Carbon and nutrient dynamics during decomposition of Chinese Fir (Cunninghamia lanceolata) stumps in subtropical plantations

The quantity of stumps within the reforested area after clear-cutting accounted for one-sixth of the total plant biomass. This is of significant importance for the carbon and nutrient cycles within the plantation ecosystem. Nevertheless, most studies relating to stump decomposition have focused on temperate and boreal plantations, creating a large gap in the understanding of carbon and nutrient dynamics in subtropical plantations. Here, we conducted a chronosequence method to investigate the carbon and nutrient dynamics associated with the decomposition of Chinese fir (Cunninghamia lanceolata) stump in subtropical China. After the Chinese fir plantation was cleared, the mean mass of the stump was 31.0 Mg/ha. The decomposition rate constant (k, year-1) was 0.01, resulting in an estimated half-life (t0.5) of 63 years. During the 35-year decay period, an average of 52.4%, 57.8%, and 61.2% of carbon, lignin, and cellulose was lost from the stumps, respectively. Interestingly, nitrogen and phosphorus stocks in stumps remained stable compared to their stocks at the initial time. In contrast, sodium and calcium stocks decreased by 11.0% and 24.1% of their initial amounts, respectively, while potassium and magnesium stocks increased by more than 1.6 and 1.8 times, respectively. These likely resulted from structural degradation, especially lignin degradation, which may promote microbial nutrient immobilization and accumulate their stocks in decomposing stumps. Furthermore, stump decomposition and nutrient return altered soil nutrient status and microbial communities. We found that the stocks of soil carbon, nitrogen, microbial biomass carbon, and microbial biomass nitrogen all significantly increased after 2 years of harvest, indicating a nutrient retention at the early stage. This study suggests that stump decomposition can function as an important carbon source and nutrient sink in subtropical plantations, especially at the early decomposition stage. Therefore, forest managers should keep stumps after forest harvest to benefit tree growth in the subsequent plantation rotation.