Using the Gini coefficient with BIOLOG substrate utilisation data to provide an alternative quantitative measure for comparing bacterial soil communities

A measure quantifying unequal use of carbon sources, the Gini coefficient (G), has been developed to allow comparisons of the observed functional diversity of bacterial soil communities. This approach was applied to the analysis of substrate utilisation data obtained from using BIOLOG microtiter plates in a study which compared decomposition processes in two contrasting plant substrates in two different soils. The relevance of applying the Gini coefficient as a measure of observed functional diversity, for soil bacterial communities is evaluated against the Shannon index (H) and average well colour development (AWCD), a measure of the total microbial activity. Correlation analysis and analysis of variance of the experimental data show that the Gini coefficient, the Shannon index and AWCD provided similar information when used in isolation. However, analyses based on the Gini coefficient and the Shannon index, when total activity on the microtiter plates was maintained constant (i.e. AWCD as a covariate), indicate that additional information about the distribution of carbon sources being utilised can be obtained. We demonstrate that the Lorenz curve and its measure of inequality, the Gini coefficient, provides not only comparable information to AWCD and the Shannon index but when used together with AWCD encompasses measures of total microbial activity and absorbance inequality across all the carbon sources. This information is especially relevant for comparing the observed functional diversity of soil microbial communities

Statistical analysis of reduction in tensile strength of cotton strips as a measure of soil microbial activity

The cotton strip assay (CSA) is an established technique for measuring soil microbial activity. The technique involves burying cotton strips and measuring their tensile strength after a certain time. This gives a measure of the rotting rate, R, of the cotton strips. R is then a measure of soil microbial activity. This paper examines properties of the technique and indicates how the assay can be optimised. Humidity conditioning of the cotton strips before measuring their tensile strength reduced the within and between day variance and enabled the distribution of the tensile strength measurements to approximate normality. The test data came from a three-way factorial experiment (two soils, two temperatures, three moisture levels). The cotton strips were buried in the soil for intervals of time ranging up to 6 weeks. This enabled the rate of loss of cotton tensile strength with time to be studied under a range of conditions. An inverse cubic model accounted for greater than 90% of the total variation within each treatment combination. This offers support for summarising the decomposition process by a single parameter R. The approximate variance of the decomposition rate was estimated from a function incorporating the variance of tensile strength and the differential of the function for the rate of decomposition, R, with respect to tensile strength. This variance function has a minimum when the measured strength is approximately 2/3 that of the original strength. The estimates of R are almost unbiased and relatively robust against the cotton strips being left in the soil for more or less than the optimal time. We conclude that the rotting rate X should be measured using the inverse cubic equation, and that the cotton strips should be left in the soil until their strength has been reduced to about 2/3

Inorganic Nutrients Increase Humification Efficiency and C-Sequestration in an Annually Cropped Soil

<div><p>Removing carbon dioxide (CO₂) from the atmosphere and storing the carbon (C) in resistant soil organic matter (SOM) is a global priority to restore soil fertility and help mitigate climate change. Although it is widely assumed that retaining rather than removing or burning crop residues will increase SOM levels, many studies have failed to demonstrate this. We hypothesised that the microbial nature of resistant SOM provides a predictable nutrient stoichiometry (C:nitrogen, C:phosphorus and C:sulphur–C:N:P:S) to target using supplementary nutrients when incorporating C-rich crop residues into soil. An improvement in the humification efficiency of the soil microbiome as a whole, and thereby C-sequestration, was predicted. In a field study over 5 years, soil organic-C (SOC) stocks to 1.6 m soil depth were increased by 5.5 t C ha^-1 where supplementary nutrients were applied with incorporated crop residues, but were reduced by 3.2 t C ha^-1 without nutrient addition, with 2.9 t C ha^-1 being lost from the 0–10 cm layer. A net difference of 8.7 t C ha^-1 was thus achieved in a cropping soil over a 5 year period, despite the same level of C addition. Despite shallow incorporation (0.15 m), more than 50% of the SOC increase occurred below 0.3 m, and as predicted by the stoichiometry, increases in resistant SOC were accompanied by increases in soil NPS at all depths. Interestingly the C:N, C:P and C:S ratios decreased significantly with depth possibly as a consequence of differences in fungi to bacteria ratio. Our results demonstrate that irrespective of the C-input, it is essential to balance the nutrient stoichiometry of added C to better match that of resistant SOM to increase SOC sequestration. This has implications for global practices and policies aimed at increasing SOC sequestration and specifically highlight the need to consider the hidden cost and availability of associated nutrients in building soil-C.</p></div

Nutrient availability limits carbon sequestration in arable soils

Soils are the largest reservoir of global terrestrial carbon (C). Conversion from natural to agricultural ecosystems has generally resulted in a significant loss of soil organic-C (SOC, up to 50% or ~30-40tha) and 'restoring' this lost C is a significant global challenge. The most stable component of soil organic matter (SOM), hereafter referred to as fine fraction SOM (FF-SOM), contains not only C, hydrogen (H) and oxygen (O), but substantial amounts of nitrogen (N), phosphorus (P) and sulphur (S), in approximately constant ratios. The availability of these associated nutrients is essential for the formation of FF-SOM. Here we show, in short term (56 day) incubation experiments with C labelled wheaten straw added to four soils with differing clay content, that conversion of straw into "new" FF-SOM is increased by up to three-fold by augmenting the residues with supplementary nutrients. We also show that the loss of "old" pre-existing FF-SOM increased with straw addition, compared to soils with no straw addition, but that this loss was ameliorated by nutrient addition in two of our soils. This finding may illuminate why the build-up of SOC in some productive agricultural soils is often much less than expected from the amounts of C-rich residues returned to them because optimum C sequestration requires additional nutrients above that required for crop production alone. Moreover, it provides greater understanding of short-term dynamics of C turnover in soil, and in the longer term, may have important implications for global strategies aimed at increasing soil C sequestration to restore fertility and help mitigate climate change

N, P, S balance (kg ha^-1) for the (−) and (+) supplementary nutrient treatments in 2012 compared to the 2006 starting values.

<p>N, P, S balance (kg ha^-1) for the (−) and (+) supplementary nutrient treatments in 2012 compared to the 2006 starting values.</p

Mean FF-N, -P, -S concentrations (%) to 1.6 m depth in the <2 mm soil fraction in 2006 and the (+) nutrient treatment in 2012.

<p>Data are means and SEM, <i>n = 4</i>. (* indicate a significant difference between 2006 value and 2012 value from (+) nutrient treatment P<0.05; ns = not significant)</p

Measured monthly (blue shaded bars) and long-term mean (red crossed line) rainfall (mm) during the fallow and crop growth periods at the Harden field site.

<p>Measured monthly (blue shaded bars) and long-term mean (red crossed line) rainfall (mm) during the fallow and crop growth periods at the Harden field site.</p

FF-C, -N, -P and -S stocks (t ha^-1) at the Harden field site in 2006 and for the (−) and (+) nutrient treatments in 2012.

<p>Data are means and SEM. <i>n = 4</i>.</p

Correlation between the changing FF-C:N, -C:P and -C:S ratios with depth (0 to 1.6 m) in 2006 and for the (−) and (+) nutrient treatments in 2012, <i>n = 16</i>.

<p>Correlation between the changing FF-C:N, -C:P and -C:S ratios with depth (0 to 1.6 m) in 2006 and for the (−) and (+) nutrient treatments in 2012, <i>n = 16</i>.</p

Mean FF-C concentration (%) to 1.6 m depth in the <2 mm soil fraction in 2006 and in the (+) and (−) nutrient treatments in 2012.

<p>Data are means and SEM, <i>n = 4</i>. (# and * indicate a significant difference between the 2006 value and the 2012 value for the (−) and (+) nutrient treatments, respectively P<0.05; ns = not significant)</p