Gaseous Loss And Balance Of Nitrogen From Paddy Field In Irrigation Area Of The Upper Yellow River

Input of nitrogen is essential for high crop yields, but losses of these same nutrients diminish environmental quality. Over-fertilization application in the irrigation area of the Yellow River where agricultural production is more developed are all faced with the serious situation of N pollution due to excessive fertilizer usage. Gaseous loss of N is a main way of nitrogen loss in the paddy fields of the irrigation area. Excessive application of nitrogen fertilizer increased the loss of gaseous nitrogen, as result of the increase of atmospheric nitrogen deposition into surface water, which has been an important source of nitrogen pollution in the irrigation area. All of these draw the high attention to the greenhouse effects by gaseous nitrogen emissions. Considering the high food production and the minimum environmental threat, we need to find an effective way to control the loss of gaseous nitrogen in the paddy fields of the irrigation area.The randomized split-plot experiments and the (15)^N tracing micro-plot experiment at the Lingwu farm of Ningxia irrigating area from 2009 to 2010 were conducted to get the patterns of the ammonia volatilization (AV) and N₂O emissions from the paddy field . The amount of the ammonia volatilization (AV) and N₂O emissions during the rice-growing season were also calculated. The fate of fertilizer derived nitrogen in the paddy field were estimated by the technique of stable isotope (15)^N-traced nitrogen fertilizer. With the mass balance method, we also estimated the nitrogen balance and risk of loss of N in the paddy field ecosystem.The five N treatments of field experiment were conducted, including two treatments of the conventional N application amount(300 kg/hm2), i.e the application of only urea (N300) and the application of the organic fertilizer (manure) with the urea nitrogen (N300-OM), two treatments of the optimized N application amount(240 kg/hm2), i.e, the application of only urea (N240) and the application of the organic fertilizer (manure) with the urea nitrogen (N240-1/2OM), and no nitrogen fertilizer application plot (CK). Three treatments were conducted for the (15)^N tracing method with the micro-plot experiment, i.e, the application of only urea N300, N240, and CK.A batch-type airflow enclosure method was used to measure the AV, and analyzed with the boric acid absorption and the standard acid titration method. The static chamber-gas chromatograph method was used to measure the N₂O emission from the paddy field. Nitrogen from rainfall and irrigation water was measured in the experimental site as well. Based on both of the measuring data and the data from the experiments, we estimated the cumulative amount of AV and N₂O emissions in the rice growing season, and analyzed the nitrogen transport characteristics, nitrogen use efficacy and its loss from the rice field system.The main results are summed as follows:1. The AV is one of major way of nitrogen loss from the paddy fields of the irrigation area of the Yellow Rive. High nitrogen fertilizer can increase the N loss through AV. The application of organic fertilizer and reducing fertilizer use by optimized fertilization will reduce the loss of AV. The cumulative loss amount of AV reached 94.1 kgN/hm2 for conventional N application (N300). Compared with N300, cumulative amount of AV of N300-OM treatment decreased by 17.4 kg/hm2 during the rice growing season. Compared with N300 treatment, cumulative loss of AV with N240-1/2OM and N240 treatmentwas reduced by 22.7 kg/hm2 and 26.5 kg/hm2, respectively. Nitrogen loss percentage through AV loss was reduced by 3.9 % to 5.5%. Under the conventional irrigation practice and high nitrogen fertilizer of 300kg/hm2, annual N losses due to AV from the rice filed in the irrigation area reached 1693.8×104 kg N/a, accounted for 22.2% of the fertilizer N applied.2. N₂O emissions from the paddy field is not an important way of nitrogen loss at the Yellow River irrigation area, but the warming potential through N₂O emissions in the irrigation area are not neglectable. Excessive application of N fertilizer in the paddy field will significantly increase N₂O emissions, and at the same level of nitrogen application, organic fertilizer amendment could increase N₂O emissions from the paddy soil (P < 0.01). Optimization of nitrogen application in the irrigated paddy field during the rice growing season could reduce N₂O emissions. At the rice growing stage, N₂O emissions mainly occurred before the tiller stage or at the pre- and late- rice growth stages and more N₂O emissions were measured after rice planting and irrigation. At different nitrogen levels, the total amount of N₂O emissions during the whole rice growing season varied among 2.64 3.87kg/hm2, and the loss of fertilizer nitrogen by N₂O emissions was only 0.43% 0.64%. With the conventional irrigation practice and high nitrogen fertilizer of 300kg/hm2, N₂O emissions from paddy fields in N300-OM treatment reached 3.87 kg/hm2. In the scales of 100a, the global warming potential (GWPs) was 20.76×107 kg CO₂/hm2. Compared with N300-OM treatment, cumulative N₂O emissions in N240 and N240-1/2OM treatments decreased by 1.18 kg/hm2 and 0.57 kg/hm2, respectively. In the scales o100a, GWPs caused by N₂O emissions in the paddy field decreased 3.30×107 kg CO₂/hm2 and 3.06×107 kg CO₂/hm2, respectively.3. Ammonia volatilization during the rice growing season mainly occurred 2⁵d after the tillering and jointing fertilier applied. Nitrogen loss through AV in N300 treatment at tillering and jointing stages accounted for 30.4% and 29.9% of N application, respectively; Nitrogen loss through AV in N300-OM treatment at tillering and jointing stages accounted for 16.8%¹8.5% of N application. Compared with N300 treatment, the percentage of fertilizer N losses decreased by 13.6% and 11.4%. Ammonia volatilization loss from basal fertilizer was relatively large, the percentage lost of different treatments were 11.2% to 14.5%, AV rate reached its maximum at 2³d after fertilization or irrigation at different stages, at a rate of 5.8⁹.14 kg N / (hm2 ? d), which was affected mainly by N application amount, and temperature, sunshine time, rainfall and pH value also have significant influence on it.4 The results with the (15)^N tracing technique showed that the high N fertilizer application increased the N uptake by rice from fertilizer, and the amount of N rice absorbed from soil reduced correspondingly, which resulted in the higher N surplus in soil. Under the conventional irrigation and fertilizer management level, the (15)^N-labbled fertilizer recovery in rice plant (ηpf) was 26.05%-30.43%. In the paddy soil profiles of 0-90cm, the residual of (15)^N-labeled fertilizer in soil (Nsf) were 53.70-69.93 kg/hm2, and N residual rate in soils were 17.9%²3.31%. Coupled with the method of N balance, the apparent loss of N from the paddy field was 47%-52%, of which N loss as gaseous from the paddy fields was 22.6%. Optimization of nitrogen fertilizer can significantly reduce the amount of N surplus and N loss from the paddy field. Compared with N300, optimized N fertilizer application could decrease the loss of fertilizer N by 26.74-32.83 kg/hm2, and reduce the amount of N surplus by 22- 31 kg/hm2.5. The distribution of (15)^N abundance variability in different soil profile indicated that in the N300 treatment, 55% of the nitrogen left in the 0-90cm soil enriched in the 0-30cm topsoil. As a result of continuous yearly rice planting, fertilizer N leached into deep soil layers along with irrigation water. The (15)^N-labelled fertilizer was detected in the 60-90cm soil layer, indicating that nitrogen fertilizer has leached below 90cm. The nitrogen fertilizer may have entered the shallow groundwater during the growing season. The analysis of changing characteristics of inorganic N accumulation showed high-level nitrogen fertilizer application significantly increased the concentration of ammonium N and nitrate content in top 0-20cm soil. Optimization can reduce the N accumulation in soil layer. Application of organic manure significantly increased nitrate accumulation in 0-100cm soil, and increased the leaching risk of nitrogen loss, as well as N₂O emissions. The N application reduction, optimization of organic manure treatment and other measures can effectively reduce nitrogen loss through AV and N₂O emissions in the paddy field at the irrigation area of Yellow River, and significantly increase nitrogen recycling rate in the paddy field.