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Cambodia has faced drastic agrarian changes over the last twenty years with a rapid and massive transformation of the uplands from forest into arable land. Soil quality depletion is one of the main drivers of the resulting decline in crop productivity and increasing production costs in the uplands of Battambang province. The objective of the study was to assess the early effects of conventional plough-based (20 cm) management (CT) and two conservation agriculture-based (CA) cropping systems (the use of a single—CAS vs. multiple cover crop species—CAM) on a maize monocropping system using three soil organic carbon (SOC) fractions on Mollisols in Battambang province. SOC content, permanganate oxidizable carbon (POXC) and soil CO2 basal respiration through SituResp® were recorded at a depth of 0–10 cm. Twelve sampling periods were used over the growth period of the cover crops and during the maize cycle. POXC and SituResp® were highly sensitive to management practices with, across the 12 sampling periods, higher values (p

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.