Study On Release Characteristics Of Nutrient Of Slow-release Compound Fertilizers And Response Of Crops

Slow/controlled release fertilizers, a hotspot of fertilizer research in China, pave the way for the future development in the fertilizer industry. So far, slow/controlled release fertilizers mainly include the coating (film-capsule) and the un-coated. The coating materials of coated fertilizer which are mainly organic-polymer cost and some of them could cause the secondary environment pollution. Therefore, the application of them is limited in the filed crops. The major technology of un-coated slow/controlled release fertilizer is using inhibitors and organic & inorganic compound controlled release materials. Organic & inorganic controlled release compound materials are of low price and advantage of improving soil fertility. However, the existing types of slow/controlled release fertilizers have some obvious problems, such as single nutrient elements (nitrogen) and single nutrient form (inorganic), so they could not fulfill the fertilizer demand of modem agriculture. The main evaluation methods of nutrients release characteristics of slow/controlled release fertilizers are still limited in laboratory, and disregard relationships between nutrient release of fertilizers and composition of nutrient forms, the characteristics of soil and plant nutrient.An un-coated organic-inorganic slow/controlled release fertilizer compound (slow/controlled release fertilizer compound, SRF) produced by the Southwest University was used as the experiment materials in this study. Firstly, experiments of water dissolution, soil-quartz leaching and soil fertilizer packets incubation and pot experiments of rice and wheat were employed to study the release characteristics of the nitrogen, phosphorus and potassium nutrients of SRF, and particularly correlation between the release rates of nutrients in the soil and the nutrient uptake rates of wheat, in order to evaluate the release characteristics of SRF in soil-crops system. Secondly, the dynamics release characteristics of different forms of Nitrogen (NH₄⁺-N, NO₃^--N, Urea-N, DON and Total N) of SRF were researched in different conditions (water and soil). Furthermore, the correlation between the ratios of the four forms of N and Total N in SRF and N uptake by rice was analyzed, and initially established the evaluation indicators in the soil-crop system. Lastly, the regulation of soil nutrients and soil biology characteristics (enzymes activity) through soil inorganic-organic-microbes-enzymes composite colloidal system was studied after nitrogen, phosphorus and potassium nutrients of SRF released into the soil. The results showed:Slow release capability of the nitrogen, phosphorus and potassium of SRF was better than those of the common compound fertilizers (CCF) in simulation soil incubation experiment, and the release cumulative quantities of three nutrients were less than CCF, by18.7%～43.3%, 26.3%～64.0% and 15.5%～43.6%, respectively. The release quantities of there nutrients were in the order of N＞P＞K and were similar to nutrient characteristics of wheat. Compared with the CCF treatment, SRF treatment increased the nitrogen; phosphorus and potassium use efficiency of wheat by 23.6%, 15.4% and 63.7%, respectively, and yield of wheat by 14.9 %.In the soil-plant system, the dynamics of nutrient cumulative release characteristic of SRF and the nutrient uptake of wheat could be quantitatively described by cubic regression equation (y=b₀+b₁x+b₂x²+b₃x³) (r=0.9285^**～0.9968^**), with the best fitting by nitrogen. As a result, the nitrogen release characteristic of SRF was better than CCF to simultaneously fulfill the demand for nitrogen during the growth stages of wheat.The transformation characteristic of different nitrogen forms of SRF showed both release of NH₄⁺-N and NO₃^--N increased with simultaneous decomposition of DON and Urea-N under soil conditions. The first-order kinetics equation (N_t=N₀(1-e^-kt)), Elovich equation (q_t=a+blnt), and parabola equation (qt=a+bt^0.5) could be used to describe the N release characteristics of SRF, as evidenced by high correlation coefficients with the best for the first-order kinetic equation. The maximum releases(N₀ values) of different N estimated by the first-order kinetics equation were in order of Total N＞NH₄⁺-N＞DON＞Urea-N＞NO₃^--N, being correlative with their actual cumulative releases. The release rate coefficients (k value) followed the order ofUrea-N＞DON＞NH₄⁺-N＞Total N＞NO₃^--N, that is the k value for urea-N was the greatest among different N forms, which is attributed to rapid decomposition and transformation of urea, whereas the k value for total N was the lowest, indicating slow release of nitrogen from the SRF.Correlation analysis showed that there were significant and very significant correlations between the ratios (N_x/N_T) and N uptake by rice plants at different growth stages. The N_x/N_T ratio for NH₄⁺-N and Urea-N obtained from the water dissolution experiment had very significant positive correlations with N uptake by rice (r=0.8437^**～0.9347^**), whereas significant negative correlation was found for DON. The N_x/N_T for NH₄⁺-N and NO₃^--N obtained from soil incubation experiment showed very significant positive correlation with nitrogen uptake by rice (r=0:8947^**～0.9540^**), but very significant negative correlation was observed for Urea-N and DON (r=-0.9146^**～0.9619^**). These results showed that the quantity and ratios of different N released from the SRF at different periods impacted N uptake by the plant, having implications for development of uncoated slow and controlled release fertilizers with nutrient configuration.In soil, different nitrogen forms sustain the constant transformation and a dynamic balance. With the viewpoint of the amount of each nitrogen form and their effect within soil nitrogen cycle, this paper defined soil microbial biomass nitrogen and fixed ammonium as "the ammonium ion flow", a new activity of nitrogen. During the early growth of wheat (from early tillering stage to full tillering stage), there was a significant decrease in the amount of "the ammonium ion flow" by 371.3mg·kg^-1 to 259.1mg·kg^-1 of the CCF treatments, whereas the ones of SRF increased slightly by 306.5mg·kg^-1 to 324.5mg.kg^-1 in the corresponding stage. In the elongation stage, the uptake nitrogen amount of wheat increased. The margin of "the ammonium ion flow" between full tillering stage and elongation stage only was 34.18mg·kg^-1 in CCF treatments, while SRF reached by 77.21mg·kg^-1, and surpassed by 125.9% than CCF.The results showed "the ammonium ion flow" of SRF fixed more ammonium ion in the early growth of wheat and reduced ammonium ions losses than CCE However, when wheat needs higher nitrogen, "ammonium ion flow" of SRF would release more ammonium ions to demand on the supply of wheat. Thereby, the effect of SRF could enhance nitrogen uptake for wheat by 13.52%, compared with the CCF. The above change is the similar to the one of nitrogen uptake of wheat. The nutrients release rates of CCF in the early wheat growth were faster and nitrogen content of wheat were higher. But with the slowly release nutrients of SRF, the N nutrient uptake by wheat were improved in the middle and late stages. There were obviously different for the change trends of different nitrogen forms of tested fertilizer between the laboratory cultivation and the biological pot conditions. Therefore, to objectively present the biological effects, environmental benefits and economic benefits of fertilizer products in the practical application, the nutrient release of slow/controlled release fertilizer should be the evaluation of the soil-crop system.Correlation analysis showed that the dynamic changes of amount of soil microbial biomass nitrogen, fixed ammonium and "the ammonium ion flow" in SRF treatment reached significant or very significant negative relations with the changes of wheat nitrogen uptake (r=-0.8728^*～-0.9006^**). So the capability of un-coated slow/controlled release compound fertilizer was better than CCF on "the ammonium ion flow", and coordination the balance of between soil nutrient supply and wheat to absorb nutrients.In soil cultivation test, the variation of urease activity among each treatment was similar to the variation of soil effective nitrogen content. At the beginning of culture, compared to CCF, the urease activity was inhibited by SRF. With the mineralization of organic P, SRF increased the phosphatase activity of soil. The catalase activity of soil in SRF treatment was higher than the CCF treatment. In pot experiment of wheat, the soil urease activity of different treatments showed a trend to decrease, and still were similar to the changes of soil effective nitrogen content unanimously. In harvest period, the urease activity of each treatment dropped to the lowest level in rank of SRF＞CCF＞CK. Phosphatase and catalase activity of soil increased in the early growth of wheat, then caused peak values in the medium-term growth and last declined gradually, in the decrease rank of SRF＞CCF. The implementation of slow/controlled-release mechanism of slow/controlled release compound fertilizers was mainly through synergies of dissolvable release of inorganic N and the biochemistry relay-release of organic N. The organic nutrients controlled the effective nutrients of soil by effecting soil enzymes activity. In general, slow/controlled release compound fertilizer could effectively control of the balance between dynamic changes of soil nutrients contents and the nutrient demand of crop, through "soil inorganic-organic-microbes-enzymes" composite colloidal system, and ultimately improved fertilizer use efficiency.