Absence of significant effect of nitrogen addition on soil respiration in a subtropical Chinese fir plantation

Chinese fir ( Cunninghamia lanceolata (Lamb.) Hook) is one of the most dominant tree species in southern China. However, the effect of different nitrogen (N) levels on soil respiration in subtropical Chinese fir plantations remains unclear. We studied soil respiration, soil physicochemical properties, and microbial biomass in Chinese fir plantations under four N treatments (control, low-N, moderate-N, and high-N). The results showed that soil respiration under low-N, moderate-N, and high-N treatments increased by 9%, 50%, and 79%, respectively, compared to the control, but the increases did not reach statistical differences. N addition significantly decreased soil microbial biomass and soil pH, and tended to increase soil aluminum ions (Al 3+ ). These results indicated that soil microorganisms are inhibited by enhanced Al 3+ content under increased N addition, which therefore reduced the soil microbial biomass. The negative correlation between soil microbial biomass and soil respiration suggests that reductions in soil microbial biomass may inhibit microbial respiration. N addition significantly increased soil ammonium nitrogen, decreased soil moisture, combined with the lower soil microbial biomass in the middle of the growing season compared with in the end of the growing season. This indicated that N addition stimulated plant growth to absorb more nutrients, intensified plant-microbe competition, and may lead to an increase in plant autotrophic respiration, which was higher than the decrease in heterotrophic respiration. Our study highlights that isolating and quantifying the source of soil respiration will be necessary in future studies.