Relating net primary productivity to soil organic matter decomposition rates in pure and mixed Chinese fir plantations

Abstract

In a 5-year field trial, we examined plant productivity and soil organic matter decomposition on plots with a mixture of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) and broadleaved trees. We compared pure fir (PF) plots to two mixed plots (2:1 ratio of fir to broadleaved trees): MP1 (C. lanceolata and Liquidambar formosana Hance) and MP2 (C. lanceolata and Alnus cremastogyne Burk). The mixed plots differed in that the MP2 plots incorporated a nitrogen-fixing tree (A. cremastogyne). We hypothesized that the mixed plots would have higher soil organic matter decomposition rates than the PF plots as a result of increased primary productivity. The increased productivity would increase carbon input into soils, thus resulting in greater microbial biomass and soil basal respiration. We measured tree biomass, soil organic matter decomposition rates, microbial biomass carbon, total organic carbon, metabolic quotient and microbial quotient for each plot. The results showed that the productivity, microbial biomass carbon, and total carbon in the MP2 plots were significantly higher than in the PF and MP1 plots. Path analyses suggested that soil respiration varied with the amount of tree biomass produced. However contrary to our hypothesis, soil basal respiration was higher in the PF plots than in the MP2 plots.