Studies On Rules Of Nitrogen Transformation In Ecological Planting And Farming Models In Paddy Fields

Sampling on fields, laboratory incubation and determination and field experiments were conducted to study dynamics and availability of soil fixed ammonium, ecological effects of soil soluble organic N, interactions among different formations of N in floodwater and soil, N losses in the ecosystems and correlations between N transformation and crop growth by a static chambers technique, a closed acid trap method, and a field lysimeter and so on. The main results are summarized as follows.1. Compared with conventional rice paddies (CK), due to the presence of ducks and fish, integrated rice-duck ecosystem (RD) and rice-fish ecosystem decreased soil pH, and significantly increased the contents of 0.02-1 mm clays and significantly decreased the contents of <0.002mm clays, which enhanced the amounts of water-stable aggregates in soil, and thus improved the texture of soil.Compared with CK, RD and RF significantly increased the contents of total N and total P, exchangeable NH₄⁺, and available P, and had no effects on the content of NO₃^-.2. Fertilization and rice growth are of influencing factors to control fixation of soil ammonium. Application of N fertilizer promoted fixation of soil ammonium while rice growth boosted the release of fixed ammonium. During rice growth, most of recently fixed ammonium was released while native fixed ammonium was not released.Compared with CK, RD and RF significantly increased the content of fixed ammonium. Exchangeable NH₄⁺, pH and texture in soil affected the content of soil fixed ammonium. Close correlation existed between soil NH₄⁺, pH and fixed ammonium content at the significant level of p<0.01; Soil temperature (at 5cm depth) was not associated with fixed ammonium content; Soil fixed ammonium content was positively related to >0.2mm clay content and negatively related to <0.002mm clay content at the significant level of p<0.05 or p<0.01, and not associated with 0.02-0.2mm and 0.002-0.02mm clay content. The study also showed that N uptake of rice was significantly negatively related to the content of soil fixed ammonium and positivelyrelated to the release amount of soil fixed ammonium.Compared with CK, RD and RF increased the content of soil N and N uptake of rice, and decreased the release of soil fixed ammonium, and thus created a more significant N sink for added fertilizer.3. The experimental results showed that during rice growth, soil microbial biomass nitrogen (MBN) content increased firstly after transplantation, and subsequently decreased and slightly increased at rice autumn. Moreover, the soil enzymes activities ascended at the early stage and then declined slightly except that catalase activity slightly changed. Linear regression analysis showed that no close correlations existed between soil MBN and soil enzymes activities, soil nutrients (soil available N (NH₄⁺ + NO₃^-), total N and total P) and N uptake of rice, and between soil enzymes activities and soil total N and total P. Moreover, Soil urease and dehydrogenase activities were closely related to soil available N, and N uptake of rice was closely related to soil urease, dehydrogenase and protease activities.Compared with CK, RD and RF significantly enhanced soil MBN content and urease, dehydrogenase and protease activity but not affected catalase activity.4. Soil soluble organic nitrogen (SON) constituted a predominant reservoir of soluble N in paddy fields. Correlation analysis indicated that during rice growth SON was significantly positively related to SIN (p<0.01), and significantly negatively related to N uptake of rice. Due to N-uptake by rice and N leaching, soil SON was not closely related to MBN. The results also showed that due to the presence of ducks and fish, compared with CK, RD and RF significantly decreased the content of soil SON.In the forepart periods of rice growth, contents of different formations of N all were highest. Dissolved organic N (DON) was the main formation of N in percolation water in paddy fields; moreover, statistical analysis indicated that compared with CK, RF significantly decreased the leakage of SON while RF slightly decreased the leakage of SON.In conclusion, soil SON was regulated by the interactions of N uptake of rice plants, microbial uptake and leaching in paddy fields during rice growth.5. NH₄⁺ was the predominant form of nitrogen in floodwater while NO₃^- was the predominant form of nitrogen in percolation water; moreover, fertilization could increase content of all form N in floodwater and percolation water. Compared with CK, RD and RF significantly increased the content of NH₄⁺, slightly enhanced the content of TN in floodwater, and significantly decreased pH in floodwater and didn't affect the content of NO₃^- in floodwater. Moreover, RD significantly increased the concentration of DO in floodwater while RF significantly decreased the concentration of DO in floodwater. Furthermore, RD and RF decreased the concentrations of NO₃^- and TN in percolation water and didn't change the concentration of NH₄⁺ in percolation water. In the 3rd day after fertilization, the ratio of NH₄⁺/TN was the most, and thereafter decreased. In addition, mean NH₄⁺/TN ratios of different treatments were almost equivalent.The results also indicated that the potential maximal radios of N fertilizer leaching in RD and RF were 2.72% and 2.58%, respectively, lower than 2.99% in CK, which demonstrated that RD and RF could reduce leakage of N fertilizer and the effect of reducing.Leakage of N fertilizer was better in RF than in RD.6. The results indicated that the P concentrations of soil and floodwater reached peaks immediately after P fertilizer applied, and then decreased in 1 week after fertilization. Compared with CK, RD and RF could significantly enhance total P concentrations of floodwater, dissolved P concentrations of floodwater, available P concentrations of soil and P uptake of rice plants, and slightly increase total P concentrations of soil. Moreover, close relationships existed between dissolved P concentrations of floodwater, available P concentrations of soil and P uptake of rice plants. Analysis on environmental impacts of floodwater P showed that it was a pivotal time in 1 week after fertilization to control P losses by leaching from paddy fields. Moreover, during ducks and fishes reared, drainage of water from paddy fields and leaching due to runoff should be controlled. Since RD and RF could enhance contents of soil available nutrient, leading to decrease the consumptions of fertilizer, which reduced detrimental impact on the environment.7. The results showed that with the same amount of urea applied as basal fertilization, N2O emission fluxes from these treatments followed a similar seasonal variation trend. Our experimental data showed that in 2 weeks after urea application and after drainage peaks of N₂O emission flux occurred. Compared with CK, N₂O emissions in RD significantly enhanced, which were 1.08-1.13 times of that in CK, and N₂O emissions in RF siginificantly decreased, which were 94%⁹5% of that in CK. Correlation analysis indicated that during the flooding seasons, N₂O emission flux was not correlated with temperature, but significantly related to soil inorganic nitrogen (SIN) (p<0.01) and soil pH (p<0.01). After drainage, N₂O emission flux was not correlated with temperature, SIN and soil pH. These observations showed that during the flooding season N₂O emission might be influenced by nitrification and denitrification due to the presence of oxidized and reduced soil layers. On the other hand, after drainage, N₂O emission might be affected by the interreactions of soil pH, water content and temperature during Nitrification.We evaluated the integrated global warming potentials (GWPs) of integrated rice-duck cultivation system based on N₂O emission, which showed RD could enhance the GWP based on N₂O while RF could reduce the GWP based on N₂O Compared with CK.8. NH₃ volatilization was measured by a closed acid trap method in the ecosystems. The results indicated that urea application could promote volatilization of NH₃. It is the key time to control volatilization of NH₃ in the first week of urea application. Correlation analysis showed that volatilization flux of NH₃ was closely related to pH in soil and floodwater and NH₄⁺ in floodwater, and not related to temperature and soil urease activities.Compared with CK, due to a drop of pH in floodwater by the presence of ducks and fish, RD and RF reduced volatilization of NH₃. Analyses on N losses from paddy fields indicated that most of nitrogen was lost by NH₃ volatilization in paddy fields; moreover, RD and RF could decrease fertilizer N loss rates, and thus promote fertilizer N use efficiency.9. The water balance analysis indicated that the amount between water input and output was imbalance due to seepage. Moreover, the N balance analysis showed that fertilization and N-uptake by rice were the main factors of N balance while N outputs via NH₃ volatilization and N input via precipitation and irrigation were also significant. In contrast to CK, due to the presence of ducks and fish, the apparent N balances were positive in RD and RF, suggesting that the presence of ducks and fish accelerated the turnover of soil organically bound nutrients and thus significantly increased N output from rice uptake.The results also showed that the presence of ducks and fish did not reduce the total yields of rice grain.