Open Incubation Research Articles

Nitrogen mineralisation rates from soil amended with dairy pond waste

An open incubation and leaching study was conducted under controlled temperature (25°C) and moisture conditions to measure the N mineralisation rate in soil amended with dairy pond sludge. The dairy pond sludge was applied at 3 different rates equivalent to 0, 200, and 400 kg N/ha. The incubation was conducted at 3 different soil moisture potentials (0, -3, and -13 kPa). Following each 2-week period of incubation, the soil was leached with 2 pore volumes of deionised water to remove the mineralisation products. Mineralisation products in the leachate and enzyme activities, microbial biomass C and N, pH, and water-soluble C in the soil were determined. The incubation lasted 18 weeks. Rapid release of nitrate occurred during the first 6 weeks of incubation, followed by a slow release over the remainder of the incubation period. Although the total amount of N released in the 200 kg N/ha treatment (169 mg N/kg soil) was less than in the 400 kg N/ha treatment (206 mg N/kg soil), when expressed as a percentage of the organic N applied, the amount of N released at the lower rate (18·4%) was greater than that at the higher rate of sludge treatment (13·0%). Rapid nitrification decreased the soil leachate ammonium concentration and the soil pH. Soil microbial biomass, water-soluble C, and deaminase activity were significantly increased after the addition of dairy pond sludge. The increase in soil microbial biomass observed was probably due to the increased water-soluble C and nutrients that stimulated the soil microbial growth. The rapid N release and nitrification rates observed were attributed to the low C : N ratio (12·7), high ammonium content (145 mg N/kg) of the dairy pond sludge used, and the optimum moisture and temperature conditions. The narrow range of soil water potential conditions did not have any significant effect on N release rate or amount.

Comparison of sulfur mineralisation in open and closed incubation systems and from field moist and conditioned soils

Abstract The amounts of sulfur (S) mineralised in three soils incubated in both open (i.e. leached) and closed systems increased with time over the period of the incubation. However, the rates and patterns of S release differed with the soil type and incubation technique used. The pattern of S mineralisation during the open incubation was a rapid release of S in the first two weeks, followed by a slightly slower almost linear rate of release for the remainder of the incubation. The pattern of S release during the closed incubation was slightly curvilinear. In the short term, the closed system released similar or greater amounts of S than the open system. However, in the long term, more S would be mineralised in the open than that in the closed system. Conditioning (i.e. pre‐incubation of air‐dried samples) did not significantly affect the amounts of S mineralised in the soils incubated in the open system, but did in two soils incubated in the closed system.

Mobilization of recently-formed soil organic sulphur

Recently-formed soil organic sulphur was labelled with radioactive 35S by incubating samples of soil with carrier-free 35S-sulphate. Different amounts of 35S incorporation and differing proportions of 35S incorporation into hydriodic acid (HI)-reducible and carbon-bonded forms of S were obtained by varying the nature of the soil treatment (conditioned, air-dried, glucose treated) and duration of incorporation period. Mobilization of the recently-formed 35S-labelled organic S was measured using an open incubation system. There was a substantial difference between samples in the proportions of incorporated 35S mobilized during a 10 week incubation. Irrespective of the nature of the original soil treatment, the proportion of incorporated 35S mobilized during open incubation decreased with an increased duration of the incorporation period. Recently-formed organic S was much more readily mobilized than the bulk of the organic S present in the soil. For samples where incorporation of 35S had taken place in the absence of added glucose, mobilized 35S was derived from both HI-reducible and C-bonded forms of organic S. There appeared to be no difference in the ease of mobilization of S from these two forms. For samples where incorporation of 35S had taken place in the presence of glucose, mobilized 35S appeared to be derived exclusively from C-bonded forms of S and some small increases were observed in 35S-labelled HI-reducible S during the 10 week incubation. The observed decrease in mobilization of recently-formed organic S with increased duration of the incorporation period is intrpreted as a gradual conversion of incorporated S to less-easily mobilized forms.

Effects of lime and two incubation techniques on sulfur mineralization in a forest soil

Limed and unlimed mor humus was incubated at 15°C in an open system and a closed one to evaluate the sulfur mineralization capacity in a Norway spruce stand in southern Sweden. Samples were collected 6yr after dolomitic lime (8800 kg ha −1) had been applied to determine the long-term effect of liming on sulfur mineralization. The open-incubation columns were periodically leached with a simulated throughfall solution, lacking sulfate. Soil in the open and closed incubation vessels was destructively sampled and extracted with distilled water. Extracts and percolates were analyzed for NO 3 −1, SO 4 2− and total S. Soil microbial activity was monitored as CO 2 evolution. The open incubation resulted in a higher net S mineralization in both the limed and control humus; i.e. 24.47 vs 14.17 and 13.60 vs —0.60μg SO 2− 4-S g −1 dw 82 days −1, in the control and limed treatment respectively. Nitrate formation was mainly affected by the liming itself, and differences related to incubation technique were only observed in the control treatment. Limed humus had a lower net S mineralization regardless of the incubation technique used. Organic S leaching from the open incubation columns was higher in the limed treatment than in the control, 17.6 and 6.9μg Sg −1dw 82 days −1, respectively. Limed humus had the highest microbial activity irrespective of the incubation technique. Immobilization of sulfate seemed to be greater in the limed incubation than in the control. The fact that the organic C-to-S ratio was lower in the limed percolation water than in the control also indicates that such a difference exists.

Sulphur mineralization and release of soluble organic sulphur from camp and non-camp soils of grazed pastures receiving long-term superphosphate applications

Topsoils (0–75 mm) from four soil types with different sulphate retention capacities were collected from stock camp and non-camp (main grazing area) sites of grazed pastures in New Zealand which had been annually fertilized with superphosphate for more than 15 years. These soils were analysed for different S fractions and incubated at 30°C for 10 weeks using an open incubation technique in order to assess the extent of S mineralization and the release of soluble soil organic S from camp and non-camp soils during incubation. The soils were preleached with 0.01 M KCl, followed by 0.04 M Ca(H2PO4)2 before being incubated. Pre-incubation leachates and weekly 0.01 M KCl leachates were analysed for mineralized S (i.e., hydriodic acid-reducible S) and total S. Soluble organic S was estimated as the difference between these two S fractions. Results obtained show higher cumulative amounts of all three S fractions in leachates over a 10-week incubation period in camp than in non-camp soils, suggesting that higher mineralization occurred in camp soils. Cumulative amounts of mineralized S from camp and non-camp soils showed a linear relationship with duration of incubation (R 2≥0.985***), while the cumulative release of soluble organic S followed a quadratic relationship (R 2≥0.975***). A significant proportion (14.6%–40.8%) of total S release in KCl leachates was soluble organic S, indcating that organic S should be taken into account when assessing S mineralization. Mineralized S and soluble organic S were best correlated with 0.01 M CaCl2-extractable soil inorganic S (R 2=0.767***) and 0.04 M Ca(H2PO4)2-extractable soil inorganic S(R 2=0.823***), respectively. Soil sulphate retention capacity was found to influence amounts of mineralized S and soluble organic S, and thus periodic leaching with KCl to remove mineralized S from soils may not adequately reflect the extent of soil S mineralization in high sulphate-retentive soils. In low (<10%) sulphateretentive soils, increasing the superphosphate applications from 188 to 376 kg ha−1 year−1 increased S mineralization but not amounts of C-bonded and hydriodic acid-reducible soil S fractions.

Sulphur mineralisation and transformations in soils as influenced by additions of carbon, nitrogen and sulphur

The mineralisation of soil organic sulphur was examined for two soils using an open incubation system. Combinations of glucose-carbon, nitrate-nitrogen and sulphate-S were added to the incubated soils at regular intervals to examine the effects of these nutrients on S mineralisation. Soil microbial activity was monitored by CO 2 evolution from the incubated samples. At the end of the 14-week incubation, the incubated soils were analysed to determine the sourcesof S mineralised during the incubation. In both soils, the addition of C, N and S had a considerable effect on S mineralisation. Additions of C or S decreased S mineralisation whereas N additions slightly enhanced mineralisation.The mineralised S appeared to be derived almost exclusively from C-bonded forms of soil organic S. In both soils, there were substantial increases in hydriodic acid (HI)-reducible forms of S during the incubation.Additions of C, N and S influenced the amounts of S mineralised or transformed from or to C-bonded and HI-reducible forms of soil organic S. The results from this study are discussed in relation to the dichotomous model of S cycling in soil as proposed by McGill and Cole ( Geoderma, 26, 267–286, 1981). In some respects, the S mineralisation data appeared to support the model. However, in relation to the source of mineralised S, there were large differences between what the model would predict for certain treatments and what was actually observed. This was particularly true with regard to the transformations of HI-reducible forms of organic S.

Accumulation of soil organic matter and the forms, mineralization potential and plant-availability of accumulated organic sulphur: Effects of pasture improvement and intensive cultivation

The amount and distribution of organic C, N and S in the soil from a 37 yr old grazed pasture supplied with superphosphate at rates of 0, 188 and 376 kg ha −1 annually was investigated. Results were compared with a wilderness site which had not been used for agriculture and an arable site that had been intensively cultivated for 11 consecutive years. Levels of organic C, S and total N in the 0–4 cm soil layer followed the order 376 = 188 > control ⩾ wilderness > arable. Compared with the control, accumulation of total N and organic S in the 188 and 376 treatments was proportionately greater than that of C reflecting the inputs of fertilizer sulphate-S and increased clover growth and N 2 fixation in superphosphate-treated plots. Under wilderness and grazed pasture there was a rapid decline in soil organic matter content, biomass C and N and arylsulphatase activity with increasing soil depth whilst in the arable soil these properties were uniformly distributed within the surface 20 cm. Mineralization of S from the various soils (0–4 cm layer) as measured in an open incubation followed the order 376 > 188 > wilderness > arable > control. When measured by plant uptake in a pot experiment the same order of mineralization was followed except S uptake from control exceeded that from arable. This was attributed to poor soil physical properties limiting growth and S uptake from the arable soil. Differences in total organic S content, hydriodic acid-reducible and C-bonded S and arylsulphatase activity between treatments did not explain the magnitude of S mineralization for the various treatments. Bicarbonate-extractable and heat-soluble organic S appeared to be better indicators of potentially mineralizable organic S than other indices since they showed the trend 376 > 188 > control and the amount extracted from wilderness was greater than that from control and arable.

Sulphur mineralisation in some New Zealand soils

An open incubation technique was used to measure S mineralisation in a range of New Zealand soils. For most of the soils studied, the release of S as sulphate was curvilinear with time, and during a 10-week incubation, the amounts of S mineralised ranged from less than 3 μg S g-1 soil to more than 26 μg S g-1 soil. The best predictor of mineralised S appeared to be the amount of C-bonded S in the soil (explaining 59% of the variation in mineralised S between soils). Examination of the soils after incubation also revealed that the bulk of the mineralised S was derived from the C-bonded S pool. Hydriodic acid-reducible forms of organic S appeared to make little contribution to mineralised S.

Open-incubation, diffusion methods for measuring solute reaction rates in sediments

Sedimentary solute distributions and fluxes are determined in part by the kinetics of production/consumption reactions in a deposit. It is possible to estimate rates and investigate kinetic relations in undisturbed or manipulated sediments by documenting build-up or depletion patterns of solutes allowed to diffuse either between relatively thin sections of sediment and a well-stirred water reservoir or through a large section of sediment without an overlying reservoir. The time-dependent concentrations in the sediment in both cases depend on reaction rates, kinetics, diffusion coefficients, and geometric scaling of the sediment and contacting reservoir. The water reservoir size and sediment thickness can be altered to allow rapid estimates of reaction rates at near steady-state or to examine nonsteady-state behavior. Nonsteady-state models are always required for the whole-core incubation method, which has the advantages that it is less labor-intensive than many other rate measurement methods and the incubations can be performed in situ. Experimental comparisons between open-incubation and more traditional closed-incubation estimates of reaction rates show good agreement for solutes such as HN{sub 4}(+), SO4(--), HPO{sub 4}(-) and I(-). In some cases, e.g., Mn(++), Fe(++), and HPO{sub 4}(-) production, where major back-reactions with sediment occur, open-incubations without substantial build-up of solutes may providemore » the most accurate method for estimating production rates. In principle, the open incubation methods described can be used for any diffusable species. 31 refs., 19 figs., 1 tab.« less