Cactus crop as an option to reduce soil C–CO₂ emissions in soils with declining fertility

Arable soils tend to lose total organic carbon, thus contributing to the increase of CO₂ 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–CO₂ 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 CO₂ 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–CO₂ 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–CO₂-reduced emissions in agricultural zones with declining soil fertility.