| The return flow widely existing in many irrigation regions is becoming a major pollutant deteriorating the quality of the Yellow River; agricultural return flow plays a contributory role. To ensure the water quality of the Yellow River as well as the social and economic sustainability of the Yellow River region, controlling agricultural return flow is urgent and strategically meaningful. Focusing on the effect on water quality of such typical pollutants like nitrogen (N) and phosphorusus (P) from the irrigated areas in the upper reaches of the Yellow River, a study was carried out at WuZhong National Science and Technology Park in Ningxia irrigation region, with rice being selected as a typical crop. Through the determination of the amounts of evapotranspiration and vertical leachate using lysimeter, the collection of lateral return flow and soil solution of 0-200 cm soil profile and lateral leachate of 0-80 cm profile, and the calculation of N and P loss during the process of agricultural return flow, this study has investigated the patterns of agricultural return flow during rice growing season as well as the pollution characteristics of nitrogen and phosphorusus during the process of drainage, and revealed the movement and polluting characteristics of N and P under the effect of different irrigation and fertilizer regimes. At last, the research calculated the load of N and P in the agricultural return flow. The major findings are as follows:(1) The composition of agricultural return flow and its factors were analyzed. Results indicated that the quantities of agricultural return flow are mainly affected by the amount of irrigation and groundwater level, with the corresponding correlation coefficient being 0.88 and -0.61, respectively. The impact of the amounts of precipitation and evapotranspiration was relatively little. Reducing irrigation during rice growth period obviously decreased agricultural return flow quantity. The volume of return flow was significantly different among the three irrigation levels, accounting for38%, 35% and 30% of the irrigation amount, respectively. The volume of return flow of traditional irrigation treatment was 1.46 times and 2.19 times of the W1 and W2. Return flow mainly occurred before tassel during the whole growth period of rice accounting for 80% of the total return flow volume. This is the appropriate time to modulate the agricultural return flow. Based on the rather obvious relation between the return flow volume and its factors, a multivariate regression model was built.(2) The main factors that impact the loss of N and P in surface return flow was examined; the concentrations of N and P as well as the return flow volume were found to be two major factors. The dynamic curve showed the concentration of N and P in surface water were affected by the quantities of irrigation and fertilizer applied. Under the same N treatment, the concentrations of N and P were much lower at a high irrigation level. Under the same irrigation level, the concentrations of N and P were much higher at a high application rate of fertilizer. The concentrations of N and P in surface water decreased with time. When fertilizer was further applied, a peak was observed, but the occurrence time was different. The amount of total N (TN) and total P(TP) peaked on the first day after fertilizer application, but 1-2 days and 3-4 days for ammonium (NH4+-N) and nitrate (NO3--N), respectively. Under the impact of fertilization amount and crop growth conditions, the concentrations of N and P were highest after the application of basal fertilizer. The characteristics of such dynamics reflected that the occurrence time of surface return flow was directly related to the amount of N and P carried in the flow. The nearer the surface flow was to the time of fertilizer application, the greater the loss of N and P.(3) The vertical return flow and the concentrations of N and P in the vertical leachate were closely related. The critical period to reduce agricultural return flow pollution is before the heading stage of rice. The regular analysis of the soil solution in 0-200 cm showed that the concentrations of N and P decreased with time and peaked after further application of fertilizer. The critical occurrence time of vertical flow pollution is within 80 days after sowing, i.e. before heading. At this period, the concentrations of N and P were the highest within the growing period as a result of the high fertilizer application rate and continual flooding condition. Under traditional irrigation level, the losses of TN, NH4+-N, NO3--N and TP in return flow accounted for 81.4%, 73.6%, 87.2% and 70% of the total amount of loss, respectively, and the corresponding percentages were basically the same under other irrigation treatments. The concentrations of TN,NH4+-N,NO3--N and TP decreased with depth, but there was a turning point at the interface at 80 cm, after which their concentrations increased. The concentrations of TN, NH4+-N, NO3--N and TP did not alter much below 120 cm. The concentrations of N and P in vertical leachate were affected by the amount of irrigation. The correlation coefficient between irrigation volume and the mean concentration of TN, NH4+-N, NO3--N and TP was 0.76, 0.48, 0.74, and 0.37, respectively. The further the travel distance of NH4+-N and TP driven by agricultural return flow,. the higher the risk of pollution.(4) The loss of N and P were affected by the amount of irrigation and fertilizer applied. Under the traditional N application rate, reducing irrigation during paddy growth period obviously reduced the loss of N and P. The amount of loss of TN, NH4+-N, NO3--N and TP under W3 is respectively 1.29, 1.20, 1.35 and 0.79 times, and 1.70, 1.59,1.73 and 1.14 times higher than under W1 and W2. Decreasing the amount of N-fertilizer applied can substantially reduce the loss of N and P. While the loss of TN in N3 was 14.11, 5.42 and 2.35 times higher than N0, N1 and N2, respectively, that of NH4+-N was 14.1, 5.27 and 2.31 times, NO3--N was 16.34, 5.74 and 2.45, and TP was 0.82, 0.50 and 0.65 times, respectively. The greatest loss of N and P among all treatments was observed under W3N3, followed by W1N3 and W2N3. This suggests that the pollution load of N and P was impacted by both the amount of irrigation and fertilizer applied with a significant interaction. Nonetheless, the amount of fertilizer played a more important role.(5) The lateral return flow occurred intermittently and first appeared in the 40 cm and 80 cm of soil profile, with the loss of NO3--N being the most serious. Lateral return flow is impacted by the quantities of irrigation and rainfall. The concentrations of TN, NH4+-N, NO3--N and TP in lateral return flow followed the same trend with time, and were the highest in soil solution 20 days after sowing. The concentrations declined subsequently and increased slightly again when fertilizer was further applied. The concentrations of TN, NH4+-N, NO3--N and TP decreased with depth. The concentrations TN, NH4+-N, NO3--N and TP during the growth period of paddy averaged 0.84-17.99, 0.03-1.28, 0.3-13.21 and 0.001-0.14mg/L, respectively. The concentrations of TN, NH4+-N and NO3--N covered a wider range than TP. The amounts of N and P in lateral return flow were impacted by the irrigation volume and fertilizer application rate. Under the traditional irrigation level in N3, the loss amount of TN, NH4+-N, NO3--N and TP was higher than in N2, N1 and N0 by 1.08, 1.34 and2 times,1.17, 1.46 and 2.11 times, and 1.21, 1.40 and2.43 times.(6) The load of N and P was estimated in different water and fertilizer treatments. Results indicated that the effect of irrigation and N application levels on the amount of TN and NO3--N in agricultural return flow was remarkable. There was a significant interaction between irrigation norm and N application level. Nitrate N, which accounted for about 70% of the TN, was mainly drained in the agricultural return flow. Among all treatments, the load of N and P per unit area was the greatest under traditional irrigation and N application. The load of TN per unit area was 12.84kg/hm2 in surface return water, while that of TN, NH4+-N, NO3--N and TP per unit area was respectively 7.59, 0.57 and 0.17 kg/hm2. The load of TN, NH4+-N, NO3--N and TP in vertical return water and lateral return water was respectively 84.66, 13.54, 61.80 and 0.73 kg/hm2,and 17.95, 2.55, 12.65 and 0.047 kg/hm2 .The correlation coefficient between irrigation and the load of TN in agriculture return water and that with TP was 0.91 and 0.88, respectively.(7) The load of TN in rice return water from rice paddy reached 8100 tons in 2008 in Ningxia irrigation region, while that of NH4+-N, NO3--N and TP was1600, 5200 and 66 tones, respectively.
|
PDF Full Text Request