Pyroligneous Acid Maximizes Soil Biological Activity

Introduction

In an era where sustainable agricultural practices are increasingly prioritized, researchers are exploring alternatives to synthetic chemicals that can harm human health and the environment. Pyroligneous acid (PA), also known as wood vinegar or wood distillate, has emerged as a promising natural solution. This yellowish-brown to dark brown liquid is produced during the combustion of woody biomass when gases from the oven are channeled to allow steam condensation.

A team of researchers from the University of Pisa, Italy, led by Roberto Cardelli, conducted a study to evaluate the effects of pyroligneous acid on soil microbial communities and activities. Their research provides valuable insights into the appropriate application rates of this natural substance and its potential benefits for soil health and agricultural sustainability.

Experimental Design and Application

The researchers collected surface soil (0-15 cm) classified as Typic Xerorthent from an agricultural area near Pisa, Italy. The soil had a loamy sand texture with 77% sand, 14% silt, and 9% clay, with a pH of 7.9.

The pyroligneous acid used in the study was produced by RM Impianti srl (Arezzo, Italy) from native forest plant essences through pyrolysis. The main characteristics of the PA included: pH 2.8, total organic carbon 33.8 g/L, total nitrogen 0.43 g/L, organic acid 3.23%, phenolic compounds 13.0 g/L, and methanol 13.4 g/L. Importantly, the PA contained no significant levels of toxic compounds such as polychlorobiphenyls or polycyclic aromatic hydrocarbons.

Five treatments were established to test different concentrations of PA:

  • Control: Water only
  • Low (L): 0.5% PA solution
  • Medium-low (ML): 1% PA solution
  • Medium-high (MH): 2% PA solution
  • High (H): 5% PA solution

The soil samples were watered with these solutions at 60% of the maximum water-holding capacity, which was considered optimal for soil biological activities. The samples were then incubated at 25°C for 10 days before analysis.

Key Findings

Effects on Soil Respiration

The respirometric test revealed fascinating results. Initially, the highest concentration (5%) of PA inhibited microbial activity, indicating a temporary stress on the soil microflora. However, after this initial phase, all PA treatments increased carbon dioxide (CO2) emission compared to the control, with the highest levels observed in the 5% treatment.

Importantly, the researchers determined that this increased CO2 emission was not due to accelerated decomposition of native soil organic matter but rather to the mineralization of the organic carbon added through the PA. This suggests that PA application does not destabilize existing soil carbon.

Effects on Soil Microbial Biomass

The microbial biomass carbon (MB-C) measurements showed that the lowest PA doses (0.5% and 1%) actually increased the microbial biomass content compared to the control. However, higher concentrations (2% and 5%) reduced the microbial population size. This indicates that while low doses of PA can stimulate microbial growth, excessive applications may have inhibitory effects.

The specific respiration of biomass (qCO2), which represents microbial respiration per biomass unit, showed an inverse relationship with the biomass content. The highest qCO2 values were found in the 5% treatment, suggesting that the remaining microorganisms in high-dose treatments were more metabolically active but less efficient in carbon utilization.

Effects on Soil Enzyme Activities

The study examined several soil enzymes involved in nutrient cycling:

  • Dehydrogenase and β-glucosidase: These enzymes showed significantly lower activity in the 2% and 5% PA treatments compared to the control and lower doses. This suggests that higher concentrations of PA inhibited these enzymes, likely due to the toxic effects of phenolic compounds and organic acids.
  • Alkaline phosphatase: Interestingly, this enzyme's activity was not affected by PA application at any concentration, indicating that phosphorus cycling remained unaltered.
  • Arylsulfatase: Similar to dehydrogenase and β-glucosidase, this enzyme showed decreased activity at higher PA concentrations.

The researchers calculated a geometric mean (GMea) of the enzyme activities as an overall indicator of soil quality. The GMea was significantly lower in the 2% and 5% PA treatments, confirming that high doses of PA can reduce soil biological quality.

The hydrolysis rate of fluorescein diacetate (FDA), an indicator of overall microbial activity, decreased notably only in the 5% treatment. Catalase activity remained unaffected across all treatments, while urease activity increased significantly in the 5% treatment.

Implications

This research offers valuable guidance for the agricultural use of pyroligneous acid. The study demonstrates that PA can enhance soil biological activity when applied at appropriate concentrations. Specifically, doses up to 1% showed no negative effects on soil biology and even improved some parameters.

However, higher application rates (2% and 5%) resulted in decreased enzyme activities and reduced soil quality. The manufacturer's recommended application rate of 0.5% aligns well with the findings of this study, although the researchers suggest that concentrations up to 1% could also be beneficial.

This study contributes to the growing body of evidence supporting the use of PA as an environmentally friendly alternative in sustainable agriculture. Further research is needed to investigate the application of PA under field conditions and its effects on different soil types and crops.

Article based on: Cardelli, R., Becagli, M., Marchini, F., & Saviozzi, A. (2020). Soil biochemical activities after the application of pyroligneous acid to soil. Soil Research.