several decades. Based on the distribution of soil erosion rates derived from detailed national surveys and soil carbon inventories, here we show that water erosion in China displaced 180 ± 80 Mt C·y −1 of soil organic carbon during the last two decades, and this resulted a net land sink for atmospheric CO 2 of 45 ± 25 Mt C·y , equivalent to 8-37% of the terrestrial carbon sink previously assessed in China. Interestingly, the "hotspots," largely distributed in mountainous regions in the most intensive sink areas (>40 g C·m ), occupy only 1.5% of the total area suffering water erosion, but contribute 19.3% to the national erosion-induced CO 2 sink. The erosion-induced CO 2 sink underwent a remarkable reduction of about 16% from the middle 1990s to the early 2010s, due to diminishing erosion after the implementation of large-scale soil conservation programs. These findings demonstrate the necessity of including erosion-induced CO 2 in the terrestrial budget, hence reducing the level of uncertainty.land−atmosphere CO 2 flux | soil carbon displacement | water erosion | national scale T errestrial ecosystems are a net sink of anthropogenic CO 2 globally (1, 2) but can be net sources or sinks regionally [e.g., Northeast Region of China (3)]. Knowledge of the distribution, magnitude, and variability of land carbon fluxes and underlying processes is important both for improving model-based projections of the carbon cycle and for designing ecosystem management options that effectively preserve carbon stocks and enhance carbon sinks. Despite considerable efforts made by the research community, the mechanisms governing uptake or release of carbon from land ecosystems are still poorly quantified (4).Soil erosion occurs naturally but is accelerated by human cultivation of the landscape, and modifies CO 2 exchange (5) between the soil and atmosphere. Soil erosion destroys the physical protection of carbon in soil aggregates and accelerates decomposition, inducing a net CO 2 source. Continuous erosion over a long period can destabilize carbon in deeper soil horizons and trigger its decomposition e.g., as conditions of temperature and moisture become more favorable (6, 7). Soil erosion also decreases nutrient availability and reduces soil water holding capacity, affecting ecosystem productivity (8) with feedback to the ecosystem carbon balance. However, because only a fraction of eroded carbon is lost to the atmosphere, the rest may be lost to streams and rivers and eventually delivered to marine ecosystems or deposited in the landscape. With the fine and light soil particles p...

No abstract

No abstract

BackgroundBacterial communities are essential to the biogeochemical cycle in riverine ecosystems. However, little is presently known about the integrated biogeography of planktonic and sedimentary bacterial communities in large rivers.ResultsThis study provides the first spatiotemporal pattern of bacterial communities in the Yangtze River, the largest river in Asia with a catchment area of 1,800,000 km2. We find that sedimentary bacteria made larger contributions than planktonic bacteria to the bacterial diversity of the Yangzte River ecosystem with the sediment subgroup providing 98.8% of 38,906 operational taxonomic units (OTUs) observed in 280 samples of synchronous flowing water and sediment at 50 national monitoring stations covering a 4300 km reach. OTUs within the same phylum displayed uniform seasonal variations, and many phyla demonstrated autumn preference throughout the length of the river. Seasonal differences in bacterial communities were statistically significant in water, whereas bacterial communities in both water and sediment were geographically clustered according to five types of landforms: mountain, foothill, basin, foothill-mountain, and plain. Interestingly, the presence of two huge dams resulted in a drastic fall of bacterial taxa in sediment immediately downstream due to severe riverbed scouring. The integrity of the biogeography is satisfactorily interpreted by the combination of neutral and species sorting perspectives in meta-community theory for bacterial communities in flowing water and sediment.ConclusionsOur study fills a gap in understanding of bacterial communities in one of the world’s largest river and highlights the importance of both planktonic and sedimentary communities to the integrity of bacterial biogeographic patterns in a river subject to varying natural and anthropogenic impacts.Electronic supplementary materialThe online version of this article (10.1186/s40168-017-0388-x) contains supplementary material, which is available to authorized users.