To study the influences of gap size on the variation in soil nutrients, soil microbial carbon (micro-C), soil samples from Pinus massoniana forests were collected in and around different forest gaps with size ranging from 40.38 to 1321.40 m² in Guizhou Province. The soil nutrients (include SOC, TP, TK, TKN, valid-P, valid-K and hydro-N) and micro-C in soil samples were analyzed, and the artificial neural networks was employed to explore the importance of different soil nutrients and gap size concerning micro-C. The results indicate that the micro-C_gap ranged from 38.00 to 660.32 mg kg^-1 with a mean value of 221.24 mg kg^-1, and the micro-C_forest ranged from 110.70 to 685.62 mg kg^-1 with a mean value of 356.16 mg kg^-1. The micro-C_gap/micro-C_forest values increased with the increase of gap size in the range from 40 to 600 m², and this trend inversed when gap size was greater than 600 m². Micro-C was mainly associated with valid-P for both gaps and surrounding forests, large gap may lead to the loss of soil N. The results suggested that: (a) SOC, TK, TKN, hydro-N and valid-P are important factors to microbial in forest soils; (b) TP, valid- K, TKN, TK, valid-P and SOC are important factors to microbial in gap soils. In summary, gaps can increase the heterogeneity of soil nutrients and soil microbial, valid-P is a key factor affecting microbial abundance for Pinus massoniana forest. These promote the preservation of biodiversity and pedodiversity.