Dominique Masse

Global decomposition experiment shows soil animal impacts on decomposition are climate-dependent

Climate and litter quality are primary drivers of terrestrial decomposition and, based on evidence from multisite experiments at regional and global scales, are universally factored into global decomposition models. In contrast, soil animals are considered key regulators of decomposition at local scales but their role at larger scales is unresolved. Soil animals are consequently excluded from global models of organic mineralization processes. Incomplete assessment of the roles of soil animals stems from the difficulties of manipulating invertebrate animals experimentally across large geographic gradients. This is compounded by deficient or inconsistent taxonomy. We report a global decomposition experiment to assess the importance of soil animals in C mineralization, in which a common grass litter substrate was exposed to natural decomposition in either control or reduced animal treatments across 30 sites distributed from 43°S to 68°N on six continents. Animals in the mesofaunal size range were recovered from the litter by Tullgren extraction and identified to common specifications, mostly at the ordinal level. The design of the trials enabled faunal contribution to be evaluated against abiotic parameters between sites. Soil animals increase decomposition rates in temperate and wet tropical climates, but have neutral effects where temperature or moisture constrain biological activity. Our findings highlight that faunal influences on decomposition are dependent on prevailing climatic conditions. We conclude that (1) inclusion of soil animals will improve the predictive capabilities of region- or biome-scale decomposition models, (2) soil animal influences on decomposition are important at the regional scale when attempting to predict global change scenarios, and (3) the statistical relationship between decomposition rates and climate, at the global scale, is robust against changes in soil faunal abundance and diversity.

Spatial carbon, nitrogen and phosphorus budget in a village of the West African savanna—II. Element flows and functioning of a mixed-farming system

Abstract The viability of mixed farming systems in West African savannas relies largely on the management of endogenous organic resources. Assessment of the organic balance at both plot and village territory scales is needed as an indicator of this viability. Distribution of carbon (C), nitrogen and phosphorus in soil and plant biomass was thus quantified for a village in southern Senegal across the different land use systems (LUS) and farm holdings. The village exhibited ring-like organisation, including a compound ring, a bush ring, and lowland paddy fields, with positive gradients of intensification and food production from the savanna to the dwellings. Marked contrasts were found between holdings, especially in livestock availability. Clear relationships were evidenced between the spatial distribution of C and nutrients and the agricultural functions of LUS and holdings, while multi-scale diversity appeared to be the main factor that ensured the functioning of the system. Intensification schemes at the village level aimed at increasing organic resources and their cycling efficiency must thus take into account their impact on this diversity.

Interactions between ectomycorrhizal symbiosis and fluorescent pseudomonads on Acacia holosericea: isolation of mycorrhiza helper bacteria (MHB) from a Soudano-Sahelian soil.

Abstract Acacia holosericea seedlings were planted in 1-l pots filled with a soil collected from an Australian Acacia plantation in Southern Senegal. After 6 months of culture, mycorrhizosphere soil, roots, galls induced by root-knot nematodes and Rhizobium nodules were sampled from each pot. The diversity of this bacterial group was characterized by siderotyping (pyoverdine IsoElectric Focusing (IEF) analysis) and by restriction fragment length polymorphism (RFLP). The effect of these isolates on the establishment of the ectomycorrhizal symbiosis between an Australian Acacia (A. holosericea) and Pisolithus sp. strain IR100 was studied. In the mycorrhizosphere soil, the population of fluorescent pseudomonads was represented by strains of two different siderovars (groups of bacterial strains presenting an identical pyoverdine-IEF pattern): siderovar 1 (74%) and siderovar 2 (26%). The siderotyping of the isolates around galls of the root-knot nematodes revealed three siderovars (40% from siderovar 1, 40% from siderovar 2 and about 15% from siderovar 3). RFLP of 16S rDNA divided the isolates into four different groups with MspI, two with HhaI and two with HaeIII endonucleases. The establishment of the ectomycorrhizal symbiosis with A. holosericea was promoted by 14 bacterial strains isolated from the mycorrhizosphere soil, three isolates from the roots and four from the galls. Shoot biomass of A. holosericea seedlings was stimulated by eight bacterial isolates from soil, six isolates from galls and seven from roots. These mycorrhiza helper bacteria could have a great ecological importance in tropical areas through the reforestation programs.

Effect of the raw materials and mixing ratio of composted wastes on the dynamic of organic matter stabilization and nitrogen availability in composts of Sub-Saharan Africa

The effect of raw materials and their proportions in initial mixtures on organic matter (OM) stabilization and nitrogen (N) availability during pit composting in Sub-Saharan Africa was assessed using biochemical fractionation and laboratory incubations to characterize composts sampled throughout the composting process. Stabilization of OM occurred more rapidly in mixtures with slaughter-house wastes, it was progressive in mixture with household refuses while tree leaves compost remained unstable. Carbon mineralization from compost samples was positively correlated to water soluble and hemicellulose-like organic fractions. Mixtures containing large proportions of household refuses reached the highest stability and total N but available N remained weak. Slaughter-house wastes in the initial mixtures made possible to reach good OM stabilization and the largest N availability. The nature of initial mixing influenced composting parameters, OM stabilization and N availability. It is suggested mixing household refuses and slaughter-house wastes with tree leaves to reach better amending and fertilizer qualities of composts.

Effects of soil faunal activity and woody shrubs on water infiltration rates in a semi-arid fallow of Senegal.

Abstract The effects of soil faunal activity on the physical properties of the soils of a 2-year-old fallow in the sahelian zone of Senegal were studied. Factors studied included the presence or absence of woody shrubs and the effect of protection of plots from grazing and removal of fuel wood. The experimental design included a control in which termite activity was excluded by treating the soil with the persistent insecticide dieldrin. In both the protected and unprotected area of the experiment, termite burrowing activity (as measured by the number of entrance holes per square-metre) and water infiltration rates were significantly (ca. 80%) lower in termite exclusion plots than in plots not treated with insecticide 2 years after initial treatment. In protected plots where termites were present, there was a significant increase in infiltration rates in the part where woody shrubs were removed. In unprotected plots, by contrast, the presence of shrubs significantly increased infiltration rates, whether or not termites were present. Burrowing activity of earthworms was greater in the protected than in the unprotected area, and in the unprotected area burrowing in dieldrin treated plots was significantly increased by almost 65%. Activity of ants appeared to be little affected by the different treatments. The results demonstrate that the presence of healthy soil faunal populations was important in the infiltration of water in fallow soils and that protection from grazing and human activity improves faunal activity and water infiltration.

Post-fallow decomposition of woody roots in the West African savanna

Fallowing is a common practice for the management of soil fertility in low-input cropping systems of the West-African savanna, but has been threatened by the growing need for land in the sub-region for the past few decades. Proposals for alternatives to traditional fallowing must rely on a proper understanding of the soil biochemical dynamics occurring after fallow conversion to cropping. Two mesh-bag experiments were thus conducted in two sites (dry and sub-humid tropical climates) in Senegal to assess the role of site-related factors (climate, macrofaunal activity) and root-related factors (tree species, root diameter) on the decomposition of tree roots after clearing of fallow vegetation as measured from mass loss. Root decomposition was fastest – and even faster than predicted from a global model – in the wettest site (first order disappearance rate: 1.00 y−1 and 1.46–1.49 y−1 under dry and sub-humid conditions, respectively). Macrofauna accounted for half of root mass loss in the sub-humid site, with biomass removal occurring even during the dry season. Fastest disappearance for roots with ∅<5 mm occurred for Dichrostachys cinerea, and Combretum glutinosum. The influence of root chemical composition on decomposition patterns among tree species and root diameter classes was not clear, with effects of cell wall composition and nutrient content changing throughout the incubation period. Fast disappearance of dead roots suggests that cropping practices that allow conservation of live stumps, such as no-tillage and direct sowing, be promoted wherever possible to ensure soil conservation. It also suggests the possible management of tree species composition and, to a much lesser extent, of macrofauna during the fallow period to control root decomposition patterns and related nutrient transfers to crop biomass after fallow conversion.

Usefulness of TAO model to predict and manage the transformation in soil of carbon and nitrogen forms from West-Africa urban solid wastes.

The TAO model of Transformation of Added Organic materials (AOM) calibrated on AOMs and substrates of temperate areas was used to assess the transformations in soil of carbon and nitrogen forms of AOMs: raw materials, selected mixtures and composts from Ouagadougou urban wastes. AOMs were studied in terms of chemical and biochemical contents and for their C and N mineralization during incubations in a typical Ferric Lixisol of the sub-urban agriculture of Ouagadougou. The TAO model was used to predict the transformations of C (very labile, resistant and stable organic C) and N (very labile, resistant and stable organic N, produced and immobilized inorganic N) forms driven by AOM biochemical data. Without any change in calibration formulae, TAO predicted accurately the C transformations and inorganic N production of most of the tested AOMs, with a tendency to slightly overestimate C mineralization of previously well-composted materials and re-mineralization of immobilized N. Complementary adjustments using more complete data from laboratory experiments are suggested, but the model agrees with other data collected in the field and appears as a promising tool to optimise the management of urban wastes in the tropical area as well as for agro industrial organic fertilizers of the temperate zone. This application suggests ways to improve the management of urban wastes aiming to optimize agricultural yields, system sustainability and C sequestration in soil.

Effect of fallow improvement on the nematode community in the Sudanian region of Senegal

A 4-year trial was conducted in the Sudanian area of Senegal to study how the manipulation of the plant composition of a natural fallow might increase soil fertility. The influence of fencing combined with stump removal or planting of Andropogon gayanus on the nematode communities was studied. Apart from fencing, all intervention favoured the multiplication of the plant parasitic nematodes for the duration of the trial. Fencing significantly reduced nematode abundance and modified the balance between the species in the community. The population of Scutellonema cavenessi, a serious pest of food crops in the region, was dramatically reduced to the benefit of the weaker pathogen, Helicotylenchus dihystera. Planting Andropogon increased the H. dihystera population slightly. Disappearance of ligneous plant material by stump removal was associated with the greatest increase in the proportion of S. cavenessi. The results suggested that the soil disturbance when planting, removing stumps, or even grazing could affect the nematode community in the topsoil layer, resulting in a decrease in H. dihystera. This species did not recover during a 4-year fallow, even if a suitable situation was promoted through fencing or the planting of Andropogon, a host plant of H. dihystera.

Near infrared reflectance spectroscopy applied to model the transformation of added organic materials in soil

Raw, mixed and composted organic materials (OM) from agricultural and urban wastes were subjected to biochemical analyses, near infrared (NIR) reflectance spectroscopy and laboratory incubations. Respiration during incubations was accurately predicted using a decomposition model [transformation of added organic materials, (TAO)] of very labile, intermediary resistant, and stable OM fractions. Calibrations using NIR spectra were developed to determine the very labile and stable fractions of OM used to predict three-month OM mineralisation in soil. This study has confirmed that OM decomposition is mainly driven by OM quality on a short-term basis. The wavelengths contributing heavily to the prediction of very labile and stable OM components and molecular functions of these fractions were identified. The resulting TAO–NIR spectroscopy model is an efficient tool to study the degradation of natural molecules and its management for plant growth and sustainability of ecosystems. As a sub-model of a more complex C cycle model, it can instantaneously simulate labile and stable fractions of various organic inputs in soil and, as a non-destructive and easily portable spectroscopic method, could be used to assess C dynamics on a regional scale.

Use of Near Infrared Reflectance Spectroscopy (NIRS) for Predicting Soil Fertility and Historical Management

This study tests the potential of near infrared reflectance spectroscopy (NIRS) for predicting soil fertility and management history from topsoil (0–10 cm deep) spectra. Soil fertility was assessed by measuring the growth of a test plant, and soil management history was determined through inquiries with farmers. Moreover, NIRS predictive value was compared with that of a group of topsoil parameters: total carbon and nitrogen, nitrate, potential respiration and denitrification, and microbial biomass. Modelling used partial and modified partial least square regressions to ensure comparisons between predictions by NIRS versus by soil parameters. Soil fertility and management history were well predicted by NIRS (Q2 = 0.78 and R2 = 0.89 both; Q2 and R2 are cross-validation and calibration coefficients of determination, respectively), as were the soil parameters (Q2 = 0.79–0.92 and R2 = 0.86–0.98). Soil fertility and management history were more accurately predicted by NIRS than by the set of soil parameters.