Tiphaine Chevallier

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.

Increasing soil carbon storage: mechanisms, effects of agricultural practices and proxies. A review

The international 4 per 1000 initiative aims at supporting states and non-governmental stakeholders in their efforts towards a better management of soil carbon (C) stocks. These stocks depend on soil C inputs and outputs. They are the result of fine spatial scale interconnected mechanisms, which stabilise/destabilise organic matter-borne C. Since 2016, the CarboSMS consortium federates French researchers working on these mechanisms and their effects on C stocks in a local and global change setting (land use, agricultural practices, climatic and soil conditions, etc.). This article is a synthesis of this consortium’s first seminar. In the first part, we present recent advances in the understanding of soil C stabilisation mechanisms comprising biotic and abiotic processes, which occur concomitantly and interact. Soil organic C stocks are altered by biotic activities of plants (the main source of C through litter and root systems), microorganisms (fungi and bacteria) and ‘ecosystem engineers’ (earthworms, termites, ants). In the meantime, abiotic processes related to the soil-physical structure, porosity and mineral fraction also modify these stocks. In the second part, we show how agricultural practices affect soil C stocks. By acting on both biotic and abiotic mechanisms, land use and management practices (choice of plant species and density, plant residue exports, amendments, fertilisation, tillage, etc.) drive soil spatiotemporal organic inputs and organic matter sensitivity to mineralisation. Interaction between the different mechanisms and their effects on C stocks are revealed by meta-analyses and long-term field studies. The third part addresses upscaling issues. This is a cause for major concern since soil organic C stabilisation mechanisms are most often studied at fine spatial scales (mm–μm) under controlled conditions, while agricultural practices are implemented at the plot scale. We discuss some proxies and models describing specific mechanisms and their action in different soil and climatic contexts and show how they should be taken into account in large scale models, to improve change predictions in soil C stocks. Finally, this literature review highlights some future research prospects geared towards preserving or even increasing C stocks, our focus being put on the mechanisms, the effects of agricultural practices on them and C stock prediction models.

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.

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.

24-h variation in soil respiration after a long dry season in a Sudano-Sahelian region

Soil respiration is a major component of the global carbon cycle which links ecosystems and the atmosphere. To evaluate the reaction of soil respiration after wetting, during a dry period, soil respiration and associated environmental factors were measured over a 24-h period, during the dry season in North Cameroon after wetting the soil. Over 24-h, soil respiration rates followed a quadratic curve during the day coming close to linear at night, while soil temperature and moisture together explained at least 73 % of the variations during the 24-h observed. These soil respiration rates increased during the morning, peaked between 11h00 and 13h00 and then decreased gradually to the minimum around 06h00. These observations were used to propose a method for estimating mean daytime and nighttime soil respiration after wetting the soil. The method proposed in this study has the advantage of being based on a small number of measurements and is, therefore, easier to implement for monitoring 24-h soil respiration after the first rains following a long dry period.

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

Methods to estimate the protection of soil organic carbon within macroaggregates, 1 : does soil water status affect the estimated amount of soil organic carbon protected inside macroaggregates ?

The additional mineralized soil organic carbon (SOC) after soil crushing is considered to be the amount of SOC protected within aggregates (>200 μm). This study investigated the effect of soil moisture in crushed and uncrushed soil samples on the calculated amounts of protected SOC in five tropical soils (Arenosol, two Ferralsols, Nitisol, and Vertisol). No differences in soil moisture optimum were observed between crushed and uncrushed soil samples, except in clayey soils with high SOC contents and high SOC mineralization rates (Nitisol and Vertisol). Crushing the soil increased soil respiration by 0.9 to 2.4 times. Soil moisture seemed to be a confounding factor in estimation of the SOC-protected amount only in soil with a high amount of protected SOC or with a low macroaggregate stability (Ferralsol and Vertisol). In these soils, the amount of protected SOC could be influenced by the method used to estimate it.

Studying the physical protection of soil carbon with quantitative infrared spectroscopy

Near infrared (NIR) and mid-infrared (mid-IR) reflectance spectroscopy are time- and cost-effective tools for characterising soil organic carbon (SOC). Here they were used for quantifying (i) carbon (C) dioxide (CO2) emission from soil samples crushed to 2 mm and 0.2 mm, at 18°C and 28°C; (ii) physical C protection, calculated as the difference between CO2 emissions from 0.2 mm and 2 mm crushed soil at a given temperature; and (iii) the temperature vulnerability of this protection, calculated as the difference between C protection at 18°C and 28°C. This was done for 97 topsoil samples from Tunisia, mostly calcareous, which were incubated for 21 days. Soil CO2 emission increased with temperature and fine crushing. However, C protection in 0.2–2 mm aggregates had little effect on the temperature vulnerability of CO2 emission, possibly due to preferential SOC protection in smaller aggregates. In general, NIR spectroscopy, and to a lesser extent mid-IR spectroscopy, yielded accurate predictions of soil CO2 emission (0.60 ≤ R2 ≤ 0.91), and acceptable predictions of C protection at the beginning of incubation (0.52 ≤ R2 ≤ 0.81) but not over the whole 21 day period (R2 ≤ 0.59). For CO2 emission, prediction error was the same order of magnitude as, and sometimes similar to, the uncertainty of conventional determination, indicating that a noticeable proportion of the former could be attributed to the latter. The temperature vulnerability of C protection could not be modelled correctly (R2 ≤ 0.11), due to error propagation. The prediction of SOC was better with NIR spectroscopy and that of soil inorganic C (SIC) was very accurate (R2 ≥ 0.94), especially with mid-IR spectroscopy. Soil CO2 emission, C protection and its vulnerability were best predicted with NIR spectra, those of 0.2 mm samples especially. Mid-IR spectroscopy of 2 mm samples yielded the worst predictions in general. NIR spectroscopy prediction models suggested that CO2 emission and C protection depended (i) on aliphatic compounds (i.e. labile substrates), dominantly at 18°C; (ii) on amides or proteins (i.e. microbial biomass), markedly at 28°C; and (iii) negatively, on organohalogens and aromatic amines (i.e. pesticides). Models using mid-IR spectra showed a negative influence of carbonates on CO2 emission, suggesting they did not contribute to soil CO2 emission and might form during incubation. They also suggested that CO2 emission and C protection related to carboxylic acids, saturated aliphatic ones especially.