Abstract:

 The soil slurry transfer methods (e.g. transferring soil slurry when it was continuously agitating or reached standing state) for extracellular enzyme activity (EEA) measurement are still different. Whether the result of EEAs is consistent between the two transfer methods remains unclear and needs to be tested. This research aimed to estimate the impact of transfer methods on EEA under different environmental conditions. Surface soil was collected in different growth stages of winter wheat. The winter wheat field was located in a free air carbon dioxide enrichment (FACE) platform in Changping district of Beijing. The FACE platform included two CO₂ concentrations, which were ambient CO₂ concentration (420±15μmol·mol⁻¹) and elevated CO₂ concentration (550±15μmol·mol⁻¹), respectively. Seven EEAs were chosen to study the influence of different transfer methods under different CO₂ concentrations. The results showed that the EEAs were significantly higher under agitating state than standing state. It might be attributed to the fact that soil slurry under agitating state contained more soil particles, which were easily to absorb extracellular enzymes. The extent of influence of transfer methods on EEA varied among different enzymes. To be specific, the carbon (C) and phosphorus (P) acquiring enzyme activities were strongly influenced by transfer methods, showing a significant increase of 191.45% to 774.09% under agitating state compared to standing state, while the nitrogen (N) acquiring enzyme activity only showed an increase of 6.66% to 30.59% under agitating state compared to standing state. The transfer methods also led to different responses of enzyme activities to elevated CO₂. Most enzyme activities showed no response to elevated CO₂ under the agitating state. By contrast, more enzyme activities decreased significantly in response to elevated CO₂ under the standing state. Furthermore, the calculated extracellular enzymatic stoichiometry and vector characteristic both indicated that microbial resource limitation was unchanged by elevated CO₂ under agitating state. However, under the standing state elevated CO₂ reduced microbial C limitation and enhanced microbial N limitation. This study provided valid evidence that transfer methods had a distinguished impact on enzyme activities, extracellular enzyme stoichiometry, vector characteristic, and their response to elevated CO₂. Consequently, the result of microbial resource limitation under elevated CO₂ differed between the two transfer methods. Based on this, authors strongly suggest to unify the transfer method to reduce study uncertainty. 

Key words:

Extracellular enzyme activity, Transfer method, Winter wheat in North China Plain, FACE platform,  , Elevated CO₂ , concentration