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 matt er 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 an d a higher rate potential for microbial processes, e.g. denitrification, th an the soil matrix not affected by macropores. This was tested by determini ng the content of NO3--N, water-soluble carbon (WSC) and the size of the mi crobial biomass in bulk soil, in soil adjacent to macropore channels (macro pore soil) and in soil not affected by macropores (matrix soil) from one ag ricultural site and one forest site located in Jutland, Denmark. Determinat ions of the bacterial biomass in the agricultural bulk soil profile (0 to 2 50 cm) generally showed the following sequence of bacterial abundance: acri dine orange direct counts > most probable numbers > total plate counts > ni trate reducers > denitrifiers. The numbers of culturable bacteria were 1 to 2 orders of magnitude lower in the forest profile than in agricultural pro file. There was a tendency for a slight decrease in the numbers of bacteria with increasing depth. The comparison of macropore soil and matrix soil cl early showed that macropore soil contained larger concentrations NO3--N and WSC and a larger bacterial biomass. In the agricultural site the average c ontents of NO3--N were 6.7 and 3.7 mu g g(-1) dry soil the macropore soil a nd the matrix soil, respectively, whereas the very low contents of NO3--N i n the forest soil (<1 mu g N g(-1) dry soil) resulted in smaller difference s between the macropore and the matrix soils. The average contents of WSC i n the macropore soil and the matrix soil were 51 and 40 mu g C g(-1) dry so il at the agricultural site and 33 and 19 mu g C g(-1) dry soil at the fore st site, respectively. With respect to the bacterial biomass the results sh owed 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 o f 44 analyses this difference was statistically significant. These results of direct measurements in macropore soil and matrix soil substantiate the r ole of macropore channels as the preferential pathways for transportation o f solutes such as nitrate and organic compounds. Consequently, the higher n umbers of bacterial cells in the macropore environments may be attributed t o a better substrate supply and the transportation of bacterial cells throu gh the macropore channels. (C) 1999 Elsevier Science Ltd. All rights reserv ed.