Excessive N fertilization in intensive agricultural areas of China has resulted in serious environmental problems because of atmospheric, soil, and water enrichment with reactive N of agricultural origin. This study examines grain yields and N loss pathways using a synthetic approach in 2 of the most intensive double-cropping systems in China: waterlogged rice/upland wheat in the Taihu region of east China versus irrigated wheat/rainfed maize on the North China Plain. When compared with knowledge-based optimum N fertilization with 30–60% N savings, we found that current agricultural N practices with 550–600 kg of N per hectare fertilizer annually do not significantly increase crop yields but do lead to about 2 times larger N losses to the environment. The higher N loss rates and lower N retention rates indicate little utilization of residual N by the succeeding crop in rice/wheat systems in comparison with wheat/maize systems. Periodic waterlogging of upland systems caused large N losses by denitrification in the Taihu region. Calcareous soils and concentrated summer rainfall resulted in ammonia volatilization (19% for wheat and 24% for maize) and nitrate leaching being the main N loss pathways in wheat/maize systems. More than 2-fold increases in atmospheric deposition and irrigation water N reflect heavy air and water pollution and these have become important N sources to agricultural ecosystems. A better N balance can be achieved without sacrificing crop yields but significantly reducing environmental risk by adopting optimum N fertilization techniques, controlling the primary N loss pathways, and improving the performance of the agricultural Extension Service.

Synthetic nitrogen (N) fertilizer has played a key role in enhancing food production and keeping half of the world's population adequately fed. However, decades of N fertilizer overuse in many parts of the world have contributed to soil, water, and air pollution; reducing excessive N losses and emissions is a central environmental challenge in the 21st century. China's participation is essential to global efforts in reducing N-related greenhouse gas (GHG) emissions because China is the largest producer and consumer of fertilizer N. To evaluate the impact of China's use of N fertilizer, we quantify the carbon footprint of China's N fertilizer production and consumption chain using life cycle analysis. For every ton of N fertilizer manufactured and used, 13.5 tons of CO 2 -equivalent (eq) (t CO 2 -eq) is emitted, compared with 9.7 t CO 2 -eq in Europe. Emissions in China tripled from 1980 [131 terrogram (Tg) of CO 2 -eq (Tg CO 2 -eq)] to 2010 (452 Tg CO 2 -eq). N fertilizer-related emissions constitute about 7% of GHG emissions from the entire Chinese economy and exceed soil carbon gain resulting from N fertilizer use by several-fold. We identified potential emission reductions by comparing prevailing technologies and management practices in China with more advanced options worldwide. Mitigation opportunities include improving methane recovery during coal mining, enhancing energy efficiency in fertilizer manufacture, and minimizing N overuse in field-level crop production. We find that use of advanced technologies could cut N fertilizer-related emissions by 20-63%, amounting to 102-357 Tg CO 2 -eq annually. Such reduction would decrease China's total GHG emissions by 2-6%, which is significant on a global scale.carbon accounting | life cycle assessment | food security | policy

The annual nitrogen (N) budget and groundwater nitrate-N concentrations were studied in the field in three major intensive cropping systems in Shandong province, north China. In the greenhouse vegetable systems the annual N inputs from fertilizers, manures and irrigation water were 1358, 1881 and 402 kg N ha(-1) on average, representing 2.5, 37.5 and 83.8 times the corresponding values in wheat (Triticum aestivum L.)-maize (Zea mays L.) rotations and 2.1, 10.4 and 68.2 times the values in apple (Malus pumila Mill.) orchards. The N surplus values were 349, 3327 and 746 kg N ha(-1), with residual soil nitrate-N after harvest amounting to 221-275, 1173 and 613 kg N ha(-1) in the top 90 cm of the soil profile and 213-242, 1032 and 976 kg N ha(-1) at 90-180 cm depth in wheat-maize, greenhouse vegetable and orchard systems, respectively. Nitrate leaching was evident in all three cropping systems and the groundwater in shallow wells (<15 m depth) was heavily contaminated in the greenhouse vegetable production area, where total N inputs were much higher than crop requirements and the excessive fertilizer N inputs were only about 40% of total N inputs.

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.

No abstract

Nitrogen dynamics and budgets in a clay loam soil (Meadow Aqualf) in the North China Plain were investigated in a winter wheat (Triticum aestivum L.) and maize (Zea mays L.) cropping system comparing the effects of four N rates (0, 120, 240 and 360 kg N ha À1 as urea) applied twice to each crop over 2 years. Ammonium nitrogen (NH 4-N) in the soil profile remained at a low and constant level (except in the surface 20 cm layer) following application of fertilizer N. In contrast, nitrate nitrogen (NO 3-N) levels were significantly altered by the rate of applied N. A strong tendency of NO 3-N to move from the surface layer to the lower layers (20-100 cm) was observed during the wheat and maize growth seasons in treatments of 240 and 360 kg N ha À1 per crop (N240 and N360). The amounts of NO 3-N accumulated in the soil profile were significantly higher in N240 and N360 than those in N0 and N120 (treatments receiving 0 and 120 kg N ha À1 per crop). After 2 years, soil NO 3-N levels at 0-300 cm depth in N120, N240 and N360 amounted to 336, 815 and 1141 kg ha À1 , respectively, with more than half of these amounts distributed in the 100-300 cm layer. The calculated total N balance indicates that most fertilizer N was available as NO 3-N in the top 300 cm of the soil profile using traditional fertilization and irrigation practices. Over the subsequent 2 years, N losses were calculated to be relatively low in N120 but significantly higher in N240 and N360. Measured gaseous N losses showed that NH 3 volatilization and denitrification comprised only a small fraction of total N losses during the 2-year rotation, while NO 3-N leaching from the top 100 cm of the soil profile accounted for most N losses across all N rates and experimental years. The N budget showed that accumulation and/or leaching of NO 3-N below 100 cm depth (beyond the reach of most roots) was the main pathway for N losses in the winter wheat-maize cropping system. The recommended N application rate of 120 kg N ha À1 minimized soil NO 3-N accumulation and leaching losses while maintaining high yields and N utilization by winter wheat and maize.

No abstract

No abstract