| Nitrogen is one of the major elements that needed for plant growth, and also one of thekey factors that determine the crop yield. N fertilizer application is of great significance inpromoting crop production and protecting China’s food security. However, excessive Nfertilization is not only adverse to the improvement of crop yield, but also leading to a seriousN loss, lower N fertilizer utilization efficiency and serious N pollution problems. GuanzhongPlain of Shaanxi Province is the major grain production bases in China and is particularlyimportant in Shaanxi Province. Excessive application of N fertilizer and burning of crop strawis widespread and local farmers are in want of scientific and rational fertilization guidance.Furthermore, due to the lack of rainfall and irrigation, previous studies have not paidsufficient attention to N leaching of farmland in this region. In this view, the field trials wasconducted in five consecutive years from2008to2012to ascertain the effect of long-term Nfertilizer application and straw mulching on crop yields, nitrate leaching and soil N budget in0—100cm soil layer. Also, a-three consecutive quarters of field trials was carried out from2011to2012to explore the optimum N fertilizer rate that will maximize crop yields andprotect the environment. Main conclusions are as follows:1. The effect of long-term N application and straw mulching on crop yields and soilpropertiesIn a5-year field experiment, effects of the long-term N application and straw mulchingon crop yields and soil properties are studied under wheat-maize rotation system on theGuanzhong Plain. The study included six treatments with three replications: No fertilizer (0kg N hm-2a-1)，reduced N application (165kg N hm-2a-1), optimum N application (330kg Nhm-2a-1), farmer customized N application (471kg N hm-2a-1), increased N application (495kg N hm-2a-1) and optimum N application+straw mulching (330kg N hm-2a-1+S). The resultsshowed that N application significantly increase the crop yields of winter wheat and summermaize, and the trends are that crop yileds increased at first and then fluctuated with Napplication rate increased. Crop yields did not decrease in the5-year successive harvests,when N application rates were reduced by41kg hm-2for winter wheat and100kg hm-2forsummer maize. Straw mulching significantly increased the yields of wheat and maize after the second planting year, and the average crop increase was17.2%and5.3%, respectively.Long-term N fertilization have on significant effect on soil organic matter, total N content andavailable potassium content, while straw mulching increased the content of soil organic matter,total N and available potassium, and reduced soil pH significantly （P<0.05）.2. The effect of long-term N application and straw mulching on nitrate leachingA5-year stationary field experiment, using the lysimeter method, was conducted toinvestigate effects of application of N fertilizer and straw mulching on nitrate leaching in the90cm soil depth in a field under wheat and maize rotation on Guanzhong Plain of ShaanxiProvince. The study included five treatments with three replications: No fertilizer (0kg Nhm-2a-1)，reduced N application (165kg N hm-2a-1), optimum N application (330kg N hm-2a-1), increased N application (495kg N hm-2a-1) and optimum N application+straw mulching(330kg N hm-2a-1+S). The results showed that nitrate leaching varied sharply in a year andoccurred mainly in July, August and September, the rainy season of the year, and could beobserved after flood irrigation, too. The annual nitrate leaching amount and ratio of nitrateleaching to N fertilizer application in the whole monitored period was2.5（±2.0） kg hm-2y-1～16.7（±12.3） kg hm-2y-1and0.8%（±0.6%）～2.9%（±2.4%）, respectively. Regression analysis ofthe data exposed an positive exponential relationship of nitrate leaching loss with annual Napplication rate, and the regression equation is y=1.7801e0.0042x（R2=0.53）, which means thatthe higher N application rate, the higher the nitrate leaching loss, and the higher theproportions of the leaching loss to the total N applied. However, no significant effects wereobserved on nitrate leaching in the straw mulching treatment.3. The effect of long-term N application and straw mulching on soil N budgetIn a5-year field experiment, effects of the long-term N application and straw mulchingon soil N budget are studied under wheat-maize rotation system in the Guanzhong Plain. Thestudy included six treatments with three replications: No fertilizer (0kg N hm-2a-1)，reducedN application (165kg N hm-2a-1), optimum N application (330kg N hm-2a-1), farmercustomized N application (471kg N hm-2a-1), increased N application (495kg N hm-2a-1)and optimum N application+straw mulching (330kg N hm-2a-1+S). The results showed that Napplication significantly enhanced the mineral-N accumulation in the0—100cm soil layer,and date showed an positive exponential relationship of the mineral-N accumulation with theannual N fertilizer inputs （y=49.046e0.0031xR2=0.69）. However, Average of annual N uptakeby wheat and maize increased at first and then fluctuated as N application rateincreased（y=0.0009x2+0.8333x+155.87R2=0.99）. The results of N budget showed thatapparent N use efficiency significantly decreased as N application rate increased, while theresidual Nmin, apparent N loss and apparent N loss ratio increased significantly. The straw mulching increased ccumulative N uptake by crops and the NUEs, decreased both theapparent N loss and apparent N loss ratio, and showed no apparent effect on mineral-Naccumulation and the ratio of mineral-N accumulated to N applied.4. Optimum-N application rate to maxmum crop yield and protect the environmentA successive three seasons of crop field experiment was conducted to determine the Napplication rate which would both maximize yield and protect the environment in awheat-maize rotation district of the Guanzhong Plain, Shaanxi Province. The study includedsix treatments replicated three times: N0(0kg N hm-2a-1), N1(248kg N hm-2a-1), N2(330kg N hm-2a-1), N3(413kg N hm-2a-1), N4(475kg N hm-2a-1) and N5(660kg N hm-2a-1).The effects of N application rate on crop yield, N fertilizer use efficiency, apparent N loss,and N budget in the0—100cm depth were determined. The results showed that annual cropyield and acumulative crop yield both increased and then decreased as N application rateincreased. In contrast, acumulative N agronomic efficiency, apparent N utilization, N uptakeefficiency, and N partial productivity decreased significantly as N application rate increased.Calculation of the N budget in the0—100cm depth showed that residual mineral-Nconcentrations were significantly higher in the N2treatment than in the N1treatment, butapparent N loss and loss rates were nearly the same in the two treatments. In contrast, residualmineral-N concentrations were nearly the same in the N3, N4, and N5treatments, butapparent N loss and loss rate increased significantly in the order N3<N4<N5. In conclusion,the application of150kg N hm-2to wheat and180kg N hm-2to maize resulted in relativelyhigh yield and N use efficiency. These application rates also maintained residual mineral-Nconcentrations and reduced apparent N losses. |
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