MICROBIAL BIOMASS AND N TRANSFORMATIONS IN 2 SOILS CROPPED WITH ANNUAL AND PERENNIAL SPECIES

A field study was undertaken to determine the effects of different pla nt species on soil microbial biomass and N transformations in a well d rained silty clay loam (Typic Dystrochrept) and a poorly drained clay loam (Typic Humaquept). The crop treatments were faba bean (Vicia faba L.), alfalfa (Medicago sativa L.), timothy (Phleum pratense L.), brom egrass (Bromus inermis L.), reed canarygrass (Phalaris arundinacea L.) , and wheat (Triticum aestivum L.). Measurements of microbial biomass C, denitrification capacity, and nitrification capacity were performed periodically in the top 2-10 cm of soil. On most sampling dates, all three parameters were higher under perennial than under annual species . The nitrification capacity was positively affected by the level of N applied to each species (r = 0.65* for the silty clay loam and 0.84* * for the clay loam) and not directly by the plant. The differences f ound in microbial biomass C were significantly correlated with the wat er-soluble organic C present under each plant species (r = 0.74** for the silty clay loam and 0.90** for the clay loam), suggesting differ ences in C deposition in the soil among plant species. In the silty cl ay loam, the denitrification capacity was positively related to the am ount of organic C found under each plant species, while in the clay lo am, it was dependent on the amount of N applied to each species. There was less denitrification activity per unit biomass under legume speci es than under gramineae, suggesting that, depending on their compositi on, root-derived materials may be used differently by soil microbes.