Effect of Mosses and Long-Term N Addition on δC and δO Values of Respired CO Under a Temperate Forest Floor.

Static chambers combined with isotopic (δC and δO) and flux (CO and CH) 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 yr) on δC and δO values of respired CO across three autumn seasons under a temperate forest (northeastern China) and their relationships with CO and CH fluxes and with soil properties. Mosses generally depleted δC and enriched δO in respired CO, likely by altering soil microenvironments or/and substrate use. The effect of N addition on the δC and δO values of respired CO varied with years, and its interaction with mosses had no effects on the isotopic values. The removal of mosses decreased CO fluxes and the addition of N at a high dose increased CH fluxes. The δC and δO values of respired CO decreased at soil moisture levels below and above an optimum, and the moisture-dependent effect became more pronounced for the δO than for the δC. The results of structural equation modeling showed that 70% of the variability of δC values of respired CO was accounted for by the N addition, mosses, soil moisture, and CH and CO fluxes, while only 22% of the variability of δO values of respired CO 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 δO values of respired CO, but its drivers remain poorly understood. Future work should target processes influencing the δO shifts of respired CO and deep soil property interactions.