Legado de los cultivos cubierta sobre la estructura de la comunidad microbiana, la micorrización y el estado nutricional de los cultivos de trigo y de maíz

Resum

Cover crops (CC) provide important ecosystem services that are increasingly demanded for sustainable agroecosystems. However, the legacy effects of CC on soil microbiota and their interactions with subsequent main crops (MC) are still poorly understood, especially for CCs composed of a mixture of grasses and legumes. Two trials were established, the first under semi-controlled conditions in a greenhouse and the second in an open field. In the first study, five CC (barley, vetch and melilotus in monoculture and the mixtures barley+vetch and barley+melilotus) were selected and grown for two rotation cycles with two MC, in order to investigate whether monocultures and CC mixtures differed in their legacy effects on soil and subsequent crop variables, and whether the identity of the MC modulated these effects. The two most widely consumed crops globally, wheat and maize, were planted separately as MC after CC. In the second study, two forms of CC introduction with increasing legume input (a barley-vetch as winter CC and an interseeded vetch in addition to the first) were established under two types of tillage (traditional tillage and minimum tillage), in a biannual irrigated rotation with maize and spring wheat as MC, to evaluate the effects of both factors and their interaction on mycorrhization and crop parameters at an early stage of maize. In the first study and for both cycles, the legacy effects of CC on the microbiological variables studied in the microcosms were highly dependent on the interaction with CP, thus showing the importance of the identity of the subsequent crops. In the first cycle, developed in autumn-winter, vetch and the barley-vetch mixture stood out for providing the microbiological conditions that enhanced the uptake of macro and micronutrients, to finally obtain the highest wheat biomass (>80% more than the control without CC). In maize, the effects of CC on soil microbiota were more limited because temperatures during this cycle were far from the optimum for their development. Soil microbiological responses for the CC mixtures were complex and sometimes opposite. Thus, in wheat, the barley-vetch mixture behaved similarly to the barley monoculture, while in maize, the barley-vetch mixture behaved like the vetch monoculture. In both MC, the barley-melilotus mixture differed completely from their monocultures, mainly by changes in the abundance of archaea and Glomeromycota, and in the ratio Fungi:Bacteria. In the second cycle (spring-summer), the effect of CC on soil microbiota and MC development was influenced by increased temperature, which improved maize development. In maize, the microbiological response did not show a clear pattern, but overall the positive effect of vetch and barley in monoculture compared to the no-CC control stands out, while melilotus, alone or in mixture, performed worse. In wheat, melilotus also showed a poor microbiological response, but mixed with barley it was one of the CCs with the best microbiological (and nutritional) impact, followed by vetch. Thus, the mixtures again stood out for their individual behavior, both with respect to their pure species and with respect to the identity of the subsequent MC. The type of tillage, traditional or minimal, with the associated CC termination, mowing and incorporation or crimping and spreading on the surface, modulated the effect of the different forms of CC introduction on variables such as mycorrhizal colonization and Glomeromycota abundance. Minimal tillage tended to minimize the differences between the forms of CC introduction in the rotation for certain variables, while traditional tillage favor them so that variables such as aerial biomass and soil electrical conductivity tended to be higher as the legume input in the rotation increased, i.e. when a vetch was interseeded with maize in addition to the winter CC barley-vetch between wheat and maize. Elements such as Mn, Cd and to a lesser extent Cu and Zn showed this pattern while macronutrient content was higher with the higher legume input irrespective of tillage type. The choice of CC species, the species mix, their form of introduction and their interactions with the subsequent main crop can have large effects on soil microorganisms, at least in the short term, with a potential impact on key soil functions and agronomic aspects. More research is needed to understand these interactions, especially with regard to mixtures, in order to support decisions on which CC is most appropriate in each case.