Epidemiological aspects of bacterial wilt of fodder grasses1

In order to explain the occurrence of very severe or more moderate wilt of grasses attacked by Xanthomonas campestris pv. graminis, various observations on the factors possibly influencing the epidemics of the disease are summarized in this article. Although natural vectors exist, the main means of transmission is certainly mowing tools. Resistance of the plants can be genetically predetermined or induced by non-pathogenic phyllosphere bacteria, if these are inoculated at the same time as the pathogen and in sufficient numbers. Young seedlings are particularly susceptible. Strains of X.c. graminis vary in their pathogenicity. Non-pathogenic strains enhance host resistance. Very severe attacks are related to extremely virulent strains. Crop management can influence the disease which is favoured by various stress situations. Disease spreads extensively when grass is mown under wet conditions. In conclusion, the following measures are recommended to avoid heavy attacks: use of resistant grass cultivars; avoiding contaminated mowing tools, especially for the first cut; cutting under dry conditions.     

Chlorfenprop-methyl: its hydrolysis in vivo and in vitro and a new principle for selective herbicidal action

Chlorfenprop-mèthyl is hydrolysed completely as soon as it penetrates the leaf in sensitive as well as in resistant plants. The product of hydrolysis chlorfenprop presumably is the herbicidally active compound. The in vitro hydrolysing activity has been characterized. The behaviour of the herbicide in a system for the detection of rapid herbicidal activity (leaching system) and in three auxin-dependent systems leads to the conclusion, that a difference at the site of action is the basis for the observed herbicidal selectivity.     

Ligno-aliphatic complexes in soils revealed by an isolation procedure: implication for lignin fate

For the last decades, the fate of lignins in soil was analyzed mainly with cupric oxide (CuO) oxidation, which is traditionally used to quantify soil lignin content and characterize its state of degradation. This method presents limitations due to incomplete depolymerization of the lignin structure. In this study, we used a physicochemical soil lignin isolation procedure, which permits recovery of a milled wall enzymatic lignin (MWEL) fraction. Elemental composition and chemical structure of MWEL isolated from plants and soil were characterized. Its incorporation rate into an agricultural loamy soil was studied using stable isotope analyses of MWEL isolated from soils after 0 to 9 years of maize cultivation after wheat. Comparison of MWEL isolated from maize tissues and soil provided information on evolution of the lignin structure once incorporated into soil. We observed aromatic–aliphatic complex formation, which could lead to its sequestration in soil evidenced by increasing MWEL content after 9 years of maize cultivation. The ¹³C natural abundance of isolated MWEL showed faster incorporation of MWEL (17.4 % of renewed lignins after 9 years) compared to total soil organic matter (9 % of total soil organic carbon (SOC) was renewed). This faster incorporation rate of MWEL compared to bulk soil organic matter is in agreement with lignin turnover observed by CuO oxidation. Radiocarbon dating of MWEL suggested a mean age of around 50 years. We conclude that lignin isolation allows (1) access to a different fraction compared to CuO oxidation and (2) a detailed characterization of lignin transformation in soil. We suggest that interaction with aliphatic compounds could be one possible pathway of lignin preservation in soil.     

Organic matter stabilization in two Andisols of contrasting age under temperate rain forest

Recent studies with Andisols show that the carbon (C) stabilization capacity evolves with soil age relative to the evolution of the mineral phase. However, it is not clear how soil mineralogical changes during pedogenesis are related to the composition of soil organic matter (SOM) and ¹⁴C activity as an indicator for the mean residence time of soil organic matter (SOM). In the present study, we analyzed the contribution of allophane and metal–SOM complexes to soil C stabilization. Soil organic matter was analyzed with solid-state ¹³C nuclear magnetic resonance spectroscopy. Additionally, the soil was extracted with Na-pyrophosphate (Al_p, Fe_p) and oxalate (Al_o, Si_o, and Fe_o). Results supported the hypothesis that allophane plays a key role for SOM stabilization in deep and oldest soil, while SOM stabilization by metal (Al and Fe) complexation is more important in the surface horizons and in younger soils. The metal/C_p ratio (C_p extracted in Na-pyrophosphate), soil pH, and radiocarbon age seemed to be important indicators for formation of SOM–metal complexes or allophane in top- and subsoils of Andisols. Changes in main mineral stabilization agents with soil age do not influence SOM composition. We suggest that the combination of several chemical parameters (Al_p, Fe_p and C_p, metal/C_p ratio, and pH) which change through soil age controls SOM stabilization.     

Stabilization of organic matter by soil minerals — investigations of density and particle‐size fractions from two acid forest soils

We tested the hypothesis whether organic matter in subsoils is a large contributor to organic carbon (OC) in terrestrial ecosystems and if survival of organic matter in subsoils is the result of an association with the soil mineral matrix. We approached this by analyzing two forest soil profiles, a Haplic Podzol and a Dystric Cambisol, for the depth distribution of OC, its distribution among density and particle‐size fractions, and the extractability of OC after destruction of the mineral phase by treatment with hydrofluoric acid (HF). The results were related to indicators of the soil mineralogy and the specific surface area. Finally, scanning electron microscopy combined with energy dispersive X‐ray spectroscopy (SEM‐EDX) was used to visualize the location of OC at mineral surfaces and associations with elements of mineral phases. The subsoils (B and C horizons) contained 40—50% of the soil OC including the organic forest floor layers. With increasing depth of soil profiles (1) the radiocarbon ages increased, and (2) increasing portions of OC were either HF‐soluble, or located in the density fraction d >1.6 g cm^—3, or in the clay fraction. The proportions of OC in the density fraction d >1.6 g cm^—3 were closely correlated to the contents of oxalate and dithionite‐citrate‐bicarbonate‐extractable Fe (r² = 0.93 and 0.88, P <0.001). SEM‐EDX analyses suggested associations of OC with aluminum whereas silicon‐enriched regions were poor in OC. The specific surface area and the microporosity of the soil mineral matrix after destruction of organic matter were less closely correlated to OC than the extractable iron fractions. This is possibly due to variable surface loadings, depending on different OC inputs with depth. Our results imply that subsoils are important for the storage of OC in terrestrial ecosystems because of intimate association of organic matter with secondary hydrous aluminum and iron phases leading to stabilization against biological degradation.     

Fate of lignins in soils: A review

Lignins are amongst the most studied macromolecules in natural environments. During the last decades, lignins were considered as important components for the carbon cycle in soils, and particularly for the carbon storage. Thus, they are an important variable in many soil-plant models such as CENTURY and RothC, and appeared determinant for the estimation of the soil organic matter (SOM) pool-size and its stabilization. Recent studies challenged this point of view. The aim of this paper was to synthesise the current knowledge and recent progress about quantity, composition and turnover of lignins in soils and to identify variables determining lignin residence time. In soils, lignins evolve under the influence of various variables and processes such as their degradation or mineralization, as well as their incorporation into SOM. Lignin-derived products obtained after CuO oxidation can be used as environmental biomarkers, and also vary with the degree of degradation of the molecule. The lignin degradation is related to the nature of vegetation and land-use, but also to the climate and soil characteristics. Lignin content of SOM decreases with decreasing size of the granulometric fractions, whereas its level of degradation increases concomitantly. Many studies and our results suggest the accumulation and potential stabilization of a part of lignins in soils, by interaction with the clay minerals, although the mechanisms remain unclear. Lignin turnover in soils could be faster than that of the total SOM. Different kinetic pools of lignins were suggested, which sizes seem to be variable for different soil types. The mechanisms behind different degradation kinetics as well as their potential stabilization behaviour still need to be elucidated.     

Lignin turnover kinetics in an agricultural soil is monomer specific

Lignin transformation and decomposition products are generally considered a major source of stable soil organic matter (SOM). Nevertheless this process remains poorly understood in part because lignin is a heterogeneous biopolymer composed of several types of phenol monomers, which potentially display specific and contrasting decomposition kinetics in soils. Here, we compared the specific in situ turnover kinetics of individual lignin monomers in a Paris-basin loamy soil through natural ¹³C labeling of SOM generated in a series of 1-9-year chronosequences of maize monoculture (C4, δ¹³C −12‰) replacing wheat (C3, −27‰). Lignin monomers were released by CuO oxidation, quantified by gas chromatography (GC)/flame ionization detection (FID) and their ¹³C content was determined by GC coupled via a combustion interface to isotope ratio mass spectrometry (GC/C-IRMS). We calculated the proportion of C4-derived OC in lignin monomer pools by applying the isotopic mass balance equation to each lignin monomer.Individual C4-derived phenols displayed contrasting accumulation rates in soils over time, confirming the monomer-specific nature of their transformation kinetics. In proportion to total lignin phenols in soils, syringyl (S) and cinnamyl (C) phenols from maize accumulated faster than their vanillyl (V) counterparts. Consequently, the turnover kinetics of lignin-derived V-moieties may be slower than those of S and C ones. Incorporation of maize-derived carbon was faster in the aldehyde than in the acid pool for V-type units, while the opposite was observed for S-units. These in situ trends for phenol monomers and V-, S- and C-moieties were remarkably similar to the trends described in the literature with laboratory incubation or litterbag studies.None of the observed kinetics had a linear shape. Using only the extreme points of the chronosequence to calculate the kinetic parameters would result, for all the lignin monomers, in underestimating the turnover kinetics at the beginning of the kinetics and overestimating it for longer times. This observation underlines the importance of comprehensive ¹³C time-sequence labelling experiments such as provided at the Closeaux site, to accurately compute the kinetic parameters of SOM for the 1st years after the vegetation change.