The natural abundance of soil nitrogen (N) stable isotope (δ15N) is a good proxy indicating the integrative soil N cycling processes and fluxes over long time scale. 
　　However, its spatial pattern in the globe is not well understood, mostly due to the inconsistent relationships between soil δ15N and the relevant environmental variables found in different regions. In addition, the association with soil water content (SWC), which is a critical factor regulating soil N cycling process, has drawn less attention. 
　　Therefore, researchers led by Prof. ZHU Qing from Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences investigated the relationship between soil δ15N and SWC at global scale, and accordingly explored the key controls of soil δ15N in different climate zones. 
　　The hypothesis is that investigating the relationship between soil δ15N and SWC can help to explain the controls of the spatial pattern of soil δ15N across regions, as SWC was also controlled by interactions among climate, soil and vegetation. 
　　This work was recently published on Catena. 
　　Results revealed an upward-concave relationship between soil δ15N and SWC based on the soil δ15N and SWC data in 910 grids of the globe. Higher soil δ15N existed at both the dry and wet ends of the SWC. In addition, inconsistent relationships in five Koppen-Geiger climate zones constituted this upward-concave relationship. “Our findings are expected to deepen understandings on the spatial pattern of soil δ15N and the indicated N cycle at the global scale,” said Prof. ZHU. 
　　Positive relationship between soil δ15N and SWC in the Tropical zone determined the increasing trend of soil δ15N at the wet end in the globe. It was mainly casued by the hydrologically regulated soil N losses. Increased SWC under the warm and moist condition would promote the denitrification and thus a lot of N gaseous losses as N2O and N2, and also trigger surface runoff and subsurface flow, and thus drive dissolved and particulate N losses.
　　Negative relationship in the Arid zone determined the decreasing trend at the dry end in the globe. Increased proportions of abiotic N losses like ammonia volatilization and N supplement through bedrock weathering (higher δ15N) with the decrease of SWC under the dry, hot and infertile soil condition may be the main reasons. 
　　Poor relationships between soil δ15N and SWC in the Temperate, Cold and Polar zones constituted the transition range of the upward-concave relationship in the globe. Not hydrologically regulated but climatically limited determined the low soil N losses and thus low soil δ15N in these zones.
　　(a) Global distributions of five Koppen-Geiger climate zones and the classifications of soil δ15N according to the climate zones. (b) Boxplots of the soil δ15N and SWC in five climate zones. (c) Biplot of the standardized soil δ15N and SWC. (Image by LAI Xiaoming) Links: https://www.sciencedirect.com/science/article/pii/S0341816222008657 Contact TAN Lei Nanjing Institute of Geography and Limnology E-mail: ltan@niglas.ac.cn

　　Visitors walk through the East Lake National Wetland Park in Wuhan, Hubei province, in February. [Photo by Zhang Liewen/For China Daily]
　　Data, surveys display improvement in quality of lakes, wetlands, air over decade
　　China's ecology and environment have undergone historic and profound improvement over the last decade, according to a series of reports published by the Chinese Academy of Sciences on Tuesday.
　　The five reports feature the latest data and survey results of national freshwater resources, wetlands, mountains, water conservation in arid northwest areas, and environmental conditions in the country's three major urban clusters: Beijing-Tianjin-Hebei, the Yangtze River Delta, and Guangdong-Hong Kong-Macao Greater Bay Area.
　　According to the reports, China had 2,670 natural lakes over 1 square kilometer in size in 2020, totaling 80,662 sq km. The water quality of 70 percent of mid-size to large lakes has improved over the decade.
　　This has also led to the gradual increase of biodiversity in lake systems. In Poyang Lake, China's largest freshwater lake, the number of wintering waterfowl rose from 357,000 to 689,000, from some 50 species of birds.
　　Wetlands are a major ecosystem rich in biodiversity and also regulate water and filter waste from the landscape, hence their nickname of the "kidneys of the Earth".
　　China has one of the world's largest and most diversified wetland ecosystems, the reports said. In 2020, it had 412,000 sq km of wetlands, the most in Asia.
　　Chinese wetlands currently hold over 55.5 billion metric tons of fresh water, and each hectare of wetland can remove 1,000 kilograms of nitrogen and 130 kg of phosphorus on average per year.
　　From 2015 to 2020, the total area of wetlands underwent a net increase of 903 sq km. By the end of 2020, there were nearly 1,200 artificial wetlands, 750 more compared to a decade ago.
　　China's wetlands are rich in biodiversity, hosting over 1,691 species of flora and 296 wetland bird species. About 73 of these plants and 91 of the bird species are rare and protected.
　　"The role of wetlands in safeguarding water and ecological security is becoming more prominent. They also play an important role in addressing global climate change," the reports said.
　　With nearly two-thirds of its land covered by mountains and plateaus, these ecosystems play a crucial role in providing natural resources, biodiversity, and safeguarding environmental security, the reports said.
　　Over the past decade, natural mountain conservation areas have increased by 14.4 percent to around 1.17 million sq km. As for water conservation in the northwest, the surface water area has increased by 161 sq km on average annually for nearly a decade. Water use in agriculture has also decreased from 95.8 percent in 2012 to 90.9 percent in 2021.
　　Beijing-Tianjin-Hebei, the Yangtze River Delta, and the Guangdong-Hong Kong-Macao Greater Bay Area urban clusters account for around 5 percent of China's land area, but are home to 25 percent of its population and contribute nearly 40 percent of its GDP.
　　"They are the core engines of economic growth, but intense levels of human activity have placed enormous stress on the ecology and environment," the reports said.
　　Over the last decade, environments in the three regions have steadily improved, most notably in terms of air quality.
　　From 2012 to 2021, the annual PM2.5 density decreased by 53.1 percent in Beijing-Tianjin-Hebei, 54.2 percent in the Yangtze River Delta, and 47.2 percent in the Guangdong-Hong Kong-Macao Greater Bay Area.

　　Satellite ocean color instruments have been used to characterize physical, chemical, and biological variabilities in oceanic, coastal, and inland waters. However, the massive loss and large uncertainty results of remote sensing reflectance data in difficulty in monitoring nearshore coastal and inland waters. The ocean color community has been pursuing easy-access and reliable reflectance products for observing nearshore coastal and inland waters.
　　Most of the coastal and inland waters are generally turbid; in these areas, the contribution of water to the signals of the top of the atmosphere is raised, suggesting the possibility reality for oceanographers and limnologists to use land reflectance products for water monitoring. MODIS surface reflectance product (R_land) has been used to monitor water clarity and suspended solids in waters yet the applications to retrieve phytoplankton pigments and colored dissolved organic carbon has rarely been addressed. To date, its applicability in aquatic remote sensing has not been sufficiently assessed. Some fundamental questions such as the following need to be addressed: How does the R_land product perform in global inland and coastal waters? What water color parameters can be mapped using R_land?
　　Recently, a research group led by Prof. Hongtao Duan and Ronghua Ma from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences provided a comprehensive evaluation of the performance of MODIS R_land products against a field optical dataset containing 4143 reflectance spectra, 2320 chlorophyll-a samples, and 1467 suspended particulate matter samples across global nearshore coastal and inland waters.
　　This work was published in Earth-Science Reviews.
　　"Despite the ease of using this product and its higher spatial resolution than the MODIS ocean bands, according to our assessment, R_land might not be an optimal data source for monitoring inland and coastal waters," said Prof. Duan.
　　The results showed that R_land significantly overestimated remote sensing reflectance, particularly in 469 nm and 859 nm bands. Such a global assessment was consistent with the results published in Lake Taihu and Chesapeake Bay before. In addition, land reflectance showed evident overestimations compared to the ocean color products derived using SeaDAS software in the east China Sea.
　　The study also reported noticeable negative values and patchiness in the R_land imagery. The proportion of R_land at 555 nm is even beyond 5% in the coastal area of Australia and Africa. R_land was frequently negative in the pixels covered by cyanobacterial scums, e.g., >20% negatives in Lake Taihu. "The negatives and patchiness in R_land possibly resulted from the unsuitable mechanism to remove aerosols in generating R_land over waters, " said Prof. Ma.
　　Existing algorithms did not estimate satisfactory Chla and suspended solids from R_land across the global inland and coastal waters. Machine learning models outperformed the state-of-the-art algorithms for SPM retrievals in global turbid waters from R_land. But, all models, including machine learning models, cannot retrieve reliable chlorophyll-a from R_land with approximately 55% uncertainty due to the limited spectral information and uncertainty of R_land products. This implicated that R_land might be able to quantify the parameters closely related to suspended solids (e.g., water clarity and extinction coefficients) in most waters; however, it is challenging to quantify pigments like Chla in waters from R_land.
　　This study comprehensively evaluated the accuracy of R_land for the first time in global inland and coastal waters. MODIS R_land does not contain sufficient information that makes these existing algorithms usable. Consequently, various water color parameters from R_land are difficult to retrieve except for several parameters, such as SPM, turbidity, and water clarity. The results are anticipated to provide a benchmark of R_land in various waters worldwide.
　　 
　　Link at https://doi.org/10.1016/j.earscirev.2022.104154
　　 
　　Contact
　　TAN Lei
　　Nanjing Institute of Geography and Limnology
　　E-mail: ltan@niglas.ac.cn