Zheng-Qin Xiong

Methane emissions from a rice agroecosystem in South China: Effects of water regime, straw incorporation and nitrogen fertilizer

To quantitatively assess the effects of agricultural practices on methane (CH4) emissions from rice fields, a two-year (2005/2006) field experiment with 23 factorial designs was conducted to assess the effects of three driving factors on CH4 emissions in South China: continuously flooded (W0) and mid-season and final drainages (W2), straw (S1) and nitrogen fertilizer (N1) applications and their controls (S0, N0). Results showed that averaged across all the treatments about 75xa0% of the seasonal total CH4 occurred between the rice transplanting and booting stage, while constituted only 33xa0% of the seasonal total rice biomass during the same period. Averaged across the treatments in 2006, CH4 emissions were substantially decreased by mid-season drainage up to 60xa0% (15.6 vs. 39.0xa0gxa0m−2). The decreased CH4 emissions represented almost all of the decrease in the total global warming potentials. Without straw incorporation CH4 emissions substantially decreased up to 59xa0% (15.9 vs. 38.7xa0gxa0m−2). The stimulating effects of straw were significantly greater for W0 than W2 treatment, being also greater in the 2005 than in the 2006 season. A significant inter-annual difference in CH4 emissions was found when averaged across straw incorporation and N fertilizer applications for the W2 treatment (42.8 and 15.4xa0gxa0m−2 in 2005 and 2006, respectively). Moreover, N fertilization has no significant effect on CH4 emissions in this study. Our results demonstrate that although straw effects varied greatly with specific management, both straw managements and water regimes are equally important driving factors and thus being the most promising measures attenuating CH4 emissions while achieving sustainable rice production.

Impacts of population growth, changing food preferences and agricultural practices on the nitrogen cycle in East Asia

While increasing population and changing food preferences have changed agriculture in some East Asian countries to high input systems with greater use of fertilizer nitrogen and greater numbers of animals, the changes and the effects on the environment in the different countries have varied considerably. Many areas still do not use sufficient nitrogen to maximize crop yields. In China, fertilizer nitrogen input has increased from 0.54xa0Tg in 1961 to 28xa0Tg in 2005, and the animal population increased dramatically, from 27 to 1,013xa0million. As a result 13xa0Tg N was lost to the environment in 2005 as nitrous oxide, ammonia or nitrate. In Mongolia, no fertilizer nitrogen was recorded as having been used until 1970, and current use is only ∼4xa0Gg. The animal population has increased from 23xa0million in 1961 to 28xa0million in 2005 and adverse effects on the environment are small (96xa0Gg N lost). However, a combination of over-ploughing and overgrazing has resulted in soil erosion from wind and rain in both countries and loss of soil nitrogen. These and other effects of changing agricultural systems on the nitrogen cycle in East Asian countries and some approaches to reduce the impact of nitrogen on the environment are reported in this paper.

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.

Greenhouse gas emissions during the seedling stage of rice agriculture as affected by cultivar type and crop density

Methane (CH4) and nitrous oxide (N2O) emissions from a paddy nursery at the rice seedling stage were measured on a daily basis by using the conventional rice cultivar Nangeng 56 under both conventional (NG-C) and reduced (NG-R) sowing density, and the hybrid rice Changyou 3 under both conventional (CY-C) and reduced (CY-R) sowing density. High N2O and CH4 emissions were observed during the first and last 2xa0weeks, respectively. Cumulative CH4 emissions were significantly (Pu2009<u20090.001) affected by sowing density rather than by the rice cultivar. Cumulative CH4 emissions reached 68.2xa0kgxa0Cxa0ha−1 in the CY-C treatment and 121.6xa0kgxa0Cxa0ha−1 in the NG-C treatment, which were significantly (Pu2009<u20090.001) higher than the emissions at reduced sowing densities (15.9xa0kgxa0Cxa0ha−1 in the CY-R treatment and 20.9xa0kgxa0Cxa0ha−1 in the NG-R treatment). Under the conventional sowing density, cumulative CH4 emissions during the seedling stage were comparable to data of rice-growing season. Both the rice cultivar and the sowing density significantly (Pu2009<u20090.05–0.01) affected cumulative N2O emissions. Relative to the CY cultivar, the NG cultivar increased global warming potential (GWP) over a 100-year horizon by 62.1% and 70.7% under the reduced and conventional sowing densities, respectively. The GWP of N2O and CH4 during the seedling stage was equivalent to the GWP of the entire rice-growing season in this region, indicating that the seedling stage is an important greenhouse gas emission source of rice agriculture.

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.