Subsurface irrigation as a microbial delivery tool for bioaugmentation: transport, distribution and survival in large packed soil columns.

Traditionally, soil surface inoculation and surface irrigation are used for delivery, transport and distribution of bacteria for agricultural and in-situ environmental decontamination applications. The objective of this study was to test whether subsurface irrigation with a water table management (WTM) system, successfully used previously for nutrient delivery, could also be used to deliver bacteria to soil depths. Twelve stainless steel columns, 1000 mm in length and 200 mm in diameter, were packed with a sandy loam soil. Four experimental treatments, two subsurface irrigation and two surface irrigation, were randomly allocated, in triplicate, to the twelve columns. The transport, implantation and survival of Sinorhizobium (Rhizobium) meliloti, strain A-025, in these columns were monitored at different depths and times after surface and subsurface irrigation. Even though the transport of bacterial cells was slower with the subsurface irrigation regime than with surface irrigation, more bacteria were implanted at different depths with subsurface irrigation. The numbers of bacterial cells distributed with subsurface irrigation were 1.6x10(5), 1.3x10(5), 2.6x10(5) and 2.9x10(5) cfu (colony forming units) x g(-1) of soil at 60, 300, 500 and 700 mm depths, respectively, whereas with surface irrigation the numbers were 2.0x10(4), 3.0x10(4), 1.9x10(5), 9.0x10(2) cfu x g(-1) of soil. These results dearly indicate that subirrigation can be used effectively to bioaugment a sandy loam soil matrix.