Microbial nutrient limitation and catalytic adjustments revealed from a long‐term nutrient restriction experiment

Introduction

Microbial abundance and activities in soils are predominantly determined by soil carbon (C), nitrogen (N) and phosphorus (P) availability. Much research has focused on the effects of soil N than P availability on soil microbial biomass and enzyme activities as sensitive proxies of microbial ecophysiology highlighting the need to investigate how microbes will respond to P availability in soil, especially in cropping systems.
Materials and Methods

The effect of P fertilisation on microbial biomass-C, -N and -P, and the kinetic parameters (maximal velocity [Vmax], Michaelis constant [Km] and catalytic efficiency [Ka]) of β-1,4-glucosidase (BG; C-acquiring), leucine-aminopeptidase (LAP; predominantly N-acquiring) and acid phosphomonoesterase (PHO; P-acquiring) were measured in a nutrient-poor agricultural soil (devoid of fertiliser application since 1942).
Results

This study showed that P fertilisation led to a 65% and 56% increase in microbial biomass-N and -P, respectively, indicating severe P limitation and inefficient N acquisition by microbes without P availability. Increased Ka values of LAP with P fertilisation further hint toward the production of efficient isoenzymes to avoid resource tradeoffs for nutrient acquisition.
Conclusions

Overall, these results decipher microbial metabolic and catalytic adjustments mediated by soil P availability. Increased microbial biomass-N and -P with P fertilisation indicated microbial N and P colimitation that was partly overcome by the production of efficient enzymes for N acquisition with P fertilisation. We argue to incorporate microbial enzyme activities as a response to different management strategies to better inform us about soil biogeochemical cycles in cropping systems.