7. Conclusions

noted that this approach would not give information on runoff volume, and it is not practical for dryland crops. Further research is needed with respect to achieving cost-effective monitor-

In China, paddy systems are attributed to an important cause for local and regional water eutrophication (e.g., [4, 5]). Both its wide distribution and intensive nutrient and water inputs point to the need for improved management to minimize its impacts on water quality. A number of studies have emphasized the importance of adopting "4R" nutrient stewardship, i.e., Right source, Right rate, Right timing, and Right placement in paddy systems (e.g., those summarized in Table 5). For example, Fujisawa et al. [30] proposed the use of thermoplastic resin-coated fertilizers that allow the application of the fertilizers at full rates to rice seedlings and thereafter the release of nutrients in line with crop needs. Liu [31] found that adopting this technology could substantially increase nitrogen use efficiency by the rice crop as compared to the urea fertilizer and conventional management practices. The technology decreased peak nitrogen concentrations in field ponding water by 85–91%, postponed the appearance of peak concentrations by a week, and reduced total nitrogen (nitrate nitrogen plus ammonium nitrogen) losses in leachate by 36–55%. As discussed earlier, overuse of fertilizers should be avoided because the nutrient surplus contributes to both short-term and long-term nutrient losses. Liu et al. [19] and Hua et al. [27] pointed out that phosphorus fertilizer applications to paddy systems should be at rates balancing crop phosphorus removal, and that phosphorus needs to be managed for both rice and the non-rice crop in a rotation to minimize phosphorus losses in the rice-growing season. Fertilizer rate management should go hand in hand with management of fertilizer application timing. Planning fertilizer timing based on local/regional weather patterns and real-time weather forecast to avoid coincidence with rainfall storms is an important timing management approach [19, 20]. In the regions where the coincidence of fertilizer application and rainfall storm is difficult to avoid, one of the optional management practices

6. Mitigation options to improve water quality in paddy systems

Category Mitigation options References

Avoid fertilizer application during rainstorm period; split and reduce

Use slow-release fertilizer and apply all fertilizers at the seedling stage Fujisawa et al. [30]; Liu et al. [31] Reduce fertilizer application rate Liu et al. [19]; Hua et al. [27]

Control irrigation and drainage water volume Zhang et al. [5]; Gao et al. (2017)

Apply fertilizer as side bars Yang and Yang [32]

Alternate drying and wetting Peng et al. [33]

Guo et al. [20]; Liu et al. [31]; Liu

et al. [19]

ing methodologies.

116 Irrigation in Agroecosystems

Nutrient management

Water management basal fertilizer dose

Control irrigation water quality

Table 5. Mitigation options to improve water quality in paddy systems.

Grand challenges exist in improving water quality in paddy systems. China is the largest rice-producing country in the world and also has the most intensive use of nutrients and water in rice production. Challenges to minimize the impacts of paddy nutrient losses on the water environment in China are even greater than anywhere else. Past research related to paddy systems has proved the importance of nutrient and water management on improving water quality. A combination of management in nutrient source, rate, timing and placement, and management in irrigation and drainage of water shows a great potential to reduce nitrogen and phosphorus losses from paddy fields. Even so, more research is needed to identify cost-effective monitoring approaches and mitigation options. Furthermore, extension and policy enforcement is needed beyond research to achieve water quality goals. Nonetheless, it should be noted that management of paddy water quality needs to be placed in a larger context of environmental protection. Our ongoing work has estimated that nutrient loss from paddy fields is smaller than that from intensively managed non-paddies such as vegetable fields. In some regions with high nutrient concentrations in surface water, paddy fields have even smaller nutrient losses in surface runoff than the nutrient inputs to the fields through irrigated and precipitated water.
