Regulation of urease production in soil by microbial assimilation of nitrogen

Summary Studies of the effects of different forms of N on urease production in soils amended with organic C showed that although microbial activity, as measured by CO₂ production, was stimulated by the addition of NH₄ ⁺ or NO₃ ^- to C-amended soils (200 mol glucose-C g^–1 soil), urease production was repressed by these forms of N. The addition of L-methionine sulfoximine, an inhibitor of inorganic N assimilation by microorganisms, relieved the NH₄ ⁺ and NO₃ ^- repression of urease production in C-amended soil. The addition of sodium chlorate, an inhibitor of NO₃ ^- reduction to NH₄ ⁺ by microorganisms, relieved the NO₃ ^- repression of urease production, but did not eliminate the repression associated with NH₄ ⁺. These observations indicate that microbial production of urease in C-amended soils is not directly repressed by NH₄ ⁺ or NO₃ ^-, but by products formed by microbial assimilation of these forms of N. This conclusion is supported by our finding that the biologically active L-isomers of alanine, arginine, asparagine, aspartate, and glutamine, repressed urease production in C-amended soil, whereas the D-isomers of these amino acids had little or no influence on urease production. This work suggests that urease synthesis by soil microorganisms is controlled by the global N regulon.     

Effects of the urease inhibitor N-(n-butyl)phosphorothioic triamide in low concentrations on ammonia volatilization and evolution of mineral nitrogen

Laboratory incubation experiments were conducted to study the influence of increasing concentrations of N-(n-butyl)phosphorothioic triamide (NBPT) on NH₃ volatilization and rate of urea hydrolysis and evolution of mineral N in Ozzano, Rimini and Carpi soils with different physicochemical characteristics. Low concentrations of NBPT reduced NH₃ losses due to volatilization after urea fertilization and the effectiveness of the inhibitor was related to the soil characteristics (e.g. high concentrations of organic C and sand). After 15 days of incubation, no significant reductions of losses were found for any of the NBPT concentrations employed in Rimini soil. The application of NBPT led to a considerable reduction of the formation of nitrite. This process was completely annulled with the highest dose of NBPT (0.5% w/w_urea) in the Carpi soil after 15 days. In Rimini soil, however, the use of NBPT was less effective in influencing nitrite formation. The use of NBPT favoured accumulation of nitrate proportional to the NBPT concentration employed while it had no influence on the NH _inf4 ^sup+ fixation by 2:1 layer silicates. The data obtained support previous evidence that NBPT is effective in reducing the problems encountered in using urea as fertilizer. However, environmental conditions and soil physicochemical characteristics may have an important influence on the effectiveness of NBPT.     

The effect of phenyl phosphorodiamidate on urease activity and ammonia volatilization in flooded rice

Summary The behaviour of urease activity, ammoniacal N concentrations and pH in flood water and that of ammonia flux was investigated in a water-logged soil either in the presence or in the absence of rice and with three different treatments (control, urea and urea + phenyl phosphorodiamidate). In the presence of the phenyl phosphorodiamidate (PPD), that is a urease inhibitor, increases in ammoniacal N concentrations and in ammonia evolution were delayed but not eliminated. The degradation and/or the inactivation of PPD might have occurred, thus removing the inhibition of the enzyme activity.     

Microbial populations,enzyme activities and nitrogen-phosphorus-potassium enrichment in earthworm casts and in the surrounding soil of a pineapple plantation

Summary Total populations of bacteria and fungi, dehydrogenase activity (as a measure of total potential microbial activity), and urease and phosphatase activities were determined in earthworm casts and surrounding laterite soils planted to pineapple. The casts contained higher microbial populations and enzyme activities than the soil. Except for fungal populations, statistically significant (P = 0.05) increases were found in all other parameters. Microbial populations and enzyme activities showed similar temporal trends with higher values in spring and summer and lower values in winter. The earthworm casts contained higher amounts of N, P, K and organic C than the soil (P = 0.05). Selective feeding by earthworms on organically rich substrates, which break down during passage through the gut, is likely to be responsible for the higher microbial populations and greater enzyme activity in the casts.     

Urease activity in two cultivated and non-cultivated arid soils

Summary Since urease activity in soil is believed to be relatively constant, the present study was designed to examine the effects of incubation, soil depth and the effect of cultivation on the persistence of urease activity in arid soils. Two soils were used, a Harkey (coarse, silty, mixed, calcareous, thermic, Typic Torrifluvent) and a Saneli (Clayey over sandy skeletal, montmorillonitic, calcareous, Vertic Torrifluvent), each consisting of a cultivated field and a non-cultivated roadbed site. Urease activity was much lower and more varable in the roadbed soils (40 years without cultivation) than in the cultivated field soils. Pre-incubation for 24 h with urea (with toluene) and without urea (without toluene) greatly reduced the total urease activity in all cases in relation to cell free urease activity (with toluene). Urease activity in the two field soils decreased slightly with profile depth but the decrease was greatest below the plow depth (33 cm). Protease activity or some inactivation processes must have lowered the urease content since there was substantially reduced urease activity after most pre-incubations. The extent of the urease activity decrease was so great that the addition of urea would have been required to increase the production of urease enzyme.     

Effect of tillage and residue management on enzyme activities in soils

Recent interest in soil tillage and residue management has focused on low-input sustainable agriculture. In this study we investigated the effect of three tillage systems (no-till, chisel plow, and moldboard plow) and four residue placements (bare, normal, mulch, and double mulch) on the activities of four amidohydrolases (amidase, L-asparaginase, L-glutaminase, and urease) in soils from four replicated field-plots. Correlation coefficients (r) for linear regressions between the activities of each of the enzymes and organic C or pH and between all possible paired amidohydrolases were also calculated. The results showed that the effects of tillage and residue management on pH in the 28 surface soil (0–15 cm) samples were not significant. The organic C content, however, was affected significantly by the different tillage and residue-management practices studied, being the greatest in soils with notill/double mulch treatment, and the least with no-till/bare and moldboard/normal treatments. Within the same tillage system, mulch treatment resulted in greater organic C content compared with normal or bare treatment. The activities of the amidohydrolases studied were generally greater in mulch-treated plots than in non-treated plots, and were significantly correlated with organic C contents of soils, with r values ranging from 0.70^*** to 0.90^***. Linear regression analyses of enzyme activities on pH values (in 0.01 M CaCl₂) of the 28 surface soils showed significant correlations for L-asparaginase, L-glutaminase, and urease, with r values of 0.74^***, 0.77^***, and 0.72^***, respectively, but not for amidase (r=0.24). The activities of the four amidohydrolases studied in the 40 soil samples tested were significantly intercorrelated, with r values ranging from 0.72^*** to 0.92^***. The activities of the four amidohydrolases decreased with increasing soil depth of the plow layer, and were accompanied by a decrease in organic C content.     

脲酶/硝化抑制剂在土壤N转化过程中的作用

综述了脲酶抑制剂/硝化抑制剂对土壤氮的转化过程:尿素水解过程、硝化过程、硝酸盐淋溶过程、反硝化过程、微生物固持过程、N矿化过程及气体挥发过程的影响和抑制剂的作用机理,并提出今后研究的发展方向,为今后如何施用抑制剂来提高土壤中氮素利用率和减少环境污染提供一定的参考价值。     

汞镉对游离和固定化脲酶活性的影响

通过对脲酶与汞镉间关系的研究 ,结果表明 :不同状态 (溶液态、粘粒态和土壤态 )脲酶对汞、镉的反应表现出类似的规律性变化 ,即汞、镉抑制脲酶活性 ,其中汞镉复合污染的影响幅度最大 ,汞的生态毒性最强 ;相关分析显示脲酶活性均可表征样品汞、汞镉污染的程度 ;在汞、镉共存条件下 ,脲酶活性不仅受到汞、镉单因素 ,而且受到它们间交互作用的影响 ,但仍以单独抑制为主 ;由于不同状态脲酶载体的差别 ,导致受影响幅度有较大差异 ,其主要取决于载体对脲酶保护作用和重金属缓冲作用的大小 ,溶液态脲酶无载体 ,而粘粒态和土壤态则相反 ,故溶液态脲酶更易受到重金属的破坏 ,酶的反应也较敏感。温度和尿素浓度均增强了汞镉的抑制作用     

土壤脲酶抑制剂正丁基硫代磷酰三胺的作用基团研究

土壤脲酶抑制剂正丁基硫代磷酰三胺(nBPT)是抑制土壤中尿素水解的最有效的化合物之一。分析大连工业大学合成的土壤脲酶抑制剂nBPT抑制脲酶活性的影响因素及作用机理,结果表明:在50°C,pH=5.91时,nBPT的抑制活性达到最大值;在nBPT各结构基团中,正丁基(-NH(CH2)3CH3)、硫基(-S)对nBPT与脲酶的结合起辅助作用,胺基(-NH2)是nBPT与脲酶结合的关键基团,与脲酶活性部位巯基(-SH)结合。土壤脲酶抑制剂nBPT与脲酶的具体结合机理还有待继续研究。     

阿维菌素对蔬菜地土壤微生物及土壤酶的生态毒理效应

研究了阿维菌素对蔬菜地土壤微生物和土壤酶的生态毒理效应.实验结果表明,阿维菌素在低浓度时（1～10 mg kg^-1）对土壤脲酶活性和脱氢酶活性有轻微的激活作用,而对土壤微生物呼吸强度没有明显的影响;在高浓度时（50～100 mg kg^-1）对土壤微生物呼吸强度、脲酶活性以及脱氢酶活性均有明显抑制作用;不同浓度阿维菌素在不同程度上均会造成土壤微生物生物量的减少和过氧化氢酶活性被强烈激活.