Cover crops (CC) provide important ecosystem services that are demanded to achieve more sustainable agrosystems. However, the legacy effects of CC on the microbial community structure and its interactions with the subsequent cash crops (CaC) are still poorly understood, especially when CC mixtures are involved. In this work, five CC (3 monocultures and 2 mixtures) were selected in an experiment under semi-controlled conditions to investigate if CC monocultures and mixtures differed in their effects on soil and crop variables and if the identity of the subsequent crop modulates these effects. The two most consumed crops worldwide, wheat and maize, were sown separately after CC. The legacy effects of CC on the studied microbial variables largely depended on the interaction with the CaC. The vetch and the barley-vetch mixture stood out by providing the microbial conditions that enhanced the absorption of macro- and micronutrients, to finally seek the highest wheat biomass (>80% more than the control). In maize, the effects of CC on soil microbiota were more limited. The soil microbial responses for CC mixtures were complex and contrasting. In wheat, the barley-vetch mixture behaved like barley monoculture, whereas in maize, this mixture behaved like vetch monoculture. In both CaC, the barley-melilotus mixture differed completely from its monocultures, mainly through changes in archaea, Glomeromycota, and F:B ratio. Therefore, it is necessary to deepen the knowledge on the CC-CaC-microbial interactions to select the CC that most enhance the sustainability and yield of each agrosystem.

Soil aggregates can be considered massive microbial incubators. Thus, the study of the bacteria and fungi colonizing different soil aggregate fractions helps to elucidate the factors influencing the associations between microhabitats in soil aggregates and microbial functioning. The main objective of this study was to evaluate the total bacterial and total fungal abundances and soil quality-related parameters of four aggregate size fractions (>2 mm, 2-0.25 mm, 0.25-0.053 mm and <0.053 mm) at two depths (0-5 cm and 7-12 cm) under three different management regimes (no cultivation, direct seeding [DS] and traditional tillage [TT]). We analysed the pH, electrical conductivity and organic carbon (OC) and nitrogen (N) concentrations in both bulk soil samples and aggregate fractions. For DNA extracted from each aggregate fraction,we performed qPCR analysis of 16S rRNA (an estimate of bacterial abundance) and internal transcribed spacer (an estimate of fungal abundance). The TT decreased the proportion of macroaggregates (>2 mm), which constituted only 16.7% (with 0.17 OC/100 g soil) under this treatment at 0-5-cm depth, compared with 50.7% (with 1.34 g OC/100 g soil) in uncultivated soil. The abundances of bacteria and fungi exhibited contrasting responses to soil management across aggregate fractions, which differed by depth. The abundance of bacteria, but not fungi, was closely related to OC and N concentrations, with higher values in macroaggregates than in microaggregates for TT and DS at 0-5 cm depth. Uncultivated soil showed no OC differences across aggregate fractions and the bacteria were distributed uniformly among them. Fungal abundance was higher in microaggregates, mainly in DS, concurrent with lower bacterial abundance. Therefore, bacteria and fungi responded differently to changes in properties at the aggregate scale induced by soil management within the soil surface horizon. Highlights• Management practices affect aggregate size distribution and their bacterial and fungal abundances • Tillage reduced the OC stored in macroaggregates >2 mm by 87% compared to uncultivated soil

Cover crops (CC) promote soil health, but the termination method can condition the benefits for soil microorganisms. In a greenhouse experiment, we evaluated the legacy effects of four common CC termination methods on mycorrhization, soil microbial abundance, structure, and activity, as well as other soil properties, and its interaction with water levels (well-watered and water deficit). Mowing and residue incorporation (INC), glyphosate (GLY), roller crimper (ROL) and glyphosate + roller crimper (RGL) were evaluated, together with no CC, at two sampling dates of a subsequent maize. The water level modulated the soil microbial response to CC termination methods, especially in the glyphosate methods. Legacy effects on soil microbial attributes were notable and evolved differently from maize, from pre-emergence to ~3 months later. At final sampling, INC showed the best microbial response at both water levels, enhancing most microbial attributes. ROL was the second most beneficial method, especially in well-watered soil, promoting fungi but nullifying the CC positive effect on bacteria. Regardless of water level, GLY and RGL showed a similar microbial response. In well-watered soil, GLY and RGL had a negative effect on the total fungi, which separated the RGL response from the ROL. Overall, the time since CC termination and water level modulated the soil microbial response to the termination methods. Further research is needed to investigate CC termination impacts under different environmental conditions, in order to better understand the processes involved and provide farm-level recommendations.

Legumes provide important benefits in rotations. Interseeding cover crops (CCs) allows an additional legume CC in case of a short window after the main crop. However, legume input level and management could modify the expected benefits. In a Mediterranean irrigated agroecosystem, we evaluated the responses of topsoil (0–10 cm) and early maize development to increasing legume CC input in a biannual maize–wheat rotation under traditional tillage (TT; CC incorporated) and minimum tillage (MT; CC rolled-crimped). In the third year, at two early maize stages, we tested three legume input levels: (i) R0, non-CC; (ii) R1, barley–vetch CC; (iii) R2, vetch interseeded into maize in addition to the CC mixture. Overall, MT enhanced soil properties, but frequently conditioned to legume input level. The tillage system affected R1 the most, with MTR1 showing the better overall soil response while TTR0 showed the poorest. MTR2 was the best combination for early maize development, but not for soil health. Moreover, a better overall soil health did not lead to a better early maize performance in the short term. In this alkaline soil, CC favored early maize growth, whereas mycorrhization, enhanced under TT, favored crop nutrition. Increased legume input under MT should be monitored to avoid negative effects in soil in the mid–long term.

Cover crops (CC) have great potential to enhance the sustainability of agroecosystems. However, the wide range of possible rotations of CC and cash crops (CaC) means that important knowledge gaps persist on how CC affects CaC. We investigated the legacy effects of five common CC (three monocultures: vetch, melilotus, and barley, and two mixtures: barley-vetch and barley-melilotus) on two of the most important CaC, maize and wheat. A microcosm, semi-controlled experiment was established simulating warm, low-income Mediterranean conditions. After two cycles, soil physicochemical and microbial properties, as well as plant growth and nutrition variables, were measured at the CC early growth CaC stage. In maize, barley CC had the best soil microbial and nutritional legacy effects, which resulted in the highest biomass and nutrient status. In contrast, barley produced the worst results on wheat, showing the disadvantages of growing two crops from the same tribe consecutively. CC mixtures also did not offer a productive advantage over pure CC. Additionally, our findings suggest that archaea seem to play a role in increasing N and Zn content in maize shoots. Furthermore, shoot B contents showed highly significant regressions with the CaC biomass. These results can help select the appropriate CC in each case.