Alain Albrecht

Spatial and temporal changes of soil C after establishment of a pasture on a long-term cultivated vertisol (Martinique)

Abstract In 1991 in Martinique (F.W.I), a Digitaria decumbens pasture was established on a vertisol that had supported a market-gardening culture for more than 10 years. Organic matter stock restoration was investigated by measuring carbon contents (C contents) and carbon/nitrogen (C/N) ratios each year from 1992 to 1997. Relations between and soil properties (particle-size distribution, soil depth) and C contents were studied. Furthermore, geostatistical analyses of C contents were realised in order to characterise the C storage in soil at plot scale. The increase of C contents from 1992 ( Y 0 ) to 1997 ( Y 5 ) was 5 g C (kg soil) −1 in the topsoil (0–10 cm) and 2.5 g C kg soil −1 , or 7.5 Mg C ha −1 , in the 0–30-cm layer. The intensity of organic C storage had a spatial pattern, although the C/N ratio remained homogeneous across the plot. However, there was no correlation between the C increase and the particle-size distribution or the depth of the soil. In the topsoil, the local variability of the C contents increased with time until 1995 and then there was a gradually spreading of this local variability. Plant-cover distribution and physical structure of vertisol could explain the evolution of spatial structure of the soil C content.

Diversity of plant-parasitic nematodes and their relationships with some soil physico-chemical characteristics in improved fallows in western Kenya

Abstract A survey was conducted in the food crop production area of western Kenya to characterise the nematode communities in different cropping systems and to monitor the effect of improved fallows on the abundance, diversity and community structure of plant-parasitic nematode assemblages. Soil samples were taken from short-term natural fallows ( 20 years), maize/beans cultivated fields and improved fallows with Crotalaria grahamiana , Sesbania sesban and Tephrosia vogelii . The Shannon diversity index and evenness were used to assess nematode diversity in the different cropping systems. The relationships between plant-parasitic nematodes and the soil physico-chemical properties were investigated with principal component analysis and co-inertia analysis. Planting trees/shrubs as improved fallows in croplands dramatically increased the populations of nematodes in the soil. However, the diversity of plant-parasitic nematodes was reduced. The distribution of plant-parasitic nematodes was correlated with that of some soil physical and chemical properties. Scutellonema spp. were dominant in the improved fallows where the highest values of magnesium and potassium were recorded. The plant-parasitic Dorylaimida ( Xiphinema spp. and Paratrichodorus minor ) were more abundant in the maize/beans plots and this abundance seemed related to high soil bulk density. Meloidogyne spp. and Rotylenchulus borealis had a strong positive correlation with soil organic matter and clay, and were abundant in the long-term natural fallows. The correlations between soil physico-chemical properties and nematodes indicate that, apart from the direct influence of the host plant, the soil characteristics play an important role in the abundance, distribution and structure of nematode communities. This validates the potential of nematodes as bio-indicator organisms of soil status.

Soil organic carbon sequestration in tropical areas. General considerations and analysis of some edaphic determinants for Lesser Antilles soils

Some general notions on soil organic carbon (SOC) sequestration and the difficulties to evaluate this process globally are presented. Problems of time- and space- scales are emphasized. SOC erosion, which is generally difficult to evaluate in relation to land use changes, is discussed in detail. Different aspects of SOC sequestration on the Lesser Antilles are presented for a wide range of soil types. Comparisons between soils revealed that the SOC stocks in the Lesser Antilles are highly dependent upon the mineralogy: higher stocks for allophanic (ALL) soils than for low activity clay (LAC) and high activity clay (HAC) soils. But in terms of potential of SOC sequestration (pSeq-SOC, differences between permanent vegetation and continuous cultivation situations), there are no differences between ALL and LAC soils (22.9 and 23.3 tC. ha−1, respectively). On the other hand, the potentials of SOC sequestration were higher for HAC soils (30.8 – 59.4 tC. ha−1, with the higher levels in the less Mg- and Na-affected Vertisol). Sheet erosion is a serious problem for Vertisol with high Mg and Na on exchange complex, causing high dispersability of fine elements. Thus, the lower SOC levels in these soils may be partly due to erosion losses. Laboratory incubations have shown that 37 – 53% of the protected SOC in these soils was located in aggregates larger than 0.2 mm. The effect of agricultural practices on SOC sequestration was studied for the Vertisols. Intensification of pastures led to higher plant productivity and higher organic matter restitutions and SOC sequestration. The gain was 53.5 and 25.4 tC. ha−1 for the low and high-Mg Vertisol, respectively (0–20 cm layer). SOC sequestration with pastures also depends upon the plot history with lower mean annual increase in SOC for the initially eroded (1.0 gC . kg−1 soil . yr−1) than for the non-degraded (1.5 gC . kg−1 soil . yr−1) Vertisol. Loss of SOC in a pasture-market gardening rotation was 22.2 tC . ha−1 with deep (30–40 cm) and 10.7 tC . ha−1 with surface (10–15 cm) tillage. It was unclear whether the differences in SOC losses were due to mineralization and/or to erosion.

Modelling of planted legume fallows in Western Kenya using WaNuLCAS. (I) Model calibration and validation

Poor soil fertility is the biggest obstacle to agricultural productivity in Sub-Saharan Africa. Improved fallows can help to raise agricultural productivity in these systems of low financial capital, however, experimental testing of their potential application domain and design is costly and time consuming. Models can evaluate alternative systems relatively quickly and at relatively low cost, but must first be validated to assess satisfactory simulation of the target systems. Specific climatic, edaphic, crop and fallow growth data was used from five sites in Western Kenya to calibrate and validate simulations of maize and improved fallow growth using the Water, Nutrient and Light Capture in Agroforestry Systems (WaNuLCAS) model. The model predicted continuous maize yields across the sites with an R2 of 0.72, an EF (model efficiency) of 0.66 and a CD (coefficient of determination) of 2.73, although the default pedotransfer functions (PTF) for volumetric soil water content used in the model had to be substituted for a tropical soils specific PTF before this was achieved. Predicted maize yield was consistently related to fallow biomass (i.e. higher fallow biomass correlated with higher subsequent maize yields) at two sites and the model predicted maize yields following fallow growth from this subset of the data with an R2 of 0.42. This relationship of fallow biomass to subsequent maize yield was not observed across the whole data set due to incomplete fallow litterfall data, factors not included in the model and associated poor model prediction of recycled tree biomass. After site and tree calibration, the model can thus be applied to assess fallow management strategies for sites limited by water and nitrogen.

Crop damage by nematodes in improved-fallow fields in western Kenya

The rotation of leguminous shrubs and crops is being tested on farms and recommended as a means of improving soil fertility and increasing crop yield in eastern and southern Africa, including western Kenya. However, this improved fallow practice may also increase the nematode population in the soil. An experiment was conducted to monitor the effects of plant-parasitic nematodes on crops after improved fallow. Soil was collected from a maize (Zea mays L.)/bean (Phaseolus vulgaris L.) field, a natural fallow, a Crotalaria (Crotalaria grahamiana Wight & Arn.) fallow, a Tephrosia (Tephrosia vogelii Hook. f.) fallow and a Crotalaria — Tephrosia mixed fallow and used to fill plastic pots placed in a shade. Three successive crop cycles of 2 months were tested in these pots using maize and beans, the most important staple foods in western Kenya. In the first cycle, beans grew poorly on the Tephrosia and Crotalaria — Tephrosia soil due to the high incidence of root knot nematodes, Meloidogyne spp., while maize did not suffer any loss. Although the populations of root knot nematodes reduced drastically in the second and third cycles, both maize and beans experienced heavy losses on the soil under improved fallow probably due to the spiral nematodes, Scutellonema spp., which became dominant in the nematode communities. Despite the use of fertilisers (N, P, K), both crops became highly sensitive to spiral nematodes in the third cycle because of the degradation of the soil physical properties. The study showed that the benefits of improved fallows in terms of crop production may be limited by the high number of plant-parasitic nematodes they help develop in the process.

Texture and organic carbon contents do not impact amount of carbon protected in Malagasy soils

Soil organic carbon (SOC) is usually said to be well correlated with soil texture and soil aggregation. These relations generally suggest a physical and physicochemical protection of SOC within soil aggregates and on soil fine particles, respectively. Because there are few experimental evidences of these relations on tropical soils, we tested the relations of soil variables (SOC and soil aggregate contents, and soil texture) with the amount of SOC physically protected in aggregates on a set of 15 Malagasy soils. The soil texture, the SOC and water stable macroaggregate (MA) contents and the amount of SOC physically protected inside aggregates, calculated as the difference of C mineralized by crushed and intact aggregates, were characterized. The relation between these variables was established. SOC content was significantly correlated with soil texture (clay+fine silt fraction) and with soil MA amount while protected SOC content was not correlated with soil MA amount. This lack of correlation might be attributed to the highest importance of physicochemical protection of SOC which is demonstrated by the positive relation between SOC and clay+fine silt fraction.

Impact of pasture establishment on CO2 emissions from a Vertisol : Consequences for soil C sequestration (Martinique, West Indies)

Establishing pasture on cultivated tropical Vertisols can increase soil organic carbon (SOC), but it is not known whether this increase results solely from enhanced inputs or also from suppressed mineralization. We measured CO2 emissions from a Vertisol under market gardening, and under “young” and “old” Digitaria decumbens pastures. Emissions of CO2-C increased in pastures, compared to market gardening, but relative SOC mineralization (CO2-C/SOC) decreased, implying the protection of SOC against mineralization with pasture establishment. Key words: Tropical pasture, carbon fluxes, soil organic carbon, physical protection, C storage

Influence of some agroforestry practices on the temporal structures of nematodes in western Kenya

The influence of agroforestry practices on the temporal fluctuation of nematodes was studied in western Kenya. The experiment comprised a fallow phase, which had the following treatments: (1) maize/beans intercropping; (2) maize/beans intercropping with rock phosphate; (3) Crotalaria fallow; (4) Crotalaria fallow with rock phosphate; (5) Crotalaria fallow with rock phosphate (+ Calliandra and napier hedges); and a cultivation phase when all plots were planted to maize and beans. There were strong seasonal fluctuations in the abundance of both free-living and plant-parasitic nematodes. The abundance of plant-parasitic nematodes did not vary among cropping systems during the fallow phase, but varied significantly during the cultivation phase of the experiment. Pratylenchus spp. appeared to be stimulated by the application of phosphorus while Scutellonema spp. had higher populations in the maize crops, which were planted after a Crotalaria fallow. In both the fallow and the cultivation phases, the sampling date had a significant impact on nematode abundance. There were more plant-parasitic nematode species during the fallow phase of the study, but the evenness of the different nematode communities was significantly greater in the cultivation phase of the experiment.