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This study quantified soil organic carbon (SOC) and soil total nitrogen (STN) changes in three long-term (14 years) experiments comparing conventional tillage (CT) and different no-till (NT) cropping systems using a diachronic approach and an equivalent soil mass approach. The three experiments were established in 2009 in a randomized complete block design with three replicates and comprise maize (MaiEx)-, soybean (SoyEx)-, and cassava (CasEx)-based cropping systems. Soil samples were collected in 2021, 10 years after the first sampling (2011), at 7 depths: 0–5, 5–10, 10–20, 20–40, 40–60, 60–80, and 80–100 cm. At 0–10 cm depth, NTs significantly increased SOC stock at a rate of 0.35, 0.51 and 0.73 Mg C ha-1 y-1 for CasEx, SoyEx and MaiEx, respectively. CT exhibited a positive trend only on CasEx with an accumulation rate of 0.14 Mg C ha-1 y-1 (0-10 cm). A significant SOC accumulation was observed under NT cropping systems for CasEX and MaiEx up to 60-80 and 80-100 cm depth, respectively. By contrast, STN stock significantly increased under NT cropping systems only at 0–5 cm depth with an accumulation rate of 13.9, 22.9 and 34.0 kg N ha-1 y-1 under CasEx, SoyEx and MaiEx, respectively. A depletion of N stocks was observed under CT and NT cropping systems at deeper soil layers. Over 10 years of time, SOC accumulation rate of NT, at 0-100 cm depth, reached 1.08 and 1.73 Mg C ha-1 yr-1 for CasEx and MaiEx, respectively while non-significant change was observed under NT SoyEx and CT of the three experiments. The findings suggest that long-term NT cropping systems with high amount of biomass-C inputs and crop rotation enhance SOC sequestration even at deeper soil layers. However, these results raise questions about the dynamics of N under NT cropping systems.

Agriculture in the Southeast Asia region has been changing from traditional subsistence farming to modern commercial farming practices at various rates and this has led to specialized commercialized farming with mechanization, intensive tillage, and increased agrochemical use. The use of high inputs and labor-saving technologies has resulted in serious land degradation and vulnerable to more frequent and severe climate impacts, particularly drought and flood. Conservation Agriculture has been introduced in the region for the last two decades to conserve, improve, and make more effective use of natural resources through the integrated management of available soil, water, and biological resources, combined with purchased external inputs. Significant benefits of CA have been observed and technically proved, but the adoption is still in very limited areas and mostly in research and demonstration plots. For wider adoption, understanding the constraints that hinder adoption and addressing for specific situations including intellectual, social, financial, biophysical, technical, and infrastructure constraints, or policy related support.

The political and territorial reintegration strategy that had been implemented in Cambodia to establish peace and order in the late 1990s caused the degraded evergreen forestlands to be allocated to the demobilized Khmer Rouge families in the western regions of the country. The increasing regional demand for cereals and tubers and the highland saturation in central rice areas have driven massive immigration of smallholder farmers. Almost half a million hectares of those forestlands were thus converted in less than 15 years for annual upland cash crops development. This dramatic expansion of agricultural area, without any plan for sustainability, has exerted tremendous pressures on the natural forest resources and on biodiversity. Its effects rapidly spread on the water and soil resources of Cambodia. With conventional practices and more frequent flooding and incidents of drought, smallholder farmers could hardly sustain their livelihoods, which are mainly based on annual upland farming. Farmers with investment capacity have shifted to planting tree crops and/or to animal production in order to cope with the hazardous phenomena. This case story presents the collaborative R&D program between farmers and researchers in Battambang and Kampong Cham provinces in Cambodia. The program aimed to restore soil fertility and build the resilience of smallholder farmers to the effects of climate change while improving crop productivity and profitability of the smallholder farmers. Using the Diagnosis, Design, Assessment, Training and Extension (DATE) methodology, the project implementers designed, tested, and evaluated crop production systems that are grounded on the principles of conservation agriculture (CA). DATE is a multi-scale, multi-stakeholder participatory approach. It integrates scientific and tacit knowledge, and is composed of four components: agrarian systems diagnosis, field experiment, on-farm assessment, and pre-extension. A number of CA-based cropping systems have been designed and validated in the program: (1) mono-cropping of maize in association with pigeon pea or mungbean as relay crops, (2) biannual rotation cropping of maize with soybean or cassava, and (3) intensified cropping of maize and cassava. Synergizing this with the benefits of CA, each system has the capacity for climate change adaptation and mitigation, to retain soil fertility, and to increase smallholder farmers' profitability. Based on the results, pigeon pea is the most suitable crop for mollisols used with maize since it can improve the water retention capacity of the soil, reduce soil evaporation, and reduce mineral nitrogen inputs. Moreover, its grain can be sold or used as animal feed to augment farmers' income, a characteristic that smallholders look for in an agricultural production system. Likewise, shifting mungbean to be sown by hand broadcast after harvesting early maize significantly reduces farmers' risks and costs, thereby improving their productivity. Shifting to CA-based cassava production (a key annual crop) using chisel to operate strip tillage on planting rows after the early maize harvest also enables farmers to significantly minimize risks and costs. These risks and costs are estimated to be about USD (United States Dollar) 300–400 per hectare and USD 200 per hectare, respectively. Designing CA-based cropping systems based on the DATE methodology presents clear benefits. DATE is a holistic approach for identifying technical, socioeconomic, and institutional elements for a sustainable and more inclusive intensification of smallholder farmers' agricultural production systems. However, designing such innovative techniques is a combination of context-specific and context-generic features. These issues need to be taken into account should such innovation be replicated in other regions. In addition, this action-research program should be a continuous process; the agro-technical performances of the introduced cropping systems should be continuously validated in multiple locations and for several years. The outcomes of the cropping systems should also be continuously monitored such that their impacts on natural resources (e.g., soil organic carbon, nutrient cycles, xenobiotic dynamic, etc.) can be determined and measured accurately.

Cambodia has been identified to be one of the most affected countries by climate change. In the past two decades, migration from the central plains to the peripheral areas changed drastically the development of the uplands, inducing tremendous degradation of natural resources. The promotion of cropping systems based on ecological intensification is of paramount importance to strengthen the resilience of small-scale farming, to mitigate the effect of climate change and to advance in farm sustainability. The aim of this study was to quantify the short-term impact of conservation agriculture (CA, direct seeding + use of cover/relay crops), on soil organic C (SOC), soil total N (STN), labile C fractions (particulate organic C - POC and permanganate oxidizable organic C - POXC), β-glucosidase and arylsulfatase activities. Established in 2009, the experimental plots were laid out in a randomized complete block design (3 replicates) with: (i) soybean under conventional tillage (CT), (ii) soybean under conservation agriculture (CA1), and (iii) soybean in bi-annual rotation with maize under conservation agriculture (CA2). Several cover/relay crops were associated prior to and with the main crops. Soil sampling was conducted in 2011 and 2013 at 0-5, 5-10, 10-20, and 20-40 cm depths. Biomass-C inputs ranged from 2.2 to 7.8 Mg C ha-1 yr-1 under CT and CA respectively, emphasizing the difference of supply of fresh organic matter. On average, CA increased SOC and STN stocks at 0-5 cm depth by 20% and 25% when compared with CT. β-glucosidase and arylsulfatase activities at 0-5 cm depth was 28% and 36% greater in CA1 and CA2 soils. In addition, CA2 (rotational sequence and higher biomass-C inputs) showed a better increasing trend of labile C fraction (HWEOC and POXC) and β-glucosidase and arylsulfatase activities than CA1. Conservation agriculture shows potential for the long term improvement of Cambodia uplands. (Texte intégral)