Determinants of soil organic matter chemistry in maritime temperate forest ecosystems

While the influence of climate, vegetation, management and abiotic site factors on total carbon budgets and turn-over is intensively assessed, the influences of these ecosystem properties on the chemical complexity of soil organic matter (SOM) remains poorly understood. This study addresses the chemical composition of NaOH-extracted SOM from maritime temperate forest sites in Flanders (Belgium) by pyrolysis-GC/MS. The studied forests were chosen based on dominant tree species (Pinus sylvestris, Fagus sylvatica, Quercus robur and Populus spp.), soil texture and soil-moisture conditions. Differences in extractable-SOM pyrolysis products were correlated to site variables including dominant tree species, management of the woody biomass, site history, soil properties, total carbon stocks and indicators for microbial activity. Despite of a typical high intercorrelation between these site variables, the influence of the dominant tree species is prominent. The extractable-SOM composition is strongly correlated to litter quality and available nutrients. In nutrient-poor forests with low litter quality, the decomposition of relatively recalcitrant compounds (i.e. short and mid-chain alkanes/alkenes and aromatic compounds) appears hampered, causing a relative accumulation of these compounds in the soil. However, if substrate quality is favorable, no accumulations of recalcitrant compounds were observed, not even under high soil-moisture conditions. Former heathland vegetation still had a profound influence on extractable-SOM chemistry of young pine forests after a minimum of 60 year

Agro-ecological niche of bacterial wilt (Xanthomonas campestris pv. musacearum) of enset (Ensete ventricosum (Welw.) Cheessman) in Gamo highlands of Ethiopia

In Ethiopia, nearly 20 million people depend on Enset (Ensete ventricosum (Welw.) Cheessman) for food, animal feed and fibre. Most of its cultivation is concentrated in the South and South-Western parts of the country. Enset based farming system is common in the Gamo highlands, with 11, 150 ha currently covered by it. Enset is supplemented with the cultivation of cereals, pulses and other root and tuber crops. Diverse high yielding clones of different ages are available all year round and can withstand dry spells, making Enset the most important food security crop in these densely populated highlands. Despite its relevance, Enset remains very poorly studied. One of the most important problems is Bacterial Wilt disease caused by Xanthomonas campestris pv. musacearum, destructive to Enset at all growth stages and present in all Enset growing areas of the country. As a plant takes up to 8 years to mature, an attack of Bacterial Wilt can leave a family vulnerable to famine for years. It is most likely spread by infected farm tools, harvesting knives, plant debris and repeated transplanting of corms. Adoption of the control package by farmers has been less effective and all known varieties are susceptible, putting a great challenge on controlling the epidemic. Despite being present in the entire Enset belt, the severity of a Bacterial Wilt infection varies greatly from region to region. Hence, this study aims at correlating the distribution and severity of the disease to environmental factors such as soil type, fertility, climate and altitude, and to the socio-economical status of the farm households. A better knowledge of the influence of these factors would help in developing more targeted measures to control the disease

Organic matter of subsoil horizons under broadleaved forest: Highly processed or labile and plant-derived?

Between 30 and 63% of the soil organic matter (SOM) is stored below 30 cm, making subsoil-SOM an important source and sink in the global carbon cycle. Nevertheless, detailed information on the composition of subsoil-SOM remains scarce. This study aims to evaluate the chemical composition of SOM in topsoil and subsoil horizons in broadleaved forests on acid loamy soils. Six sites were chosen in Northern Belgium under beech, oak and hybrid poplar, on Gleysols, Umbrisols, Cambisols and Albeluvisols on loamy Quaternary deposits. Analytical pyrolysis–gas chromatography/mass spectrometry (pyrolysis–GC/MS) was performed on the dialyzed alkaline extract, which represents between 41 and 90% of the total organic carbon for the selected sites. All extracts show a significant shift in chemical composition between the topsoil and the subsoil. While topsoil-SOM mainly differs according to input and nutrient status, subsoil-SOM shows high relative amounts of alkanes and alkenes or polysaccharides for coarse and fine textured soils respectively. Lignins, lignin-derived phenols or aromatics were not major contributors to subsoil-SOM, regardless of soil type. Furthermore, results show that very labile plant-derived molecules are present in the subsoil, i.e. long-chain aliphatics and (cellulose-derived) anhydrosugars. The organic matter signature of the subsoil samples was evaluated for typical indications of fresh material, decay, podzolisation and anaerobic processes, and indicates root input and stabilization of certain labile plant-derived compounds against microbial decay to be important in the subsoil

Paleo-pedological record of the Rocourt Pedosequence at Veldwezelt–Hezerwater

As the environment influences soil formation, loess–paleosols are important archives of former climatic conditions. However, this record is highly complex and all features need to be evaluated in order to obtain consistent information on soil formation. This study revises the pedological record of the Rocourt-paleosol complex, an important pedo-stratigraphic marker for the Eemian and Early Weichselian, at an exceptionally well-preserved in-situ sequence (Veldwezelt–Hezerwater, Belgian loess belt). Contrasting hypotheses exist considering the evolution of the parent material after the deposition of the Saalian loess and on the importance of aggrading aeolian deposition. In this study, soil morphological features and depth functions of X-ray diffraction, grain-size distribution, CEC, base saturation, total Fe, Al, K, Na and Ca content, pH and electrical conductivity were established to reconstruct deposition dynamics, weathering and carbonate content of the parent material. Texture analysis confirms the petrostratigraphic homogeneity of the parent material and suggests that it developed under a regime of fluctuating rates of primary dust deposition intermixed with periods of (fluvial) reworking of the dust. Strong weathering was identified in the lower stratigraphic unit, supporting the general consensus that it formed during the Eemian in Saalian loess. Less information is present to confirm the origin of the upper, less developed illuvial units, which may have formed by welding or by pedogenesis in locally redistributed or aeolian material. The overlying A-horizons are clearly less weathered, and evidence for early recalcification indicates that deposition of unweathered, calcareous sediments gained importance during the formation of these horizons

Differences in chemical composition of soil organic matter in natural ecosystems from different climatic regions: a pyrolysis-GC/MS study

Soil organic matter (SOM) is a key factor in ecosystem dynamics. A better understanding of the global relationship between environmental characteristics, ecosystems and SOM chemistry is vital in order to assess its specific influence on carbon cycles. This study compared the composition of extracted SOM in 18 topsoil samples taken under tundra, taiga, steppe, temperate forest and tropical forest using pyrolysis¿GC/MS. Results indicate that SOM from cold climates (tundra, taiga) still resembles the composition of litter, evidenced by high quantities of levosugars and long alkanes relative to N-compounds and a clear odd-over-even dominance of the longer alkanes. Under temperate conditions, increased microbial degradation generally results in a more altered SOM chemistry. SOM formed under temperate coniferous forests shows an accumulation of aromatic and aliphatic moieties, probably induced by substrate limitations. Tropical SOM was characterized by an SOM composition rich in N-compounds and low in lignins, without any accumulation of recalcitrant fractions (i.e. aliphatic and aromatic compounds). Lignin composition moreover varies according to vegetation type. Results were validated against 13 new samples. The humic signature of topsoil organic matter formed under different biomes indicates a dominating effect of (i) SOM input composition related to vegetation, and (ii) SOM breakdown reflecting both climate and input quality. No evidence was found for a chemically stabilized SOM fraction under favorable decomposition conditions (temperate or warm climate with broadleaved vegetation