Claire Chenu

Relationship of soil organic matter dynamics to physical protection and tillage

Tillage has been reported to reduce organic matter concentrations and increase organic matter turnover rates to a variable extent. The change of soil climate and the incorporation of aboveground C inputs within the soil lead to no unique effect on biodegradation rates, because of their strong interaction with the regional climate and the soil physical properties. The periodical perturbation of soil structure by tools and the subsequent drying‐rewetting cycles may be the major factor increasing organic matter decomposition rates by exposing the organic matter that is physically protected in microaggregates to biodegradation. This paper reviews the assessed effects of tillage on organic matter, the scale, extent and mechanisms of physical protection of organic matter in soils. # 2000 Elsevier Science B.V. All rights reserved.

Clay— or sand—polysaccharide associations as models for the interface between micro-organisms and soil: water related properties and microstructure

Abstract Many soil micorganisms are able to produce extracellular polysaccharides (EPS). Electron microscopic observations of soils demonstrated that EPS are produced in soils and are closely associated with the surrounding clay particles. In the present study, experimental clay-polysaccharide associations were taken as models for the soil/biota interface, and their microstructure and physical properties were investigated. In the methodology, special attention was given to control the water potential and to preserve, as far as possible, the organizations of the original hydrated conditions. EPS increased the water retention of clay minerals or sands on desiccation and on rehydration, and reduced desiccation and rehydration rates. This was explained by the strong water-holding properties of EPS. Cryo-scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used, and showed that the polysaccharides occurred as a network of strands in the interparticle porosity of clay minerals. Polysaccharides changed the clay microstructure into an organo-mineral network with extensive interparticle bridging. The present results show that the clay-polysaccharide associations exhibit specific physical properties and microstructures that should affect biological functions and survival, for example through storage of water and buffering against water potential fluctuations. Clay-polysaccharide sheaths also take part in the binding of aggregates by soil biota.

The dynamics of carbon in particle-size fractions of soil in a forest-cultivation sequence

Cultivation of forest and grassland soils induces heavy changes in soil organic matter (SOM) dynamics. To better predict the effect of cultivation, there is a need to describe which organic pools are affected and to which extent. We used a chronosequence of thick humic forest soils converted to maize cultivation for 40 yr in southwest France. The dynamics of soil carbon was investigated through particle-size fractionation and the use of 13C allowed to distinguish forest-derived organic matter and new crop-derived organic matter. This partitioning of soil carbon by size on one hand and by age on the other provided a precise description of carbon turnover. The level towards which tend the organic pools under cultivation showed that the decay rates of soil carbon were one order of magnitude higher under cultivation than under forest. SOM can thus be considered as deprotected under cultivation. All size fractions appeared to be deprotected to the same extent. A progressive transfer of silt-sized C to clay-sized C was nevertheless suspected and attributed to the decreasing stability of fine silt-sized microaggregates with cultivation. SOM furthermore contained some very stable C present as silt-sized and possibly clay-sized particles. The turnover times of maize-derived organic matter was the same as that observed in similar soils cultivated for centuries. This indicated that the new conditions induced by cultivation were reached in the very first years after forest clearing and that the high initial SOM content and high mineralization rate of initial organic matter did not affect the dynamics of newly incorporated carbon.

A novel pathway of soil organic matter formation by selective preservation of resistant straight-chain biopolymers: chemical and isotope evidence

A resistant soil organic residue, “humin”, has been analyzed by solid-state 13C-NMR and pyrolysis followed by molecular and 13C determination of the pyrolysate alkane–alkene fraction. The results show the occurrence of highly aliphatic, straight-chain biopolymer material as a substantial part of soil organic matter. They confirm the hypothesis by which a part of soil organic matter can be formed by selective preservation of resistant highly aliphatic microbial polymers. This pathway is additional to the longstanding neogenesis hypothesis involving recondensation of small polar molecules such as amino acids, carbohydrates and phenols.

Influence of organic amendments on copper distribution among particle-size and density fractions in Champagne vineyard soils

The intensive use for over 100 years of copper sulfate (Bordeaux mixture) to fight against mildew in vineyard soils has led to an important, widespread accumulation of Cu (100 to 1500 mg Cu kg-1 soil). In Champagne vineyards, organic amendments are used currently to increase soil fertility and to limit soil erosion. Organic amendments may have a direct effect on the retention of Cu in the soil. To assess the influence of the organic management on the fate of Cu in calcareous Champagne vineyard soils, we studied Cu distribution (1) in the soil profile and (2) among primary soil particles, in vineyard parcels with different amendments. Amendments were oak-bark, vine-shoots and urban compost. The results were compared with the amount and the distribution of Cu in an unamended calcareous soil. Physical soil fractionations were carried out to separate soil primary particles according to their size and density. Cu has a heterogeneous distribution among soil particle fractions. Two fractions were mainly responsible for Cu retention in soils: the organic debris larger than 50 microns or coarse particulate organic matter (POM) issued from the organic amendments, and the clay-sized fraction < 2 microns. The POM contained up to 2000 mg Cu kg-1 fraction and the clay fraction contained up to 500 mg Cu kg-1 fraction. The clay-sized fraction was responsible for almost 40% of the total amount of Cu in the four parcels. POM was predominantly responsible for the differences in Cu contents between the unamended and the three amended parcels. Our results attested that methods of soil particle-size fractionation can be successfully used to assess the distribution of metal elements in soils.

The role of roots, fungi and bacteria on clay particle organization. An experimental approach

Experimental studies were conducted to investigate the role of different living organisms (roots, fungi, bacteria) on clay microfabric. For this purpose various microorganisms (bacteria, fungi) and roots were allowed to grow on kaolinite or montmorillonite pastes under controlled hydric conditions (−0.01 to −102 MPa). Microstructures were studied with a cold-stage SEM, that allowed the preservation of the organizations that characterized the wet states. With fungi, three main effects were observed in moist conditions: orientation of clay particles around the cells; secretion of extracellular polysaccharides that induced local binding of clay particles, and a general packing effect by hyphae. These effects lead to a new microstructure, in the immediate surrounding of the cell, designated as a microenvironment. A modified microstructure was recognized with all species, the size depending on the size of organism. With bacteria, polysaccharide-mediated aggregation was predominant. With grass roots, modifications of the microstructure were more complex than with fungi and occurred at a larger scale. The present results thereby confirm and complete the existing models on biologically mediated aggregation in soils.

Influence of a fungal polysaccharide, scleroglucan, on clay microstructures

Abstract The microorganization of complexes formed between clay minerals (Ca-kaolinite, Ca-montmorillonite) and a fungal polysaccharide (scleroglucan) was studied with water content and apparent volume measurements. X-ray diffractometry and scanning electron microscopy. Scleroglucan did not appear to intercalate montmorillonite. The fabric of the complexes was a card-house structure for koalinite and a three-dimensional network of quasicrystals for montmorillonite, both with enhanced porosities. The present results indicate that the strong water-stabilizing effect of fungal polysaccharides occurs without major microstructural rearrangements but could be related to the formation of stable organo-mineral networks.

Soil Security: Solving the Global Soil Crisis

Soil degradation is a critical and growing global problem. As the world population increases, pressure on soil also increases and the natural capital of soil faces continuing decline. International policy makers have recognized this and a range of initiatives to address it have emerged over recent years. However, a gap remains between what the science tells us about soil and its role in underpinning ecological and human sustainable development, and existing policy instruments for sustainable development. Functioning soil is necessary for ecosystem service delivery, climate change abatement, food and fiber production and fresh water storage. Yet key policy instruments and initiatives for sustainable development have under-recognized the role of soil in addressing major challenges including food and water security, biodiversity loss, climate change and energy sustainability. Soil science has not been sufficiently translated to policy for sustainable development. Two underlying reasons for this are explored and the new concept of soil security is proposed to bridge the science–policy divide. Soil security is explored as a conceptual framework that could be used as the basis for a soil policy framework with soil carbon as an exemplar indicator.

Ecosystem services provided by soils of urban, industrial, traffic, mining, and military areas (SUITMAs)

PurposeThe sustainable use and management of global soils is one of the greatest challenges for the future. In the urban ecosystem, soils play an essential role with their functions and ecosystem services. However, they are still poorly taken into consideration to enhance the sustainable development of urban ecosystems. This paper proposes a categorization of soils of urbanized areas, i.e., areas strongly affected by human activities, according to their ecosystem services.Materials and methodsFocus is put first on ecosystem services provided by non-urban soils. Then, the characteristics and number of services provided by soil groups of urbanized areas and their importance are given for each soil group.Results and discussionSoils of urbanized areas are here defined as SUITMAs, because they include soils of urban, sensu stricto, industrial, traffic, mining, and military areas. This definition refers to a large number of soil types of strongly anthropized areas. SUITMAs were organized in four soil groups, i.e., (1) pseudo-natural soils, (2) vegetated engineered soils, (3) dumping site soils, and (4) sealed soils. For each soil group, examples for ecosystem services were given, evaluated, and ranked.ConclusionsThis proposal contributes to foster the dialogue between urban spatial planning and soil scientists to improve both soil science in the city and recognition of SUITMAs regarding their role for the sustainable development of urban ecosystems and, in particular, to enhance multifunctional soils in urban areas.

Diffusion of glucose in microbial extracellular polysaccharide as affected by water potential

Abstract Extracellular polysaccharide secretion (EPS) is a salient feature of many soil bacteria. This study investigates whether an EPS shell modulates the diffusion rate of nutrients in the immediate microenvironment of soil microorganisms. The diffusion of glucose through pure microbial polysaccharides xanthan and dextran, and through kaolinite and EPS-amended kaolinite was measured at several water potentials using a steady state method. A specific device was developed to simultaneously monitor water potential (Ψ) using polyethylene glycol (PEG) solutions, and measure diffusion rates. The diffusion rate of glucose increased with increasing Ψ and volumetric water content in both polysaccharides. At all Ψ values studied, glucose diffused faster in EPS or in EPS-amended clay than in pure clay, due to the higher volumetric water content of EPS and their water-saturated porosity. Water retention and diffusion characteristics in EPS may thus help soil bacteria to maintain physiological functions at low water potential.