Land use intensification of naturally thin and nutrient poor soils has often been invoked as the prime explanation for reductions in food production and food security in tropical mountains. Little is known about the relative importance of land use legacies and current practices (like agrodiversity reduction) on soil functioning. For Los Tuxtlas, Mexico we (1) used the BGBD-Mex database to statistically model the cumulative effects of plot use history on soil fertility (10 indicators) and biodiversity (10 soil functional groups) and (2) experimentally investigated the consequences of aboveground crop diversity reduction (intra e interspecific) on the genetic diversity and function of native mycorrhiza and nitrogen fixing bacteria in indigenous maize polycultures (BioPop project). Finally, we will summarize preliminary results of the new DeMano project that aims at modelling and optimizing the linkages between micro symbiont diversity, soil health, food diversity and human health.
The legacy of cropping years explained 8-22% of the variability in soil fertility and 5- 22% of belowground taxa richness (Linear Models, R2=0.3-0.8, P<0.05). The capacity of soils to establish mycorrhizal symbiosis diminishes with decreasing diversity of cultivated plants (Mixed model F(2,6)= 6.38, P<0.05), while available P (Bray) increases with the diversity of crops (Mixed model F(2,6)=8.63, P<0.05). Compared to an introduced hybrid, some native maize landraces and their symbionts were much more efficient in obtaining P from very deficient soils. Many bacterial isolates from nitrogen-fixing nodules corresponded to bradyrhizobia closely related to native bradyrhizobia from the forest and novel groups were found. This being the first report of nodule bacteria from P. lunatus in its Mesoamerican site of origin and domestication. The interactions of land use legacies with current agricultural practices may be hampering the natural mechanisms that native polycultures have to cope with naturally thin and nutrient poor soils. The immense variety of locally developed crops benefits from symbiotic relationships with an equally diverse array of coevolved soil microorganisms. Understanding such a network is allowing us now to develop locally tailored technologies aimed at improving food security and conserving an invaluable indigenous below-above ground heritage.