The present study evaluated nitrogen (N) input and output in mainland China using updated data of temporally and spatially-based land use maps and statistical data at national and provincial scales. The total N inputs increased from 3,081 kg km -2 in 1985 to 5,426 kg km -2 in 2007. Chemical fertilizer dominated the N input and showed an increasing trend. Biological N fixation was the second important N input till 1990 and atmospheric deposition became the second most important source after that, accounting for 24.0% in 2007. There was no net N input through food/feed import in 1985, but it accounted for 3.5% of the total N input in 2007. According to a mass balance model, we assumed total N input equal to output. The results showed that more than half of the total N was denitrified or stored in the system. Ammonia volatilization accounted for 18.9-22.9% of the total N input, and N export to water bodies accounted for 17.9-20.7%. About 5.1-7.7% of the N input was emitted to the atmosphere through biomass burning. When calculated per unit area, total N input, N export to water bodies, denitrification and storage could be very well explained by human population density. Nitrogen input and major outputs were also positively related to per capita gross domestic product and the percentage of total land area used as cropland. The N budget is compared to that of some other countries and the environmental impacts of the N cycle is discussed.

Using soil slurry-based (15)N tracer combined with N2/Ar technique, the potential rates of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA), and their respective contributions to total nitrate reduction were investigated in 11 typical paddy soils across China. The measured rates of denitrification, anammox, and DNRA varied from 2.37 to 8.31 nmol N g(-1) h(-1), 0.15 to 0.77 nmol N g(-1) h(-1) and 0.03 to 0.54 nmol N g(-1) h(-1), respectively. The denitrification and anammox rates were significantly correlated with the soil organic carbon content, nitrate concentration, and the abundance of nosZ genes. The DNRA rates were significantly correlated with the soil C/N, extractable organic carbon (EOC)/NO3(-) ratio, and sulfate concentration. Denitrification was the dominant pathway (76.75-92.47%), and anammox (4.48-9.23%) and DNRA (0.54-17.63%) also contributed substantially to total nitrate reduction. The N loss or N conservation attributed to anammox and DNRA was 4.06-21.24 and 0.89-15.01 g N m(-2) y(-1), respectively. This study reports the first simultaneous investigation of the dissimilatory nitrate reduction processes in paddy soils, highlighting that anammox and DNRA play important roles in removing nitrate and should be considered when evaluating N transformation processes in paddy fields.

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Denitrification is the primary process that regulates the removal of bioavailable nitrogen (N) from aquatic ecosystems. Quantifying the capacity of N removal from aquatic systems can provide a scientific basis for establishing the relationship between N reduction and water quality objectives, quantifying pollution contributions from different sources, as well as recommending control measures. The Lake Taihu region in China has a dense river network and heavy N pollution; however, the capacity for permanent N removal by the river network is unknown. Here, we concurrently examined environmental factors and net N2 flux from sediments of two rivers in the Lake Taihu region between July 2012 and May 2013, using membrane inlet mass spectrometry, and then established a regression model incorporating the highly correlated factors to predict the N removal capacity of the river network in the region. To test the applicability of the regression model, 21 additional rivers surrounding Lake Taihu were sampled between July and December 2013. The results suggested that water nitrate concentrations are still the primary controlling factor for net denitrification even in this high N loading river network, probably due to multicollinearity of other relevant factors, and thus can be used to predict N removal from aquatic systems. Our established model accounted for 78% of the variability in the measured net N2 flux in these 21 rivers, and the total N removed through N2 production by the river network was estimated at 4 × 10(4) t yr(-1), accounting for about 43% of the total aquatic N load to the river system. Our results indicate that the average total N content in the river water discharged into Lake Taihu would be around 5.9 mg of N L(-1) in the current situation, far higher than the target concentration of 2 mg of N L(-1), given the total N load and the N removal capacity. Therefore, a much stronger effort is required to control the N pollution of the surface water in the region.

The nitrogen (N) budget calculation approach is a useful means of evaluating the impact of human activity on the N cycle. Field scale N budget calculations may ignore the interactions between landscapes, and regional scale calculations rely on statistical data and indirect parameters. Watershed scale budget calculations allow for a more direct quantification of N inputs and outputs. We conducted N budget calculations for a rice paddydominated agricultural watershed in eastern China for 2007-2009, based on intensive monitoring of stream N dynamics, atmospheric deposition, ammonia (NH 3 ) volatilization and household interviews about Nrelated agricultural activities. The results showed that although total N input to the watershed was up to 280 kg N ha -1 year -1 , riverine discharge was only 4.2 kg N ha -1 year -1 , accounting for 1.5% of the total N input, and was further reduced to 2.0 kg N ha -1 year -1 after reservoir storage and/or denitrification removal. The low riverine N output was because of the characteristics of the rice paddydominated landscape, which intercepts run-off and enhances soil denitrification. The watershed actually purified the N in rainwater, as N concentrations in river discharge were much lower than those in rain water. Major N outputs included food/feed export, NH 3 volatilization from chemical fertilizer and manure, and emissions from crop residue burning. Net reactive gaseous emissions (emissions minus deposition) accounted for 5.5% of the total N input, much higher than riverine discharge. Therefore, the agricultural N cycle in such paddy-dominated watersheds impacts the environment mainly through gas exchange rather than water discharge.Keywords Eastern China Á Nitrogen budget Á Reservoir Á Riverine discharge Á Watershed Abbreviations NANI Net anthropogenic nitrogen input N Nitrogen TN Total dissolved nitrogen

The Loess Plateau (LP) of China is a good representative area for critical zone (CZ) science studies. The LP is famous for its deep loess. In most areas, the thickness of the loess profile is deeper than 100 m, and two-thirds of the area is arid and semiarid. With the Grain-for-Green project, the vegetation of the plateau has recovered gradually. However, with the increase in vegetative coverage, especially the planted vegetation, the water content of the soil profile has decreased and the soil is much drier. In this review, particular emphasis is paid to the dry conditions of deep soil, drought, regional restoration of vegetation, and effective management of soil moisture. We reviewed the progress of research on dried soil layers (DSLs) that resulted from soil drought in the past decades on the Plateau, and then we summarized the development of the concept and models of soil water carrying capacity for vegetation (SWCCV). This review is helpful for understanding the development of DSLs, optimizing soil water management through vegetation mediation, and designing a long-term sustainable framework for water-limited ecosystems.

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