Xiaotang Ju

Reducing environmental risk by improving N management in intensive Chinese agricultural systems

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.

Nitrogen Fertilization, Soil Nitrate Accumulation, and Policy Recommendations in Several Agricultural Regions of China

Abstract Excessive nitrogen (N) fertilization and decreasing N recovery rates by crops have caused dramatic increases in non-point source pollution from agriculture in China. The rate of N fertilization across the country varies widely among regions and crops, depending on the stage of economic development. For example, N application rates in the eastern regions and on cash crops are far higher than in western regions of the country and on cereal crops. Moreover, N application rates in wealthier regions are higher than recommended by the Chinese Academy of Sciences. To successfully achieve environmental protection as well as high crop yields, China must formulate relevant agricultural policies to encourage farmers in economically developed areas to reduce their N fertilization rate while also issuing conventional fertilization recommendations for small-scale farming systems and the expanding cultivation of cash crops.

New technologies reduce greenhouse gas emissions from nitrogenous fertilizer in China

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 CO2-equivalent (eq) (t CO2-eq) is emitted, compared with 9.7 t CO2-eq in Europe. Emissions in China tripled from 1980 [131 terrogram (Tg) of CO2-eq (Tg CO2-eq)] to 2010 (452 Tg CO2-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 CO2-eq annually. Such reduction would decrease China’s total GHG emissions by 2–6%, which is significant on a global scale.

Integrated reactive nitrogen budgets and future trends in China

Significance China is the world’s largest producer of reactive nitrogen (Nr), and Nr in the form of synthetic fertilizer has contributed substantially to increased food production there. However, Nr losses from overuse and misuse of fertilizer, combined with industrial emissions, represent a serious and growing cause of air and water pollution. This paper presents a substantially complete and coherent Nr budget for China and for 14 subsystems within China from 1980 to 2010, evaluates human health/longevity and environmental consequences of excess Nr, and explores several scenarios for Nr in China in 2050. These scenarios suggest that reasonable pathways exist whereby excess Nr could be reduced substantially, while at the same time benefitting human well-being and environmental health. 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.

Ammonia Volatilization Loss from Surface-Broadcast Urea: Comparison of Vented- and Closed-Chamber Methods and Loss in Winter Wheat–Summer Maize Rotation in North China Plain

Abstract Ammonia (NH3) volatilization is an important pathway for fertilizer nitrogen (N) loss from soil and is also a major source of air and environmental pollution. On calcareous soils in North China Plain, application of N fertilizer in the form of urea under intensive cropping with winter wheat (Triticum aestvum L.) and summer maize (Zea mays L.) rotation can lead to serious NH3 loss. The objective of this study was to compare a modified vented-chamber method with the traditional closed-chamber method to measure NH3 volatilization loss under laboratory and field conditions and to determine in situ NH3 volatilization in the field from surface broadcast urea at 0, 120, 240, and 360 kg N ha−1 rates to each crop (for winter wheat, one-half at sowing and the other half at the elongation growth stage; for summer maize, one-half at the 3-leaf and the other half at the 10-leaf growth stage) in a winter wheat and summer maize rotation at the northern edge of North China Plain from October 1998 to September 1999. Urea was surface applied after irrigation before sowing, or prior to irrigation during the growing season. Compared to the closed chamber method, the vented chamber method was found to be simpler in structure, easier to operate, more suitable for in situ determination of NH3 volatilization in the field, and had higher recovery of emitted NH3 (99.5 vs. 70.8%). For surface broadcast urea after first irrigation prior to sowing of winter wheat, the NH3 volatilization rate reached, a maximum on the second to fifth day after application. Total NH3 loss from soil during the October 8 to November 18, 1998, period was 2.9, 4.8, 10.5, and 35.7 kg N ha−1 at the 0, 60, 120, and 180 kg N ha−1 rates, respectively. When urea was top-dressed prior to irrigation at the elongation growth stage of the winter wheat in midspring, NH3 loss was relatively low (i.e., 1.5, 2.1, 2.4, and 2.7 kg N ha−1 at 0, 60, 120, and 180 kg N ha−1 rates, respectively). The NH3 loss from urea for the entire winter wheat–growing season accounted for 2.1, 3.6, and 9.5% of the applied N at 120, 240, and 360 kg N ha−1 rates, respectively. For summer maize where urea was top-dressed at the 3-leaf and 10-leaf growth stages, the NH3 volatilization rate increased more quickly and maximized on the first or second day after N application. During the summer maize–growing season, total NH3 loss from urea accounted for 5.6, 4.8, and 4.9% of the applied N at 120, 240, and 360 kg N ha−1 rates, respectively. Based on the results of this study, total NH3 loss was estimated to range from 163,000 to 477, 200 Mg of N from soil in a winter wheat–summer maize rotation in North China Plain. Of these, about 68,400 to 382,600 Mg of N loss were from the N fertilizer surface applied as urea.

Greenhouse gas emissions from a wheat–maize double cropping system with different nitrogen fertilization regimes

Here, we report on a two-years field experiment aimed at the quantification of the emissions of nitrous oxide (N2O) and methane (CH4) from the dominant wheat-maize double cropping system in North China Plain. The experiment had 6 different fertilization strategies, including a control treatment, recommended fertilization, with and without straw and manure applications, and nitrification inhibitor and slow release urea. Application of N fertilizer slightly decreased CH4 uptake by soil. Direct N2O emissions derived from recommended urea application was 0.39% of the annual urea-N input. Both straw and manure had relatively low N2O emissions factors. Slow release urea had a relatively high emission factor. Addition of nitrification inhibitor reduced N2O emission by 55%. We conclude that use of nitrification inhibitors is a promising strategy for N2O mitigation for the intensive wheat-maize double cropping systems.

Ammonia-oxidation as an engine to generate nitrous oxide in an intensively managed calcareous Fluvo-aquic soil

We combine field observations, microcosm, stoichiometry, and molecular and stable isotope techniques to quantify N2O generation processes in an intensively managed low carbon calcareous fluvo-aquic soil. All the evidence points to ammonia oxidation and linked nitrifier denitrification (ND) being the major processes generating N2O. When NH4+-based fertilizers are applied the soil will produce high N2O peaks which are inhibited almost completely by adding nitrification inhibitors. During ammonia oxidation with high NH4+ concentrations (>80 mg N kg−1) the soil matrix will actively consume oxygen and accumulate high concentrations of NO2−, leading to suboxic conditions inducing ND. Calculated N2O isotopomer data show that nitrification and ND accounted for 35–53% and 44–58% of total N2O emissions, respectively. We propose that slowing down nitrification and avoiding high ammonium concentrations in the soil matrix are important measures to reduce N2O emissions per unit of NH4+-based N input from this type of intensively managed soil globally.

Spatial and temporal variation of atmospheric nitrogen deposition in the North China Plain

Abstract A monitoring network of nine sites was established to determine the spatial and temporal variation of atmospheric nitrogen (N) deposition in the North China Plain (NCP) over a two-year period. The annual bulk deposition of inorganic N in the North China Plain ranged from 18.4 to 38.5 kg/hm 2 and averaged 28.0 kg/hm 2 . The concentration of NH 4 + -N and NO 3 − -N in rainwater averaged 3.76 and 1.85 mg/L, respectively, which were significantly higher than the values at background sites in China (normally less than 0.5 mg/L). Annual bulk deposition of inorganic N in the Beijing area (32.5 kg/hm 2 ) was higher than that in Shandong and Hebei provinces (21.2 kg/hm 2 on an average). Also bulk N deposition was much greater in Dongbeiwang and Fangshan than in Yanqing and Shunyi counties. Significant spatial variation of bulk deposition was observed in the Beijing area because of variation of precipitation, and 60% of bulk deposition occurred from June to September. Bulk deposition of NH 4 + -N was 2.0 times that of NO 3 − -N deposition at the rural monitoring sites. However, the situation was reversed at the Beijing Academy of Agricultural-Forestry Sciences (BAAFS), the unique urban monitoring site. The results suggest that reduced N in precipitation is dominant in rural regions, but oxidized N is the major form in urban regions. The positive relationship between inorganic N deposition and precipitation can be fitted by a power equation ( r 2 = 0.67), showing an increase of NH 4 + -N and NO 3 − -N inputs with increased precipitation. Wet deposition of N accounted for 73% of the bulk deposition, implying that dry deposition of N, particularly NH 4 + -N from dust, is important in the North China Plain.

Nitrous oxide and methane emissions from optimized and alternative cereal cropping systems on the North China Plain: A two-year field study

The impacts of different crop rotation systems with their corresponding management practices on grain yield, greenhouse gas emissions, and fertilizer nitrogen (N) and irrigation water use efficiencies are not well documented. This holds especially for the North China Plain which provides the staple food for hundreds of millions of people and where groundwater resources are polluted with nitrate and depleted through irrigation. Here, we report on fertilizer N and irrigation water use, grain yields, and nitrous oxide (N2O) and methane (CH4) emissions of conventional and optimized winter wheat-summer maize double-cropping systems, and of three alternative cropping systems, namely a winter wheat-summer maize (or soybean)-spring maize system, with three harvests in two years; and a single spring maize system with one crop per year. The results of this two-year study show that the optimized double-cropping system led to a significant increase in grain yields and a significant decrease in fertilizer N use and net greenhouse gas intensity, but the net greenhouse gas N2O emissions plus CH4 uptake and the use of irrigation water did not decrease relative to the conventional system. Compared to the conventional system the net greenhouse gas emissions, net greenhouse gas intensity and use of fertilizer N and irrigation water decreased in the three alternative cropping systems, but at the cost of grain yields except in the winter wheat-summer maize-spring maize system. Net uptake of CH4 by the soil was little affected by cropping system. Average N2O emission factors were only 0.17% for winter wheat and 0.53% for maize. In conclusion, the winter wheat-summer maize-spring maize system has considerable potential to decrease water and N use and decrease N2O emissions while maintaining high grain yields and sustainable use of groundwater.

Nitrogen Recommendation for Winter Wheat Using Nmin Test and Rapid Plant Tests in North China Plain

Nitrogen (N) over‐fertilization is a common phenomenon in cereal crop production in North China Plain in order to meet the food requirement by increasing population. It is of significance to optimize N fertilization in winter wheat according to crop N need using suitable N recommendation methods. Thus a field experiment with split‐plot design was carried out in this region to evaluate the role of Nmin (mineral N) test and rapid plant tests on N recommendation for winter wheat. The results showed that Nmin sollwert (NS, Nmin + fertilizer N) 90 kg N ha− 1 at regreening stage (early March) and N rate 90 kg N ha− 1 at shooting stage (middle April) could meet N requirement of winter wheat with the target yield of 6000 kg ha− 1. Plant nitrate concentration in the stem base of wheat as well as SPAD readings (using SPAD‐502 chlorophyll meter from Minolta, Japan) of wheat functional leaves positively correlated with the NS levels at shooting stage, indicating that plant nitrate and SPAD tests could reflect the N nutritional status at this stage. Under treatment of NS 90 + 90, the 0–90 cm soil residual Nmin (53.2 kg N ha− 1) and apparent N loss (12.4 kg N ha− 1) were controlled at the environmental safe levels without yield loss compared with other treatments with higher N rates, suggesting total N supply of 180 kg N ha− 1 may be the optimal fertilization under the experimental condition.