| Nitrogen and phosphorus loss from agricultural lands has been an important source ofagricultural non-point pollution. The loss ways, loss fluxes and pollution potential of N&Pfrom paddy field was closely related with N&P transformation in the field especially in thefloodwater. Therefore, the influence of large amounts of duckweed and algae in floodwaterand enzymes in topsoil on N&P transformation in water-soil interface is very necessary to beclarified. In this research, the typical paddy field in Hangjiahu plain area was chosen as theresearch object. By the aid of regional investigation, on-farm experiment combined withlaboratory experiment, the interresponse between duckweed, algae, soil enzymes and N&Ptransformation and loss was illuminated. There are the main conclusions:In rice-wheat rotation system, nitrate leaching and ammonia volatilizaiton were twomajor ways of N loss. In rice season, the leaching rates of TN accounting for total appliedurea-N were from 2.82％to 5.07％, in wheat season, the rates were much higher with therange of 8.37％-13.11％. For NH3 volatilization, the loss percentages in rice season were from19.24％-40.75％, which were much higher than the corresponding rates from 5.48％to24.35％in wheat season. Due to the comprehensive effects of rainfall & fertilizationconditions, surface runoff is not the most important way for N&P loss.Based on the regional investigation, it was found that the growth of duckweed and algaewas very common in paddy fields of Hangjiahu plain area. Among the investigated 18 sites,duckweed density in the range of 0-200 g m-2, 201-400 g m-2, 401-600 g m-2 accounted for33.3％, 38.9％and 27.8％, respectively. Chlorophyll-a concentrations ranged from 4.36-55.20μg l-1. Liner negative correlation was found between chlorophyll-a concentrations andduckweed densities.In the on-farm investigation, duckweed densities increased with the increasing of fertilizer levels. The results from simulativie experiment showed that duckweed accelerated ureahydrolysis and decreased N and P concentrations in floodwater. About 10 days afterfertilization, TN and TP concentrations increased back in duckweed treatment, which wasrelated with N release from duckweeds through decomposition.Based on 15N-labled laboratory experiment, urea hydrolysis and NH3 volatilization arethe major progresses in the first 1-2 days, which resulted in the sharply increase and quicklydecrease of NH4+-N in floodwater. From day 3 to day 8, the uptake of NH4+-N by duckweedwas the dominant progress, which caused faster NH4+-N decrement in duckweed treatment.From day 9 to day 20, the decomposition induced the increase of NH4+-N concentrations induckweed treatment. Through calculating the atom% of 15N, 75.0% of fertilized N was savedin duckweed in the first 8 days while 60.8% of fertilized N was in duckweed, indicating that14.2% of fertilized N was released back to floodwater. It was approved that the uptake andrelease of N by duckweed regulated N transformation and loss in paddy fields.The algae population and chlorophyll-a concentrations increased with optimal fertilizationlevel while over dose of fertilizer inhibited algae growth. The diversity of algae in floodwaterwas simplex, especially in unfertilized plots, the dominant algae species in flood water wasbacillariophyta. With the N&P fertilizer application, the percent of cyanophyta andchlorophyta was increased; N fertilizer especially induced the growth of cyanophyta.The growth of algae accelerated urea hydrolysis in floodwater, while algae death slowedthis progress. NH4+-N, NO3--N and PO43- concentrations were lower in with algae growingdue to N&P uptake by algae, while were higher when large amount of N&P was released fromalgae through algae death.The presence of duckweed cover decrease NH3 volatilization rate through assimilating N,decreasing floodwater pH and temperature and inhibiting algae growth and etc. At different Nlevels, NH3 volatilization was reduced by 19.0-53.8% in duckweed-covered plots, compared to the corresponding uncovered plots. In addition, rice grain yields were increased by0.64-0.74 t ha-1 in duckweed-covered plots.Fertilizer application could exert complicated influences on soil enzyme activities.Certain amount of urea can enhance the urease activities, while excess urea would inhibit theenzyme activities. There were obvious correlation relationship between NH4+-Nconcentration in water and soil urease activity as well as between NH3 volatilization rates andsoil urease activity. Therefore, soil urease had significant influence on nitrogen transformationbetween soil-water interface. The relationship between TP, PO43- concentrations in floodwaterand soil neutral phosphatase was not significant.Compared with urea alone treatment, the combined application of new nitrificationDMPP with urea decreased the population of ammonia oxidizing bacteria, the activity ofnitrate reductase and nitrite reductase by about 24.5%-30.9%, 14.9%-43.5% and14.7%-31.6%, respectively. NH4+-N concentrations in leachate and soil were enhanced by19.1%-24.3% and 11.3%-24.2%, respectively, while NO3--N concentrations were decreasedby 44.9%-56.6% and 23.4%-53.7%, respectively. Total mineral-N loss flux by leaching weredecreased by 1.96-9.46 kg N ha-1 in DMPP plots. There was no significant difference in NH3volatilization loss between DMPP and UA plots. It is proposed that DMPP has the potential toreduce either NO3--N leaching by inhibiting ammonia oxidization or N losses fromdenitrification, which was in favor of N conversations in the rice-rape cropping system. |
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