The Loess Plateau is an important site for many ecological restoration studies in China. Different forest types can have a great influence on the soil environment, including carbon and nitrogen biogeochemical cycles. The soil microbial biomass can have a large effect on soil processes and soil microorganisms after vegetation restoration in the loess hilly area of the Loess Plateau. Understanding the relationships between soil physicochemical properties and soil microbial biomass under different forest types can provide useful information for vegetation restoration in the Loess Plateau. This study sampled soils in four different forest types to investigate the soil composition and correlation with soil microbial biomass carbon (MBC), soil microbial biomass nitrogen (MBN), soil dissolved organic nitrogen (DON) and soil dissolved organic carbon (DOC), the sum of DOC and MBC (DOC+MBC) and the sum of DON and MBN (DON+MBN). The sample sites were located on Ziwuling Mountain of the Loess Plateau. The four different forests were all established 15 years ago. The results showed that in the 0–5 cm soil layer of the 4 forest types, MBC ranged between 350 and 693.15 mg/kg, MBN ranged between 52.21 and 93.61 mg/kg, DON ranged between 53.49 and 89.07 mg/kg, DOC ranged between 302.33 and 460.87 mg/kg, DOC+MBC ranged between 543.72 and 1114.76 mg/kg and DON+MBN ranged between 105.70 and 182.68 mg/kg. All values at 0–5 cm were higher than values obtained in the 5–20 cm soil layer. Quercus liaotungensis and Biota orientalis had the highest MBC and MBN, Pinus tabulaeformis Carr. and Robinia pseudoacacia had the lowest SMBC and MBN. Soil DOC was not significantly different between forest soils for the soil layer 0-5 cm. DOC+MBC of Q. liaotungensis and B. orientalis were significantly higher than for P. tabulaeformis Carr. and R. pseudoacacia, with R. pseudoacacia having the lowest values of DOC+MBC. Soil DON was highest for Q. liaotungensis and was significantly different from the other three forest soils. Soil DON+MBN were highest for Q. liaotungensis, lowest for P. tabulaeformis Carr. and R. pseudoacacia, and there were no significant differences between P. tabulaeformis Carr. and R. pseudoacacia. Soil DON+MBN was lowest for B. orientalis in the 5-20 cm soil layer. The ratio of MBC to MBN (MBC/MBN) ranged from 4 to 8, the ratio of MBN to total nitrogen (TN) (MBN/TN) ranged from 3.21% to 5.03% and the ratio of MBC to the total organic carbon (SOC) (MBC/SOC) ranged from 2.02 to 3.29%. MBC and MBN were correlated or significantly correlated with MBC/MBN, DOC, DON, DOC+MBC, DON+MBN, TN and SOC. MBC was significantly correlated with MBN. DOC+MBC better reflected soil available carbon than MBC and DOC alone, and similarly DON+MBN better reflected soil available nitrogen. Different forests have a significant influence on the microbial biomass and water dissolved carbon and nitrogen. Q. liaotungensis and B. orientalis had much greater effects on the soil organic carbon and soil nitrogen content, which could improve the soil quality. R. pseudoacacia is a native species in this region, and this species had lower effects on the contents of soil carbon and nitrogen. For the process of vegetation restoration in the Loess Plateau, Q. liaotungensis and B. orientalis were more suitable forest species than R. pseudoacacia.