Availability of carbon (C) often limits microbial activity in soil. Thus when C is added to soil, microbial activity usually increases and this rise coincides with a decrease in extractable mineral nitrogen (N). With increased activity, there is a greater microbial demand for mineral N. However, in theory, not all C forms should illicit the same microbial response. Molecules such as glucose are easily metabolized by most all microbes, while larger compounds such as lignin, can only be degraded by a subset of microbes with the necessary enzymatic capacity. Thus the additions of glucose versus lignin should not only cause differences in soil microbial activity but in mineral N immobilization as well. Nitrogen immobilization by microbes should be greater following the addition of glucose than the addition of lignin.

Glucose, soluble starch and lignin were added in solution to A-horizon soils collected from adjacent fertilized and unfertilized loblolly pine (Pinus taeda) plantations. During a 4-week period, the soils were routinely measured to determine mineral N (NO₃^- and NH₄⁺), dissolved organic N (DON) and C (DOC) and microbial N and C. Only glucose caused a net immobilization of N (NO₃^-, NH₄⁺ and DON) over the 4-week period. Total extractable N was greatest in the control (water additions), starch and lignin treated soils. There were no consistent trends in microbial biomass N during the experiment, but microbial biomass N in the fertilized soils was always higher than in the unfertilized soils. In the fertilized soils microbial biomass C increased in response to all the C additions, but decreased following C additions to the unfertilized soil. Therefore, although soil N in response to the small (glucose) and large (lignin) molecular weight C compounds support the hypothesized high C - low N response, the trends measured in the microbial biomass are not easily explained.