Long-Term Forestry Restoration Projects Have Changed the Spatial Variation of the Forest Soil Organic Carbon, Nitrogen, and Phosphorus Stocks Across Southwestern China

Abstract

Southwestern China ranks second in forest area nationally. Meanwhile, it covers the upper catchments for many important waters such as the Yangtze and Pearl Rivers and crosses the eastern edge of the Tibetan Plateau. Despite over three decades of the National Forestry Ecological Project's implementation, comprehensive records detailing the discrepancies between plantations and natural forests regarding soil organic carbon, nitrogen, and phosphorus stocks have been lacking. To bridge this gap, we gathered forest soil data from 223 published articles encompassing the eastern edge of the Tibetan Plateau. We aimed to elucidate the diverse responses of plantation forests and natural forests to forestry ecological engineering, unveiling the underlying driving factors. Our findings revealed plantation forests exhibited significantly lower soil organic carbon (SOC) density (mean = 11.10 Mg C ha-1) but higher soil total nitrogen (TN) density (mean = 1.20 Mg N ha-1) than natural forests (12.60 Mg C ha-1 and 1.15 Mg N ha-1). However, soil total phosphorus (TP) pools showed no significant variance between the two forest types. Natural forests in this region showcased a limitation in soil nitrogen (indicated by lower N:P ratios) than plantations. Moreover, natural forests demonstrated a more pronounced sensitivity to the geographical environment concerning soil carbon and nutrient accumulation, exhibiting stronger spatial patterns than plantation forests. These outcomes suggest that large-scale forestry ecological projects bring about shifts in forest soil's spatial distribution and diminish the influence of environmental factors. Consequently, there is a pressing need for enhanced management strategies aimed at augmenting the soil carbon sequestration capacity of plantation forests.

Keywords: soil carbon stocks, soil nutrient stocks, spatial pattern, forest restoration