Microbial biomass and numbers of denitrifiers related to macropore channels in agricultural and forest soils

In clay and loamy soils the vertical flow of dissolved and particulate matter primarily takes place through macropore channels supplying the attached microorganisms with nutrients and substrates for their growth. Consequently the walls of the macropore channels may have a higher microbial biomass and a higher rate potential for microbial processes, e.g. denitrification, than the soil matrix not affected by macropores. This was tested by determining the content of NO3−-N, water-soluble carbon (WSC) and the size of the microbial biomass in bulk soil, in soil adjacent to macropore channels (macropore soil) and in soil not affected by macropores (matrix soil) from one agricultural site and one forest site located in Jutland, Denmark. Determinations of the bacterial biomass in the agricultural bulk soil profile (0 to 250 cm) generally showed the following sequence of bacterial abundance: acridine orange direct counts>most probable numbers>total plate counts>nitrate reducers>denitrifiers. The numbers of culturable bacteria were 1 to 2 orders of magnitude lower in the forest profile than in agricultural profile. There was a tendency for a slight decrease in the numbers of bacteria with increasing depth. The comparison of macropore soil and matrix soil clearly showed that macropore soil contained larger concentrations NO3−-N and WSC and a larger bacterial biomass. In the agricultural site the average contents of NO3−-N were 6.7 and 3.7 μg g−1 dry soil the macropore soil and the matrix soil, respectively, whereas the very low contents of NO3−-N in the forest soil (<1 μg N g−1 dry soil) resulted in smaller differences between the macropore and the matrix soils. The average contents of WSC in the macropore soil and the matrix soil were 51 and 40 μg C g−1 dry soil at the agricultural site and 33 and 19 μg C g−1 dry soil at the forest site, respectively. With respect to the bacterial biomass the results showed that with only four exceptions, the numbers of bacteria were higher in the macropore soil samples than in the matrix soil samples and in 19 out of 44 analyses this difference was statistically significant. These results of direct measurements in macropore soil and matrix soil substantiate the role of macropore channels as the preferential pathways for transportation of solutes such as nitrate and organic compounds. Consequently, the higher numbers of bacterial cells in the macropore environments may be attributed to a better substrate supply and the transportation of bacterial cells through the macropore channels.