Nitrogen loss through lateral seepage from paddy fields: A case study in Sanjiang Plain, Northeast China

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1 WFL Publisher Science and Technology Meri-Rastilantie 3 B, FI Helsinki, Finland info@world-food.net Journal of Food, Agriculture & Environment Vol.11 (1): Nitrogen loss through lateral seepage from paddy fields: A case study in Sanjiang Plain, Northeast China Hui Zhu 1, Baixing Yan 1 * and Shahbaz Khan 2 1 Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun , China. 2 Water and Sustainable Development Section, Global Coordinator Ecohydrology Programme, UNESCO, Paris 75732, France. * yanbx@neigae.ac.cn Received 9 October 2012, accepted 28 January Abstract This paper provides details of a study to track the lateral seepage of nitrogen through the bunds of a paddy field in Sanjiang Plain, Northeast China. Field simulation experiments were conducted during the rice growing season in 2010 at the experimental plots of the Sanjiang Mire Wetland Experimental Station of the Chinese Academy of Sciences. Measurements revealed that the lateral seepage rates were significantly affected by the irrigation water amount. Once the lateral seepage began, its rate increased with the increasing water level in the paddy field. The concentrations of nitrogen in lateral seepage from the bund showed good responsiveness to the nitrogen application rates and the fertilization time in the paddy field. The nitrogen concentrations increased with increasing urea application rates, and these concentrations decreased sharply within the first few days (10~15 d) after fertilization, and then presented a stable trend, suggesting that the first few days (10~15 d) after fertilization were critical period for pollution control. The bund could intercept some of the nitrogen in seepage water, and it presented higher interception ability to total nitrogen (N t ) and ammonium nitrogen (NH 4+ -N) than that of nitrate nitrogen (NO 3- -N). For the three urea treatments, the seasonal N t output loads were estimated to be 7.82, and kg ha -1, respectively, taking about 8.69, and percent of the total nitrogen fertilizer applied. The lateral seepage contributed to more than 35 percent of the nitrogen loss load, and it is one of the most important pathways for nitrogen loss from paddy field. Key words: Lateral seepage, nitrogen, non-point source pollution, paddy field, Sanjiang Plain. Introduction The combined effects of point source (PS) and non-point source (NPS) pollutions of water environment make it difficult to improve water quality significantly 1. The effective abatement of PS pollution in recent years has highlighted the increasing relative contribution of NPS agricultural pollution to surface water in China 2, 3. On the other hand, the increasing population and the resulting food production pressure makes it necessary to apply large amount of fertilizer on the croplands, which may aggravate the NPS pollution and cause serious threat to water quality. Therefore, the control of NPS pollutants escape from croplands is of great significance for the protection of recipient environment. Rice (Oryza sativa L.) is the major staple crop in Asia and is predominantly cultivated with irrigation water, as rice is an intensive water use crop 4, 5. Detailed hydrological studies and water losses from rice fields 6, 7 show the role of the underlying soil and groundwater conditions on the net vertical and lateral seepage to the underlying groundwater systems. Some hydrology studies have shown that bunds (also referred to as dikes, banks or levees) surrounding paddy rice fields provide one of the major pathways for water loss because of the saturation of the bunds due to long-term flooding of paddy field 4, 8. This makes it possible for the pollutants to accompany this seepage water as water is a key vehicle of pollutants transportation. However, the previous studies focused mainly on vertical leaching, runoff and drainage of nitrogen, phosphorus and pesticides from croplands Lateral seepage did not gain much attention from environmental researchers until recent years due to its unnoticeable and slow occurrence 2, 13, 14. However, the neglect of lateral seepage pathways may cause the imprecise estimation of pollution load, which leads to the underestimation of threat to the receiving water quality and ecological condition of the surrounding areas. In China, previous investigations concerning nitrogen loss via lateral seepage from paddy field had been performed in Taihu Lake Region 2, 13, but very little information is available for the North China. As is known, the mode of cultivation, the climate and hydrology conditions as well as the soil types varied largely between South and North China due to different local agroclimate conditions. The different agro-climate conditions can lead to different pollution mechanisms requiring customised management. In this study, we investigated the mechanism and the influencing factors of nitrogen loss through lateral seepage from paddy fields in North China, and estimate the total nitrogen (N t ), the ammonium nitrogen (NH 4+ -N) and nitrate nitrogen (NO 3- -N) pollution load. This work is important for controlling NPS agricultural pollution in this area and is significant for providing guidance for other locations under the similar agro-climatic conditions. 841

2 Materials and Methods Experimental site: The experiments were conducted at the experimental plot of Sanjiang Mire Wetland Experimental Station of the Chinese Academy of Sciences (CAS), which is located in Sanjiang Plain. The Sanjiang Plain (43 49'-48 27' N, ' ' E) is a low-lying alluvial floodplain formed by Amur River, Songhua River and Ussuri River. Now it is one of Chinas major commodity grain bases 15. The mean annual precipitation of Sanjiang Plain is around 600 mm, and the mean annual temperature is 1.91 C. Its main soil types are albic soil (light coloured subsurface horizon), meadow soil and bog soil 16. The experimental paddy plot used in the current study was cultivated from marshy meadow in Soil type of the experimental plot is albic soil, which takes about 23.67% of the paddy field area in Sanjiang Plain. The physical and chemical properties of experimental plots soils are summarized in Table 1. Cold weather in Sanjiang Plain limits rice production to one crop per year. Table 1. Physical and chemical properties of the experimental rice paddy soil. Chemical properties Particle Size Fraction (%) Total-Carbon (%) 2.58 Sand 6.25 ph 1:2.5 KCl 6.85 Silt Total-N (mg kg -1 ) Clay Total-P (mg kg -1 ) CEC ( cmol(+) kg -1 ) Field experiment: A near trench paddy field was chosen from the experimental plots. Twelve 1 m 1 m plots were separated with bunds in field, and all plots bordered a 1.5 m-deep trench on one side. The other three sides of each plot were lined with impermeable plastic to a depth of 800 mm, which can ensure that the lateral seepage was allowed only through the near-trench side of each plot. The height of water level in the trench was mm lower than that in the field throughout the growing season. Paddy fields were flooded and maintained 40~100 mm floodwater, the common agronomic practice in Sanjiang Plain. Water level was measured by using a wood stick with scales, which was buried in each plot. The plots were irrigated in the evening if the water level was lower than 40 mm and the quantity of irrigating water was recorded. Irrigating water was got from a pond near the paddy field, with the N t concentrations in the range of 0.48 mg kg-1 to 2.05 mg kg -1, the NH 4+ -N concentrations in the range of 0.15 mg kg -1 to 1.67 mg kg -1, and 0.29 mg kg -1 to 1.87 mg kg -1, respectively. An in situ collector was specifically invented for collecting lateral seepage water (Fig.1). The container was made of rotresistant plastic, and was primarily composed of four parts: the insert board (a), the suck-surface (b), the transport tube (c), and the container (d). Each plot has a separate collector. The collector was installed outside of the bund. Width of the bund was 600 mm. The insert board was inserted into the bund soil to ensure that the contact of bund soil and the suck-surface was tight, in this way, all the seepage water can be collected. After installing the collector, it was buried in the soil to fix its position. The suck-surface was drilled, and the diameter of the holes was about 5 mm. Then a fiber cloth was fixed on the suck-surface to avoid the inburst of soil. Seepage water from the bund soil passed through the suck-surface and then flowed into the container via Figure 1. Sketch of the seepage water collector (letter a refer to the insert board, letter b refer to the suck-surface, letter c refer to the transport tube, letter d refer to the container ). the transport tube. Water in the container was eliminated every evening to make sure that the water in container was new seepage water. The operation of emptying water and water sampling was conducted by using a siphon. Quantity of the seepage water was calculated by the change of water level in the container. Urea is the most commonly used N fertilizer in paddy fields of Sanjiang Plain. Thus, four urea concentrations (0, 90, 180 and 270 kg N ha -1 ) with three replicates were applied in the experimental plots. Sixty percent of the urea fertilizer was applied on 3 June as the basic fertilizer, and forty percent was applied on 29 June as top-dressing. All plots received 40 kg P 2 O 5 ha -1 and 150 kg KCl ha -1 on 3 June. Rice seedlings (28 d old) were transplanted at 150 mm 150 mm spacing on 1 July and were harvested on 25 September in On 25 August, all of the floodwater in paddy field was discharged into the trench artificially (also refer to artificial drainage), and the paddy field surface was kept dry till the rice was harvested. The data of rainfall and evaporation of the experimental field were supplied by the Sanjiang Mire Wetland Experimental Station. Sample collecting and analysis: The paddy field floodwater and the lateral seepage water for each plot were collected daily within five days after the application. Thereafter, samples were collected at two to seven days intervals. Surface runoff water sample from each plot was collected only when the runoff happened under intense rainfall. The runoff water was collected and calculated following the method presented by Liang et al. 2, 13. The drainage water sample was collected on 25 August when the floodwater in paddy field was discharged. Quantity of drainage water was calculated by measuring the original depth of paddy field surface water before discharging the water. Water samples were filtered with diameter >11µm to remove particles, the filtrates were preserved at 4 C for later analysis. Concentrations of N t, NH 4+ -N were determined by using the automatic chemical analyzer (Mode Smartchem 200, Italy). Data analysis: The lateral seepage rate of water (mm d -1 ) was calculated by dividing the volume of lateral seepage water removed from the container at each sampling time by the plot area and dividing this value by the time interval (d). Daily surface runoff (mm d -1 ) was similarly calculated from the volume of runoff 842

3 water divided by the area of the plot and the time interval. Seasonal nitrogen loss load via lateral seepage and surface runoff were calculated by: d i1 C V i Load( N) (1) A plot where C is the nitrogen concentrations in lateral seepage or surface runoff, V is the volume of lateral seepage through lateral seepage or surface runoff, A is area of the field plot, and I plot means the i-day after the rice paddy field irrigating (i= 1~ 92 in this experiment). The interception ability of bund to nitrogen was represented by the concept of interception ratio calculated by the following formula: C paddywater Cseepagewater R 100% (2) C paddywater where R = the interception ratio of bund to nitrogen (%), C paddywater = nitrogen concentrations in the surface paddy water (mg L -1 ), and C seepagewater = nitrogen concentrations in lateral seepage water (mg L -1 ). Statistical analysis of the data was accomplished using the SPSS software package (SPSS 16.0). The correlation between paddy field surface water and lateral seepage rate was analyzed according to the Pearson correlation coefficients method preceded by the Kolmogorov-Smirnov (K-S) examination of normal distribution. Results and Discussion The relation between paddy field surface water and lateral seepage rate: The relation between paddy field surface water and the lateral seepage can be described at two stages. The first stage was the period in the short-term after the first irrigation. In this stage, no lateral seepage water can be observed through the bunds, and the seepage face flow did not develop until two days after the first irrigation. After flowing into the paddy field, water moved along both horizontal and vertical directions firstly, and then diffused into the soil layers which turned from unsaturated to saturated status as the water infiltrated through the soil profile. Paddy fields are typically flooded before rice seedlings are transplanted, and seasonal cycles of puddling (wet tillage) and drying, over the long term, a hard pan of 50 to 100 mm thickness often emerges at the interface between the topsoil and subsoil 17. This hard pan has been reported as the least permeable soil layer in the rice paddy, which can limit the infiltration of ponding water into the subsoil 18, 19, promoting the infiltration of water in horizontal direction. Furthermore, slow rate of travel of water (<0.2 mm d -1 ) was found in subsoil of the experimental paddy field due to the existence of clay 20, which also exaggerated the water removal through bunds. When the diffused water travelling through the bunds reached a certain level, and all the bund soil layers turned saturated, the lateral seepage of water began. Thereafter, it advanced to the second stage, in which the paddy field surface water level affected the lateral seepage rate in a distinct manner. As shown in Fig.2, there are significant positive correlation between the paddy field surface water level and the lateral seepage rate (R = 0.75, P<0.01). With the increasing of water level, the lateral seepage rate increased. The correlation can be attributed to the Darcy s Linear Flow mechanism through the bund. Higher surface water level means greater head difference, and more ability of water removal. The results indicated that the irrigating water level in the paddy field affected the NPS pollution of paddy field to some extent, and less irrigating water level would cause less NPS pollution. Lateral seepage rate (mm d -1 ) Floodwater level (mm) Figure 2. Correlation of the floodwater level and the lateral seepage rate. Nitrogen concentrations in seepage water: According to the observed data, concentrations of both N t and the two inorganic forms (NH 4+ -N) in seepage water presented a good responsiveness to the N application rates. As shown in Fig.3, with the increase of N application rate, the concentrations of N t, NH 4+ -N in lateral seepage increased, demonstrating that the N application rate affect the N loss via lateral seepage significantly, and the control of N fertilizer amount is important for reducing the lateral seepage of nitrogen from paddy field. During the experiment, the peak values of N t - N in lateral seepage water appeared about one day after each N fertilizer application, and then the concentrations decreased sharply in the first few days after each peak value appearing. The duration of this period varied with the N application rate, and in general, it lasted about 10~15 d. Thereafter, the concentrations presented a stable value. According to these results, the application of N fertilizer was suggested to be carried out at least 10 days before intense rainfall and/or drainage activity in order to reduce N concentrations in lateral seepage and trench water. For the three treatments with N fertilizer application, the mean percentages of NH 4+ -N in lateral seepage water concentrations were % and 16.80%, respectively, and of equal significance, the two forms of inorganic nitrogen (NH 4+ -N) in lateral seepage water appeared rapidly after each N fertilizer application, which can be explained by the rapid hydrolysis of urea in the floodwater 21. Interception ability of bund to nitrogen: In most of the monitoring period, especially in the earlier period of each fertilization cycle, the bund showed stronger interception ability -N (with a biggest ratio of more than 95%) than that to NO 3- -N (with a biggest ratio of less than 75%). Different 843

4 N t concentrations (mg L -1 ) NH 4 + -N concentrations (mg L -1 ) NH 3 --N concentrations (mg L -1 ) (a) (b) (c) Figure 3. Change of nitrogen concentrations in lateral seepage water after fertilizing(vertical bars above each data point indicate SE. Arrows represent N fertilizer application dates). interception mechanisms lead to the variation of interception ability. As is known, soil grains with negative electrical charge tend to attract the cations 22. As a result, when the nitrogen in the paddy field surface water transferred through the bund, the soil grain with large amount of negative charge in the bund can easily fix the ammonium because of the physical adsorption. While, in contrast to ammonium, the nitrate with negative charge are susceptible to transfer in soil. Therefore, the bund presented weaker interception ability to NO 3- -N. The above results suggested that the form of nitrogen in paddy field water affected the nitrogen pollution via lateral seepage as well. In addition, as it is shown in Fig.4, the interception ratios of bund -N presented the similar results during the whole experiment cycle. In the earlier period of each fertilization cycle, the bunds showed strong interception ability, and then, with the decrease of nitrogen concentrations in the Interception ratio of bund to NO 3 --N (%) Interception ratio of bund to NH 4 + -N (%) Interception ratio of bund to Nt (%) (a) (b) (c) Figure 4. The interception ratio of bund to nitrogen represented by vertical drop line (Vertical bars above each data point indicate SE. Arrows represent N fertilizer application dates). surface paddy water, the interception ratios decreased, particularly in the later period of each fertilization cycle. The Fig. 4 shows that the downtrend continued in the later period, and the interception ratios even became negative. According to this result, we inferred that some of the NH 4+ -N fixed by the soil grains previously in the earlier period might be released and be washed out by the low-nitrogen containing paddy water in the later period, leading to the higher nitrogen concentration in seepage water than that in paddy water. This also shows that the nitrogen pollution via lateral seepage should not be neglected even after 10~15 d after fertilization. The nitrogen concentration in lateral seepage water might decrease, while it was still higher than that we can anticipate due to washing out of the fixed nitrogen in bund soil. Of course, some of the NH 4+ -N fixed in the bund soils can transform into NO 3- -N by nitrification, explaining the similar trend of NO 3- -N -N. Hence, it can be concluded that the fixation and release process, the form transformation, the transferring and washing out process as well as all the other biogeochemical processes continued and 844

5 Table 2. The seasonal nitrogen output load under each urea application rates (kg ha -1 ). Total output load Pollutants (runoff + drainage + lateral seepage) Lateral seepage load 90 kg N ha kg N ha kg N ha kg N ha kg N ha kg N ha -1 N t NH + 4 -N NO - 3 -N made up the complex environmental activity of nitrogen in the bund soil and its solution. Nitrogen loss load through lateral seepage: The seasonal NPS pollution load (sum of the surface runoff, drainage and lateral seepage) of nitrogen from paddy field and the nitrogen loss load via lateral seepage were calculated in Table 2. The seasonal output loads of N t -N from paddy field were in the range of 7.82~33.35 kg ha -1, 2.52~7.83 kg ha -1 and 1.89~4.25 kg ha -1, respectively. For the three experimental treatments, the N t output load accounted for about 8.69, and 12.35% of the N fertilizer applied, respectively, indicating a big problem of N loss in the rice cultivation process of Sanjiang Plain. The N t output load through lateral seepage was in the range of 2.92~12.74 kg ha -1 for the three treatments, took more than 35 percents of the seasonal NPS pollution load. On the one hand, this result proved that the old method for estimating the nitrogen output load in paddy field were not precise due to the lack of consideration of the lateral seepage pathway. On the other hand, the farmers, managers and researchers should raise their awareness of nitrogen loss through lateral seepage. Conclusions The principal conclusions of this research are summarized as follows: (1) Higher ponded water levels in paddy field might lead to more water seepage and greater loss of nitrogen through bunds. (2) Nitrogen concentrations in seepage water presented a good responsiveness to both the nitrogen application rates and the fertilization time, and the first few days (10~15 d) after fertilization are critical period for pollution control. (3)The bunds of paddy field played an important role in controlling of nitrogen loss through lateral seepage, and it showed a stronger interception ability of bund -N than that of NO 3- -N. (4) The nitrogen loss from paddy field in Sanjiang Plain is a serious issue worth highlighting, and as one of the most important pathways for nitrogen loss from the paddy field, lateral seepage contributes to more than 35 percents of the seasonal nitrogen output load. Acknowledgements This research was under the auspices of the National Natural Science Foundation of China (No ), the Open Research Fund of Key Laboratory of Mollisols Agroecology, Chinese Academy of Sciences (No.2012ZKHT-01), and the National Key Basic Research Program of China (No. 2013CB430401). References 1 Yang, Y. H., Yan, B. X. and Shen, W. B Assessment of point and nonpoint sources pollution in Songhua River Basin, Northeast China by using revised water quality model. Chin. Geogr. Sci. 20(1-4): Liang, X. Q., Chen, Y. 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