SignificanceOveruse of agricultural chemicals has resulted in enormous damages to environmental quality and human health in China. Reducing the use of agricultural chemicals to an optimal level is a crucial challenge for the sustainable development of agriculture. We demonstrate that small farm size (in China, typically ∼0.1 ha for each parcel) is strongly related to overuse of agricultural chemicals. Farm size increases with economic development in many other countries, but this is not observed in China due to national policies. Increasing farm size by removing policy distortions would substantially decrease both the use of agricultural chemicals and their environmental impact, while increasing rural income in China.

Reactive nitrogen (Nr) plays a central role in food production, and at the same time it can be an important pollutant with substantial effects on air and water quality, biological diversity, and human health. China now creates far more Nr than any other country. We developed a budget for Nr in China in 1980 and 2010, in which we evaluated the natural and anthropogenic creation of Nr, losses of Nr, and transfers among 14 subsystems within China. Our analyses demonstrated that a tripling of anthropogenic Nr creation was associated with an even more rapid increase in Nr fluxes to the atmosphere and hydrosphere, contributing to intense and increasing threats to human health, the sustainability of croplands, and the environment of China and its environs. Under a business as usual scenario, anthropogenic Nr creation in 2050 would more than double compared with 2010 levels, whereas a scenario that combined reasonable changes in diet, N use efficiency, and N recycling could reduce N losses and anthropogenic Nr creation in 2050 to 52% and 64% of 2010 levels, respectively. Achieving reductions in Nr creation (while simultaneously increasing food production and offsetting imports of animal feed) will require much more in addition to good science, but it is useful to know that there are pathways by which both food security and health/environmental protection could be enhanced simultaneously.

Four species of Spartina (Spartina anglica, S. alterniflora, S. patens and S. cynosuroides) have been introduced to China, but currently only the first three are present and only the first two successfully reproduce on the Chinese coast. Spartina anglica and S. alterniflora were introduced to China from England in 1963 and from the United States in 1979, respectively. Today, S. alterniflora has expanded its coverage to more than 112 000 ha and S. anglica has declined to <50 ha. This is compared with only 260 ha of S. alterniflora and over 36 000 ha of S. anglica in 1985. The fates of Chinese Spartina, with dramatic expansion of S. alterniflora and significant decline of S. anglica, were different from those in other locations throughout the world. Factors affecting the growth of the two naturalized Spartina species in China include differences in artificial plantation strategy, impacts of tideland reclamation, species competition ability and genetic diversity. Several methods for Spartina control in China, such as harvesting, herbicide application and freshwater irrigation, have been developed, but more research is needed to verify their effectiveness.

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Human activities have intensely altered the global nitrogen cycle and produced nitrogenous gases of environmental significance, especially in China where the most serious atmospheric nitrogen pollution worldwide exists. We present a comprehensive assessment of ammonia (NH(3)), nitrogen oxides (NO(x)), and nitrous oxide (N(2)O) emissions in China based on a full cycle analysis. Total reactive nitrogen (Nr) emission more than doubled over the past three decades, during which the trend of increase slowed for NH(3) emissions after 2000, while the trend of increase continued to accelerate for NO(x) and N(2)O emissions. Several hotspots were identified, and their Nr emissions were about 10 times higher than others. Agricultural sources take 95% of total NH(3) emission; fossil fuel combustion accounts for 96% of total NO(x) emission; agricultural (51%) and natural sources (forest and surface water, 39%) both contribute to the N(2)O emission in China. Total atmospheric Nr emissions related health damage in 2008 in China reached US$19-62 billion, accounting for 0.4-1.4% of China's gross domestic product, of which 52-60% were from NH(3) emission and 39-47% were from NO(x) emission. These findings provide policy makers an integrated view of Nr sources and health damage to address the significant challenges associated with the reduction of air pollution.

Soil nitrate is important for crop growth, but it can also leach to groundwater causing nitrate contamination, a threat to human health. Here, we report a significant accumulation of soil nitrate in Chinese semi-humid croplands based upon more than 7000 samples from 141 sites collected from 1994 to 2015. In the 0–4 meters depth of soil, total nitrate accumulation reaches 453 ± 39, 749 ± 75, 1191 ± 89, 1269 ± 114, 2155 ± 330 kg N ha−1 on average in wheat, maize, open-field vegetables (OFV), solar plastic-roofed greenhouse vegetables (GHV) and orchard fields, respectively. Surprisingly, there is also a comparable amount of nitrate accumulated in the vadose-zone deeper than 4 meters. Over-use of N fertilizer (and/or manure) and a declining groundwater table are the major causes for this huge nitrate reservoir in the vadose-zone of semi-humid croplands, where the nitrate cannot be denitrified due to the presence of oxygen and lack of carbon sources. Future climatic change with more extreme rainfall events would increase the risk of accumulated nitrate moving downwards and threatening groundwater nitrate contamination.

Weekly water samples were taken to measure stable isotope composition (d 13 C and d 15 N) of particulate organic matter (POM) in Lake Wauberg, Florida, from June 1994 to May 1995. The average d 13 C of POM was 219.3%, consistent with an autochthonous origin from phytoplankton production, and exhibited a seasonal pattern that coincided with changes in water temperature, pH, CO 2 concentration, and phytoplankton biomass in the surface water. The 13 C enrichment in POM was attributed to reduced isotope fractionation due to carbon (C) limitation and the use of an isotopically heavy dissolved inorganic carbon pool supported mainly by atmospheric invasion and anaerobic respiration. Intermittent declines in d 13 C of POM were related to the frequent collapses of phytoplankton blooms and increases in CO 2 concentration resulting from both increased community respiration and terrestrial loading. Average d 15 N of POM was 1.3% and varied within a narrow range (20.1% to 2.5%). No significant correlation between phytoplankton biovolume and the d 15 N of POM was found. The low d 15 N is indicative of strong N 2 fixation, which is in line with the low concentration of dissolved inorganic nitrogen and the presence of high biovolume of N 2 -fixing cyanobacteria in the surface water. This study suggests that stable C isotopes are good proxies for surface water CO 2 concentration and primary production, while stable N isotopes can be used to indicate N 2 fixation.

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