Short-term impact of irrigation on soil microbiome in a Mediterranean maize cropping system

Soil is a complex environment, where microorganisms play a crucial role in the maintenance of soil structure, nutrient cycling and thus soil quality, relevant for croplands productivity. Soil microbial activity, however, is largely determined by water availability, and both are affected by agricultural management. This study aims to investigate the impact of irrigation, i.e. different amount of water and application scheduling, on soil microbial biomass and functional structure of soil microbiome (number of microorganisms in various ecological-trophic groups) in a Mediterranean maize farm. In 2023, six irrigation treatments were applied in a maize farm located in the Mondego Agricultural Valey, centre region of mainland Portugal. The irrigation treatments included three different amounts of water per week applied with drip irrigation: i) 100 mm, the average optimal amount over the last 30 years, based on APSIM crop model), ii) 55 mm, selected to simulate water scarcity conditions, and iii) the amount recommended by the local farmers’ association, based on weekly weather forecast and water balance modelling (ranged between 24 mm and 66 mm over the study period). The water was applied once or split in two applications during the week in different treatments. Each treatment was applied in triplicated plots, each plot covering five maize rows and extending over 10 m length. Nine composite soil samples (0-15 cm depth) per treatment were collected immediately before and after the irrigation period (~ 6 to 17 weeks after sowing). The soil samples were analyzed in sterile conditions using solid growth media: Nutrient Agar, Agar-Agar, Jensens Medium, Soil Agar, and Czapek-Dox Medium. The serial dilutions of the samples were provided until the suspension contained a microorganism titer within the range of 10⁻³–10⁻⁵ CFU/mL. The content of general microbial biomass (Сmic) in the soil was determined using the rehydration method. The results show that after the irrigation period, Cmic increased between 14% to 48%. The number of different nitrogen fixing bacteria and ammonifiers (nitrogen-mineralizing bacteria) increased, whereas the number of micromycetes, spore forming bacteria, oligotrophic, and pedotrophic bacteria groups decreased in all the treatments, which are good indicators about soil quality. Generally, these changes are slightly higher in the treatments where irrigation was applied twice instead of once a week (e.g. 28-31% vs 20-34% increase in nitrogen fixing bacteria, and 33-50% vs 12-36% decrease in oligotrophic bacteria - often associated to nutrient-poor soils). This highlights the relevance of providing a more uniform soil moisture content over the crop season (through smaller amounts of water, applied more often) to support soil microbial communities. Microbial biomass was lowest in plots receiving the less water (55 mm per week), but it was similar between plots receiving 100 mm of water per week and adjusting the amount of water to the recommendations of local farmers’ association. Long term average data would be useful to support decision on the amount of water to apply in areas where technical recommendations are not available. Adequate irrigation management in croplands can support soil biodiversity.