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 [V_max], Michaelis constant [K_m] and catalytic efficiency [K_a]) 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 K_a 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.