Root systems and soil microbial biomass under no-tillage system

Some root parameters such as distribution, length, diameter and dry matter are inherent to plant species. Roots can influence microbial population during vegetative cycle through the rhizodeposits and, after senescence, integrating the soil organic matter pool. Since they represent labile substrates, especially regarding nitrogen, they can determine the rate of nutrient availability to the next crop cultivated under no-tillage (NT). The root systems of two crop species: maize (Zea mays L.) cultivar Cargill 909 and soybean [Glycine max (L.) Merr.] cultivar Embrapa 59, were compared in the field, and their influence on spatial distribution of the microbial C and N in a clayey-textured Typic Hapludox cultivated for 22 years under NT, at Tibagi, State of Paraná (PR), Brazil, was determined. Digital image processing and nail-plate techniques were used to evaluate 40 plots of a 80 ´ 50 ´ 3 cm soil profile. It was observed that 36% and 30% of the maize and soybeans roots, respectively, are concentrated in the 0 to 10 cm soil layer. The percent distribution of root dry matter was similar for both crops. The maize roots presented a total of 1,324 kg C ha-1 and 58 kg N ha-1, with higher root dry matter density and more roots in decomposition in the upper soil layer, decreasing with depth. The soybean roots (392 kg C ha-1 and 21 kg N ha-1) showed higher number of thinner roots and higher density per length unity compared to the maize. The maize roots enhanced microbial-C down to deeper soil layers than did the soybean roots. The microbial N presented a better correlation with the concentration of thin active roots and with roots in decomposition or in indefinite shape, possibly because of higher concentration of C and N easily assimilated by soil microorganisms