Municipal solid waste compost application improves productivity, polyphenol content, and antioxidant capacity of Mesembryanthemum edule

Abstract

Organic wastes were successfully used as soil amendment to improve agrosystems productivity. Yet, the effectiveness of this practice to enhance plant antioxidant capacities has received little attention. Here, we assess the effect of municipal solid waste (MSW) compost (at 40 t ha⁻¹) on growth, polyphenol contents and antioxidant activities of Mesembryanthemum edule. MSW compost application significantly increased the soil contents of carbon, nitrogen, calcium, phosphorus and potassium. This was associated with higher nutrient (N, P, and K) uptake, which likely led to the significant improvement of the plant biomass and relative growth rate (RGR) (+93% on average) as compared to the control. In the same way, the fertilizing effect of the added organic matter significantly enhanced the antioxidant potential M. edule, assessed by radical scavenging activity, iron reducing power and β-carotene bleaching capacity. This was associated with significantly higher antioxidant contents, mainly total phenols and flavonoids. Heavy metal (Pb, Cd, Cu, and Zn) concentrations were slightly increased upon compost application, but remained lower than phytotoxic values. Overall, our results point out that short-term MSW compost application at 40 t ha⁻¹ is efficient in enhancing the productivity together with the antioxidant potentiality of M. edule without any adverse environmental impact.

Introduction

Degraded soil quality, as poor surface soil aggregation, high bulk density, low porosity, and slow infiltration, is considered as a major environmental factor restricting plant growth and productivity [1]. This environmental issue, mainly observed in arid and semi-arid areas, is to a great extent due to the scarcity of precipitation and the intensive land exploration. Meanwhile, the growing global demand for food implies the need to increase crop production through specific agricultural fertilization practices. Therefore, there is growing interest in developing simple methodologies to increase the productivity in such affected areas. Various organic amendments, such as manures, have been investigated for their effectiveness in soil remediation [2]. Increasing soil organic matter content through the addition of organic amendments has proven to be a valuable practice for maintaining or restoring soil quality [3]. Interestingly, Hadas and Portnoy [4] showed that a 20-year compost application period (yearly average compost load 15 t ha⁻¹) led to the increase of humus content by 0.4–0.5%. Comparatively, the above-mentioned parameter decreased by 0.5% in the case of inorganic fertilization. Besides improving crop yield, the utilization of organic amendments could be extended to enhance the productivity of plant rich in active substances used as antioxidants, pharmaceutical, and cosmetic industries. Many spontaneous plants contain significant levels of biologically active compounds that provide health benefits and basic nutrition. In the biomedical area, these plants are highly attractive mainly for their bioactive substances used as antioxidant, antimicrobial, antiviral, and anti-tumor drugs [5].

Application of MSW compost has beneficial impacts on soil fertility and physico-chemical properties [6]. MSW compost utilization may promote nutrient availability, plant growth, stimulate respiration, photosynthesis, and chlorophyll content [2]. Interestingly, the yield of the essential oils and main components of medicinal plant Origanum dictamnus cultivated on soil amended with organic amendment was increased [7]. Recently, Jin et al. [8], reported higher antioxidant capacities, polyphenol and flavonoid contents of strawberries produced from organic culture compared to conventional culture. Hence, one may reasonably hypothesize that MSW compost could be beneficial for Mesembryanthemum edule (M. edule) (Aizoaceae family), an edible plant naturally rich in bioactive substances, commonly found in the semi-arid zones of Tunisia [5]. This species is known for its pharmacological potential, mainly as antiseptic poultices for sores, burns, scalds, and as gargled to treat mouth infections [5]. It can also been taken orally to treat dysentery and tuberculosis, and as a diuretic in South African traditional medicine [9]. It is also worth mentioning that a number of phenolic compounds including rutin, hyperoside, quercitrin and kampferol were isolated from M. edule shoots [5].

However, the agricultural excessive application of composts and/or their low quality can be environmentally hazardous because of the accumulation of micro-pollutants such as heavy metals (Pb, Cd, Cu, …). This aspect must be taken into account to ensure a safe use of this byproduct as a soil conditioner [9].

Interestingly, data related to the impact of the supply of MSW compost on plant phenolic compounds and related risks are relatively scarce. Thus, the main objective of this study was to investigate the impact of MSW compost on the medicinal plant M. edule. Plant growth, mineral nutrition, polyphenol contents (total polyphenol, flavonoid, and condensed tannin), antioxidant activities, and Pb, Cd, Cu, and Zn concentrations were emphasized.