The low dose of L-lactic acid as loess soil amendment enhances wheat rhizosphere microecosystem

Abstract

Loess covers ∼12% of the global continental area. However, soil salinization, due to chemical or physical weathering of rock, overgrazing and inappropriate agricultural practices, etc., threatens the quality of loess and the yield of crops. Exploring sustainable, accessible, and affordable soil amendments is consequently required to upgrade loess soil quality. Herein, L-lactic acid, as the main by-product in the process of biowaste fermentation, has been investigated due to its affordability and accessibility for this purpose. The interactions of L-lactic acid with loess soil and rhizosphere microecosystem were explored to unveil its capacity to ameliorate salinity-related concerns and rejuvenate the agricultural capabilities of the soil. The effects of L-lactic acid addition on soil rhizosphere microorganisms and growth of wheat seedlings were investigated at the L-lactic acid dosages of 0.07–2.99 mg-C/g-soil with determination of the physiological characteristics of wheat seedlings, bulk soil, and rhizosphere microorganisms. The low dose of L-lactic acid (0.07–0.75 mg-C/g-soil) was demonstrated to abate the salt ion content (Na+, K+, and Ca2+) in bulk soil and alleviate the effects of salt stress on plants. By contrast, the addition of a high dose of L-lactic acid (1.49–2.99 mg-C/g-soil) boosted the salt ion content (Na+, K+, and Ca2+) in bulk soil and exacerbated salt stress. In accordance with the K+:Na+ ratio and chlorophyll content indicators of wheat seedlings, a low dose of L-lactic acid enhanced plant photosynthesis and tolerance to salt stress. The L-lactic acid–induced rhizosphere microecosystem established an environment that supported microbial growth with elevated soluble phosphorus levels, which provided the basis for the plant–microbe–soil nutrient cycle and metabolism. Gas chromatography–high resolution Orbitrap mass spectrometry used plant volatile organic compounds (VOC) as the indicators of specific and sensitive signals to reveal that a low dose of L-lactic acid supported the enrichment of VOC in the aerial parts of wheat by heightening plant disease resistance and thereby increasing the aboveground biomass of plants. A high dose of L-lactic acid diminished the K+:Na+ ratio and chlorophyll content in wheat and reduced plant photosynthesis resistance to salt stress; furthermore, the number of disease-resistant VOC available to wheat was curtailed, inhibiting wheat growth. Our study suggests that L-lactic acid added at an appropriate dose can effectively benefit the rhizosphere microecosystem and plant growth, thereby enhancing crop defense and yield. Thus, a soil amendment derived from L-lactic acid demonstrates significant potential in the application for loess soil.