Maintaining rice yield and reducing N pollution by substituting winter legume for wheat in a heavily-fertilized rice-based cropping system of southeast China
Introduction
Intensive cereal cropping and synthetic N fertilizer have been acknowledged for their roles in increasing cereal yields to feed the growing world population (Tilman et al., 2002). Nevertheless, the injudicious and excessive use of synthetic N fertilizers in agriculture has actually resulted in decreased soil fertility and N-utilization efficiency by crops, as well as increased N loss to the environment, polluting both the atmosphere and water systems (Galloway et al., 2008, Vitousek et al., 1997). Legumes are an efficient N source due to their ability to fix atmospheric N2 in symbiosis with compatible rhizobia (Hargrove, 1986, Postgate, 1970). Considering the long-term benefits that legumes can have on N fertilizer savings, improvement of C/N cycling, and soil fertility (Drinkwater et al., 1998), international emphasis on sustainable agriculture (Conway and Barbier, 1990) has focused on the potential role of introducing legumes into cereal cropping systems over the past twenty years (Crews and Peoples, 2004, Peoples et al., 1995, Peoples et al., 2009, Voisin et al., 2014). In rice-based cropping systems, planting legumes in the fallow seasons followed by incorporation into paddy soils was reported to maintain soil fertility and reduce volatilization losses due to the partial replacement of synthetic N fertilizer for rice by legume green manure (Becker et al., 1994, Buresh and De Datta, 1991, Diekmann et al., 1993). However, when using legumes as an option for sustainable rice production and reducing environmental impacts, more information is needed about the incorporation of legumes into different rice-based agroecosystems under varying crop rotation, management practice and climate worldwide.
The Taihu Lake Plain is one of the most important rice-based agroecosystems in southeast China. This rice ecosystem is characterized by flooded summer rice/rain-fed winter wheat cropping rotation and high rates of synthetic N fertilizer application (c. 240–300 kg N ha−1 for rice and 200–250 kg N ha−1 for wheat). Significant concerns have been raised about the environmental risks generated from reactive N output (mainly N runoff and leaching, NH3 volatilization, and N2O emission) in this rotation system during the past decade due to the accelerated N losses driven by heavy fertilization and strong water flow under intensive flooding/draining cycling (Ju et al., 2009, Zhao et al., 2009, Zhu et al., 1997). Despite the adoption of double cropping and heavy fertilization, yields of winter wheat (c. 5.0 t ha−1) in the rice/wheat rotation are commonly far lower than for either summer rice (c. 7.0 t ha−1), or irrigated wheat (commonly beyond 6.0 t ha−1) in the important wheat producing area of China – the North China Plain (Zhu and Zhang, 2010). This is because waterlogging that results from uneven distribution of abundant rainfall during the wheat growing season in the subtropical monsoon climate always depresses wheat growth and reduces production (Dickin and Wright, 2008). In order to minimize yield losses, local farmers prefer to dig drainage ditches to prevent waterlogging that might injure wheat plants in addition to the high N application rate used for wheat. However, these practices usually lead to decreasing N-utilization efficiency due to increased N runoff loss to the environment. From a 3-year field observation employing local farming practices, we found that wheat yields were as low as 4.2–4.6 t ha−1 at 200 kg N ha−1 urea application; in contrast, the export of N via runoff and leaching from the wheat season was as high as 34–69 kg N ha−1 (Zhao et al., 2012a). This considerable N export via water flow during the wheat seasons can be primarily attributed to high N fertilization and heavy water runoff driven by precipitation and drainage ditches (Zhao et al., 2012b). In terms of integrating agronomic and environmental aspects into sustainable agriculture, the current cropping system that has poor grain yield but high N pollution potential during the winter wheat season is obviously unsustainable in the Taihu Lake Plain. Yu et al. (2013) recently reported the positive effects of using legumes as a winter crop in rice–bean and rice–vetch combinations on reducing N runoff loss and improving rice yield and soil N supply. When using legumes to replace wheat in the conventional rice–wheat crop rotation, not only could chemical N for the winter crop be eliminated, but also the amount of chemical N fertilizer needed for the subsequent rice crop can be reduced due to the incorporation of residue. It is important to investigate the agronomic and environmental effects of rice/legume green manure rotations in comparison with conventional rice/wheat cropping systems, with the aim of addressing the feasibility of winter legume substitution in the Taihu Lake Plain to achieve rice production security and environmental sustainability.
Thus, the objectives of our research were, (1) to determine the effects of substituting winter legume for wheat on crop yield, chemical N input, N runoff and leaching, NH3 volatilization, and N2O emission using a continuous 3-year, field-scale observation spanning six consecutive summer rice/winter crops of the rice/wheat, rice/fava bean, and rice/milk vetch rotation types; (2) to qualify the yield benefits and environmental costs relating to reactive N export from the three rotation types based on the field observation results. The data from this study will provide useful information for future comprehensive evaluations of the sustainability of substituting winter legumes for wheat in the heavily fertilized rice-based cropping systems of the Taihu Lake Plain.
Section snippets
Field plot design and management
The experiment was conducted at the Yixing Base for Agri-Environment Research, Changshu National Agro–Ecosystem Observation and Research Station, Chinese Academy of Sciences (Fig. 1a). This location has a subtropical monsoon climate with an average temperature of 15.7 °C and an annual rainfall of 1177 mm. The paddy soil is classified as Gleyi-Stagnic Anthrosols, and originated from a Lacustrine deposit as parent material. The top 20 cm soil layer in the fields consists of 81.5% silt, and 10.2%
Crop yield and chemical N input
There were yearly variations in crop yield. Under the three rice/wheat rotations from 2009 to 2012, grain yield was 7155, 8150, and 6228 kg ha−1 for rice, while it was 4860, 6270, and 5411 kg ha−1 for wheat. For the rice/fava bean, rice grain yield was 7079, 8170, and 6303 kg ha−1, respectively, while fava bean grain yield was 1358, 1835, and 718 kg ha−1. For the rice/milk vetch, no grain was obtained in the winter seasons because milk vetch was used as green manure; the three rice seasons produced
Discussion
The conventional rice/wheat crop rotation produced an average of 7178 and 5514 kg ha−1 grain yield for rice and wheat, respectively. These yield benefits were gained at the great expense of 119 kg ha−1 N losses (NH3 volatilization + N runoff and leaching + N2O emission), which accounted for 27.0% of the annual chemical N input (440 kg N ha−1) (Fig. 7c). This demonstrated the high N pollution potential of the rice–wheat rotation system to introduce a considerable amount of reactive N into the atmosphere
Conclusions and future implications
This 3-year field study provides support for our conclusion that substituting winter legumes (fava bean/milk vetch tested in this study) for wheat in a heavily chemical N fertilized rice-based cropping system is an effective way to reduce environmental impacts relating to N losses without compromising rice yield. A key reason for this is more than 50% reduction in annual fertilizer N input due to the replacement of wheat with a legume and incorporation of N rich residue. The most obvious
Acknowledgments
We specially thank Dr. Bolun Li in ISS, CAS for technical support on ArgGIS map. We also would like to acknowledge anonymous reviewers and editors for their valuable comments. This work was funded by the Knowledge Innovation Program of the Chinese Academy of Sciences (KSCX2-EW-B-1-3).
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