Increase of soil nitrogen availability and recycling with stand age of Chinese-fir plantations

Soil nitrogen (N) is frequently limiting forest productivity, especially in plantations. To investigate the soil N dynamics and the consequences of N accumulation as well as the losses in the ecosystem, we studied a chronosequence of Chinese-fir (Cunninghamia lanceolata [Lamb.] Hook) plantations with stands aged 3, 16, 25, 32, and >80 years. Total N (TN), dissolved organic N (DON), microbial biomass N (MBN) and mineral N (i.e., ammonium (NH4+) and nitrate (NO3−)) in the top 0–5 cm and 5–15 cm soils were compared with the annual N accumulation by trees, annual litter N return and microbial N transformation rates. Stronger changes with stand age were observed in all N forms in the top 5 cm compared to the 5–15 cm layer. Topsoil TN and DON increased steadily in stands from ages 3- to >80-years-old by 1.9-times and 2.1-times, respectively. MBN increased from 206 mg kg−1 to 327 mg kg−1 in the first 25 years and stabilized thereafter. NH4+ content increased sharply from 3 to 16 years and then stabilized, while NO3− increased linearly with stand age. Both N demand of Chinese-fir and litter N input were clearly age dependent and defined the soil N status: rapid tree biomass accumulation in younger stands greatly depleted soil N, while the older stands with larger litter return and slower growth enabled N re-accumulation in the topsoil. The close positive correlation between total mineral N and gross mineralization rate, as well as between NO3− and gross nitrification rate indicated that the mineralization of soil organic matter and the nitrification of released NH4+ were the two primary microbial processes controlling the available N supply. The topsoil NO3−: NH4+ ratio in older stands (32- and >80 years) was greater than 1.0, and the MBN: (NH4+ + NO3−) ratio was greatly reduced. This indicates that N availability increase and N cycling is accelerated with stands age.