Zhaoliang Zhu

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 losses from fertilizers applied to maize, wheat and rice in the North China Plain

Ammonia volatilization, denitrification loss and total nitrogen (N) loss (unaccounted-for N) have been investigated from N fertilizer applied to a calcareous sandy loam fluvo-aquic soil at Fengqiu in the North China Plain. Ammonia volatilization was measured by the micrometeorological mass balance method, denitrification by the acetylene inhibition – soil core incubation technique, and total N loss by 15N-balance technique. Ammonia loss was an important pathway of N loss from N fertilizer applied to rice (30–39% of the applied N) and maize (11–48%), but less so for wheat (1–20%). The amounts of unaccounted-for fertilizer N were in the order of rice > maize > wheat. Deep placement greatly reduced ammonia volatilization and total N loss. Temperature, wind speed, and solar radiation (particular for rice), and source of N fertilizer also affect extent and pattern of ammonia loss. Denitrification (its major gas products are N2 and N2O) usually was not a significant pathway of N loss from N fertilizer applied to maize and wheat. The amount of N2O emission (N2O is an intermediate product from both nitrification and denitrification) was comparable to denitrification loss for maize and wheat, and it was not significant in the economy of fertilizer N in agronomical terms, but it is of great concern for the environment.

Nitrogen Runoff and Leaching Losses During Rice-Wheat Rotations in Taihu Lake Region, China*

Abstract Although nitrogen (N) loss through runoff and leaching from croplands is suspected to contribute to the deterioration of surrounding water systems, there is no conclusive evidence for paddy soils to prove this hypothesis. In this study, field plot experiments were conducted to investigate N losses through runoff and leaching for two consecutive years with 3 N fertilization rates in rice (Oryza sativa L.)-wheat (Triticum aestivum L.) rotations in the Taihu Lake region, China. A water collection system was designed to collect runoff and leachates for both the rice and wheat seasons. Results showed that dissolved N (DN), rather than particulate N (PN), was the main form of N loss by runoff. The NO3−-N concentration in runoff was between 0.1 and 43.7 mg L−, whereas the NH4+-N concentration ranged from below detection limit to 8.5 mg L−. Total N (TN) loads by runoff were 1.0–17.9 and 5.2–38.6 kg ha− during rice and wheat seasons, respectively, and the main loss occurred at the early growing stage of the crops. Nitrogen concentrations in leachates during the rice seasons were below 1.0 mg L− and independent of the N application rate, whereas those during the wheat season increased to 8.2 mg L− and were affected by the fertilizer rate. Annual losses of TN through runoff and leaching were 13.7–48.1 kg ha-1 from the rice-wheat cropping system, accounting for 5.6%–8.3% of the total applied N. It was concluded that reduction in the N fertilization rate, especially when the crop was small in biomass, could lower the N pollution potential for water systems.

Nitrous Oxide and Methane Emissions as Affected by Water,Soil and Nitrogen

Specific management of water regimes, soil and N in China might play an important role in regulating N2O and CH4 emissions in rice fields. Nitrous oxide and methane emissions from alternate non-flooded/flooded paddies were monitored simultaneously during a 516-day incubation with lysimeter experiments. Two N sources (15N-(NH4)2SO4 and 15N-labeled milk vetch) were applied to two contrasting paddies: one derived from Xiashu loess (Loess) and one from Quaternary red clay (Clay). Both N2O and CH4 emissions were significantly higher in soil Clay than in soil Loess during the flooded period. For both soil, N2O emissions peaked at the transition periods shortly after the beginning of the flooded and non-flooded seasons. Soil type affected N2O emission patterns. In soil Clay, the emission peak during the transition period from non-flooded to flooded conditions was much higher than the peak during the transition period from flooded to non-flooded conditions. In soil Loess, the emission peak during the transition period from flooded to non-flooded conditions was obviously higher than the peak during the transition period from non-flooded to flooded conditions except for milk vetch treatment. Soil type also had a significant effect on CH4 emissions during the flooded season, over which the weighted average flux was 111 mg C m−2 h−1 and 2.2 mg C m−2 h−1 from Clay and Loess, respectively. Results indicated that it was the transition in the water regime that dominated N2O emissions while it was the soil type that dominated CH4 emissions during the flooded season. Anaerobic oxidation of methane possibly existed in soil Loess during the flooded season.

Assessment of Nitrogen Pollutant Sources in Surface Waters of Taihu Lake Region

Abstract The nitrogen (N) pollution status of the 12 most important rivers in Changshu, Taihu Lake region was investigated. Water samples were collected from depths of 0.5–1.0 m with the aid of the global positioning system (GPS). The seasonal variations in the concentrations of different N components in the rivers were measured. Using tension-free monolith lysimeters and 15 N-labeled fertilizer, field experiments were carried out in this region to determine variations of 15 N abundance of NO 3 − in the leachate during the rice and wheat growing seasons, respectively. Results showed that the main source of N pollution of surface waters in the Taihu Lake region was not the N fertilizer applied in the farmland but the urban domestic sewage and rural human and animal excreta directly discharged into the water bodies without treatment. Atmospheric dry and wet N deposition was another evident source of N pollutant of the surface waters. In conclusion, it would not be correct to attribute the N applied to farmlands as the source of N pollution of the surface waters in this region.

Nitrogen Balance and Loss in a Greenhouse Vegetable System in Southeastern China

Abstract High rates of fertilizer nitrogen (N) are applied in greenhouse vegetable fields in southeastern China to maximize production; however, the N budgets of such intensive vegetable production remain to be explored. The goal of this study was to determine the annual N balance and loss in a greenhouse vegetable system of annual rotation of tomato, cucumber, and celery at five N (urea) application rates (0, 348, 522, 696, and 870 kg N ha −1 year −1 ). Total N input to the 0–50 cm soil layer ranged from 531 to 1 053 kg ha −1 , and N fertilizer was the main N source, accounting for 66%–83% of the total annual N input. In comparison, irrigation water, wet deposition, and seeds in total accounted for less than 1% of the total N input. The fertilizer N use efficiency was only 18% under the conventional application rate of 870 kg N ha −1 and decreased as the application rate increased from 522 to 870 kg N ha −1 . Apparent N losses were 196–201 kg N ha −1 , of which 71%–86% was lost by leaching at the application rates of 522–870 kg N ha −1 . Thus, leaching was the primary N loss pathway at high N application rates and the amount of N leached was proportional to the N applied during the cucumber season. Moreover, dissolved organic N accounted for 10% of the leached N, whereas NH 3 volatilization only contributed 0.1%–0.6% of the apparent N losses under the five N application rates in this greenhouse vegetable system.

Gaseous nitrogen losses from urea applied to maize on a calcareous fluvo-aquic soil in the North China Plain

Gaseous nitrogen losses, by NH3 volatilisation and denitrification, are mainly responsible for the low recovery of N fertiliser applied to irrigated maize on the North China Plain. Two field experiments were conducted to measure NH3 volatilisation and nitrification-denitrification losses from urea applied to maize (Zea mays L.) grown on a calcareous fluvo-aquic soil (Aquic Inceptisol) in Fengqiu County, Henan Province. The first was carried out in June 1998 (urea applied at 75 kg N/ha 3 weeks after sowing), and the second in July 1998 (urea applied at 200 kg N/ha 6 weeks after sowing). Each experiment included 3 treatments-control, surface-broadcast (SB), and deep point placement (DP) or broadcast followed by irrigation (BI). NH3 loss was measured by a micrometeorological method (NH3 sampler). Denitrification (N2 + N2O) was measured by the acetylene inhibition-intact soil core technique, and N2O emission was also measured in the absence of acetylene. The recovery of applied N was measured by a 15N balance technique. When urea was surface broadcast (SB) 3weeks (75 kg N/ha) and 6weeks (200 kg N/ha) after sowing, 44 and 48% of the applied N was lost by NH3 volatilisation, respectively. The corresponding losses from the BI and DP treatments were only 18% and 11%, respectively. Denitrification was a significant process in this well-drained sandy soil, with average loss rates of 0.26-0.43 kg N/ha.day in the controls (from resident soil N), compared with 0.52-0.63 kg N/ha.day in the surface fertiliser treatments. Deep placement of urea reduced the denitrification rate to an average of 0.3 kg N/ha.day. The net denitrification loss from the fertiliser was <2% of the applied N, except for the SB urea treatment in the second experiment. The application of N fertiliser as urea increased N2O emissions from c. 0.3 to c. 2.3 kg N/ha over 57 days in the second experiment, with average N2O emission rates in the control and SB treatment of 0.006 and 0.042 kg N/ha.day, respectively. The significantly lower ratio of N2 /N2O in the urea treatments compared with the control suggested that nitrification of applied N may have contributed to N2O production. Alternatively, the ratio of N2 /N2O during denitrification may have changed with the greater supply of NO3 -. denitrification, maize, NH3 volatilisation, N2O emission.

Optimizing Nitrogen Fertilizer Application for Rice Production in the Taihu Lake Region, China

Abstract To determine the optimal amount of nitrogen (N) fertilizer for achieving a sustainable rice production at the Taihu Lake region of China, two-year on-farm field experiments were performed at four sites using various N application rates. The results showed that 22%–30% of the applied N was recovered in crop and 7%–31% in soils at the rates of 100–350 kg N ha−1. Nitrogen losses increased with N application rates, from 44% of the applied fertilizer N at the rate of 100 kg N ha−1 to 69% of the N applied at 350 kg N ha−1. Ammonia volatilization and apparent denitrification were the main pathways of N losses. The N application rate of 300 kg N ha−1, which is commonly used by local farmers in the study region, was found to lead to a significant reduction in economic and environmental efficiency. Considering the cost for mitigating environmental pollution and the maximum net economic income, an application rate of 100–150 kg N ha−1 would be recommended. This recommended N application rate could greatly reduce N loss from 199 kg N ha−1 occurring at the N application rate of 300 kg N ha−1 to 80–110 kg N ha−1, with the rice grain yield still reaching 7 300–8 300 kg DW ha−1 in the meantime.

Impacts of population growth and economic development on the nitrogen cycle in Asia

Asia is the major consumer of fertilizer nitrogen and energy in the world, and consequently shares a considerable proportion of the world creation of reactive nitrogen (Nr). However, if estimated on per capita basis, Asia is characterized by a lower arable land area, fertilizer nitrogen consumption, energy consumption, and gross domestic product, as well as lower daily protein intake. To meet the increasing needs for food and energy for the growing population combined with the improvement of living standards, Nr will inevitably increase. The present study estimates the creation of Nr and the emissions of various N compounds into environment in Asia currently and in 2030. In comparison with the world averages, the lower fertilizer nitrogen and energy use efficiencies, and the lower use of animal wastes for agriculture imply that there is potential for moderating the increase in Nr and its impacts on the environment. Strategies for moderating the increase are discussed.

Nitrate and ammonium leaching in variable- and permanent-charge paddy soils.

Abstract A variable-charge (VC) and a permanent-charge paddy soil (PC) were selected to study nitrate-N (NO−3-N) and ammonium-N (NH+4-N) leaching with N isotopes for one consecutive year. An irrigation and intermittent drainage pattern was adopted to mimic natural occurrence of rainfall during the upland crop season and drainage management during the flooded rice season. Treatments to each soil type were no-N controls (CK), 15N-labeled (NH4)2SO4 (NS), and milk vetch (NV) applied at a rate equivalent to 238 kg N ha−1 to unplanted lysimeters, totaling six treatments in triplicates. Results indicated that the soil type dominated N leaching characteristics. In the case of PC, NO−3-N accounted for 78% of the total leached inorganic N; NS was prone to leach three times more than the NV, being 8.2% and 2.4% of added 15N respectively; and > 85% of leached NO−3-N came from native N in the soil. In the case of VC, NH+4-N made up to 92% of the total inorganic N in leachate. Moreover, NH+4-N particularly high during the flooded season. NO−3-N leaching in VC occurred later at a lower rate compared to that in PC. The findings of this study indicate that NO−3-N leaching during the drained season in permanent-charge paddy soils and NH+4-N leaching in variable-charge soils deserve more attention for regional environmental control.