RESUMO
In order to reveal the mechanism of silicon (Si) fertilizer in improving nitrogen (N) and phosphorus (P) nutrient availability in paddy soil, we designed a series of soil culture experiments by combining application of varying Si fertilizer concentrations with fixed N and P fertilizer concentrations. Following the recommendations of fertilizer manufacturers and local farmers, we applied Si in concentrations of 0, 5.2, 10.4, 15.6, and 20.8 μg/kg. At each concentration of added Si, the availability of soil N and P nutrients, soil microbial activity, numbers of ammonia-oxidizing bacteria and P-decomposing bacteria which means that the organic P is decomposed into inorganic nutrients which can be absorbed and utilized by plants, and urease and phosphatase activity first increased, and then decreased, as Si was added to the soil. These indicators reached their highest levels with a Si application rate of 15.6 μg/kg, showing values respectively 19.78%, 105.09%, 8.34%, 73.12%, 130.36%, 28.12%, and 20.15% higher than those of the controls. Appropriate Si application (10.4 to 15.6 µg/kg) could significantly increase the richness of the soil microbial community involved in cycling of N and P nutrients in the soil. When the Si application rate was 15.6 μg/kg, parameters for characterizing microbial abundance such as sequence numbers, operational taxonomic unit (OTU) number, and correlation indices of microbial community richness such as Chao1 index, the adaptive coherence estimator (ACE) index, Shannon index, and Simpson index all reached maximum values, with amounts increased by 14.46%, 10.01%, 23.80%, 30.54%, 0.18%, and 2.64%, respectively, compared with the control group. There is also a good correlation between N and P mineralization and addition of Si fertilizer. The correlation coefficients between the ratio of available P/total P (AP/TP) and the number of ammonia-oxidizing bacteria, AP/TP and acid phosphatase activity (AcPA), AP/TP and the Shannon index, the ratio of available N/total amount of N (AN/TN) and the number of ammoniated bacteria, and AN/TN and AcPA were 0.9290, 0.9508, 0.9202, 0.9140, and 0.9366, respectively. In summary, these results revealed that enhancement of soil microbial community structure diversity and soil microbial activity by appropriate application of Si is the key ecological mechanism by which application of Si fertilizer improves N and P nutrient availability.
RESUMO
The nitrate-N content in KNO3 solution and soil was rapidly predicted using techniques of mid-infrared spectroscopy, in which 15 NO-3 and 14 NO-3 were distinguished and predicted. The results showed that the characteristic band of nitrate in solution and soil was 1200-1500 cm-1 , and compared with 14 NO-3 , the red shift of characteristic band of 15 NO-3 was about 35 cm-1 . In the characteristic band of nitrate, absorption band increased with the nitrate nitrogen concentration with less interference absorption. The linear regression was made between the first principal component of characteristic band and nitrate-N content, and correlation coefficient was more than 0 . 9840 , indicating that the technique of mid-infrared attenuated total reflectance spectroscopy could be applied for rapid monitoring of nitrate in solution and soil. Meanwhile, based on the red shift characteristic of 15 NO-3 absorption band, the method of partial least squares were involved to predict the nitrate-N of different N-isotope labeled in solution and soil, resulting that all the prediction models reached excellent levels. For 14 NO3-N and 15 NO3-N in solution, the correlation coefficients ( R2 ) were 0. 9980 and 0. 9982 respectively, and ration performance to standard deviations ( RPD ) were 6. 44 and 4. 76, respectively. While for 14 NO3-N and 15 NO3-N in soil, the correlation coefficients ( R2 ) were 0. 9794 and 0. 9679, and RPD were 5. 75 and 4. 78, respectively. Therefore, the technique of mid-infrared attenuated total reflectance spectroscopy can be applied for rapid monitoring different N-isotope labeled nitrate in solution and soil, to provide a new in situ and fast time method to study nitrification process in soil.