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The rice root nematodes, Hirschmanniella spp., are considered the predominant plant-parasitic nematode in the clay soils of Battambang’s lowland rice fields in Cambodia. In this study, we compared the nematode population dynamics, rice yield parameters, and soil organic matter content in lowland rice fields under conservation agriculture (CA) system with conventional tillage systems with green manure management (GMCT) or with plough-based tillage (CT) systems. Results demonstrated that GMCT for one year (GMCT1) and the long-term CA for seven years (CA7) reduced nematode densities in both soil and in rice roots, almost throughout the study period, compared to the CT. In the GMCT with tillage for two years (GMCT2), however, the Hirschmanniella spp. densities were high at the beginning, but reduced at later stages of the cycle. For rice yield components and soil fertility, CA7 proved to be effective in increasing plant height, the number of tillers/plant and soil organic matter. Based on molecular, morphometric and morphological features, the nematodes were identified as H. mucronata. The phylogenetic trees of three nuclear markers displayed similarity among 18S, 18S-ITS1-5.8S and D2D3 regions, identifying nematodes as H. mucronata clade I (bootstrap values of 79–100) and related to H. kwazuna and H. loofi sisters. Morphologically the long body (1,512–2,665 µm), long stylet (25–29 µm) and obvious mucron at the end of terminus matched with H. mucronata. Therefore, long term CA, with reduced tillage and using leguminous cover crops provides a promising system to control H. mucronata and promote rice yields and soil organic matter in Cambodian lowland rice fields.

Highlights Soil type, operational speed, machinery weight, and power size affected corn seeding rate and seeding efficiency Corn growth and yield was affected by seeder type, so utilization of appropriate machinery is very important Having cover crop residue on the land can maintain soil moisture for a longer time than tilled land Abstract.Conservation agriculture (CA) is gaining popularity in Cambodia to sustain soil fertility, productivity, and profits. However, the adoption remains slow due to the low engagement so far of local services into CA machinery and technology. Therefore, this paper aimed to compare different no-till seeders and residue retention methods for corn (Zea mays L.) sowing and to determine their effects on growth and yield. The experiment was conducted in Rattanak Mondul District, Battambang Province, in the wet season of 2019. Sunn hemp (Crotalaria juncea L.) was grown as a cover crop for 74 days prior to corn. Each main plot was 14 × 34 m and was subdivided lengthwise with 4.0-m buffers for operations of two different seeders, namely one-row Morrison seeder mounted on 14 kW Oggun tractor and four-row Brazilian seeder mounted on 56 kW John Deere tractor. The results show that using Brazilian seeder saved around 50% of seed and 80% of time, when compared with Morrison seeder. Plant population, plant diameter, and kernel characteristics were also significantly greater with Brazilian seeder, when compared to the Morrison seeder. A combination of the Brazilian seeder and Cambodia made crimper plots had larger ear size and weight. Volumetric Moisture Content (VMC) was significantly affected by residue retention with higher VMC for rolled and crimped residue compared to disked/incorporated residue. The study demonstrates that using no-till equipment such as roller crimpers and no-till planters is feasible in Cambodia’s small farming systems although more research is needed to study long-term effects on crop yields and soil health. Keywords: Battambang Province, Conservation agriculture, Green manure, Soil improvements

Sandy soils are widely distributed in Brazil and occur in most states. However, they have serious limitations for agricultural use due to low natural fertility, low water retention capacity, low resilience, and high susceptibility to erosion. Management of these soils through a no‐till (NT) system following its basic principles (i.e., elimination of plowing, diversification of crop rotation, retention of crop residues, and maintenance of permanent soil cover) is a practice that can restore the soil organic carbon (SOC) stock, and restore productivity and economic viability of these soils. We hypothesized that: (a) we can recover the carbon (C) stock in Typic Quartzipisamment in subtropical and tropical climates to the level of native vegetation; (b) the adoption of NT during the initial phase, in conjunction with intensive cropping systems with high plant biomass input, is an efficient strategy to recover SOC in Typic Quartzipisamment soils; and (c) long‐term NT (over 20 years—the maintenance phase), and low/medium input of plant biomass, is an efficient strategy for SOC recovery in Typic Quartzipisamment soils. Thus, the present study aimed at assessment of the recovery of SOC stocks in soil profile (0–1.0 m depth) in a Typic Quartzipisamment in subtropical and tropical climates, in comparison with the neighboring soil under native vegetation (NV). The sites selected for this study are located in two Brazilian grain‐producing regions: (a) Manoel Viana City—Rio Grande do Sul State (MV—site 1) that represents the subtropical climate condition, and (b) Luiz Eduardo Magalhaes City—Bahia State (LEM—site 2), which represents the tropical climate. Soil samples were collected from soil profile for 0–1.0 m depth. At site 1, the predominant crop rotation over the years was wheat/soybean/oats + forage turnip/soybean. At site 2, the crop rotation includes cultivation in two periods designated as the crop and off‐seasons, and the succession consisted of soybean/Brachiaria/cotton/millet/soybean/maize‐Brachiaria. In both sites, soils under the neighboring NV were also sampled. SOC content was determined by the dry combustion method using an elemental C and N analyzer. Soil C stock was calculated based on the equivalent soil mass. Approximately 31 and 23% of the SOC stock was stored in the surface at 0–0.20 m, compared with 69 and 77% in 0.20–1.0 m layer of a Typic Quartzipisamment with high and low/medium input of crop residues, respectively. The Typic Quartzipisamment with low/medium input of crop residues fully recovered the stock of C in the surface soil layer (0–0.05 m). The Typic Quartzipisamment with high input of crop residues fully recovered the stock of C in the cultivated (0–0.20 m) and also in deeper soil layers (0–0.40 m). Thus, adoption of NT associated with intensification of cultivation systems and high addition of biomass‐C is an effective strategy for C restoration in a Typic Quartzipisamment, while also playing a crucial role in restoring ecosystem productivity, soil quality, and environment.

Meloidogyne spp. and Hirschmanniella spp. are among the most damaging plant-parasitic nematodes (PPNs). They threaten rice production, the main staple food in Asia. Cropping systems that promote natural biocontrol and plant tolerance to diseases are put forward as sustainable solutions to protect rice from these pests. In particular, cropping systems managed under conservation agriculture (CA) are promising because they improve soil health and functioning. We investigated the effects of two cropping system components in a Cambodian field, (i) CA practices, i.e., no-tillage with a cover crop Stylosanthes guianensis (cv. Nina), versus conventional plow-based tillage with no cover crop, and (ii) using IR504, IR64, Azucena and Zhonghua 11 rice varieties, on PPNs in roots and on communities (bacteria, fungi and nematodes) in the rhizosphere. We used a sequencing approach via amplicon barcoding to target microbial marker genes (16 S and ITS rRNA gene) and a microscopic approach to identify and quantify nematodes in the rhizosphere compartment. The variety had less effect than agricultural practices on the infection by PPNs and on the assembly of the three rhizosphere communities. Under CA, the abundance of PPNs extracted from the roots was reduced by 88%. Soil quality was substantially improved (+83% of total Kjeldahl nitrogen, +34% of available phosphorus, +10% of exchangeable potassium, +110% of soil organic carbon, +30% for the cation exchange capacity), thus providing more basal resources for microbial decomposers, especially fungi (+164% putative saprotrophs). Characterization of the three rhizosphere communities revealed a shift in the structure associated with soil enrichment. Both microbial richness (+3% for bacteria and +38% for fungi) and diversity (Shannon index, +11% for fungi and +5% for nematodes) increased. The relative abundance of taxa was modified by CA with notably more mycorrhizal fungi (+329% Glomeromycota spp.) and fewer Pratylenchidae nematodes (−92% Hirschmanniella spp.) in the rhizosphere. The reassembly of the communities using CA was associated with regulation of PPN populations. The reduction in Meloidogyne spp. abundance in roots (−64%) was correlated with the maturity of the food web (maturity index, +10% under CA) and with the increase in the relative abundance of omnivorous nematodes in the rhizosphere (+68% under CA). Seven years of CA in this field enabled the whole soil food web to mature thus creating a favorable niche for potentially predatory nematodes and microbes antagonistic against PPNs. This study confirms that CA is an alternative to nematicides to limit infection by PPNs in rice cropping systems.

International audience; Conservation agriculture (CA) is one strategy with which both sustainability and productivity can be achieved by improving soil health. However, linkages between practices, soil health and cropping system performance remain poorly disentangled. We assessed the relationships between soil health and cropping system performance for three maize-based cropping systems in New Caledonia. Two CA systems, one with direct seeding into a mixed species dead mulch (CA-DM) and one into a stylo living mulch (CA-LM), were compared to a conventional tillage (CT) system. CA vs. CT experiment started in 2011, whereas the differentiation between CA-DM and CA-LM was initiated in 2017 only. In 2018, soil health was evaluated using Biofunctool (R), a set of ten in-field tools that assess soil carbon transformation, structure maintenance and nutrient cycling functions. The performance of the three cropping systems were assessed by monitoring weeds, maize growth and yield components. Structural equation modelling (SEM) was used to disentangle the links between agricultural management, soil health and cropping system performance. Soil structure maintenance and nutrient cycling functions were higher under CA-DM and CA-LM than under CT, and carbon transformation function was higher under CA-DM than under CT and CA-LM. Overall, the soil health index (SHI) was 1.3-fold higher under CA systems than under CT. Cropping system management had both direct and indirect effects on soil functioning and crop productivity leading to a 1.3-fold higher yield under CA than under CT. The direct and indirect effects of CA systems on soil health had positive impacts on ecosystem services (i.e., productivity, weed regulation and soil ecosystem services). Such integrative approaches that account for the relationships and possible trade-offs between cropping system components enable a better understanding of the effects and the performance of practices, and support adaptive agricultural management.

Conservation agriculture through no-till based on cropping systems with high biomass-C input, is a strategy to restoring the carbon (C) lost from natural capital by conversion to agricultural land. We hypothesize that cropping systems based on quantity, diversity and frequency of biomass-C input above soil C dynamic equilibrium level can recover the natural capital. The objectives of this study were to: i) assess the C-budget of land use change for two contrasting climatic environments, ii) estimate the C turnover time of the natural capital through no-till cropping systems, and iii) determine the C pathway since soil under native vegetation to no-till cropping systems. In a subtropical and tropical environment, three types of land use were used: a) undisturbed soil under native vegetation as the reference of pristine level; b) degraded soil through continuous tillage; and c) soil under continuous no-till cropping system with high biomass-C input. At the subtropical environment, the soil under continuous tillage caused loss of 25.4 Mg C ha

The conversion of native vegetation (NV) into agricultural land by clearing and tillage disrupts the soil structure, and depletes soil organic carbon (SOC) pool. Data on changes in SOC pools are needed to enhance scientific knowledge regarding the effects of land use and Conservation Agriculture (CA) on soil fertility, agronomic productivity, and soil C sink capacity. The objective of this study was to quantify changes in SOC fractions due to conversion of NV to agricultural land, and to assess the rate of recovery of SOC fractions and the resilience index of CA cropping systems under sub-tropical (Ponta Grossa/PR — PG) and tropical (Lucas do Rio Verde/MT — LRV) regions of Brazil.

The adoption of Conservation Agriculture (CA) and the maintenance of crop residues on the soil surface result in the long-term increase of carbon (C) in the system, promoting C sequestration and reducing C-CO2 emissions to the atmosphere. The present study conducted in subtropical and tropical regions in Brazil aimed to assess the impact of the continuous plow-based conventional tillage (CT) on soil organic carbon (SOC) stock vis-a-vis native vegetation (NV) as baseline, compare SOC balance among CT, CA cropping systems, and NV; and evaluate the redistribution of SOC stock in soil profile in relation to soil resilience.

Nutrient cycling is considered as a key agroecosystem service of Conservation Agriculture (CA). CA brings together major nutrients and organic matter fluxes within the soil-micro-organism-plant system. It also generates organic and inorganic nutrient pools accessible through new biological pathways. This chapter addresses nutrient management practices and benefits in CA systems. Current trends in CA systems design are discussed, as is integrated nutrient management. The concept of the “forest model” is introduced and discussed in detail. Sections on nutrient pools, bioavailability of elements, and mineral use efficiency in CA systems are also included. The chapter concludes with two case studies from contrasting agroecosystems: one from a Brazilian Fazenda transitioning from "generic" CA to CA based on multifunctional mix species cover, and one from a French dairy farm, illustrating how CA, associated with important and diversified organic matter inputs from cover crops and crop residues, impacts and modifies plant nutrition management.

Soil organic carbon (SOC) pools, particularly labile pools, and soil enzymes are good indicators of short-term impacts of soil management. We hypothesized that labile SOC pools drives C accumulation and enzyme activity can be an efficient indicator for C build up in short-term conservation agriculture. The aims of this study were to quantify the impacts of tillage and crop rotations with diverse crop residue inputs on changes in SOC labile pools and enzymatic activities in rice-, soybean- and cassava-based cropping systems designated as RcCS, SbCS and CsCS, respectively. The four treatments in each cropping system consisted of: conventional tillage (CT), no-till in which main crops (rice, soybean and cassava) were grown in a one-year frequency pattern (NT1) and, no-till in which the main crops were grown in bi-annual rotations with maize (NT2 and NT3). After 5 years experiment period, greater hot-water extractable organic C (HWEO-C) stocks of 61%, 55% and 53%, and permanganate oxidizable C (POX-C) stocks of 23%, 21% and 32% were attributed to NT than those in CT soils under RcCS, SbCS and CsCS, respectively, at 0–5 cm soil layer. The pyrophosphate extractable organic C (PEO-C) and chemically stabilized organic C (CSO-C) stocks were almost constant in each depth among treatments, except 0–5 cm in CsCS. The β-glucosidase activity was 18%, 28% and 49% greater in NT than those in CT soils at 0–5 cm under RcCS, SbCS, CsCS, respectively. Arylsulfatase activity was 36% and 39% greater in NT than in CT under SbCS and CsCS, respectively but no significant differences in RcCS. A strong and positive correlation (P

In the Northwestern uplands of Cambodia, the commodification of agriculture in the 2000 s substituted the traditional rotational and diversified cropping systems with monocropping of commercial crops such as maize and cassava. Driven by high market demand, this transition was associated with deforestation, erosion of soils and biodiversity as well as pollution from increased use of chemical inputs. Land degradation observed after a few years of intensive monocropping undermined the sustainability of the overall agricultural system. However, lessons learned from previous failures of crop boom-bust cycles did not materialize as an incentive to adopt alternative sustainable practices. Along with local villagers we developed a role-play game to investigate farmers’ decision-making in relation to land-use transitions and their participation in a Conservation Agriculture (CA) initiative aimed at mitigating land degradation. The game revealed that farmers were still trapped in the boom-bust cycle with commercial crops. Market opportunities and high, short-term economic returns are key parameters in the decision-making process, which mostly overrides environmental aspects. This study shows the importance of opportunity windows for development interventions, the crucial role of farming communities in co-designing alternative cropping systems and the potential of social learning devices to bring CA to scale.

As a response to the worldwide challenge raised by soil degradation, Conservation Agriculture (CA) was proposed to help restoring the three main soil functions, i.e. carbon transformation, nutrient cycling and structure maintenance. However, there is still a lack of integrative studies that assess the overall impact of CA on soil health. To fill the gap, Biofunctooll (R), a set of in-field indicators, was developed to monitor changes in soil biological functioning. In this study, Biofunctool (R) was used to assess the impact of a conventional tillage (CT) and three CA annual-based cropping systems on soil health on a Cambodian Oxisol. Eight indicators related to the three soil functions were monitored and integrated into a Soil Quality index (SQI), i.e. the Biofunctool (R) Index. Overall, we found that soil health was twice higher under the CA treatments than under CT treatment. Although it was similar in the three CA treatments, the contribution of each soil function to the soil health diverged. An analysis of soil carbon dynamics also showed that CA support short-term soil organic carbon stabilization compared to CT. This study demonstrates that Biofunctool (R) is a robust, relevant, time-and cost-effective in-field assessment tool that can be used in multiple ways including cropping system management, capacity building of local stakeholders, and policy dialogue.

At the end of the 1990s, the Northwestern uplands of Cambodia were one of the last forest frontiers of the country. In a region that was the last Khmer Rouge stronghold, the opening of former conflict zones after a peace agreement initiated a vast movement of agricultural colonization. This movement was economically triggered by high market demand for agricultural commodities such as maize and cassava and fueled by a massive flow of spontaneous in-migration of land-poor farmers from lowland regions around the entire country. Focused on four upland districts along a pioneer front of Northwestern Cambodia, we analyzed historical trajectories of land use/cover changes using a chronological series of Landsat data from 1976 to 2016. We identified key drivers of deforestation using demographic data and qualitative information from local actors and other relevant stake-holders. We found a 65 percent forest cover loss due to conversion by smallholders into agricultural land for maize and cassava cultivation over a period of 15 years. The underlying mechanisms of land use change were further investigated to understand the diversity of individual farm trajectories and decision-making processes in relation to land conversion. These elements of diagnosis are essential to engage farming communities in innovative land use systems and to develop sustainable alternatives to boom crops that have led to the current situation of land degradation and economic instability.

No-till (NT) cropping systems have the potential to enhance soil aggregation, providing physical protection and soil C sequestration. The existence of discrepancies in the impact of tillage on soil aggregation and soil C sequestration warrants further studies, particularly for different crop rotations. We hypothesized the following: a) NT biannual crop rotations tend to be more effective in restoring large macroaggregation and the concentrations of soil organic C (SOC), total N and permanganate oxidizable C (POXC) associated with macroaggregates than NT systems with a one-year frequency pattern and conventional tillage (CT); b) the continuous biomass-C inputs via crop residues in large macroaggregates under NT tend to increase the proportion of aliphatic C than those under CT. Therefore, the objectives of this study were: (i) to assess changes in the aggregate size distribution and levels of aggregate-associated total SOC, total N and POXC and (ii) to characterize humic acid (HA) using 13C CP-MAS nuclear magnetic resonance (NMR) spectra of 8- to 19-mm soil aggregate size class in a reference vegetation (RV) and in rice-, soybean- and cassava-based cropping systems (RcCS, SbCS and CsCS, respectively) in a clayed Oxisol after tillage and crop rotation management. We evaluated four treatments in each cropping system: 1) CT, and 2) three NT systems in a randomized complete block design with three replicates. Soil aggregate samples were collected at depths of 0–5, 5–10 and 10–20 cm. The conversion of RV to agricultural land influenced the distribution of aggregate size classes, soil aggregation indices and aggregate-associated SOC, total N and POXC in the two surface layers. The formation of large macroaggregates (8–19 mm) dominated the aggregate size distribution with a relatively higher proportion under RV and NT than under CT. Across all soil depths, the proportions of the 8- to 19-mm aggregate size fraction were 59% (NV), 43% and 47% (RcCS), 45% and 53% (SbCS) and 34% and 37% (CsCS) for the CT and NT systems, respectively. Among the three NT systems, the biannual crop rotations in the three cropping systems (NT2-Rice, NT2-Soybean, NT2-Cassava; NT3-Rice, NT3-Soybean and NT3-Cassava) indicated better performance than the one-year frequency pattern in restoring large macroaggregation and the concentrations of SOC, total N and POXC associated with large macroaggregates. Additionally, in the surface (0–5 cm) and subsurface (10–20 cm) soil layers, the SbCS with a high rate (7.32 Mg C ha−1 year−1) and diversity [Pennisetum typhoides) (Pearl millet)/maize + Brachiaria ruziziensis (Brz), Stylosanthes guianensis (St)] of biomass-C inputs reached the highest levels of lability of SOC and POXC in the macroaggregate size classes of 0.25–0.5 and 8–19 mm, respectively. The CP-MAS 13C NMR measurement suggests that the continuous and high biomass-C inputs with diverse crop residues under NT, such as millet, maize, Brz, St and Crotalaria juncea, tended to increase the proportion of aliphatic C than under CT; an opposite trend was observed for aromatic C. 13C NMR revealed an advance caused by the association between the quantity and quality of C addition via cultural residues in the discrimination of the composition of C in the macroaggregation in the tropical region.

Conservation agriculture (CA) is an effective tool that is used to increase soil C sequestration and enhance soil quality and agronomic productivity. However, rigorous empirical evidence from Southeast Asia, particularly in the Cambodian agro-ecosystem, is still scarce. We hypothesized that high and diversified biomass-C inputs in CA might be the first step toward to increase SOC in the topsoil by creating the C flow to support C storage overtime. Thus, the aim of this study was to quantify the short-term (i.e., five year) impacts of soil management and cropping systems on soil organic C (SOC), soil total N (STN), particulate organic C (POC) and mineral-associated organic C (MAOC). There were three distinct experiments comprised of a combination of cover and main crops including rice-, soybean- and cassava-based cropping systems, hereafter designated as RcCS, SbCS and CsCS, respectively. The experimental plots were laid out in a randomized complete block design with three replicates. Soil management treatments included conventional tillage (CT) and no-till (NT) and a selected adjacent area of reference vegetation (RV). Soil sampling was conducted in 2011 and 2013 at seven depths (0–5, 5–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm). Soil management and crop sequences significantly affected SOC and STN stocks in all three cropping systems. On average, NT SOC stocks at 0–5 cm depth was greater than those of CT by 10%, 20% and 18% and STN stocks by 8%, 25% and 16% for RcCS, SbCS and CsCS, respectively. SOC levels followed the order RV > NT > CT. SOC stocks in the subsoil layers were consistently lower in NT than in CT in all three cropping systems. POC stocks at 0–5 cm depth in NT were on average 22%, 20% and 78% greater than those in CT in RcCS, SbCS and CsCS, respectively. However, significant differences were detected only in RcCS and CsCS. The major POC stocks were found at 0–20 cm depth. NT treatments in SbCS stored 9% greater MAOC stocks at 0–5 cm depth than those in CT, and an increasing trend of NT was observed in RcCS and CsCS. In all three cropping systems, NT systems with diversified crop species significantly increased SOC stocks ranging by 6 to 28% and POC stocks by 56–127% in the surface soils and tended to restore SOC and POC in the subsoil layers after five years. The results leads to accept the hypothesis that short-term CA associated with high biomass-C inputs (particularly bi-annual rotations) promotes SOC recovery in the topsoil layer and creates a potential to increase SOC in the subsoil layers when deep-rooting cover crops are included in crop rotations.

The assessment of the impacts of land-use and management on soil organic carbon (SOC) dynamics is a major environmental concern, as the soil carbon cycle underpins key ecosystem services. However, assessments based on short-term SOC dynamics face methodological and experimental difficulties. Hurisso et al. (2016) proposed a method to assess SOC dynamics by coupling two methods: Permanganate Oxidizable Carbon (POXC) and Basal Soil Respiration (BSR). This method has been used in laboratory on dried and re-wetted soil samples from temperate regions mainly. In our study, we adapted this method to the field and proposed a cost-effective in-field indicator combining the POXC and in situ Basal Soil Respiration (SituResp (R) method). We tested the indicator at four study sites (n = 169 points) within various tropical land-use and management contexts based on rubber, soybean and oil palm cropping systems respectively in Thailand, Cambodia and Indonesia. The results demonstrated the relevance, sensitivity and robustness of the POXC-SituResp (R) indicator to characterize the impact of a gradient of disturbance on SOC dynamics. The results also highlighted the potential of conservation agriculture (no-tillage and crops residues) and compost amendments to accumulate SOC. Rock-Eval (R) analysis showed that POXC-SituResp (R) indicator is negatively linked to excess of potentially mineralizable labile carbon. Carbon pools targeted by the POXC were specified by Rock-Eval (R) pyrolysis measurements to be a rather thermal resistant pool of SOC. Our study confirms that the integrated indicator based on POXC and BSR assess a relative carbon stabilization of SOC pools. This indicator can be measured in the field by a rapid and cost-effective method.

Rice production in Southeast Asia is significantly affected by root-parasitic nematodes (RPN). The Green Revolution has encouraged new agricultural practices (e.g. intensive monoculture, high yielding rice variety) to respond to the high rice demand; however, these methods have promoted the spread of these pests. The recent banning of chemical nematicides resulted in a need for alternative sustainable control strategies. In the present study, we assessed the effects of a direct-seeding mulch-based cropping system (DMC) vs conventional plough-based tillages (CT) on soil properties, rice yield and RPN communities during a two-year trial in Cambodia. Our results show that on average the population densities of RPN were significantly higher in DMC than in CT. Molecular identification revealed only two RPN species associated with roots: Meloidogyne graminicola, not previously reported from Cambodia, was predominant and was present throughout the plant’s development, whereas Hirschmanniella mucronata was only found at the tillering and milky stages. We conclude that DMC had a significant positive impact on rice yield, despite higher RPN short-term pressure. In order to increase the efficiency of such cropping systems, further studies and an evaluation of the long-term relationships between DMC, the nature of cover crops used, the soil biota including RPN, and rice yield should be conducted.

Research data on the mechanisms of C stabilization and saturation affected by soil management systems in highly weathered soils remain scarce. Past studies have revealed the fundamental role of aggregation promoted by soil conservation practices in the physical protection of C fractions. This study is based on the hypothesis that the increased physical C protection provides sufficient time to strengthen the interaction between C fractions and soil minerals, as being the pathway for C stabilization and accumulation in highly weathered soils. Thus, the objectives of this study were to: i) evaluate the C stocks including labile and mineral-associated C fractions in soil under conventional (CT), no-till (NT) and native vegetation (NV), and, ii) assess the C saturation level in different C fractions through the use of contrasting mathematical models of C accumulation. Soil samples were collected (0–100 cm depth) from agroecosystems established in tropical (Lucas do Rio Verde) and subtropical (Ponta Grossa and Londrina) regions of Brazil. The data show that all C fractions were affected by soil management systems. However, the impact was more pronounced with the labile C fractions than with the mineral-associated C fractions. The depletion of C stock of labile fractions in the 0–5 cm layer upon conversion of NV to CT accounted for 86, 89 and 72% of total C in soil of Ponta Grossa, Londrina and Lucas do Rio Verde, respectively. On the other hand, compared to CT, restoration of 89, 15 and 12% of these labile fractions was observed at these respective sites with adoption of NT. The mineral-associated C fraction was the best fit to a C saturation model at all sites. The estimated C saturation level for this fraction was 98.1, 60.2 and 39.1 g C kg− 1 silt + clay at the Ponta Grossa, Londrina and Lucas do Rio Verde sites, respectively, which is still far from the current C content. Thus, the long-term use of NT might be the pathway for physical protection of the labile C fractions as well as strong organo-mineral associations. Together, these processes contribute to C stabilization and accumulation in highly weathered soils.

Conclusions based on studies of the impacts of soil organic carbon (SOC) fractions and soil texture on macroaggregation and SOC stabilization in long-term (> 20 years) no-till (NT) fields remain debatable. This study was based on the hypothesis that the amount and frequency of biomass-C input associated with NT can be a pathway to formation of macroaggregates and to SOC buildup. The objectives were to: 1) assess the macroaggregate distribution (proportional mass, class mass) and the SOC and particulate organic carbon (POC) stocks of extra-large (8–19 mm), large (2–8 mm) and small (0.25–2 mm) macroaggregate size classes managed for two decades by NT, and 2) assess the recovery of SOC stocks in extra-large macroaggregates compared to adjacent native vegetation (Andropogon sp., Aristida sp., Paspalum sp., and Panicum sp.). The crop rotation systems were: soybean (Glycine max L.), maize (Zea mays L.) and beans (Phaseolus vulgaris L.) in summer; and black oat (Avena strigosa Schreb), white oat (Avena sativa), vetch (Vicia sativa L.), black oat. + vetch (Avena strigosa Schreb + vetch) and wheat (Triticum aestivum L.) in winter. The experimental was laid out as 2 × 2 randomized block factorial with 12 replicates of a NT experiment established in 1997 on two highly weathered Oxisols. The factors comprised of: (a) two soil textural types: clay loam and sandy clay, and (b) two sampling depths: 0–5 and 5–20 cm. The three classes of macroaggregates were obtained by wet sieving, and the SOC content was determined by the dry combustion method. The extra-large macroaggregate classes in 0–20 cm depth for sandy clay (SdC) and clay loam (CL) Oxisol represented 75.2 and 72.4% of proportional mass, respectively. The SOC and POC stocks among macroaggregate classes in 0–5 and 5–20 cm depths decreased in the order: 8–19 mm > 2–8 mm ≈ 0.25–2 mm. The SdC plots under soybean/maize at 3:1 ratio recovered 58.3%, while those at 1:1 ratio (high maize frequency) in CL recovered 73.1% of SOC stock in the extra-large macroaggregates compared with the same under native vegetation for 0–20 cm depth. Thus, partial restoration of the SOC stock in original extra-large macroaggregate confirms the hypothesis that NT through higher maize cultivation frequency can be a pathway to fomation of macroaggregates and SOC buildup.

Rapid changes in agricultural systems call for profound changes in agricultural research and extension practices. The Diagnosis, Design, Assessment, Training and Extension (DATE) approach was developed and applied to co-design Conservation Agriculture-based cropping systems in contrasted situations. DATE is a multi-scale, multi-stakeholder participatory approach that integrates scientific and local knowledge. It emerged in response to questions raised by and issues encountered in the design of innovative systems. A key feature of this approach is the high input of innovative systems which are often although not exclusively based on conservation agricultural practices. Prototyping of innovative cropping systems (ICSs) largely relies on a conceptual model of soil–plant–macrofauna–microorganism system functioning. By comparing the implementation of the DATE approach and conservation agriculture-based cropping systems in Madagascar, Lao PDR, and Cambodia, we show that: (i) the DATE approach is flexible enough to be adapted to local conditions; (ii) market conditions need to be taken into account in designing agricultural development scenarios; and (iii) the learning process during the transition to conservation agriculture requires time. The DATE approach not only enables the co-design of ICSs with farmers, but also incorporates training and extension dimensions. It feeds back practitioners’ questions to researchers, and provides a renewed and extended source of innovation to farmers.

No-till (NT) cropping systems have been widely promoted in many regions as an important tool to enhance soil quality and improve agronomic productivity. However, knowledge of their long-term effects on soil organic carbon (SOC) stocks and functional SOC fractions linking soil resilience capacity and crop yield is still limited. The aims of this study were to: (i) assess the long-term (16 years) effects of tillage systems (i.e., conventional - CT, minimum - MT, no-till with chisel - NTch, and continuous no-till cropping systems - CNT) on SOC in bulk soil and functional C fractions isolated by chemical (hot water extractable organic C - HWEOC, permanganate oxidizable C - POXC) and physical methods (light organic C - LOC, particulate organic C - POC, mineral-associated organic C - MAOC) of a subtropical Oxisol to 40 cm depth; (ii) evaluate the soil resilience restoration effectiveness of tillage systems, and (iii) assess the relationship between the SOC stock enhancement and crop yield. The crop rotation comprised a 3-year cropping sequence involving two crops per year with soybean (Glycine max, L. Merril) and maize (Zea mays L.) in the summer alternating with winter crops. In 2005, the soil under CNT contained 25.8, 20.9, and 5.3 Mg ha?1 more SOC (P < 0.006) than those under CT, MT, and NTch in 0-40 cm layer, representing recovery rates of 1.61, 1.31, and 0.33 Mg C ha?1 yr?1, respectively. The relative C conversion ratio of 0.398 at CNT was more efficient in converting biomass-C input into sequestered soil C than NTch (0.349), MT (0.136), and CT (0.069). The soil under CNT in 0-10 cm depth contained ?1.9 times more HWEOC and POXC than those under CT (P < 0.05), and concentrations of LOC and POC physical fractions of SOC were significantly higher throughout the year under CNT. Considering CT as the disturbance baseline, the resilience index (RI) increased in the order of MT (0.10) < NTch (0.43) < CNT (0.54). Grain yield was positively affected by increase in SOC stock, and an increase of 1 Mg C ha?1 in 0-20 cm depth resulted in an increase in yield equal to ?11 and 26 kg grain ha?1 of soybean (R2 = 0.97, P = 0.03) and wheat (R2 = 0.96, P = 0.03), respectively. The data presented emphasizes the role of labile fractions in the overall SOC accumulation processes in soils managed under CNT and their positive impacts on the soil resilience restoration and on agronomic productivity. (Resume d'auteur)

In north-eastern Laos, the savannah grasslands of the Plain of Jars cover vast areas of potentially cultivable land. However, soil acidity, low inherent fertility, and the absence of alternatives to tillage represent significant constraints to the development of sustainable smallholder agriculture. Our objective was to evaluate the potential for conservation agriculture (CA) to enhance soil productivity and farming system profitability. A three-year rotation of rice/maize/soybean was tested under three fertilization levels and four agricultural systems: one conventional tillage-based (CT) system and three CA systems based on no-tillage with cover crops. After four cropping seasons, our results show that, compared with CT, CA systems led to similar-to-higher grain production, similar-to-higher profits, higher opportunity of livestock system intensification, and higher labour productivity regardless of fertilization levels. While CA represents a relevant alternative to current practices, our results suggest that its contribution to the emergence of a sustainable smallholder agriculture is conditioned by broader institutional transformations, including the enrolment of local manufacturers and traders for deploying no-till implements and seed market channels for cover crops, long-term public support to maintain active research and technical mentoring to farmers, and possibly the integration of ecosystem services in agricultural policy.

The continuous use of plowing for grain production has been the principal cause of soil degradation. This project was formulated on the hypothesis that the intensification of cropping systems by increasing biomass-C input and its biodiversity under no-till (NT) drives soil restoration of degraded agro-ecosystem. The present study conducted at subtropical [Ponta Grossa (PG) site] and tropical regions [Lucas do Rio Verde, MT (LRV) site] in Brazil aimed to (i) assess the impact of the continuous plow-based conventional tillage (CT) on soil organic carbon (SOC) stock vis-a-vis native vegetation (NV) as baseline; (ii) compare SOC balance among CT, NT cropping systems, and NV; and (iii) evaluate the redistribution of SOC stock in soil profile in relation to soil resilience. The continuous CT decreased the SOC stock by 0� 58 and 0� 67MgCha � 1 y � 1 in the 0- to 20-cm depth at the PG and LRV sites, respectively, and the rate of SOC sequestration was 0� 59 for the PG site and ranged from 0� 48 to 1� 30MgCha � 1 y � 1 forthe LRVsite. ThefractionofC inputby cropresiduesconvertedintoSOC stockwas ~14� 2% atthe PGsiteand~20� 5%at the LRV site. The SOC resilience index ranged from 0� 29 to 0� 79, and it increased with the increase in the C input among the NT systems and the SOC sequestration rates at the LRV site. These data support the hypothesis that NT cropping systems with high C input have a large potential to reverse the process of soil degradation and SOC decline. Copyright © 2013 John Wiley & Sons, Ltd.

Soil organic carbon (SOC) plays an essential function in global agroecosystems. Conservation agriculture (CA) associated with diverse and high C-input is an important tool to increase SOC, improve soil quality, and increase agronomic productivity. However, the information about the potential of highly weathered soils under CA to accumulate SOC, and when the SOC saturation may occur, is scarce. This study was based on the hypothesis that in highly weathered soil from tropical and subtropical agro-ecosystems, the potential to store SOC lies more in the sub-soil than in surface layers and is determined by nutrient scarcity. Thus, the aim of this study, performed in a long-term incubation experiment (30 months), was to: (i) assess the SOC flow and mineralization based on CO 2 -C emissions for estimating SOC accumulation; (ii) evaluate the impact of nutrient scarcity on C accumulation efficiency by soil layers; and (iii) determine when C saturation occurs in these soils. The incubation study was performed in three Brazilian Oxisols under long-term CA, and was comprised of four amounts of C-inputs (0, 6, 12 and 24 Mg C ha − 1 ) added at 0, 10 and 20-months to three soil layers (0–20, 20–40 and 40–100 cm). The CO 2 -C emission was 19.0, 9.0 and 7.0% higher in the 0–20 cm than that in 40–100 cm layer for Ponta Grossa, Londrina and Lucas do Rio Verde sites, respectively, which was associated with higher antecedent SOC content and fertility status. A higher SOC accumulation efficiency was observed for the 0–20 cm layer than in deeper layers. Nutrient scarcity in deep soil layers; especially that of P, Ca 2 + and Mg 2 + ; was the driving force limiting SOC accumulation. Carbon saturation was not achieved indicating a high SOC storage capacity in these soils. Because these and similar soils cover a large global area, they possess a large C sink to mitigate atmospheric CO 2 -C . The potential SOC storage estimated for 20–100 cm layer based on this study and upscaling for 1/3 Brazilian Oxisols (100 million ha) may offset 0.06 to 0.36 Pg C yr − 1 or 5.5% to 32.7% of the global annual greenhouse gas emissions by land use change.

Agricultural practices affect the physical and chemical characteristics of the soil, which in turn may influence soil microorganisms with consequences on soil biological functioning. However, there is little knowledge on the interactions between agricultural management, soil physicochemical properties, and soil microbial communities, notably in tropical ecosystems with few studies conducted in strongly weathered and acid soils. Here, we investigated the early effect of tillage and crop residues management on top soil physical, chemical, and microbial properties in an acid savannah grassland of northeastern Laos. We initiated a 3-year rotation of rice/corn/soybean under three no-till systems (NTs) distinguished by the cover crops associated prior to and with the main crops, and one conventional tillage-based system (CT). The effect of agricultural management was evaluated 2 years after land reclamation in reference to the surrounding natural pasture (PAS). Our results demonstrate that NTs improve soil physicochemical characteristics (aggregate stability, organic carbon, and cation exchange capacity) as well as microbial abundance (total biomass, bacterial and fungal densities). A significant discrimination of the genetic structure of soil bacterial community was also observed between NTs, CT, and PAS. Interestingly, bacterial abundance and diversity were differently influenced by soil environment changes: microbial density was affected by the quantity and diversity of crop residues, soil organic carbon, and exchangeable base contents, whereas soil bacterial genetic structure was mainly determined by exchangeable aluminum content, pH, cation exchange capacity, and C/N ratio. Altogether, our study represents one of the most complete environmental evaluations of agricultural practices in tropical agrosystems and leads to recommend no-till systems with high residue restitutions to improve the physical, chemical, and microbial properties of tropical acid soils and thus contribute to the sustainability of agriculture in these ecosystems.

Over the past half-century, major efforts have been made worldwide to develop sustainable alternatives to agricultural tillage. In line with these efforts, two main research development initiatives have supported the experimentation and dissemination of conservation agriculture (CA) in Laos. Here we present the results of a 4-year monitoring and evaluation study conducted in 21 villages targeted for dissemination. In a context of rapid transition to intensive commercial agriculture in Laos, CA has become an important constituent of agricultural landscapes. However, there are significant variations in adoption rates across the study region. Statistical and qualitative evidence suggests that experimentation and adoption are not contingent upon farm-level variables such as capital, labor, age and education. While access to land helps shape local decision-making, the land tenure threshold under which farmers are not willing to experiment with alternative cropping systems is relatively low and highly variable in both space and time. Rather, experience and awareness of land degradation, production costs, social cohesion and leadership appear to be key factors in explaining most variations in local adoption rates. These results indicate that the practice of CA is not necessarily incompatible with smallholder farming. However, while complex crop associations and rotations are necessary for integrated weed control and reduced chemical use, their diffusion would require a broader transformation of the agricultural industry and the current market demand.

9 pages; International audience; It is widely assumed that agricultural practices have a major impact on soil living organisms. However, the impact of agricultural practices on soil microbes is poorly known, notably for species richness, evenness, and taxonomic composition. The taxonomic diversity and composition of soil indigenous microbial community can be assessed now using pyrosequencing, a high throughput sequencing technology applied directly to soil DNA. Here, we studied the effect of agriculture management on soil bacterial and fungal diversity in a tropical grassland ecosystem of northeastern Laos using 454 pyrosequencing of 16S and 18S rRNA genes. We studied soil microbial diversity of agricultural soils 3 years after conversion from native grasslands. We compared five systems: one tillage, two no-tillage rotational, one no-tillage improved pasture, and one natural grassland. Our results show first that compared to the natural grassland, tillage decreases fungal richness and diversity by −40 % and −19 %, respectively and increases bacterial richness and diversity by +46 % and +13 %, respectively. This finding evidences an early impact of agricultural management on soil microbial diversity. Such an impact fits with the ecological concept of "intermediate perturbation"--the hump-backed model--leading to classify agricultural practices according to the level of environmental stress they generate. We found also that land use modified soil microbial taxonomic composition. Compared to the natural pasture, tillage decreased notably the relative abundance of Actinobacteria (by −6 %), Acidobacteria (by −3 %) and Delta-proteobacteria (by −4 %) phyla, and by contrast increased the relative abundance of Firmicutes (by +6 %), Gamma-proteobacteria (by +11 %), and Chytridiomycota (+2 %) phyla. We conclude that soil microbial diversity can be modified and improved by selecting suitable agricultural practices. Moreover no-till systems represented intermediate situations between tillage and the natural pasture and appear therefore as a fair trade-off between the need for agriculture intensification and soil ecological integrity preservation.

8 pages; International audience; Agricultural practices should modify the diversity of soil microbes. However, the precise relationships between soil properties and microbial diversity are poorly known. Here, we study the effect of agricultural management on soil microbial diversity and C turnover in tropical grassland of north-eastern Laos. Three years after native grassland conversion into agricultural land, we compared soils from five land use management systems: one till versus two no-till rotational cropping systems, one no-till improved pasture and the natural grassland. Soils were incubated in microcosms during 64 days at optimum temperature and humidity. Bacterial and fungal diversity were evaluated by metagenomic 454-pyrosequencing of 16S and 18SrRNA genes, respectively. Changes in soil respiration patterns were evaluated by monitoring 12C- and 13C-CO2 release after soil amendment with 13C-labelled wheat residues. Results show that residue mineralization increased with bacterial richness and diversity in the tilled treatment 7 days after soil amendment. Native soil organic C mineralization and priming effect increased with fungal richness and diversity in improved pasture and natural grassland. No-till cropping systems represented intermediate situations between tillage and pasture systems. Our findings evidence the potential of controlling soil microbial diversity by agricultural practices to improve soil biological properties. We suggest the promotion of no-till systems as a fair compromise between the need for agriculture intensification and soil ecological processes preservation.

Data on humification is important to assessing the rate and magnitude of soil carbon (C) sequestration. Thus, this study assessed the humification degree (HLIF) of soil organic matter (SOM) and the changes in functional C groups (aromatic-C and aliphatic-C) for contrasting land use and management practices (native vegetation (NV), conventional plow-based tillage (CT) and no-till (NT) systems) in sub-tropical and tropical Brazilian environments. Experiments were conducted at Ponta Grossa (PG) in Paraná State and Lucas do Rio Verde (LRV) in Mato Grosso State of Brazil. Laser-induced fluorescence (LIFS) and Fourier-transform infrared (FTIR) spectroscopies, were used on whole soil samples to 1-m depth, and on seven aggregate size classes (8-19, 4-8, 2-4, 1-2, 0.5-1, 0.25-0.5, 0.053-0.25 mm) obtained by wet sieving of 0-5 and 5-10 cm layers. Three functional C groups were selected based on FTIR: aliphatic-C1 (1404 cm? 1), aromatic-C (1632 cm? 1), and aliphatic-C2 (2852 and 2922 cm? 1). The HLIF was 3 to 5 times higher at the LRV site than at PG at all soil depths, indicating that selective preservation by aromaticity of SOM is the predominant mechanism in this environment. Relatively lower HFIL was observed in NT soils at both locations because of aggregation which protects most labile moieties. The depletion of C concentration in CT was related to the decrease in functional C groups (i.e., aromatic-C and aliphatic-C) and an overall increase in the humification degree, indicating that physical protection mechanisms are not sufficient to protect the labile fractions of OM. In contrast, the intensity of functional C groups under NT systems was similar to that in the soil under NV at both locations. A discriminant analysis of principal components clearly showed that soils at both locations can be clustered into three groups, corresponding to the three main land-use and management practices. Thus, soils under NV, NT, and CT differed significantly in terms of the composition of organic compounds, and in the interactions between inorganic and organic fractions. Land use changes modify the arrangement of organic compounds necessitating the diversification of agroecosystems and conversion to NT farming. Altogether, our results reveal that LIFS and FTIR are fast, efficient, and precise techniques for analyzing the degree of SOC humification, functional C groups, and hence the efficiency of NT cropping systems in promoting long-term carbon sequestration in soils.

Encapsulation of soil organic carbon (SOC) within aggregates is one of the principal mechanisms for long-term C sequestration, macroaggregate formation and stabilization. Our objectives were to quantify the changes in aggregate size distribution, aggregate-C concentrations and stocks upon conversion of native vegetation (NV) to conventional plow-based tillage (CT), and to assess the rate of aggregation and SOC recovery with no-till (NT) under diverse biomass-C inputs. The study was conducted at both sub-tropical (Ponta Grossa – PG, State of Parana) and tropical (Lucas do Rio Verde – LRV, State of Mato Grosso) sites in Brazil. The SOC content under NV was used as a baseline to evaluate the depletion rate under CT and the restoration rate under NT. A specific emphasis was given to the largest macroaggregate size class (8–19 mm) because of its importance to protecting the recently deposited labile SOC. A discriminant analysis of principal components (DAPC) indicated that NV soil is modified by conversion to an arable land use and that, mechanical tillage, biomass input, and their interactions drastically influence the distribution of aggregate-size classes, aggregation indices, and SOC distribution within aggregates. At both sites, soil aggregation indices were positively impacted by NT and associated with SOC concentration in the labile fractions (e.g., total polysaccharides (TPS), hot water extractable organic C (HWEOC), particulate organic C (POC)). At the PG site, the 8–19 mm aggregate size fraction was significantly affected by land use and tillage treatments and represented 54%, 43%, and 72%, under NV, CT, and NT in 0–20 cm depth, respectively. Furthermore, the 8–19 mm size fraction stored 55%, 45%, and 71% of the total SOC stock under NV (53.8 Mg C ha −1 ), CT (28.5 Mg C ha −1 ) and NT (51.2 Mg C ha −1 ), respectively. At the LRV site, the 8–19 mm aggregate size fraction decreased from 50% under Cerrado NV to 35% under CT, and ranged from 33% to 51% under diverse biomass-C input under NT in 0–20 cm depth. The 8–19 mm size fraction stored 52%, 37%, and 41% of the total SOC stock across all aggregate sizes under NV (25.4 Mg C ha −1 ), CT (11.7 Mg C ha −1 ), and NT (9.9–18.1 Mg C ha −1 ), respectively. The difference in SOC stock among land uses is largely attributed to storage in the 8–19 mm aggregate size class, indicating that NT cropping systems rebuilt the largest macroaggregates, which are crucial for stabilization of SOC.

Inventories of C and N footprints on a landscape scale are essential tools for estimating C offsets from agricultural emissions. Therefore, the aims of this study conducted in the subtropical humid ecosystem in southern Brazil were to: (i) conduct a soil-specific inventory of landscape soil C and N stocks with reference to soil order, soil texture, and land use/management type; (ii) estimate accretion rates for soil organic C (SOC) and total N (TN) for areas managed under no-till (NT) practices management with reference to native vegetation (NV) based on this inventory; (iii) generate a map of C stocks for each land use system; and (iv) calculate estimated C offset for the region through the use of NT compared to conventional tillage (CT). Soil samples were collected at 324 points to a 1-m depth from the entire region. Soil texture and duration of NT had a strong influence on C and N stocks. The average soil C stock across all types of soils for depths of 0-40 and 40-100 cm was 57.0 and 43.0%, respectively. The extrapolation of C stored in the 0- to 40-cm depth based on the NT management for 11 and 20 yr for 1.52 million hectare (Mha) was 9.08 ± 0.62 Tg (1 Tg = 1012 g) representing 11.9% of the C stored in all soil orders. The long-term of C sink capacity by conversion of arable land from CT to NT in this region is 33.2 Tg of CO2, with the C offset of 22.5% of all anthropogenic emissions. (Resume d'auteur)

The conversion of native vegetation (NV) into agricultural land by clearing and tillage disrupts the soil structure, and depletes soil organic carbon (SOC) pool. The data on changes in SOC pools are needed to enhance scientific knowledge regarding the effects of land use and no-till (NT) systems on soil fertility, agronomic productivity, and soil C sink capacity. Thus, the objective of this study was to quantify changes in SOC fractions due to conversion of NV to agricultural land, and to assess the rate of recovery of SOC fractions and the resilience index of NT cropping systems under sub-tropical (Ponta Grossa/PR — PG) and tropical (Lucas do Rio Verde/MT — LRV) regions of Brazil. The conversion from CT to NT was 29 and 8 years at the PG and LRV sites, respectively. Five different fractions of SOC pools were extracted by chemical methods (i.e., C in the polysaccharides — CTPS, hot-water extractable C — HWEOC, chemically-stabilized organic C — CSOC), and physical fractionation (i.e., particulate organic C — POC, and mineral-associated organic C — MAOC). Land use change primarily altered the labile (HWEOC, TPS, and POC) and also some of the stable (MAOC) pools at both sites. The CSOC pool was almost constant throughout the soil profile and represented, across land uses, 7.2 g C kg − 1 at the PG and 3.1 g C kg − 1 at the LRV sites. At the PG site, the HWEOC and CTPS concentrations in the 0–5 cm depth decreased by 56% (1.21 g kg − 1 ) and 45% (7.21 g kg − 1 ) in CT soil, respectively. At the LRV site, concentrations of HWEOC and CTPS in the 0–5 cm depth decreased by 50% (0.4 g kg − 1 ) and 42% (4.8 g kg − 1 ), respectively. In contrast, concentrations of HWEOC and CTPS fractions in soil under NT in the 0–20 cm depth were closer than those under NV, and exhibited a distinct gradient from surface to sub-soil layers. The adoption of CT reduced POC by 46% (4.7 Mg ha − 1 ), and MAOC by 21% (15.1 Mg C ha − 1 ) in the 0–20 cm depth at the PG site. Using CT for 23 years at the LRV site, decreased SOC fractions in the 0–20 cm depth at the rate of 0.25 and 0.34 Mg C ha − 1 yr − 1 for POC and MAOC, respectively. In contrast, adoption of intensive NT systems in tropical agro-ecoregions increased POC at the rate of 0.23 to 0.36 Mg C ha − 1 yr − 1 , and MAOC by 0.52 and 0.70 Mg C ha − 1 yr − 1 . An important effect to be emphasized is the possibility of recovering, at least partially, the SOC fractions by adopting high biomass-C inputs under NT management, and despite the fact that the experimental duration at the LRV site was only eight years. With a high and diversified input of biomass-C in intensive NT systems, higher resilience index was observed for CTPS, HWEOC, and MAOC. The variation in SOC among CT and NT systems was mainly attributed to the MAOC fraction, indicating that a significant proportion of that fraction is relatively labile, and that spatial inaccessibility of SOC plays a significant role in the restoration of SOC.

Monitoring C content is essential for carrying out surveys and inventories on C storage in soils under different land uses (LUs). The objectives of the present study, which was conducted in fi ve agro-ecoregions in Brazil with contrasting climates, LU managements, soil texture gradients, and soil depths, were to: (i) develop a C-equivalent correction factor (CF) between total organic carbon determined by dry combustion (TOCDC) and organic carbon by wet oxidation recorded by the Walkley-Black (OCWB) method, and (ii) assess the infl uence of LU, soil texture gradients, and sampling depth on a C-equivalent CF based on a linear model adjusted for each experimental location. The results indicated an effect of LU at almost all sites, except for one subtropical location where clay content and sampling depth were the most important factors. Additionally, there were LU × clay and LU × sampling depth interactions on the C-equivalent CF. The linear models computed to estimate the CF from this set of qualitative (LU) and quantitative variables (soil texture gradient and soil depth) differed among and within the agro-ecoregions and LU managements. The C-equivalent CF in subtropical sites ranged from 1.37 to 1.55, while for tropical sites ranged from 1.13 to 1.60. These models are site specifi c, and the results demonstrate the need to develop models calibrated for each site before extrapolating to other agroecoregions to recalculate past soil C inventories and to evaluate temporal changes of SOC stocks. (Resume d'auteur)

The effects of genotype and environment on the leaf area index of rice are well documented, but the rules governing leaf area distribution among main stem and tillers are less well understood. This study investigated the ontogenetic patterns of leaf blade area and dimensions on the main stem and tillers of three rice cultivars, IAC47, Javae ( Oryza sativa L.) and CG14 ( O. glaberrima Steud.) grown in pots without competition among plants. No differences in phyllochron were observed among culms, but leaves on tillers appeared 0.5 to 0.8 phyllochrons earlier than the genealogically corresponding leaves on the main stem (system of Katayama). Cohorts were thus not fully synchronized. All culms produced their largest leaves soon after panicle initiation (PI), despite differences in tiller age. Leaves that appeared subsequently were smaller owing to reduced length but not width, which remained constant. Three different hypothetical rules governing the behaviour of culms were tested: (1) tillers behave as clones of the main stem; (2) cohorts behave uniformly, and (3) behaviour is determined by leaf position (sum of genealogical leaf and tiller indices). The results were best explained by hypotheses (2) and (3), with leaf blade width and length showing markedly different patterns.