The relationship of soil organic matter (OM) concentration to microbial biomass concentration and mineralizable OM is central to understanding the establishment and functioning of soil nutrient cycles. These parameters are presumably related to aboveground plant biomass and plant N concentration, but the mechanisms and controls of these interactions are not well understood. To further evaluate these relationships, a field study was established in a series of soil materials whose organic C concentrations ranged from 1 to 21 g kg⁻¹. Surface soil (0-15 cm) and vegetation samples were collected from plots of each treatment during the 1983 growing season. Microbial biomass was measured by chloroform fumigation and incubation; mineralizable C and N were measured in 20-d laboratory incubations; plant growth was measured by weighing material clipped from 0.18-m² frames; and plant N concentration was measured by Kjeldahl digestion and colorimetric analysis. Microbial biomass increased linearly with soil OM concentration. Mineralizable N and plant production also increased with soil OM, but were greater with 7 than with 15 g kg⁻¹ of organic C. Even though aboveground plant biomass was greater with either 7 or 15 than with 21 g kg⁻¹ organic C, plant N concentrations were highest with 21 g kg⁻¹. Soil OM concentrations were more closely related to microbial biomass than to mineralizable C and N, or to plant biomass and plant N concentrations. Nitrogen mineralization in the laboratory corresponded to plant N concentration in the field. Soil OM concentrations controlled microbial biomass C and N concentrations. However, additional factors also influenced the activity of the microbes and the resultant OM mineralization and plant N concentrations.

The relation between urease activity and bacterial biomass has been studied in soil incubation experiments and the results obtained are discussed with respect to the use of physiological methods for the biomass calculation. A high correlation between both tested parameters has been found in the absence of fresh organic matter input into the soil. However, the correlation disappeared after soil enrichment with the substrates stimulating the growth of zymogenous bacteria, among them of ureolytic bacteria

International audience | The mineralization of microbial material of different C-to-N ratios (5.2, 7.9, 10.2, 12.7) was followed in fumigated soil. The microbial materials used were from Aspergillus,flavus cultures, grown in liquid media and labelled with [‘4C]glucose and (‘5NH,)2S0,. Three contrasting soils were used and the microbial materials incubated with the fumigated soils for 28 days at 28°C. The evolution of the added organic microbial C was fast: 80% of the [‘4C]C02 produced during the whole 28 days incubation was evolved in the first week. Microbial C mineralization was mainly related to soil type; the C-to-N ratio had small effect on the ratio (mineralized microbial carbon-to-added microbial carbon). Calculation of the KC coefficient (the fraction of the added microbial C mineralized in 7 days) shows that KC values lie between 0.38 and 0.43 in the 3 soils. Organic N in the added microbial material also breaks down quickly: between 60 and 100% of the organic nitrogen mineralized was evolved during the first week of incubation. Mineralization kinetics are related to soil type and to the C-to-N ratio of the microbial material. The proportion of N mineralized in 7 days was lower in an acid soil than in near neutral soils and lower with high C-to-N ratio material than with low C-to-N ratio material. The ratio (mineralized microbial N-to-added microbial N) depends on soil type and is negatively correlated with the C-to-N ratio of the microbial material. The KN value (the fraction of the added microbial N mineralized in 7 days) lies between 0.22 and 0.47 for the three soils and four materials investigated. The added microbial material induced a priming effect on soil native N: materials with C-to-N ratios of 10.2 and 12.7 produced negative priming effects whereas materials with C-to-N ratios of 5.2 and 7.9 sometimes produced a positive priming action. From the relationship between the C-to-N ratio of the added material and the (mineralized microbial C-to-mineralized microbial N) ratio, the soil native microbial biomass was estimated using the flush-C-to-flush-N ratio. Biomass nitrogen was then calculated from the formula biomass-N = biomass- C/(biomass C-to-N ratio). Calculated in this way, 24% of the total nitrogen in the three soils was in microbial biomass.

Six selected strains of azolla were studied in Bangladesh Rice Research Inst. field under shade and without shaded condition. For biomass production a single strain was studied with and without phosphate application at different water depths. Rate of biomass production of azolla varied among different strains and from one season to another. Under shaded condition biomass production was far better than without shade. Maximum biomass production of 43 kg dry matter/ha/day was recorded under shaded condition that corresponds to 1.16 kg N/ha/day.

Product formation equations were used for the calculation of specific maintenance energy rates for soil microbial biomass and incorporated into a conceptual model for maintenance C flow. The model separates active and sustaining biomass and includes considerations for endogenous catabolism, cryptic growth, and microbial death. Coupling of the model with product formation equations enabled the calculation of specific maintenance rates for five soils, using data from a 60-d incubation. Calculated maintenance rates were approximately half the values commonly reported for soils and used in ecosystem models. With the use of the calculated specific maintenance rates, the conceptual model and data from a 60-d incubation, the net soil N mineralized during the steady-state period could also be calculated. The resultant C/N ratio of the biomass formed during this period was 7.6. Calculations involving N provided verification of the maintenance rate calculations and of the validity of the conceptual model.

Mining process affected the soil properties by increasing sandproperties by increasing sand particle, decreasing silt and clay in Sand and Gravel zone as contrasted to grass land, Grass and Casua-rina and Clay Pan. The particle density and bulk density were lower in mining areas than in the undisturbed adjacent soil. Porosity percentage was not different from the areas before mining, while gravel percentage tended to increase only in the Scrub forest. Investigation on the chemical properties of soils revealed that in the upper soil layer, the organic matter was higher than in the lower soil layer except Sand and Gravel Zone was lowest, phosphorus, potassium and calcium in all zones were grouped into low level and in Moist evergreen forest, Scrub forest and Sand and Gravel Zones were the lowest level but other zones were low level. Sulfur in Swamp forest decreased but other zones increased following the depth of soil while Grass and Casuarina, Grass land and Clay Pan were higher level. Soil pH showed the strong acidity. All phenomena might be due to the influence of tin mining process and the decomposition of litter in that area.