The Effect Of Lanthanum, Cerium On The Hydrolyzed Conversion Of Urea And Related Enzyme Activities In Calcareous Purple Soil

Rare earth elements (REEs) were located in chemical III periodic table B. which atomic number were 57-71 and consisted of 15 kinds of lanthanides and scandium (Sc) and yttrium (Y), at a total of 17 kinds of chemical elements. Many studies had shown that the yield and quality of crops were significantly improved with the appropriate amount of REEs. In recent years, with the popularization and application of the REEs micro fertilizer, the ecological environmental problems caused by REEs into the agricultural ecological system has attracted more and more attention. Studied the effects of exogenous REEs on the transformation and utilization of nutrient elements in soil was an important part of the environmental risk assessment.Urea as the main nitrogen fertilizer, accounting for more than 80% of the world’s nitrogen application. Over-fertilization of N based on urea was one of the main reasons to cause non-point source pollution and water eutrophication. At the same time, some of the nitrogen in the form of greenhouse gases into the atmosphere to cause the greenhouse effect. But at present, the studies of the effects of REEs on soil more focused on transformation and utilization of N in soil itself and paid little attention to the exogenous nitrogen. This paper was based on the soil culture experiment, and studied the hydrolysis and conversion products of urea:NH4+-N, NO3--N, available nitrogen and the characteristics of urease, protease, nitrate reductase, nitrite reductase and catalase activities in calcareous purple soil, under different concentrations (100 mg·kg-1、200 mg·kg-1、400 mg·kg-1、600 mg·kg-1、800 mg·kg-1) of lanthanum and cerium with the same amount of urea (200 mg·kg-1) treatments.The results were as follows:(1) In various concentrations of lanthanum and cerium with urea treatments, the soil pH were lower than the single urea treatment. When at the concentration of 200 mg·kg-1 of lanthanum, the soil pH was significantly declined. With the increase of concentrations of lanthanum and cerium and the extension of incubation time, the soil pH gradually reduced; The effects of lanthanum and cerium with urea treatments on soil pH were not significantly different.(2) The application of all concentrations of lanthanum mixtured with urea effected on NH4+-N contents in soil were significantly lower than that of single urea treatment (p< 0.05). With the increase of concentrations of lanthanum, NH4+-N contents showed a trend of significantly decreased, which showed that lanthanum had inhibitory effect on soil urea hydrolysis. With the culture time extension, the NH4+-N contents was gradually decreased, but the inhibitory effects alleviated. Cerium had a similar effect with lanthanum, but at the same concentration the cerium showed a greater inhibitory effect on urea hydrolysis.(3) The application of all concentrations of lanthanum mixtured with urea effected on NO3--N contents in soil were significantly lower than that of single urea treatment (p< 0.05). With the increase of concentrations of lanthanum, NO3--N contents showed a trend of decreased, which showed that lanthanum had inhibitory effect on NH4+-N to NO3--N conversion process. With the culture time extension, the inhibiting effect of lanthanum was gradually decreased and the NO3--N contents in soil were increased at all concentrations of lanthanum treatments. Cerium showed a greater inhibitory effect on urea hydrolysis product NH4+-N to NO3--N transformation process.(4) The application of all concentrations of lanthanum mixtured with urea effected on available nitrogen contents in soil were significantly lower than that of single urea treatment (p<0.05). With the increase of concentrations of lanthanum, available nitrogen contents showed a trend of decreased. With the culture time extension, available nitrogen contents of all treatments showed a rising trend. Cerium had a similar effect with lanthanum but available nitrogen contents overall lower.(5) Urease, protease, nitrate reductase, nitrite reductase and catalase activities were obviously inhibited under the treatments of the lanthanum mixtured with urea, and on the cultured first day, were significantly lower than that of single urea treatment (p<0.05). With the increase of concentrations of lanthanum, the inhibitory effect was enhanced. With the culture time extension, urease and nitrate reductase activities both showed a gradual upward trend at the concentration of 100 mg-kg-1 lanthanum mixtured with urea. In the treatments of lanthanum>100 mg-kg-1, showed a trend of first decreasing and then increasing, and reached the lowest value in the third day, which significantly lower than that of single urea treatment (P<0.05). With the culture time extension, protease, nitrite reductase and catalase activities at all concentrations of lanthanum mixtured with urea, all showed a gradual upward trend. Under the treatment of 100 mg-kg-1, urea and catalase activities were higher than that of single urea treatment in third and 5 days respectively, which showed a promoting effect. Cerium treatments effected on urease. protease, nitrate reductase, nitrite reductase and catalase activities was similar with lanthanum but inhibition stronger.(6) Correlation analysis results showed that soil pH had a very significant positive correlation with NH4+-N (p< 0.01), and negative correlation with NO3--N; NH4+-N had a positive correlation with NO3--N, and had a very significant positive correlation with available nitrogen, urease, protease, nitrate reductase, nitrite reductase and catalase (p< 0.01); there were very significant positive correlation between NO3--N, available nitrogen, urease, protease, nitrate reductase, nitrite reductase and catalase (p<0.01).