Abstract
Soil extracellular enzyme activities and associated enzymatic stoichiometry are considered sensitive indicators of nutrient availability and microbial substrate limitation. However, many of previous studies have been focusing on uppermost soil layer with a single enzyme as representative of the whole nutrient acquisition, leading to critical uncertainties in understanding soil nutrient availability and its relationship with microbial activities in deeper soils. In the current study, we investigated C-, N- and P-acquiring enzyme activities across a range of soil layers (0–10, 10–20, 20–40 and 40–60 cm), and examined the microbial C, N and P limitation in natural secondary forests (NSF) and Chinese fir (Cunninghamia lanceolata) plantation forests (CPF) in subtropical China. The results showed that microbial C and P co-limitation was detected in the two typical subtropical forests at all soil depths, rather than microbial N limitation. Microbial C and P limitation fluctuated along soil depth, but higher N was demanded by microbes in soil under 20 cm in both forests. The present results highlight the asymmetrical patterns of microbial nutrient limitation along the whole soil profile, and provide essential information in understanding nutrient limitations in deeper soils. These vertical and asymmetrical nutrient limitation patterns should be incorporated into future research studies priority.
Highlights
Tropical and subtropical forests possess rapid growth rates compared with their temperate counterparts and on a global scale, are more productive and sequester more atmospheric CO2 [1,2,3]
The only significant difference we found between the 2 forests was for N-acquiring (NAG + leucine aminopeptidase (LAP)) enzyme activities at surface soil (0–10 cm) and natural secondary forest (NSF) activities were significantly greater than Chinese fir plantation forest (CPF) (Fig 2)
The correspondence analysis (CCA) indicated that CBH and BX activities were strongly positively correlated with most soil nutrients (Fig 5A) and this was consistent with Soil organic carbon (SOC) and total nitrogen (TN) levels that were higher in 0–10 cm in the NSF than in the CSF (Table 1)
Summary
Tropical and subtropical forests possess rapid growth rates compared with their temperate counterparts and on a global scale, are more productive and sequester more atmospheric CO2 [1,2,3]. Tropical forest ecosystems are generally phosphorus (P) limited and at high altitudes are nitrogen (N) limited as well [4, 5]. The storage and subsequent release of P and N from soil for plant use is primarily governed by the action of soil micro-organisms. Microbial processes are limited or co- limited by carbon (C) (carbon can be considered as energy source) or key nutrients (usually N or P) [6].
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