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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), NH 4 + -N, NO 3 - -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 NH 4 + -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.

• Converting natural forest to plantations reduced soil P availability. • Converting natural forest to plantations decreased soil microbial biomass. • Labile and moderately labile organic P were the potential source of available P. • Phosphatase activity and labile Pi were positively related. The large-scale conversion of natural forests to plantations profoundly alters the accumulation and transformation of soil nutrients. However, our understanding of how soil P responds to forest conversion is far from complete. This study aims to explore the changes in various soil P fractions associated with forest conversion in subtropical forest soils, where P is known to be relatively scarce. Soil samples were collected from an evergreen broad-leaved forest, a secondary forest, and two plantation forests. The soil properties, P fractions, amino sugars, and phosphatase activities were determined. The replacement of natural forests by plantation forests was associated with reductions in soil labile inorganic P (Pi) and organic P (Po), moderately labile Po, and total P. Soil amino sugars, which are indicative of historical populations of soil microbes, consistently decreased following forest conversion. We also found significant positive relationships between most P fractions and amino sugars content. A structural equation model suggested that Po, especially labile and moderately labile Po, were the potential sources of labile Pi. The positive relationship between labile Pi and acid phosphatase activity indicates that microbes are involved in soil P transformations by regulating P-hydrolytic enzymes activity. Overall, our work suggests that forest conversion significantly decreased most soil P fractions, including labile Pi, labile Po, and moderately labile Po, partly due to the declines in microbial populations that play an important role in releasing P from litter and root necromass. Organic P may play a vital role in sustaining P availability through organic P decomposition facilitated by microbial phosphatases, resulting in the release of labile P in the forest soils. These findings provide a mechanistic understanding of soil P cycling following forest conversion in low available P subtropical forests. [ABSTRACT FROM AUTHOR]

There has been an increasing interest in studying microbial necromasses and their contribution to soil organic carbon (SOC) accumulation. However, it remains unclear how the interaction among climate, plants, and soil influence the microbial anabolism and how microbial necromass contribute to SOC formation. Here, we assessed the relative contribution of microbial residues to SOC pool across a subtropical elevation gradient (ranged from 630 to 2130 m a.s.l.) representing a subtropical ecosystem on Wuyi Mountain in China, by using amino sugars as tracers. Analysis of topsoil (0–10 cm) amino sugars and the composition of microbial community across this gradient revealed that the soil total amino sugars accounting for 12.2–25.7% of the SOC pool, decreased with increasing elevation. Moreover, the linear reduction in the bacterial-derived carbon (C) and an increase in the ratio of fungal- to bacterial-derived C with increasing elevation suggested the reduction in the contribution of bacterial-derived C to SOC pool across this elevation gradient. The divergent changes in the contribution of the microbial residues to SOC infer a potential change in SOC composition and stability. The microbial-derived SOC formation and its climatic responses are influenced by the interaction of vegetation types and soil properties, with soil amorphous Fe being the determiner of soil amino sugar accrual. Our work highlights the importance of understanding ecosystem type and mineral composition in regulating microbial-mediated SOC formation and accumulation in responses to climate change in subtropical ecosystems. Unlabelled Image • Soil amino sugars were measured along a subtropical elevation gradient. • The microbial residues related to SOC reduced with increasing elevation. • Soil mineral protection impaired the microbial necromass formation in soils. • Fungal-derived carbon contributed more to SOC than the bacterial-derived carbon. [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]