David S. Powlson

The effects of biocidal treatments on metabolism in soil—V: A method for measuring soil biomass

A new method for the determination of biomass in soil is described. Soil is fumigated with CHCl3 vapour, the CHCl3 removed and the soil then incubated. The biomass is calculated from the difference between the amounts of CO2 evolved during incubation by fumigated and unfumigated soil. The method was tested on a set of nine soils from long-term field experiments. The amounts of biomass C ha−1 in the top 23 cm of soil from plots on the Broadbalk continuous wheat experiment were 530 kg (unmanured plot), 590 (plot receiving inorganic fertilizers) and 1160 (plot receiving farmyard manure). Soils that had been fallowed for 1 year contained less biomass than soils carrying a crop. A calcareous woodland soil contained 1960 kg biomass C ha−1, and an unmanured soil under permanent grass 2020. The arable soils contained about 2% of their organic C in the biomass; uncultivated soils a little more—about 3%.

Measurement of soil microbial biomass provides an early indication of changes in total soil organic matter due to straw incorporation

The straw and stubble of spring barley (Hordeum vulgare; 5t dry matter ha−1) were either burned or incorporated into soil annually for 18 yr in two field experiments in Denmark. Both experiments were on light soils situated at Studsgaard (loamy sand) and Ronhave (sandy loam). At both sites 18 yr of annual straw incorporation increased total soil organic C by only 5% and total N by about 10% but produced large increases in microbial biomass measured by the CHCl3-fumigation method. The increases in biomass C were 45 and 37% at Studsgaard and Ronhave, respectively: the corresponding increases in biomass N were 50 and 46%. Biomass measurements thus gave an early indication of slow changes in organic matter content long before these could be measured accurately against the background of organic matter already present in the soils. Increases in biomass P due to straw incorporation appeared to be even greater. However, the amounts of P released by CHC13 were small so the measurements of biomass P were less accurate than those of biomass C or N. During a 60-day laboratory incubation at 25°C, evolution of CO2-C was 55–79% greater in soil from straw incorporated plots than in soil from burned plots. Mineralization of N was 40–50% greater where straw had been incorporated, indicating thaf the long-term incorporation of straw had increased the quantity of mineralizable N in soil.

A comparison of the performance of nine soil organic matter models using datasets from seven long-term experiments

Nine soil organic models were evaluated using twelve datasets from seven long-term experiments. Datasets represented three different land-uses (grassland, arable cropping and woodland) and a range of climatic conditions within the temperate region. Different treatments (inorganic fertilizer, organic manures and different rotations) at the same site allowed the effects of differing land management to be explored. Model simulations were evaluated against the measured data and the performance of the models was compared both qualitatively and quantitatively. Not all models were able to simulate all datasets; only four attempted all. No one model performed better than all others across all datasets. The performance of each model in simulating each dataset is discussed. A comparison of the overall performance of models across all datasets reveals that the model errors of one group of models (RothC, CANDY, DNDC, CENTURY, DAISY and NCSOIL) did not differ significantly from each other. Another group (SOMM, ITE and Verberne) did not differ significantly from each other but showed significantly larger model errors than did models in the first group. Possible reasons for differences in model performance are discussed in detail.

Measurement of microbial biomass phosphorus in soil

A method for measuring the amount of P held in soil micro-organisms (biomass P) is described and the assumptions on which it is based are discussed. Biomass P is calculated from the difference between the amount of inorganic P (Pi) extracted by 0.5 (Spm) NaHCO3 (pH 8.5) from fresh soil fumigated with CHCl3 and the amount extracted from unfumigated soil. Some CHCl3-released Pi is sorbed by soil during fumigation and extraction: an approximate allowance for this is made by incorporating a known quantity of Pi during extraction and correcting for recovery. Most of the P released is in inorganic form and the proportion increases with duration of fumigation. Non-microbial P is little, if at all, affected by fumigation. Microbial biomass P is calculated from CHCl3-released Pi by dividing by 0.4, i.e. by assuming that 40% of the P in the biomass is rendered extractable as Pi by CHCl3. Measurements of biomass P must be done in fresh soil, CHCl3 releases much less P in air-dry soil.

The effects of biocidal treatments on metabolism in soil—I. Fumigation with chloroform

This series of five papers is a study of how biocidal treatments influence metabolism in soil, directed particularly towards the flush of decomposition caused by fumigation, and designed to see if the size of this flush can be used as a measure of the soil biomass. Chloroform fumigation caused an immediate increase in the amounts of ammonium and organic C extracted from a soil by 1 N K2SO4. When the CHCl3-treated soil was then inoculated with fresh soil and incubated for 10 days. it consumed 2·8 times more O2, evolved 2·2 times more CO2 and mineralised 7·3 times more N than an unfumigated soil. Extractable organic C decreased by about 40% when the fumigated soil was incubated for 10 days. A second fumigation given immediately after the first produced no further increase in the flush, but some recovery occurred if the soil was incubated between fumigations. However, this recovery was slow and incomplete; a second fumigation given 53 days after the first gave a flush only one-seventh the size of the first. Glucose (or ryegrass) added to the soil and allowed to decompose before fumigation increased the size of the flush. After a 52-day incubation, 29% of the C originally added as 14C labelled glucose remained in the soil; fumigation on the 52nd day increased the evolution of labelled CO2 during the subsequent 10-day period by a factor of 8. Fumigation of a soil that had already been sterilized by 2·5 Mrads of gamma radiation increased the flush slightly; the amount of O2 consumed in 10 days increased from 123 to 137 mg/100 g soil. It is proposed that the flush of decomposition following CHCl3 fumigation is caused by the decomposition of killed organisms by the survivors (or by organisms added in the inoculum) and that organisms are more rapidly and completely attacked after exposure to CHCl3 than after irradiation. On this hypothesis. 10% of the glucose C originally added to the soil was located in the soil biomass after 52 days.

Phosphorus in the soil microbial biomass

Phosphorus in the soil microbial biomass (biomass P) and soil biomass carbon (biomass C) were linearly related in 15 soils (8 grassland, 6 arable, 1 deciduous woodland), with a mean P concentration of 3.3% in the soil biomass. The regression accounted for 82% of the variance in the data. The relationship was less close than that previously measured between soil biomass C and soil ATP content and indicates that biomass P measurements can only provide a rough estimate of biomass C content. Neither P concentration in the soil biomass, nor the amount of biomass P in soil, were correlated with soil NaHCO3-extractable inorganic, organic or total P. The calculated mean annual flux of P through the biomass (in a soil depth of 10 cm) in 8 grassland soils was large, 23 kg P ha−1 yr−1, and more than three times the mean annual P flux through 6 arable soils (7 kg P ha−1 yr−1), suggesting that biomass P could make a significant contribution to plant P nutrition in grassland. About 3% of the total soil organic P in the arable soils was in microbial biomass and from 5 to 24% in the grassland soils. The decline in biomass P when an old grassland soil was put into an arable rotation for about 20 yr was sufficient to account for about 50% of the decline in total soil organic P during this period. When an old arable soil reverted to woodland, soil organic P doubled in 100 yr; biomass P increased 11-fold during the same period.

Chloroform fumigation and the release of soil nitrogen: The effects of fumigation time and temperature

Fumigation with CHC13 (24 h, 25°C) increased the amount of NH4-N and total N extracted by 0.5 M K2SO4 from two soils (one arable, one grassland). The amount of N released by CHC13 increased with the duration of fumigation up to 5 days, when it levelled off. Between about 10–34% of the total N released by CHC13 was in the form of NH4-N, the proportion increasing with duration of exposure. When a grassland soil that had received a field application of 15N-labelled fertilizer 1 yr previously was fumigated, the N released by CHC13 was 4 times more heavily labelled than the soil N as a whole. Prolonging the exposure of this soil to CHC13 increased the amount of total N released, but hardly altered the proportion of labelled N in the CHC13-released N, suggesting that N is being released from a single soil fraction. The most likely soil fraction is the soil microbial biomass. It is suggested that CHC13 does not alter the K2SO4-extractability of soil-N fractions other than microbial N and that the extra N released by CHC13 and extracted by K2SO4 gives a direct measure of soil microbial biomass N. In contrast to fumigation done at lower temperatures, less total N was released by soil fumigated at 60°C, or above, than was released from unfumigated soil held at the same temperature. The greater release of N in the non-fumigated soils above 60°C could have been due to soil enzymic processes which were inhibited by CHC13 in the fumigated soil.

The effects of biocidal treatments on metabolism in soil—II. Gamma irradiation, autoclaving, air-drying and fumigation

Respiration and mineralisation of N were measured in a set of contrasting soils that had either been autoclaved, air-dried, fumigated (with chloroform or methyl bromide) or exposed to gamma radiation. The soils used were a manured and an unmanured arable soil, an acid and a neutral woodland soil, an arable sandy soil and an organic soil under grass. With the exception of the acid woodland soil, the flushes of decomposition (i.e. the increases in O2 consumption, CO2 evolution and N mineralisation that occurred when the treated soil was inoculated and incubated for 10 days) were in the order: air-drying < CH3Br ⩽ CHCl3 < irradiation < autoclaving. All of the treatments, except air-drying, decreased the ratio (C mineralised after treatmcnt)/(N mineralised after treatment). All of the treatments increased the amount of 1N K2SO4 extractable organic C, autoclaving causing by far the greatest increase. Neither of the fumigants increased respiration in the acid soil over the whole 10 day period, although N mineralisation was slightly increased. Irradiation, air-drying and autoclaving did, however, produce a flush in the acid soil, the order being: irradiation < air-drying < autoclaving. A soluble substrate, extracted from yeast cells by ultrasonic disintegration, decomposed to about the same extent in neutral and in acid soil. When 14C labelled glucose was added to the acid soil and incubated for 52 days, the retention of labelled C was slightly greater (31·6%) than in a comparable near-neutral soil (28·8%). However, the flush that followed fumigation of the acid soil was only half that in the near-neutral soil, suggesting that less biomass is formed under acid conditions. Liming increased the size of the flush in an acid soil. For soils from the same field but under different management, the size of the flush caused by CHCl3 is in the order: grassland > cropped arable > bare fallow. The flush is much more sensitive to differences in soil management than is the total amount of soil organic matter; a fallowed soil lost half its organic C in 10 yr whereas the increase in respiration that followed fumigation fell to one-seventh its original value. Two Nigerian soils behaved similarly; a soil that had been 2 years under cultivation contained only 16% less total organic C than an adjacent soil still under secondary forest, yet the flush in the cultivated soil was half that in the forest soil. The amount of substrate metabolised during the flush is thus very sensitive to changes in soil management that alter the amount of fresh organic matter entering the soil each year.

Nitrogen Mineralization in Temperate Agricultural Soils: Processes and Measurement

Publisher Summary Soils form a major repository of nitrogen (N) within both natural and agricultural terrestrial ecosystems, containing, on a global basis, an estimated 2.4 x 10 11 tons of N. The soil receives N inputs through fertilizer additions and from the atmosphere in precipitation and dry deposition or via biological fixation; inputs are also made in plant and animal residues. N is removed in the harvested crop and is lost by leaching and surface run-off of soluble forms, by gaseous transfer as N gas and N oxides (during nitrification and denitrification processes), and by ammonia volatilization. In some circumstances, erosion may also be important. In addition to these interactions with the total ecosystem, internal cycles also operate within the soil, so that even if gains and losses are in balance, then N still continues to cycle in the soil. This chapter describes the current understanding of the conceptual basis of the processes involved in mineralization, relationships among the processes and other factors, and how their effects can be determined practically. The aim is to present this in a way that is relevant to current and future agricultural development and to environmental issues.

Methane oxidation in soil as affected by land use, soil pH and N fertilization☆

Net uptake of CH4 was measured in intact soil cores (6.4 cm dia, 12 cm deep) collected from an arable wheat field, from three sites left uncultivated for more than 110 years following arable cropping and from a permanent grassland with different mineral N treatments subdivided into four pH levels. Soil cores were incubated in sealed 1 litre jars at 25°C for 48 h with a CH4 -amended atmosphere of 10 μl 1−1 at the start of incubation. The decrease in CH4 concentration followed first-order-kinetics and by log-transformation individual uptake rates could be calculated for each treatment. Soil from a calcareous site (pH 7.4) under deciduous woodland (Broadbalk Wilderness wooded section) oxidized CH4 6 times faster than the arable plot (pH 7.8) with the highest activity in the adjacent Broadbalk Wheat Experiment (with uptake rates of −80 and −13 nl CH4 1−1 h−1, respectively). The CH4 uptake rate was only 20% of that in the woodland in an adjacent area that had been uncultivated for the same period but kept as rough grassland by the annual removal of trees and shrubs and, since 1960, grazed during the summer by sheep. It is suggested that the continuous input of urea through animal excreta was mainly responsible for this difference. Another undisturbed woodland area with an acidic soil reaction (pH 4.1) did not oxidize any CH4. On a permanent grassland site (Park Grass Continuous Hay Experiment), the plot without N fertilization showed a distinct pH effect: CH4 consumption decreased from −67 to − 35nl CH4 1−1 h−1 with decreasing pH in the range 6.3–5.6 and declined to zero between pH 5.6 and 5.1. Mineral N applied annually as (NH4)2SO4, at either 96 or 144 kg N ha −1 for 130 years, completely inhibited CH4 oxidation, even where lime was applied to maintain a soil pH of about 6. By contrast, the long-term application of N as NaNO3 (96 kg N ha−1 a−1) caused no decline in CH4 oxidation compared to unfertilized grassland at the same pH and, in some cases, caused a small increase. Withholding NH4-N for 3 years caused no significant recovery of CH4 -oxidizing activity; withholding NO3-N caused a slight decline. Thus, land use (arable, cut grassland, grazed grassland or woodland), soil pH, N fertilizer inputs and form of N (NH4 or NO3) all have marked and interacting effects on the extent to which aerobic soil acts as a sink for CH4. The mechanisms through which the factors operate are not known but some possibilities are discussed. The results have important implications for the planning of land use and agricultural practices that will maximize the extent to which aerobic soils can act as a sink for CH4.