Around 30% of global food is produced by smallholder farmers, yet they constitute the most food-insecure group. In Mexico, food self-sufficiency is declining. Rural policies in the country have stimulated the production of cash crops to the detriment of the traditional intercropping system, the milpa. Such a decline may have negative consequences for the food security of subsistence farmers. This study aimed to assess changes in nutritional self-sufficiency over the last 30 years and the role of milpa systems in food security for two communities in the highlands of Oaxaca, Mexico. The study used satellite images, censuses, and field data to estimate food production. Three cropping systems, monoculture of maize, monoculture of common bean, and the milpa were compared in terms of nutrients and vitamins produced. Furthermore, a household typology was developed for each community to contrast nutritional self-sufficiency levels between the different household types. Results showed that the milpa produced more volume of food per area compared to the other systems. The milpa also produced all the nutrients and vitamins (except for B12) required to feed at least 2 persons ha-1. Monocultures of maize lacked vitamins A, B9, B12, and C, and the common bean lacked vitamins A, B12, and C. While farmers recognized the importance of the milpa, they preferred monocultures due to the reduced labor demands of this system. Households that obtained most of their income from off-farm activities had the lowest nutritional self-sufficiency. Enhancing nutritional self-sufficiency through crop diversification has the potential to not only improve the nutrition of subsistence farmers, but also to enhance ecosystem service provision, promote biodiversity conservation and restoration, and improve resilience to climate change.

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SUMMARYInformation on the performance of the Rothamsted organic carbon turnover model (RothC model) in predicting changes in soil organic carbon (SOC) in short-term experiments is scarce. In Mexico, it was found that these experiments covered not more than 20 years. The purpose of the present study was to evaluate short-term SOC prediction performance of the RothC model in the following systems: (1) farming with residues added (A+R), (2) farming with no added residues (A−R), (3) pure forest stands (F), (4) grasslands (GR) and (5) rangeland (RL). Work was done in five experimental sites: Atécuaro, Michoacán; Santiago Tlalpan, Tlaxcala; El Batán, State of Mexico; Sierra Norte, Oaxaca; and Linares, Nuevo León. Carbon (C) inputs to the soil were plant residues and organic fertilizers, which need to be known to operate the RothC model. The adjustment coefficients for site modelling had R2 values of 0·77–0·95 and model efficiency (EF) was −0·60 to 0·93. When RothC performance was evaluated by a system, R2 values were 0·06–0·92 and EF was −0·24 to 0·90. The low R2 and EF values in rangelands were attributed to the fact that these systems are complex because of heterogeneous vegetation, soil and climate. In general, the evaluation of the RothC model indicates that it can be useful in simulating SOC changes in temperate and warm climate sites and in farming, forest and grassland systems in Mexico.

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Arable soils tend to lose total organic carbon, thus contributing to the increase of CO 2 emissions into the atmosphere. This process has been occurring in large areas of Mexico cultivated with maize. Perennial species such as cactus (Opuntia spp.) and agave are grown in Mexico and other parts of the world, which can contribute to the maintenance of total organic carbon in the soil (TOC). Within this context, a study was designed to compare the patterns of emissions of C-CO 2 and TOC in a highland of central Mexico. The selected management systems were the following: (1) maize monoculture with conventional tillage and fertilization, (2) maize associated with Vicia faba and manure addition, (3, 4) cactus without and with composted manure mulching, (5) soil in oak-pine forest, and (6) maize fields under 4 years of soil fallow and without weed control. Measurements of CO 2 flux pulses and volumetric moisture were performed every 15 days at 5 points of each plot. The soil in oak-pine forest showed stable C-CO 2 emissions throughout the year, while under maize fields, emissions were unstable with several respiration peaks. The soil in cactus crop showed a very close pattern of forest soil respiration. The annual patterns of soil respiration were in agreement with the results of TOC recently reported for the same plots where soil respiration was measured. Here we show, for the first time, that TOC in cactus approached the reference line of soil under forest (6 g 100 g −1), while in maize, we found a reduction greater than 50% of this value. Cactus crop represents an option in low-input maize for C-CO 2-reduced emissions in agricultural zones with declining soil fertility.