Environmental behaviors of phenolic acids dominated their rhizodeposition in boreal poplar plantation forest soils

Purpose

The deposition of phenolic acids in soils is of ecological and environmental importance for growth of trees and nutrient cycling of soil. The objective of this study was to investigate the relationship between environmental behaviors of phenolic acids and their rhizodeposition in forest soils. The results could provide more information regarding the ecological process of root exudates at the plant-soil interface.

Materials and methods

The amounts of four types of phenolic acids (i.e., p-hydroxybenzoic acid, benzoic acid, cinnamic acid, and vanillin acid) in the rhizosphere and bulk soils of two plantation forests were measured and compared using HPLC (Thermo Electron Corp., USA). The root exudates of poplar saplings under three nutrient conditions (normal (CK), nitrogen stress (low-N), and phosphorus stress (low-P)) were collected via adsorption resins (Amberlite XAD-16, USA). The phenolic acids in root exudates were then quantified by external standards. The adsorption and degradation dynamics of the phenolic acids in soils were monitored by exogenous application in the lab. Several models were employed to reveal the environmental behavior properties of phenolic acids after they entered into soils. Meanwhile, the enumeration of the culturable bacteria and fungi was conducted using soil dilution plate method to measure the abundance variation of the microbial community along with the degradation of the phenolic acids.

Results and discussion

More phenolic acids deposited in the rhizosphere soils than in the bulk soils of the poplar plantations. The difference was significant (p?<?0.05). Under normal nutrient conditions, poplar roots could secrete phenolic acids. However, in low-N and low-P conditions, poplar roots would significantly increase the secretion (p?<?0.05). Phenolic acids were quickly adsorbed by the soil within 24 h. These four phenolic acids showed varied adsorption capacities by soil, but they are generally more than 1500 μg g soil^?1. Phenolic acids were degraded over time with half-lives around 1.29–4.24 days. Among them, p-hydroxybenzoic acid showed the highest secretion capability and the longest persistence, indicating that p-hydroxybenzoic acid had higher deposition potential in soils. Environmental behaviors should be responsible for the rhizodepostion of phenolic acids in poplar plantations.

Conclusions

Roots could release a certain amount of phenolic acids into the rhizosphere soil, especially under nutrient stress. Phenolic acids could be easily adsorbed onto soil colloids. However, the degradation amount of phenolic acids was positively related with the abundance of the fungi community. Thus, the rhizodeposited phenolic acids were the trade-off between those produced by root exudation and those consumed by microbial degradation.