Abstract: The pot and field experiments were conducted to study the effects of root and fertilization on soil microbial biomass nitrogenin semi-arid farmland. The result showed that fertilization and crop root increased obviously the soil (microbial) biomass nitrogen amounts, the microbial biomass nitrogen in rhizosphere soil being as high as 1.10—2.04 times as that in non-rhizosphere soil indicated that roots played a great role in increasing its amounts. Different fertilizer or (manure) had different impacts, the effects of straw and manure with rich organic material on soil microbial biomass were (larger) than chemical fertilizer, and the soil microbial biomass nitrogen amount raised with increase of straw application rate. A long-term experiment conducted on a manual loessial soil showed that without fertilizers, the microbial biomass nitrogen at 0—20cm depth was N 102.2μg/g, and N 110.4μg/g with application of both N and P fertilizers, whereas addition of 9375kg, 18750kg, 37500kg fresh maize straw and 37500kg of organic manure per hectare in a year on the (basis) of applying N and P fertilizers raised the microbial biomass nitrogen to N 147.5, 163.2, 286.4 and 265.3μg/g, respectively. Adding organic manures or crop residues provided energy source for soil microorganisms, thereby stimulated their reproduction and growth. Applying chemical fertilizers increased the growth of shoots and roots of crops, therefore increased the organic materials in soil and the microbial biomass nitrogen also raised accordingly. The incubation experiment showed that the ability of fixation of ammonium nitrogen into microbial biomass was much higher than that of nitrate nitrogen but in case the available energy material such as glucose was applied the nitrogen immobilized by microorganisms from the 2 forms of mineral nitrogen increased in a large scale, meanwhile, the fixed amounts were almost similar.