Bernard Barthès

Aggregate stability as an indicator of soil susceptibility to runoff and erosion; validation at several levels

Abstract The evaluation of soil susceptibility to runoff and water erosion in the field is often expensive or time-consuming. Several authors have reported that susceptibility is linked to aggregate stability, whose determination is far easier. However, this susceptibility has generally been deduced from rainfall simulation experiments on sieved soil samples, whose behaviour is not always representative of field-scale phenomena. Our aim was to extend the validity of relationships between soil aggregation and erosion through comparisons of topsoil aggregate stability and field-assessed susceptibility to runoff and erosion. Susceptibility to runoff and erosion was determined at several levels: first, on a southern French Regosol, through measurements of runoff and soil loss from 1-m2 microplots under simulated rainfall; second, from 100- to 800-m2 runoff plots on a Nitosol in Benin, a Ferralsol in Cameroon and a Regosol in Mexico (with additional data on Syria from the literature); and finally, soil susceptibility to erosion was determined through semi-quantitative assessment of the frequency of erosion features on vineyard hillsides in southern France. Aggregate stability was determined by immersion in water and wet-sieving of 2-mm sieved, air-dried 0- to 10-cm soil samples, which actually tests aggregate resistance to slaking. Under simulated rainfall, runoff depth and soil loss after 30 min were negatively correlated with topsoil content in stable macroaggregates (>0.2 mm). On runoff plots, 3-year runoff rate and soil loss were negatively correlated with topsoil aggregate stability, especially stable macroaggregate content; these correlations were improved when slope gradient and climate aggressiveness were considered in addition to aggregate stability. On vineyard hillsides, the frequency index of erosion features was negatively correlated with topsoil content in stable macroaggregates, especially in the absence of conservation practices. These results confirm that aggregate stability is a relevant indicator of soil susceptibility to runoff and erosion, especially in Mediterranean and tropical areas where intense rainfall is frequent. They also confirm that simple laboratory determination can provide data closely correlated with those resulting from field investigations.

Organic matter management for soil conservation and productivity restoration in Africa: a contribution from Francophone research

Soil fertility is closely linked to soil organic matter (SOM), whose status depends on input, i.e., mainly biomass management, and output, i.e., mineralization, erosion and leaching. Preliminary results from runoff plots and lysimeters on hillslopes in West Africa indicated that carbon losses by erosion and leaching ranged between 10 and 100 kg C ha−1 yr−1, depending on annual rainfall and vegetal cover. Under natural conditions, losses may be low enough to be compensated by aerial deposits. But together with mineralization, erosion can locally be an important cause of SOM decrease in cropping systems where there is poor soil cover, steep slopes and erosive rain conditions. The effect of previous erosion on cereal production was assessed in case studies from Rwanda, Burundi, Cameroon, and Burkina Faso. On the densely populated hillslopes of Rwanda, hedges and manure reduced runoff and erosion efficiently, but did not succeed in improving grain yields due to P-deficiency of these ferrallitic soils. In Burundi, under similar conditions but under banana plantation, tree density and mulch cover had a strong influence on erosion; this previous erosion had an important effect on the next maize yield, even when the soils were amended with manure, mineral fertilizers and lime. On sandy ferruginous soils of North Cameroon, erosion increased with increasing tillage intensity. Manure application increased grain yield, but burying organic residues did not improve SOM levels and soil resistance to erosion. Mulching and tillage limited to the plant rows protected the topsoil against erosion, but did not clearly increase the yield. Manuring permitted the restoration of soil productivity, but additional mineral fertilizers (P, N) were needed to reach rapidly a high level of grain production. In the same way, experiments conducted with traditional Zaï system for restoring a degraded Entisol in Burkina Faso showed that runoff harvesting and organic matter input were not sufficient with no additional N and P fertilizers. Complementary experiments in Cameroon showed that a 4-mm selective sheet erosion and a 50-mm non-selective de-surfacing resulted in similar yield decline. Long fallowing, burning and grazing are traditional ways to utilize available biomass in Africa. Considering social habits and technical realities, it seems useful to balance ‘grazing-manuring’ and mulching in order to protect the soil and maintain its productive capacity. Minimum tillage with mulch (crop residues, weeds or legume fallow) is the new trend used for increasing crop production, with the help of herbicides. Agroforestry that produces good-quality litter is also a part of the solution.

Relationship between soil erodibility and topsoil aggregate stability or carbon content in a cultivated Mediterranean Highland (Aveyron, France)

Abstract In the Rougiers de Camares area (in the south of France), hillslopes are very susceptible to water erosion. This is the result of physical features (steep slopes, soft bedrocks, thin soils), climatic aggressiveness (frost, storms), as well as farming systems (intensive tillage, short crop cycles, land consolidation). The objective of this work was to study the relationships between soil erodibility, macroaggregate stability, and carbon content of surface samples (0–10 cm), in a Rougiers Entisol (Lithic Udorthent) under various management practices (flat or raised moldboard ploughing, superficial tillage, direct drilling, with inputs in the form of mineral fertilizers or sheep manure). The soil erodibility was assessed by field rainfall simulation (60 mm h‐1) on manually retilled bare dry soil; water‐stable macroaggregation (>0.2 mm) was assessed by wet‐sieving, after immersion in water. Runoff, turbidity and soil losses were linked to water‐stable macroaggregation and carbon content in the 0–10 c...

Determination of total carbon and nitrogen content in a range of tropical soils using near infrared spectroscopy: influence of replication and sample grinding and drying

Near infrared (NIR) reflectance spectroscopy has been receiving increased attention for the rapid and inexpensive determination of soil properties and of total carbon (Ct) and nitrogen content (Nt) in particular. However, methodological aspects such as sample grinding and drying or replication have not been addressed extensively. The objectives of the paper were, thus, to assess how NIR predictions of Ct and Nt were affected by sample grinding (2 mm sieving vs. 0.2 mm grinding), drying (air-drying vs oven-drying at 40°C during 24 h) and replication (use of one to six sub-samples to determine average spectra). This was performed on a range of tropical soils that differed widely in mineralogy (low and high activity clay soils, allophanic soils) and texture (sandy to clayey). The accuracy of the NIR predictions of Ct and Nt was higher with oven-dried compared to air-dried samples and, more markedly, with 0.2 mm ground compared to 2 mm sieved samples. Replication had a positive effect on NIR predictions when 2 mm sieved samples were used, especially for air-dried samples, but this effect was not clear with 0.2 mm ground samples. Thus, the most accurate predictions of Ct and Nt were obtained with oven-dried finely ground samples, with limited response to sample replication. Accurate predictions were, however, also obtained with four replicates on oven-dried 2 mm sieved samples. Acceptable and less tedious results could, thus, be achieved when replacing fine grinding by replication. Even with this procedure, the r2 between predicted (NIR) and measured (reference) values was 0.9 and the ratio of standard error of prediction to mean (CV%) was 20% which can be considered satisfactory for the heterogeneous sample set under study.

Determination of soil content in chlordecone (organochlorine pesticide) using near infrared reflectance spectroscopy (NIRS).

Chlordecone is a toxic organochlorine insecticide that was used in banana plantations until 1993 in the French West Indies. This study aimed at assessing the potential of near infrared reflectance spectroscopy (NIRS) for determining chlordecone content in Andosols, Nitisols and Ferralsols from Martinique. Using partial least square regression, chlordecone content conventionally determined through gas chromatography-mass spectrometry could be correctly predicted by NIRS (Q(2) = 0.75, R(2) = 0.82 for the total set), especially for samples with chlordecone content <12 mg kg(-1) or when the sample set was rather homogeneous (Q(2) = 0.91, R(2) = 0.82 for the Andosols). Conventional measures and NIRS predictions were poorly correlated for chlordecone content >12 mg kg(-1), nevertheless ca. 80% samples were correctly predicted when the set was divided into three or four classes of chlordecone content. Thus NIRS could be considered a time- and cost-effective method for characterising soil contamination by chlordecone.

Comparing near and Mid-Infrared Reflectance Spectroscopy for Determining Properties of Malagasy Soils, Using Global or LOCAL Calibration:

Nowadays, near infrared (NIR) and mid-infrared (mid-IR) reflectance spectroscopy are recognised useful approaches for quantifying soil properties, cost and time effectively. The aim of this work was to compare predictions of soil carbon (C) and nitrogen (N) content, C/N ratio, substrate-induced respiration (SIR) and denitrifying enzyme activity (DEA) using NIR and mid-IR spectroscopy over a diverse set of 360 Malagasy topsoils. Partial least square regression was used for fitting NIR and mid-IR spectra to conventional data through procedures of calibration either global (one prediction model for all samples) or LOCAL (one prediction model per sample). Prediction accuracy was assessed according to validation (r2), standard error of prediction (SEP) in proportion to the mean and ratio of standard deviation to SEP (RPD). Using both NIR and mid-IR spectroscopy, global calibration over the whole sample set yielded predictions that were excellent for C and N (r2 > 0.9, SEP <20%, RPD ⩾ 3), good for C/N, acceptable for SIR, but poor for DEA. LOCAL calibration improved C/N and SIR predictions with both NIR and mid-IR spectroscopy, while DEA prediction became acceptable with NIR spectroscopy only. Additional improvement was achieved when LOCAL calibration was carried out over the fine-textured sub-set, especially for SIR (r2 > 0.9, SEP < 20%, RPD > 3). In contrast, LOCAL calibration over the coarse-textured sub-set was clearly not useful for improving prediction accuracy. NIR outperformed mid-IR spectroscopy whatever the variable, the calibration procedure and the sample set (except for SIR over the coarse-textured sub-set, where both similar), suggesting its possible superiority for tropical soils.

Relations entre stabilité de l'agrégation et matière organique totale et soluble à l'eau chaude dans des sols ferrallitiques argileux (Congo, Brésil)

Organic matter (OM) generally plays an important role in soil aggregate stability. The objective of this work was to characterize the hot water-extractable OM and its role in the aggregate stability of clayey ferrallitic soils under different land use management. The macroaggregate (>200 µm) stability of these soils was determined before (AS) and after hot-water extraction (ASe). The contents in total organic carbon and in carbon present as carbohydrates, as well as the sugar composition, were determined on the bulk soils and their hot-water extracts.The carbon and macroaggregate contents decreased upon land-clearing and cultivation, but to a lesser extent when some cultural practices were used. Whatever the situation considered, the hot-water extract always presented a higher carbohydrate content than the bulk soil. The high values of ratios r = (galactose + mannose)/(arabinose + xylose) suggested that a large proportion of carbohydrates was of microbial origin. There were significant correlations betwee...

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.

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.