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Amino sugars (AS) are routinely used as microbial biomarkers to investigate the dynamics of soil carbon (C) and nitrogen (N) under different environments. However, the effect of any AS on soil C and N, or other AS, is not well-defined. In this study, acid soils from Dongbei (D) and Fujian (F) and alkaline soil from Henan (H) were selected to perform an incubation experiment under glucosamine addition for 36 days. In the present study, the dynamics of soil soluble organic C (SOC), NH4+-N, NO3−-N, soluble organic N (SON), and four AS: glucosamine (GluN), mannosamine (ManN), galactosamine (GalN), and muramic acid (MurN), were investigated. The results showed that AS was different among the three soils, but had similar dynamics in the same soil. The higher total C and inorganic N in the D and F relative to the H soil were related to the greater AS in two soils. With incubation, AS decreased in D soil and increased in F soil before 1 week, while after 1 week, the inverse dynamics were observed, which suggest that SOC or SOC combined with inorganic N may be a mechanism to adjust the dynamics of C from AS. Overall, glucosamine addition did not significantly affect AS in D, while the reverse was true for F and H soils. Glucosamine addition decreased AS at day 0 for D soil and at day 3 for F and H soils, and increased SOC. The lowered NH4+-N and AS in D soil, but the higher values of these, were observed in F soil after 1 week of incubation. The increase of SON in D soil with glucosamine addition might be due to the depolymerization of soil organic matter (SOM) into SON. However, the decrease of SON in F soil could be attributed to the mineralization of SON. [ABSTRACT FROM AUTHOR]

• Nitrogen addition significantly increased amino sugars and their contribution to SOC. • Nitrogen addition significantly increased Mehlich-P but decreased organic P. • Nitrogen addition increased fungal biomass and acid phosphatase activity. • Soil P availability influenced amino sugars through increasing fungal biomass. Microbial residual carbon (C) plays a crucial role in soil organic C (SOC) stabilization, and exogenous nutrients have frequently been observed to increase the proportion of microbial residual C in soils with limited nitrogen (N). However, how microbial residual C responds to N deposition and their mechanisms in subtropical forest soil with low phosphorus (P) availability remains poorly understood. To fill this knowledge gap, an experiment with 5.5 consecutive years of N addition (control, 40 and 80 kg N ha−1y−1) to a Castanopsis carlesii forest in Fujian, China was performed. Soil properties (e.g., soil C, N, P, and pH), amino sugars (biomarkers of microbial residual C), phospholipid fatty acids (PLFAs), and enzyme activities were investigated. We found that N application and soil depths had significant interactive effects on amino sugars, the PLFAs, and soil enzyme activities. N addition significantly increased the contents of amino sugars and their proportions to soil C in the 0–10 cm soil layer but not the 10–20 cm soil layer (P > 0.05). In addition, glucosamine (GluN), which was mostly derived from fungal cell walls, accounted for 64.9–73.9% of the total amino sugars across three treatments, and the proportion of GluN to total C also increased after N addition in the 0–10 cm layer. These results indicate that N addition elevated the contribution of amino sugars, especially GluN, to soil C sequestration of the 0–10 cm soil layer but not 10–20 cm soil layer. Moreover, N addition significantly enhanced the biomass of fungi and ectomycorrhizal fungi (EMF) and the activities of phosphatase (ACP), phosphodiesterase (PD), N-acetyl glucosaminidase (NAG), acid phenol oxidase (PhO x), and peroxidase (Perox), while it significantly decreased soil organic P (OP) in the 0–10 cm soil layer. These results implied that changes in N:P ratio caused by N addition contributed to the increases of EMF and fungi, which have high P acquisition ability, this in turn enhanced soil P availability. The increased microbial biomass of EMF and fungi likely attributed to the increased contribution of amino sugars to SOC accumulation in the studied forest soil. Our study revealed a novel mechanism of soil C sequestration in subtropical forest ecosystems with low soil P under ongoing N deposition. [ABSTRACT FROM AUTHOR]

The magnitude and direction of soil organic carbon (SOC) mineralization are closely regulated by exogenous nutrient addition – and, especially, by limited nutrients such as phosphorus (P) – in tropical and subtropical ecosystems. However, the response of SOC decomposition to P addition and the underlying mechanisms operating in subtropical P-limited forest soils remain unclear. In this study, four subtropical forest soils (sandstone and granite soils planted with the chinkapin Castanopsis carlesii and Cunninghamia lanceolata (Chinese fir)) in Sanming City, Fujian Province, China were selected, and an incubation experiment with three levels (0, 0.1, 0.6 mg P g−1 soil) of P addition was conducted to explore the impact of P addition on SOC mineralization in subtropical forest soils. SOC mineralization, phospholipid fatty acids (PLFAs), enzymatic activities, and soil properties were determined. Results showed that P addition significantly increased the cumulative mineralization of SOC (P < 0.001). Additionally, the microbial biomass (e.g., fungal, bacterial, and total PLFAs) and the activities of C-cycling enzymes significantly increased (by 11.95 %–570.25 %) after P addition (P < 0.001), which suggests that exogenous P input enhanced microbial activity and thus accelerating SOC mineralization. P addition also significantly increased the concentration of dissolved organic C (DOC) (P < 0.001), thereby making soil C more available to microorganisms and allowing it to become quickly decomposed. A variance partitioning analysis (VPA) showed that DOC and fungal biomass acted as the predominant abiotic and biotic factors accelerating SOC mineralization under P addition. Using the results of a partial least squares path analysis (PLM-PM), we conclude that P addition stimulated the decomposition of SOC in these subtropical P-limited forest soils. Two processes help explain these results: (i) the enhancement of microbial activity in response to the alleviation of microbial P limitation and (ii) the fact that P addition desorbed the microbial inaccessible organic C and converting it into DOC, which is easily accessible to microbes, thereby contributing to SOC mineralization. Our study reveals how P input influences organic C mineralization in subtropical forest soils where P is limited. It also provides novel insight into the uncertainty of the global soil C sink, which might be partially attributable to exogenous nutrient inputs, especially in the case of limiting nutrients. • P addition accelerates SOC mineralization in subtropical P-deficient forest soil. • P addition significantly increased microbial biomass and enzymatic activities. • P addition enhanced DOC concentration by desorbing mineral-associated organic C. [ABSTRACT FROM AUTHOR]