Global-scale nitrogen budgets developed to quantify anthropogenic impacts on the nitrogen cycle do not explicitly consider nitrate stored in the vadose zone. Here we show that the vadose zone is an important store of nitrate that should be considered in future budgets for effective policymaking. Using estimates of groundwater depth and nitrate leaching for 1900–2000, we quantify the peak global storage of nitrate in the vadose zone as 605–1814 Teragrams (Tg). Estimates of nitrate storage are validated using basin-scale and national-scale estimates and observed groundwater nitrate data. Nitrate storage per unit area is greatest in North America, China and Europe where there are thick vadose zones and extensive historical agriculture. In these areas, long travel times in the vadose zone may delay the impact of changes in agricultural practices on groundwater quality. We argue that in these areas use of conventional nitrogen budget approaches is inappropriate.
The one belt and one road initiative, an international tourism island construction and the sustainable development of the Hainan province in China, requires the global Earth observation information obtained via satellite remote sensing. However, the unique geographical location of the Hainan province and its cloudy and rainy climate causes several challenges for the acquisition of high-quality satellite remote sensing images. To address this issue, the proposed Hainan satellite constellation is aimed at building a remote sensing satellite system equipped with optical, hyperspectral, and radar payloads as well as a payload for ship automatic identification system, considering specific business requirements. The application will focus on ship detection and islands and marine environment observations. Once the complete operation of the Hainan satellite constellation is established, it will supply high-frequency and coverage observations in low-latitude regions, covering major portions of the maritime silk road areas. This will facilitate marine environment protection and services, and marine resources exploration. This paper introduces the design of the Hainan satellite constellation, including its architecture, characteristics and innovations, and potential applications for ocean observations.
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