Digestate Improves Stinging Nettle (Urtica dioica) Growth and Fiber Production at a Chlor-Alkali Site

Abstract

Marginal lands have been proposed to produce non-food crop biomass for energy or green materials. For this purpose, the selection, implementation, and growth optimization of plant species on such lands are key elements to investigate to achieve relevant plant yields. Stinging nettle (Urtica dioica) is a herbaceous perennial that grows spontaneously on contaminated lands and was described as suitable to produce fibers for material applications. Two mercury-contaminated soils from industrial wastelands with different properties (grassland soil and sediment landfill) were used in this study to assess the potential growth of stinging nettle in a greenhouse mesocosm experiment. Two organic amendments were studied for their impact on nettle growth. The solid digestate from organic food wastes significantly doubled plant biomass whereas the compost from green wastes had a lower impact. The highest doses of organic amendments significantly increased the number of fibers, which doubled following digestate application, while reducing leaf Hg concentration. Both amendments significantly improved soil respiration and enzymatic activities linked to the microbial biomass in the soil from the sediment landfill by the end of the experiment. In the context of a phytomanagement scenario, solid digestate would be a preferred amendment resource to improve nettle production on industrial wastelands.

Keywords: N fertilization; compost; digestate; enzymatic activities; fiber; marginal lands.

Figure 1

Principal component analysis (PCA) plots showing the ordination of the samples depending on the nature of amendment (C: compost; D: digestate) and rate (+; ++; +++) applied using the whole dataset for (a,b) St-Symphorien-sur-Saône and (c,d) Tavazzano soils. The ellipses represent a 95% confidence interval. Vectors are colored depending on their contribution to the overall distribution and indicate the direction and strength of each environmental variable.

Figure 2

Impact of organic amendments on plant growth parameters (n = 10). (a,d) Aboveground and belowground dry biomass (g ± SE); (b,e) relative growth (% ± SE) of nettle plants during cultivation (days) and (c,f) final plant height (cm ± SE) of Urtica dioica after 83 days of cultivation depending on the nature of the amendment (C: compost; D: digestate) and the rate (+; ++; +++) applied to (a–c) St-Symphorien-sur-Saône and (d–f) Tavazzano soils. Different letters indicate significant differences between treatments and soils for each variable (Kruskal–Wallis, p < 0.05).

Figure 3

Impact of amendments on nettle stem parameters. Mean stem diameter (mm), internode length (cm), and number of internodes (n = 10 ± SE) per stem after 83 days of cultivation depending on the nature of amendment (C: compost; D: digestate) and rate (+; ++; +++) applied to (a) St-Symphorien-sur-Saône and (b) Tavazzano soils. Different letters indicate significant differences between treatments and soils for each variable (Kruskal–Wallis test, p < 0.05).

Figure 4

Impact of amendments on the quantity of fibers in nettle stems. Mean number of fibers from Urtica dioica stems grown on St-Symphorien-sur-Saône soil (n = 3 ± SE) depending on the amendment applied (C: compost; D: digestate). Pictures represent the associated micro-computed transverse cross-sections of the stems obtained with VG StudioMax software (version 2023.1). Fibers were identified in green with MATLAB software (version R2023b). Different letters indicate significant differences between treatments (Kruskal–Wallis test, p < 0.05).

Figure 5

Impact of amendments on rhizospheric soil enzymatic activities. (a,c) Mean (n = 10) enzymatic activities (EA) expressed in nanomoles of 4-MUF·g⁻¹ or AMC·g⁻¹ soil per hour involved in carbon (C), nitrogen (N), phosphate (P), and sulfur (S) cycles and esterases depending on the nature of amendment (C: compost; D: Digestate) and rate (+; ++; +++) applied. * indicates a mean significantly different from that of the control treatment for each soil (Kruskal–Wallis or Tukey’s test, p < 0.05); - indicates a lower mean than that of the control. (b,d) Spearman’s correlation matrix between soil respiration (S.Resp), dsDNA, nitrogen (N) concentration, and enzymatic activities in (a,b) St-Symphorien-sur-Saône and (c,d) Tavazzano rhizospheric soils. Only the significant correlations are represented.

Figure 6

Impact of amendments on soil respiration. Soil respiration (μgC/day/g soil; n = 5 ± SE) after 83 days of cultivation on the (a) St-Symphorien-sur-Saône and (b) Tavazzano soils depending on the nature of amendment (C: compost; D: digestate) and rate (+; ++; +++) applied. Different letters indicate significant differences between treatments for each soil (Kruskal–Wallis test, p < 0.05).

Figure 7

Impact of amendments on leaf phenolic compounds. Mean leaf total phenolic, hydroxycinnamic acid 1 (HCA1), hydroxycinnamic acid 2 (HCA2), and flavonoid concentrations (n = 5, mg·g⁻¹DW ± SE) depending on the nature of amendment (C: compost; D: digestate) and rate (+; ++; +++) applied in (a) St-Symphorien-sur-Saône and (b) Tavazzano soils and their relative quantitative abundances in percentage (chlorogenic acid (CGA), caffeoylmalic acid (CMA) and others are related to HCA1). Different letters indicate significant differences between treatments and soils for total phenolic compounds (Kruskal–Wallis test, p < 0.05). * indicates significant differences from the control for each considered compound and for each soil (Tukey’s test, p < 0.05).