Occluded Organic Matter Research Articles

Storage effects on the community level physiological profiles of Mediterranean forest soils

Community level physiological profiles (CLPPs) have been implemented in many European soil monitoring programmes as a rapid tool for the assessment of the functional properties of soil microbial communities. However, this technique has not been harmonized among different laboratories. In this experiment, the changes of the CLPPs after storage of three Mediterranean forest soils were characterised for two sampling depths. The methods to store the samples were cooling, freezing and room temperature storage. Data presented in this study provide evidence that substantial changes can occur to the soil microbial community functions, regardless of the kind of storage. Changes in functional diversity and substrate evenness depended on the profile and sampling depth. The rate of consumption of the groups of substrates, however, shifted towards an enhanced utilisation of carboxylic acids, phenols and/or amines after 1 month storage at 4 °C, regardless of the profile or sampling depth. This was attributed to the physical disruption of soil aggregates and exposure of relatively recalcitrant occluded organic matter. These effects levelled out during 1-year storage. The great sensitivity of CLPPs to storage treatment suggests to take care for immediate analysis after sampling. In case fresh samples are not available, freezing is recommended as a convenient alternative; in that case, however, the kinetic approach is required for data analysis.

Soil Organic Matter Fractions in Humid Tropics as Influenced by Application of Crop Residues

The contribution of crop residues to soil organic matter (SOM) in the sand size (particulate organic matter, POM) and sodium (Na) Iodide light (NAL) fractions were determined from soil samples (0–20 cm) collected from an experiment (April 1997–August 1999) studying the contribution of crop residues in sustaining yields of maize (Zea mays L.) and groundnut (Arachis hypogaea L.) in a crop rotation system. The treatments were: recommended inorganic fertilizer (NPK) with crop residues (T1), recommended inorganic fertilizer without crop residues (T2), and half dose of the recommended inorganic fertilizer with crop residues and combined with 10 t ha−1 chicken manure (T3). Soil organic matter size fractionation was carried out for samples collected after harvest of the second (1998) and fourth (1999) crop whereas density fractionation was carried out for samples collected after harvest of the fourth crop. Results showed that dry matter content of the POM, organic carbon (C) and nitrogen (N) relative to total soil dry weight, total soil C and total soil N in T1 and t3 plots were statistically similar to T2 plots. For all treatments, these values were found to be 1.8–3.3%, 8.5–18.1% and 4.1–7.8% for total dry matter, content of C and N, respectively. After two years, 67% of the POM dry matter weight was recovered in the small size (0.125–0.053 mm) compared to 13% in the 2.0–0.25 mm size particles. Also for all treatments, C and N in the light fractions (LF) were 0.5–0.7%, 0.6–2.1%, 0.4–1.0% (in the free LF) and 0.03–0.08%, 0.02–0.09%, 0.04–0.09% (in the occluded LF). Dry matter content of the occluded organic matter was significantly (P≤0.05) lower in T3 plots (0.93%) compared to T1 and T2 (0.13–0.2%). It was concluded that during application of organic inputs in the humid tropics, two years study was not sufficient for positive accumulation of labile SOM.

Study of free and occluded particulate organic matter in soils by solid state 13C Cp/MAS NMR spectroscopy and scanning electron microscopy

A simple densimetric method for the separation of free and occluded particulate organic materials was developed and applied to five virgin soils. The free organic matter was isolated by suspending the soil in sodium polytungstate solution (d = 1.6 Mg m-3) and decanting the light material. The remaining soil was disaggregated by sonification for liberation of occluded organic materials. The free light fraction consisted of large, undecomposed or partly decomposed root and plant fragments. This fraction comprised 0.59-4.34% of soil dry weight and accounted for 6.9-31.3% and 5.9-22.1% of total soil carbon and nitrogen respectively. Identifiable components of the occluded fraction were small particles of incompletely decomposed organic residues, pollen grains, particles of plant tissue such as lignin coils and phytoliths. This fraction comprised 0.69-1.81% of soil dry weight and represented 9.2-17.5% and 6.2-14.1% of the total soil carbon and nitrogen. The proportion of soil organic carbon recovered as the occluded fraction was high in soils with high clay contents. The chemical composition of occluded and free organic materials was investigated by solid-state 13C CP/MAS NMR spectroscopy. Despite the differences in soils, environmental conditions and vegetation, the organic structure of the free light fraction was similar in four of the five soils. This fraction consisted of 55-63% O-alkyl C, 18-25% alkyl C, 14-18% aromatic C, and 5-7% carbonyl C. In the other soil, this fraction showed a higher proportion of alkyl C (31%) and lower O-alkyl C (46%). Most of the differences between soils were associated with organic materials contained in the occluded light fraction. The differences in chemical structure between the occluded light fraction and free light fractions were similar in all examined soils. The NMR data showed that the proportion of O-alkyl C was lower and alkyl C higher in the occluded light fractions than in the free light fractions. The proportion of aromatic and carbonyl carbon was higher in the occluded fractions of three soils while the percentage of these two types of carbon remained unchanged in the two other soils. It is considered that the occluded organic matter is an old pool of carbon that has been accreted within aggregates during decades of root growth and it is that pool which is lost due to cultivation.