5. Monitoring of water quality in paddy systems

degleyed paddy soil that ranged from 0.41 kg/ha in the control group to 0.70–1.49 kg/ha in the treatments with 50–230 kg phosphorus per hectare. Although the differences in magnitude of phosphorus losses in the two soils could result from different soil characteristics and rainfall patterns, both revealed increased phosphorus losses with greater phosphorus fertilizer application rates (Table 3). Furthermore, Liu et al. [19] found that the time interval between fertilizer application and the subsequent first large runoff event played a critical role in determining the annual phosphorus losses. Phosphorus losses greatly increased with decreasing time interval. The finding was supported by Guo et al. [20] who claimed that about 40% of total phosphorus loss from a rice-wheat rotation occurred within 10 days after

In addition to "incidental" nutrient losses, overuse of fertilizers or manure also constitutes long-term risks of nutrient losses. A number of studies have demonstrated evidential buildup of soil phosphorus status and elevated degree of phosphorus saturation due to long-term phosphorus applications at rates exceeding crop needs [21–23]. In turn, phosphorus losses in surface runoff and leaching have been found to increase with elevated soil phosphorus status or degree of phosphorus saturation [22, 24]. This has been widely referred as legacy phosphorus issues [25]. Even though most of the research on this topic has been conducted on dryland soils, a few studies have reported that long-term excessive application of nutrients could enhance environmental pollution risk in paddy fields [4, 26]. In double cropping systems where flooded rice is planted in rotation with drained dryland crop, we can expect nutrient surplus from both rice and dryland crop-growing seasons [27]. It should be noted that China has the most intensive nutrient use in paddy systems among the world's top 10 rice-producing countries (Table 4). Therefore, there is a special need of better water and nutrient management

Country Average yield (Mg/ha) Nitrogen (kg/ha) Phosphorus (kg/ha) Potassium (kg/ha)

Table 4. Rice yield and nutrient applications to rice in world's major rice-producing countries (data adapted from [28]).

China 6.2# 145 26.2 33.2 India 2.7 68 10.5 7.5 Indonesia 4.1 105 9.6 11.6 Bangladesh 3.2 72 6.5 8.3 Vietnam 4.1 115 19.6 34.9 Thailand 2.4 62 14.4 14.1 Myanmar 3.2 35 5.2 3.3 Philippines 3.0 51 6.5 9.1 Brazil 3.0 40 21.8 24.9 Japan 5.8 78 40.2 59.7

fertilizer application to paddies.

114 Irrigation in Agroecosystems

to minimize nutrient losses from paddy systems.

#: Estimation of rice yield in China is made by the authors of this chapter.

In China, the approach for monitoring water quality in paddy systems has become standardized over the past 2 decades [12, 19, 29]. In the field, research plots are separated with plastic films down to 0.9 m in the soil profile and with soil berms up to 0.2 m on the soil surface to prevent flow of surface and shallow subsurface water between the plots (Figure 6). Soil berm is a common practice to maintain field ponding water for rice production. During the ricegrowing season, irrigation water is applied to individual plots through polyvinyl chloride pipe inlets when needed. During the non-rice crop-growing season in a double-cropping system, irrigation is usually not applied. Excessive ponding water is drained through shallow open ditches. Outside each plot on the opposite side of the irrigation water inlet, a cement pond is constructed to collect runoff water from every plot. Two water outlets of polyvinyl chloride pipe are installed on the wall of the cement pond, at depths of approximately 10 cm above and 10 cm below the soil surface, for collecting runoff water during the rice-and wheat-growing seasons, respectively. The runoff water collected in the pond is measured for volume and sampled for analyses of nutrients and sediments. Usually, one sample is taken after every regular runoff event and multiple samples during a large runoff event (i.e., rain storms).

Even though the approach described earlier is widely used such as in a national program to estimate nutrient losses from paddy systems across China, there has been an increasing interest in seeking alternative, simplified monitoring approaches. One potential approach is to monitor nutrient concentrations in the field ponding water. Liu et al. [19] found that concentrations of both total phosphorus and dissolved reactive phosphorus in surface runoff were significantly correlated with their concentrations in the field ponding water (r <sup>2</sup> = 0.83– 0.88, p < 0.0001). In a follow-up study, Hua et al. [27] monitored different forms of phosphorus concentrations in field ponding water of five paddy soils over 2 years. They found that 2 weeks after fertilizer application is a critical period for phosphorus loss from paddies, which supported findings of others [19, 20]. Despite the large potential of monitoring field ponding water to save a lot of work associated with constructing runoff collection facilities, it should be

Figure 6. Monitoring of water quality in paddy systems: Research plots and field ponding water on the left and runoff collection facility on the right.

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 monitoring methodologies.

might be to split fertilizer to multiple doses and reduce the dose of basal fertilizers [31]. Furthermore, Yang and Yang [32] suggested applying fertilizers as side bars close to paddy roots, which could significantly increase nutrient use efficiency and reduce losses as compared

Water Quality in Irrigated Paddy Systems http://dx.doi.org/10.5772/intechopen.77339 117

Water management is also of great importance to reduce nutrient losses from paddy fields to surrounding water bodies. In a field study in China's Taihu Lake Region, Peng et al. [33] found that adopting an alternate drying and wetting technology reduced total phosphorus losses by up to 52% in surface runoff and 55% in subsurface drainage across an array of nutrient management practices, as compared to the conventional irrigation and drainage management. Zhang et al. [5] proposed a "zero-drainage water management" approach, which used natural field drying to replace conventional surface drainage based on the physiological water need for rice growth. They found that a combination of improved irrigation and field drying based on rainfall forecasting eliminated all drainage and phosphorus export from paddy fields (0.65 kg/ha under conventional management), while successfully meeting the physiological water requirement of plant growth. Elsewhere, Gao et al. [34] also found that appropriate control of irrigation and drainage could significantly reduce nitrogen and phosphorus concentrations in the field ponding water. Furthermore, when irrigation water is rich in nutrients such as in the scenario of wastewater irrigation [15], control of irrigation water quality is necessary to reduce paddy nutrient release to the water environments. Potentially, nutrient management practices and water management practices should be combined to achieve most desirable water quality outcomes and a sustainable

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

with broadcasting the fertilizers.

agroecosystem.

7. Conclusions

water.
