A seemingly endless stream of research has shown that plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) can significantly boost plant growth, increase plant tolerance to droughts and pests, and improve the roots’ ability to uptake water and nutrients. AMF form networks throughout the soil with their hyphae, connecting the roots of neighboring plants. During drought, these fungi are able to redistribute water between plants, reducing water stress. Soil microbes cycle carbon, nitrogen, and other nutrients throughout the soil, acting as a slow-release reservoir of these nutrients for plants. The composition of the soil microbiome is determined by organic matter content, soil nitrogen content, and water availability.

Bacterial growth is favored in moist soils, whereas fungi dominate in drier soils. When soil is saturated above 70% water-filled pore volume, reduced soil oxygen can also lead to the development of anaerobic conditions, which favors denitrification (the release of N as N2O gas). Increasing soil moisture has also been correlated with increased mineralization of organic matter. Therefore, excessive irrigation may accelerate the loss of soil nitrogen and carbon.

Irrigation management which alternates the soil through periods of wet and dry will prevent any one group of microorganisms from dominating, and will also reduce rates of decomposition and denitrification. In addition, this type of irrigation promotes long-term soil health by building the physical structure of the soil, and can promote plant growth as well. A 2008 study compared the effects of mild water deficit irrigation on maize. The authors compared conventional irrigation, alternate partial root zone irrigation (APRI; one side of the plant irrigated, alternating sides with each irrigation event), and fixed partial root zone irrigation (FPRI; one side of the plant irrigated, same side each time), the authors found that the soils in mild water deficit had more than 2x the number of soil microorganisms than the soils in conventional irrigation, with the alternate partial root zone irrigation having the greatest number of soil microbes. The plants in the APRI treatment also had the highest irrigation water use efficiency. In a similar study of tomatoes, the authors found that deficit irrigation and partial root zone drying significantly increased the water use efficiency for fruit production, and allocated a greater proportion of N to fruit production than vegetative growth.

Figure from Trost, B., Prochnow, A., Drastic, K., Meyer-Aurich, A., Ellmer, F., and Baumecker, M. (2013). Irrigation, soil organic carbon and N2O emissions. A review. HAL Open Science.

The rate of irrigation water infiltration has a significant effect on the soil structure. Slow water infiltration, such as from drip irrigation, has little effect on soil aggregates, whereas rapid infiltration from flooding or furrow irrigation can displace trapped air and cause disaggregation. This leads to the loss of soil organic matter. Heavy irrigation can also leach N from the soil, potentially increasing future fertilizer costs.

Long-term heavy irrigation can cause soil compaction, reducing pore space and the soil’s ability to retain water. But when soil alternates between states of wet and dry, swelling and shrinking in the soil can create new pore space, improving soil aeration and moisture capacity. Thus, lowered irrigation rates that allow soil drying can improve the physical structure of the soil. However, this may have negative effects on soil aggregates in certain soil types, especially in arid regions.