Spruce plantations in the upper reaches of Minjiang River were representative low-benefit forests at present, with the complex of environmental conditions, which made it hard to improve its species structure, soil conditions and ecological service function through natural regeneration. Researches in domestic and abroad reported that forest gap thinning could change the stand structure and increase the heterogeneity of micro-environment, which represented reasonable measure in forest management. In order to break the low-benefit status of spruce plantations, this paper used different sizes gaps and control canopy in spruce plantations as test objects in the upper reaches of Minjiang River, through measuration of soil chemical and microbiological parameters in gaps pre-thinning and post-thinning one year later to examine the short-term effects of small (30m2), medium (50m2) and large (100m2) gaps and gap size on microclimate(air temperature, air humidity, soil temperature, soil moisture, leaf area index and total transmittance), seasonal variations of soil microbial biomass (carbon, nitrogen and phosphorus)and soil nutrients. Additionally, the key factors affected soil microbial biomass were analysed. The results could provide consult for ecological function preservation and management of plantation in the upper reaches of Minjiang River.
The results showed that air temperature (5cm above ground), soil temperature (soil depth at 10cm) in the center of forest gaps were higher than canopies. Furthermore, they elevated with the gap size extended. Soil humidity (soil depth at 10cm) in forest gaps center was higher than canopies but did not elevate with forest gap size extended. Air humidity (5 cm above ground) at forest gaps center was higher or lower than canopies. Total transmittance at forest gaps center higher than control canopies while leaf area index at forest gaps center lower than control canopies. Soil microbial biomass carbon (MBC) in gaps and canopies in 2009 higher than in 2008, while microbial biomass nitrogen (MBN) lower than in 2008 and microbial biomass phosphorus(MBP) increased or decreased in 2009. So, we could conclude that soil microbial community structure had been altered in 2009. However, forest gap thinning and forest gap size had no significantly effect on the variation of soil microbial biomass and did not affect whole seasonal variations of soil MBC and MBN, but it changed the variation of soil MBC and MBN in specific season. Forest gap thinning and gap size had different effect on seasonal variations of soil MBP.
Soil water soluble organic carbon (WSOC), nitrate concentration (NO3--N), soil organic carbon (TOC) and total phosphorus (TP) of Topsoil and subsoil in spruce gaps and canopies had increased with different extent in 2009 while gaps had lager rise than canopies. Additionally, soil ammonium concentration (NH4+-N), water soluble organic nitrogen (WSON), total nitrogen (TN) and total potassium (TK) have decreased with different extent in 2009 while gaps had lower reduction than canopies. So, compared to canopies, forest gap thinning may increaded soil nutrients. SoilWSOC, TOC, TN and pH of subsoil and soil TOC, pH of topsoil were significantly affected by gap. Gap size had significantly influence on soil TOC and pH of topsoil and TOC, TN and pH of subsoil. Soil NH4+-N and TP in spruce gaps were increased with the extended of gap sizes and other indexes responded to gap size were not clear. Among all factors affected soil microbial biomass, light environment (leaf area index and total transmittance), NH4+-N、TOC、TN and cation exchange capacity（CEC）play a key role on decide soil microbial biomass.
All in all, despite that forest gap thinning could not significantly affect soil microbial biomass and its seasonal variation, but it could improve microclimate, stimulate the release of soil nutrients. Therefore, forest gap thinning as an alternative measure to manage mature plantation in the upper reaches of Mingjiang River are feasible in theory.