Nitrogen effects on quantity, chemistry, and decomposability of Pinus densiflora and Quercus variabilis litters under elevated CO₂ and warming

The effects of increased nitrogen (N) availability through atmospheric deposition on the quantity and quality of tree leaf litter have been widely reported. However, such N effects under co-elevated CO₂ concentration ([CO₂]) and air temperature (T_air) have not been investigated. In this study, changes in biomass and chemistry of litter of Pinus densiflora S. et Z. (pine) and Quercus variabilis Blume (oak) saplings with two levels of each N × [CO₂] × T_air were investigated for two growing seasons. The levels of N, [CO₂], and T_air were 2 and 6-fold of ambient N deposition, ambient and ambient + 285 ppmv, and ambient and ambient + 2.2 °C, respectively, on average across the two seasons. The concentrations of N, minerals, and carbon (C) compounds such as non-structural carbohydrate (NSC) and lignin as well as stoichiometry such as the ratio of C to N (C/N) and lignin to N (lignin/N) of litter were analyzed to determine litter chemistry. In addition, the legacy effect of changed chemistry on microbial decomposability of the litters were also assessed by measuring CO₂ emission in a 60-day lab incubation experiment. The effect of high N addition on increasing litter production was consistent regardless of [CO₂] and T_air for oak; however, for pine, N-induced increases in litter production were negated under elevated [CO₂] and/or T_air, probably due to efficient uptake and use of N by pine under elevated [CO₂] as well as lowered N demand via warming-induced stress under elevated T_air. High N-induced decreases in C/N and lignin/N was consistent across [CO₂]-T_air combination for both species though other chemical components such as NSC and minerals including phosphorous, calcium, aluminum, and manganese varied inconsistently. The CO₂ emission from the soils amended with litters of different chemistry was not correlated with litter chemistry, suggesting that changed litter chemistry under different N level × [CO₂] × T_air may not affect litter decomposability in the soils. This study enlarges our understanding of the effect of increased N availability on litter quantity, chemical composition, and microbial decomposability under different [CO₂] and T_air. However, a study with mature trees in a natural forest stand should further improve our understanding.