The effects of earthworms on soil structure and the resulting physical properties in natural or c... more The effects of earthworms on soil structure and the resulting physical properties in natural or cultivated systems were studied in different sites of the Macrofauna project. Four main results were obtained from these experiments: 1. In kaolinitic soils (Lamto, Manaus, Yurimaguas), irrespective of clay content, andogeic earthworms play a major role in soil structure formation and maintenance, while in smectitic soils (Martinique), the effects of earthworms on soil structure formation and maintenance are not as clear. In the Vertisols of Martinique, the effect of roots and organic materials seem predominant in soil structure development and earthworms play a secondary role. 2. Endogeic earthworm species have different effects on soil properties. Large earthworms such as Pontoscolex corethrurus or Millsonia anomala egest large and compact casts. They increase the proportion of large aggregates in soil and the bulk density ; they are called 'compacting species'. Conversely, smal...
2 Abstract The relationships between the structural variations of a fenisol and the plant parasit... more 2 Abstract The relationships between the structural variations of a fenisol and the plant parasitic nematode communities of sugarcane were studied along three transects. These transects, about 20 m long, started in horizon A and ended in a levelled area where horizon C began. Results were analysed with the Co-inertia analysis method, which allowed us to study simultaneously the soil and nematode data. Progressive variations of the content of some physico-chemical soil elements (organic matter, phosphorus, pH) appear linked to those of the abundance of some nematode species (Heniicricoloideszenioides and Prarylenclzus). This analysis also shows relations that do not vary according to a gradient along the transects. For instance, the abundance of Helicotylenclius can be correlated with the existence of high calcium grades.
Cette étude vise à tester l'effet de systèmes de culture en semis direct sous couverture végétale... more Cette étude vise à tester l'effet de systèmes de culture en semis direct sous couverture végétale permanente (SCV) pour contrôler l'érosion hydrique et améliorer le rendement de cultures pluviales sur versant des Hautes-Terres de Madagascar. L'essai a été entrepris sur un dispositif expérimental d'érosion à Andranomanelatra, sous pluies naturelles. Le dispositif comporte plusieurs traitements en SCV et en labour conventionnel (bêchage à l'angady) ainsi qu'un témoin avec sol nu et un autre en jachère herbacée spontanée. Pendant quatre campagnes, des mesures du ruissellement, des pertes en terre, de la hauteur de la pluie et de son intensité maximale ont été effectuées à chaque événement pluvial érosif. Des mesures de rendements ont également été réalisées à la fin de chaque campagne culturale.
Assessing the spatial variability of soil organic carbon (SOC) is crucial for SOC monitoring and ... more Assessing the spatial variability of soil organic carbon (SOC) is crucial for SOC monitoring and comparing management options. Topsoil (0-5 cm) SOC concentrations were surveyed in a coffee agroforestry watershed (0.9 km 2 ) on Andisols in Costa Rica with uniform farm management. We encountered high values and large spatial variations of SOC, from 48.1 to 172 g kg −1 in the dry combustion set (SOC ref ; n = 72) used for calibrating the visible-near-infrared reflectance spectroscopy (VNIRS) samples (SOC VNIRS ; 350-2500 nm; n = 520). VNIRS using partial least squares regression was effective in predicting SOC (R 2 = 0.85; a root mean square error (RMSE) = 12.3 g kg −1 ) and proved an effective proxy measurement. We assessed several topographic, vegetation and andic soil property variables, of which only the latter (metal-humus complexes and allophanes) displayed strong correlations with SOC ref concentrations. We compared Random Forest and three geostatistical approaches for the interpolation of SOC in unsampled locations. Ordinary kriging with SOC ref yielded an RMSE of 28.0 g kg −1 . Random Forest was successful in incorporating many weakly and non-linearly correlated covariates with SOC (RMSE = 14.7 g kg −1 ), provided Al p (the sodium pyrophosphate extractable aluminum), the best predictor of SOC (r = 0.85) but also the most costly variable to acquire. Co-kriging with Al p also showed high reduction in RMSE (16.0 g kg −1 ). Co-kriging with SOC VNIRS only showed marginal reduction in RMSE to 24.2 g kg −1 due to the presence of a high nugget effect. Local variability of SOC in this volcanic agroforestry watershed was dominated by andic properties whereas topographic or vegetation variables had very little impact. Estimation of SOC variability is recommended using inexpensive proxy measurements like VNIRS (RMSE = 12.3 g kg −1 ) rather than spatial interpolation techniques.
The rotation of leguminous shrubs and crops is being tested on farms and recommended as a means o... more 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
ABSTRACT The soil organic carbon plays an important role in climate change regulation through car... more ABSTRACT The soil organic carbon plays an important role in climate change regulation through carbon emissions and sequestration due to land use changes, notably tropical deforestation. Monitoring soil carbon emissions from shifting-cultivation requires to evaluate the amount of carbon stored at plot scale with a sufficient level of accuracy to be able to detect changes. The objective of this work was to map soil carbon stocks (30 cm and 100 cm depths) for different land use at regional scale using high resolution satellite dataset. The Andohahela National Parc and its surroundings (South-Est Madagascar) - a region with the largest deforestation rate in the country - was selected as a pilot area for the development of the methodology. A three steps approach was set up: (i) carbon inventory using mid infra-red spectroscopy and stock calculation, (ii) spatial data processing and (iii) modeling and mapping. Soil spectroscopy was successfully used for measuring organic carbon in this region. The results show that Random Forest was the inference model that produced the best estimates on calibration and validation datasets. By using a simple and robust method, we estimated uncertainty levels of of 35% and 43% for 30-cm and 100-cm carbon maps respectively. The approach developed in this study was based on open data and open source software that can be easily replicated to other regions and for other time periods using updated satellite images.
The influence of agroforestry practices on the temporal fluctuation of nematodes was studied in w... more 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.
... properties and nematodes indicate that, apart from the direct influence of the host plant, th... more ... properties and nematodes indicate that, apart from the direct influence of the host plant, the ... Author Keywords: Bio-indicator; Diversity; Improved fallows; Nematodes; Soil status. ... because of population pressure, gains in crop production after these short-duration weed fallows may ...
Removing atmospheric carbon (C) and storing it in the terrestrial biosphere is one of the options... more Removing atmospheric carbon (C) and storing it in the terrestrial biosphere is one of the options, which have been proposed to compensate greenhouse gas (GHG) emissions. Agricultural lands are believed to be a major potential sink and could absorb large quantities of C if trees are reintroduced to these systems and judiciously managed together with crops and/or animals. Thus, the importance of agroforestry as a land-use system is receiving wider recognition not only in terms of agricultural sustainability but also in issues related to climate change. The objective of this paper was to analyse C storage data in some tropical agroforestry systems and to discuss the role they can play in reducing the concentration of CO 2 in the atmosphere. The C sequestration potential of agroforestry systems is estimated between 12 and 228 Mg ha −1 with a median value of 95 Mg ha −1 . Therefore, based on the earth's area that is suitable for the practice (585-1215 × 10 6 ha), 1.1-2.2 Pg C could be stored in the terrestrial ecosystems over the next 50 years. Long rotation systems such as agroforests, homegardens and boundary plantings can sequester sizeable quantities of C in plant biomass and in long-lasting wood products. Soil C sequestration constitutes another realistic option achievable in many agroforestry systems. In conclusion, the potential of agroforestry for CO 2 mitigation is well recognised. However, there are a number of shortcomings that need to be emphasised. These include the uncertainties related to future shifts in global climate, land-use and land cover, the poor performance of trees and crops on substandard soils and dry environments, pests and diseases such as nematodes. In addition, more efforts are needed to improve methods for estimating C stocks and trace gas balances such as nitrous oxide (N 2 O) and methane (CH 4 ) to determine net benefits of agroforestry on the atmosphere.
Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time... more Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time consuming to measure using conventional laboratory incubation methods. Recent studies have demonstrated the ability of visible-near infrared spectroscopy (NIRS) for rapid nondestructive determination of soil organic carbon (SOC) and nitrogen (N) concentration. We investigated whether NIRS (350-2500 nm) can predict C mineralization rates in physically fractionated soil aggregates (bulk soil and 6 size fractions, n ¼ 108) and free organic matter (2 size fractions, n ¼ 27) in aerobically incubated samples from a clayey soil (Ferralsol) and a sandy soil (Arenosol). Incubation reference values were calibrated to first derivative reflectance spectra using partial least-squares regression. Prediction accuracy was assessed by comparing laboratory reference values with NIRS values predicted using full hold-out-one cross-validation. Cross-validated prediction for C respired (500 days) in soil aggregate fractions had an R 2 of 0.82 while that of C mineralized (300 days) in organic matter fractions was 0.71. Major soil aggregate fractions could be perfectly spectrally discriminated using a 50% random holdout validation sample. NIRS is a promising technique for rapid characterization of potential C mineralization in soils and aggregate fractions. Further work should test the robustness of NIRS prediction of mineralization rates of aggregate fractions across a wide range of soils and spectral mixture models for predicting mass fractions of aggregate size classes. r
We studied the biochemical and biophysical processes of carbon sequestration in an intensive agro... more We studied the biochemical and biophysical processes of carbon sequestration in an intensive agroforestry system on two soils (Feralsol -Luero; Arenosol -Teso) in W. Kenya to elucidate the mechanisms associated with long-term carbon storage. Specifically, we looked at a top-down model (macro-aggregates form around organic matter particles and micro-aggregates form within the macro-aggregates) and a bottom-up model (micro-aggregates form independently and are incorporated into macro-aggregates) of soil aggregate formation. Soil samples were collected from experiments on improved tree fallows using different species and two tillage treatments; water-stable aggregates were extracted and sorted into three size classes: macroaggregates (N 212 μm), meso-aggregates (53-212 μm) and micro-aggregates (20-53 μm). Organic matter characterization of each fraction was based on 13 C isotope abundance, Fourier transform infrared (FTIR) spectroscopy and the abundance of polysaccharides. Improved fallows increased soil C by 0.28 and 0.26 kg m 2 in the top 20 cm of the soil profile in Luero and Teso, respectively. Tillage altered the distribution of aggregates among size classes. Changes in the δ 13 C signature in each fraction indicated that more of the new carbon was found in the macro-aggregates (35-70%) and meso-aggregates (18-49%) in Luero and less (9-17%) was found in the micro-aggregates. In Teso, about 40-80% of the new aggregate C was found in the meso-aggregates, 14-45% was found in the micro-aggregates and only 4-26% was found in the macro-aggregates. The mesoaggregates and macro-aggregates to a lesser extent, in both sites, were enriched in carboxylic-C and aromatic-C, indicating the importance of OM decomposition and plant-derived C in the stabilization of larger aggregates, supporting the top-down model of aggregate formation. Microbially derived polysaccharides play a leading role in the formation of stable micro-aggregates and carboxylic-C promotes stabilization through surface occlusion. This bottom-up process is essential to promote long-term carbon sequestration in soils. Additionally, the micro-aggregates at both sites were enriched in polysaccharides and had elevated ratios of galactose + mannose:arabinose + xylose than the other aggregate fractions, indicating the importance of microbial processes in the formation of stable micro-aggregates and supporting the bottom-up model.
Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time... more Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time consuming to measure using conventional laboratory incubation methods. Recent studies have demonstrated the ability of visible-near infrared spectroscopy (NIRS) for rapid non-destructive determination of soil organic carbon (SOC) and nitrogen (N) concentration.
Poor soil fertility is the biggest obstacle to agricultural productivity in Sub-Saharan Africa. I... more 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
Short rotations of Eucalyptus plantations under coppice regime are extensively managed for wood p... more Short rotations of Eucalyptus plantations under coppice regime are extensively managed for wood production in Madagascar. Nevertheless, little is known about their biomass production and partitioning and their potential in terms of carbon sequestration. If above-ground biomass (AGB) can be estimated based on established allometric relations, below-ground (BGB) estimates are much less common. The aim of this work was to
Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time... more Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time consuming to measure using conventional laboratory incubation methods. Recent studies have demonstrated the ability of visible-near infrared spectroscopy (NIRS) for rapid nondestructive determination of soil organic carbon (SOC) and nitrogen (N) concentration. We investigated whether NIRS (350-2500 nm) can predict C mineralization rates in physically fractionated soil aggregates (bulk soil and 6 size fractions, n ¼ 108) and free organic matter (2 size fractions, n ¼ 27) in aerobically incubated samples from a clayey soil (Ferralsol) and a sandy soil (Arenosol). Incubation reference values were calibrated to first derivative reflectance spectra using partial least-squares regression. Prediction accuracy was assessed by comparing laboratory reference values with NIRS values predicted using full hold-out-one cross-validation. Cross-validated prediction for C respired (500 days) in soil aggregate fractions had an R 2 of 0.82 while that of C mineralized (300 days) in organic matter fractions was 0.71. Major soil aggregate fractions could be perfectly spectrally discriminated using a 50% random holdout validation sample. NIRS is a promising technique for rapid characterization of potential C mineralization in soils and aggregate fractions. Further work should test the robustness of NIRS prediction of mineralization rates of aggregate fractions across a wide range of soils and spectral mixture models for predicting mass fractions of aggregate size classes. r
The effects of earthworms on soil structure and the resulting physical properties in natural or c... more The effects of earthworms on soil structure and the resulting physical properties in natural or cultivated systems were studied in different sites of the Macrofauna project. Four main results were obtained from these experiments: 1. In kaolinitic soils (Lamto, Manaus, Yurimaguas), irrespective of clay content, andogeic earthworms play a major role in soil structure formation and maintenance, while in smectitic soils (Martinique), the effects of earthworms on soil structure formation and maintenance are not as clear. In the Vertisols of Martinique, the effect of roots and organic materials seem predominant in soil structure development and earthworms play a secondary role. 2. Endogeic earthworm species have different effects on soil properties. Large earthworms such as Pontoscolex corethrurus or Millsonia anomala egest large and compact casts. They increase the proportion of large aggregates in soil and the bulk density ; they are called 'compacting species'. Conversely, smal...
2 Abstract The relationships between the structural variations of a fenisol and the plant parasit... more 2 Abstract The relationships between the structural variations of a fenisol and the plant parasitic nematode communities of sugarcane were studied along three transects. These transects, about 20 m long, started in horizon A and ended in a levelled area where horizon C began. Results were analysed with the Co-inertia analysis method, which allowed us to study simultaneously the soil and nematode data. Progressive variations of the content of some physico-chemical soil elements (organic matter, phosphorus, pH) appear linked to those of the abundance of some nematode species (Heniicricoloideszenioides and Prarylenclzus). This analysis also shows relations that do not vary according to a gradient along the transects. For instance, the abundance of Helicotylenclius can be correlated with the existence of high calcium grades.
Cette étude vise à tester l'effet de systèmes de culture en semis direct sous couverture végétale... more Cette étude vise à tester l'effet de systèmes de culture en semis direct sous couverture végétale permanente (SCV) pour contrôler l'érosion hydrique et améliorer le rendement de cultures pluviales sur versant des Hautes-Terres de Madagascar. L'essai a été entrepris sur un dispositif expérimental d'érosion à Andranomanelatra, sous pluies naturelles. Le dispositif comporte plusieurs traitements en SCV et en labour conventionnel (bêchage à l'angady) ainsi qu'un témoin avec sol nu et un autre en jachère herbacée spontanée. Pendant quatre campagnes, des mesures du ruissellement, des pertes en terre, de la hauteur de la pluie et de son intensité maximale ont été effectuées à chaque événement pluvial érosif. Des mesures de rendements ont également été réalisées à la fin de chaque campagne culturale.
Assessing the spatial variability of soil organic carbon (SOC) is crucial for SOC monitoring and ... more Assessing the spatial variability of soil organic carbon (SOC) is crucial for SOC monitoring and comparing management options. Topsoil (0-5 cm) SOC concentrations were surveyed in a coffee agroforestry watershed (0.9 km 2 ) on Andisols in Costa Rica with uniform farm management. We encountered high values and large spatial variations of SOC, from 48.1 to 172 g kg −1 in the dry combustion set (SOC ref ; n = 72) used for calibrating the visible-near-infrared reflectance spectroscopy (VNIRS) samples (SOC VNIRS ; 350-2500 nm; n = 520). VNIRS using partial least squares regression was effective in predicting SOC (R 2 = 0.85; a root mean square error (RMSE) = 12.3 g kg −1 ) and proved an effective proxy measurement. We assessed several topographic, vegetation and andic soil property variables, of which only the latter (metal-humus complexes and allophanes) displayed strong correlations with SOC ref concentrations. We compared Random Forest and three geostatistical approaches for the interpolation of SOC in unsampled locations. Ordinary kriging with SOC ref yielded an RMSE of 28.0 g kg −1 . Random Forest was successful in incorporating many weakly and non-linearly correlated covariates with SOC (RMSE = 14.7 g kg −1 ), provided Al p (the sodium pyrophosphate extractable aluminum), the best predictor of SOC (r = 0.85) but also the most costly variable to acquire. Co-kriging with Al p also showed high reduction in RMSE (16.0 g kg −1 ). Co-kriging with SOC VNIRS only showed marginal reduction in RMSE to 24.2 g kg −1 due to the presence of a high nugget effect. Local variability of SOC in this volcanic agroforestry watershed was dominated by andic properties whereas topographic or vegetation variables had very little impact. Estimation of SOC variability is recommended using inexpensive proxy measurements like VNIRS (RMSE = 12.3 g kg −1 ) rather than spatial interpolation techniques.
The rotation of leguminous shrubs and crops is being tested on farms and recommended as a means o... more 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
ABSTRACT The soil organic carbon plays an important role in climate change regulation through car... more ABSTRACT The soil organic carbon plays an important role in climate change regulation through carbon emissions and sequestration due to land use changes, notably tropical deforestation. Monitoring soil carbon emissions from shifting-cultivation requires to evaluate the amount of carbon stored at plot scale with a sufficient level of accuracy to be able to detect changes. The objective of this work was to map soil carbon stocks (30 cm and 100 cm depths) for different land use at regional scale using high resolution satellite dataset. The Andohahela National Parc and its surroundings (South-Est Madagascar) - a region with the largest deforestation rate in the country - was selected as a pilot area for the development of the methodology. A three steps approach was set up: (i) carbon inventory using mid infra-red spectroscopy and stock calculation, (ii) spatial data processing and (iii) modeling and mapping. Soil spectroscopy was successfully used for measuring organic carbon in this region. The results show that Random Forest was the inference model that produced the best estimates on calibration and validation datasets. By using a simple and robust method, we estimated uncertainty levels of of 35% and 43% for 30-cm and 100-cm carbon maps respectively. The approach developed in this study was based on open data and open source software that can be easily replicated to other regions and for other time periods using updated satellite images.
The influence of agroforestry practices on the temporal fluctuation of nematodes was studied in w... more 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.
... properties and nematodes indicate that, apart from the direct influence of the host plant, th... more ... properties and nematodes indicate that, apart from the direct influence of the host plant, the ... Author Keywords: Bio-indicator; Diversity; Improved fallows; Nematodes; Soil status. ... because of population pressure, gains in crop production after these short-duration weed fallows may ...
Removing atmospheric carbon (C) and storing it in the terrestrial biosphere is one of the options... more Removing atmospheric carbon (C) and storing it in the terrestrial biosphere is one of the options, which have been proposed to compensate greenhouse gas (GHG) emissions. Agricultural lands are believed to be a major potential sink and could absorb large quantities of C if trees are reintroduced to these systems and judiciously managed together with crops and/or animals. Thus, the importance of agroforestry as a land-use system is receiving wider recognition not only in terms of agricultural sustainability but also in issues related to climate change. The objective of this paper was to analyse C storage data in some tropical agroforestry systems and to discuss the role they can play in reducing the concentration of CO 2 in the atmosphere. The C sequestration potential of agroforestry systems is estimated between 12 and 228 Mg ha −1 with a median value of 95 Mg ha −1 . Therefore, based on the earth's area that is suitable for the practice (585-1215 × 10 6 ha), 1.1-2.2 Pg C could be stored in the terrestrial ecosystems over the next 50 years. Long rotation systems such as agroforests, homegardens and boundary plantings can sequester sizeable quantities of C in plant biomass and in long-lasting wood products. Soil C sequestration constitutes another realistic option achievable in many agroforestry systems. In conclusion, the potential of agroforestry for CO 2 mitigation is well recognised. However, there are a number of shortcomings that need to be emphasised. These include the uncertainties related to future shifts in global climate, land-use and land cover, the poor performance of trees and crops on substandard soils and dry environments, pests and diseases such as nematodes. In addition, more efforts are needed to improve methods for estimating C stocks and trace gas balances such as nitrous oxide (N 2 O) and methane (CH 4 ) to determine net benefits of agroforestry on the atmosphere.
Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time... more Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time consuming to measure using conventional laboratory incubation methods. Recent studies have demonstrated the ability of visible-near infrared spectroscopy (NIRS) for rapid nondestructive determination of soil organic carbon (SOC) and nitrogen (N) concentration. We investigated whether NIRS (350-2500 nm) can predict C mineralization rates in physically fractionated soil aggregates (bulk soil and 6 size fractions, n ¼ 108) and free organic matter (2 size fractions, n ¼ 27) in aerobically incubated samples from a clayey soil (Ferralsol) and a sandy soil (Arenosol). Incubation reference values were calibrated to first derivative reflectance spectra using partial least-squares regression. Prediction accuracy was assessed by comparing laboratory reference values with NIRS values predicted using full hold-out-one cross-validation. Cross-validated prediction for C respired (500 days) in soil aggregate fractions had an R 2 of 0.82 while that of C mineralized (300 days) in organic matter fractions was 0.71. Major soil aggregate fractions could be perfectly spectrally discriminated using a 50% random holdout validation sample. NIRS is a promising technique for rapid characterization of potential C mineralization in soils and aggregate fractions. Further work should test the robustness of NIRS prediction of mineralization rates of aggregate fractions across a wide range of soils and spectral mixture models for predicting mass fractions of aggregate size classes. r
We studied the biochemical and biophysical processes of carbon sequestration in an intensive agro... more We studied the biochemical and biophysical processes of carbon sequestration in an intensive agroforestry system on two soils (Feralsol -Luero; Arenosol -Teso) in W. Kenya to elucidate the mechanisms associated with long-term carbon storage. Specifically, we looked at a top-down model (macro-aggregates form around organic matter particles and micro-aggregates form within the macro-aggregates) and a bottom-up model (micro-aggregates form independently and are incorporated into macro-aggregates) of soil aggregate formation. Soil samples were collected from experiments on improved tree fallows using different species and two tillage treatments; water-stable aggregates were extracted and sorted into three size classes: macroaggregates (N 212 μm), meso-aggregates (53-212 μm) and micro-aggregates (20-53 μm). Organic matter characterization of each fraction was based on 13 C isotope abundance, Fourier transform infrared (FTIR) spectroscopy and the abundance of polysaccharides. Improved fallows increased soil C by 0.28 and 0.26 kg m 2 in the top 20 cm of the soil profile in Luero and Teso, respectively. Tillage altered the distribution of aggregates among size classes. Changes in the δ 13 C signature in each fraction indicated that more of the new carbon was found in the macro-aggregates (35-70%) and meso-aggregates (18-49%) in Luero and less (9-17%) was found in the micro-aggregates. In Teso, about 40-80% of the new aggregate C was found in the meso-aggregates, 14-45% was found in the micro-aggregates and only 4-26% was found in the macro-aggregates. The mesoaggregates and macro-aggregates to a lesser extent, in both sites, were enriched in carboxylic-C and aromatic-C, indicating the importance of OM decomposition and plant-derived C in the stabilization of larger aggregates, supporting the top-down model of aggregate formation. Microbially derived polysaccharides play a leading role in the formation of stable micro-aggregates and carboxylic-C promotes stabilization through surface occlusion. This bottom-up process is essential to promote long-term carbon sequestration in soils. Additionally, the micro-aggregates at both sites were enriched in polysaccharides and had elevated ratios of galactose + mannose:arabinose + xylose than the other aggregate fractions, indicating the importance of microbial processes in the formation of stable micro-aggregates and supporting the bottom-up model.
Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time... more Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time consuming to measure using conventional laboratory incubation methods. Recent studies have demonstrated the ability of visible-near infrared spectroscopy (NIRS) for rapid non-destructive determination of soil organic carbon (SOC) and nitrogen (N) concentration.
Poor soil fertility is the biggest obstacle to agricultural productivity in Sub-Saharan Africa. I... more 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
Short rotations of Eucalyptus plantations under coppice regime are extensively managed for wood p... more Short rotations of Eucalyptus plantations under coppice regime are extensively managed for wood production in Madagascar. Nevertheless, little is known about their biomass production and partitioning and their potential in terms of carbon sequestration. If above-ground biomass (AGB) can be estimated based on established allometric relations, below-ground (BGB) estimates are much less common. The aim of this work was to
Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time... more Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time consuming to measure using conventional laboratory incubation methods. Recent studies have demonstrated the ability of visible-near infrared spectroscopy (NIRS) for rapid nondestructive determination of soil organic carbon (SOC) and nitrogen (N) concentration. We investigated whether NIRS (350-2500 nm) can predict C mineralization rates in physically fractionated soil aggregates (bulk soil and 6 size fractions, n ¼ 108) and free organic matter (2 size fractions, n ¼ 27) in aerobically incubated samples from a clayey soil (Ferralsol) and a sandy soil (Arenosol). Incubation reference values were calibrated to first derivative reflectance spectra using partial least-squares regression. Prediction accuracy was assessed by comparing laboratory reference values with NIRS values predicted using full hold-out-one cross-validation. Cross-validated prediction for C respired (500 days) in soil aggregate fractions had an R 2 of 0.82 while that of C mineralized (300 days) in organic matter fractions was 0.71. Major soil aggregate fractions could be perfectly spectrally discriminated using a 50% random holdout validation sample. NIRS is a promising technique for rapid characterization of potential C mineralization in soils and aggregate fractions. Further work should test the robustness of NIRS prediction of mineralization rates of aggregate fractions across a wide range of soils and spectral mixture models for predicting mass fractions of aggregate size classes. r
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Papers by Alain Albrecht