Effect of Mosses and Long-Term N Addition on δ13C and δ18O Values of Respired CO2 Under a Temperate Forest Floor

Static chambers combined with isotopic (δ13C and δ18O) and flux (CO2 and CH4) measurements were applied, to explore the effects of mosses and long-term nitrogen (N) addition at two levels (22.5 and 45 kg N ha−1 yr−1) on δ13C and δ18O values of respired CO2 across three autumn seasons under a temperate forest (northeastern China) and their relationships with CO2 and CH4 fluxes and with soil properties. Mosses generally depleted δ13C and enriched δ18O in respired CO2, likely by altering soil microenvironments or/and substrate use. The effect of N addition on the δ13C and δ18O values of respired CO2 varied with years, and its interaction with mosses had no effects on the isotopic values. The removal of mosses decreased CO2 fluxes and the addition of N at a high dose increased CH4 fluxes. The δ13C and δ18O values of respired CO2 decreased at soil moisture levels below and above an optimum, and the moisture-dependent effect became more pronounced for the δ18O than for the δ13C. The results of structural equation modeling showed that 70% of the variability of δ13C values of respired CO2. was accounted for by the N addition, mosses, soil moisture, and CH4 and CO2 fluxes, while only 22% of the variability of δ18O values of respired CO2 was explained by these factors. The results highlight that moss–soil interaction drives the isotopic shifts, which is modulated by N availability. Soil moisture regulates the δ18O values of respired CO2, but its drivers remain poorly understood. Future work should target processes influencing the δ18O shifts of respired CO2 and deep soil property interactions.