Effects of litter and straw mulch amendments on compacted soil properties and Caucasian alder (Alnus subcordata) growth

Application of mulch on compacted soil is a common engineering measure to suppress runoff and soil loss during ground-based mechanized forest operations. Despite the expanded use, efficacy of these rehabilitation treatments on soil quality as well as seedling survival and growth rate are crucial issues that require further attention. This study aimed to assess the efficacy of two soil amendment techniques including straw (1.27 kg m⁻²) and litter (1.67 kg m⁻²) mulch on soil properties and growth properties of Caucasian alder. Three treatments: straw mulch (SM), litter mulch (LM) and untreated trail (U) were applied on newly established machine operating trails subjected to low, medium, and high machine traffic intensity classes and compared to the undisturbed area (i.e., control; UND). Approximately 22 months after ground-based machine traffic and mulching application, seeds of Caucasian alder were sown in each treatment and in the undisturbed area and further extracted after the first growing season to assess their growth rates. Three years after applying mulch on the compacted soil, recovery values of soil physical and chemical properties (with the exception of soil C/N ratio) in all traffic intensities were significantly higher in LM than SM, compared to U treatment. Nevertheless, recovery values of soil physical and chemical properties were still higher than values in UND area over a 3-year period. All the measured growth and biomass responses of Caucasian alder seedlings changed significantly with increasing traffic intensities (all P ≤ 0.05). Significantly higher seed germination percentage was recorded in LM with low traffic intensity (54%). Significantly higher seedling stem height (17.3 cm), main root length (23.6 cm), seedling total (7.68 g), shoot (2.87 g), and root dry (4.81 g) biomass were observed in the UND area followed by LM with low traffic ≈ LM with medium traffic treatments, whereas the lowest values of seedling size and biomass were detected on U with low, medium, and high traffic intensities. Seedling root mass ratio (RMR; 61%) and root to shoot ratio (R/S; 1.89) were greatest in the UND area, while the RMR (40%) and R/S (0.86) were least in the U plots with high traffic intensity. We can conclude that the highest seedling growth, biomass, and allocation ratios and favorable soil physical, chemical, and bio-chemical properties can be attributed to UND, LM with low and medium traffic, and SM with low traffic treatments.