Forests are one of the most important ecosystems on earth that not only provide essential services such as climate control, habitat protection, erosion prevention, nutrient cycle, and biodiversity protection, but also maintain an important economic resource for many countries (Hume et al. 2018; Picchio et al. 2021). Based on the sustainable management of forests, there is an increasing demand for forest products, resulting in high environmental pressures towards forests (Acharya et al. 2019; Labelle et al. 2022).
Sustainable management is essential for improved protection and development of forests. In this regard, understanding soil properties is one of the principles of basic forest management (Chen et al. 2005). Ground-based mechanized operations performed by skidding machines on skid trails can create severe damage to soils and decline forest productivity (Jourgholami et al. 2018; DeArmond et al. 2020). Soil compaction and degradation following skidding operations by heavy machinery can, beyond changes to soil physical properties (Han et al. 2006), lead to significant changes in soil chemical and biological properties such as less gas exchange, reduced water penetration, reduced root and tree growth, reduced decomposition rates, alteration and disruption of the food cycle, resulting in an adverse effect on soil microbes (Crawford et al. 2021).
The use of organic and inorganic mulch (i.e., foliage, sawdust, and protective mats) and limiting the number of machine passes on skid trails as remedial treatments can mitigate soil disturbance in the short term (Han et al. 2009). However, under natural conditions, it is important to consider the type of forest stand and litter as an ecological solution to reduce the negative effects and accelerate the recovery time of soil properties after skidding operations (Jourgholami et al. 2019). In addition to the protective role of the soil surface (Li et al. 2014), forest floor litter contributes to the nutrient flow and carbon cycle (Kooch et al. 2018). In turn, high-quality litter plays a key role in stimulating soil organic carbon decomposition and the activity of soil microorganisms and microorganisms (Fang et al. 2011; Jourgholami et al. 2018).
In forest ecosystems, soil microbes govern the decomposition of organic matter, the food cycle, and the availability of plant-absorbable nutrients (Plante 2007). In the soil ambiance, the availability of usable carbon substrate is the most important factor limiting microbial activity, which results in an increase of the microbial population around the substrate once litter has been added to the soil (Kooch et al. 2020). In fact, soil microbial population is responsible for regulating the nutrient cycle in the soil layer and providing nutrients to the plant and thus, directly impacts plant growth and biomass production (Chen et al. 2007).
Trees can alter soil properties by changing the litter quantity and quality, organic matter, carbon to nitrogen ratio, moisture and acidity. These changes have a multiple effect on the abundance and diversity of earthworms, the activity of microbial communities, enzyme activities as well as nitrogen mineralization (Gei and Powers 2013; Tian et al. 2015). Meanwhile, soil microbial biomass carbon and nitrogen are the main components of a soil ecosystem (Burton et al. 2010) and are strongly influenced by the litter of forest stands. These components dictate many ecological processes such as the carbon and nutrient cycle, nitrogen mineralization, litter decomposition, and soil productivity (Gei and Powers 2013).
Furthermore, tree species with different litter quality contribute to the above-ground and underground environments, which in turn influences soil respiration. According to Thoms and Gleixner (2013), soil microbial respiration rate under beech, linden, maple, ash, and hornbeam litter is highly different and the highest value is observed in the stand of linden and maple species and the lowest value in beech stands. Soil enzyme activities can be a suitable indicator to quantify and monitor changes in the structure and activity of microbial communities as well as the dynamics of soil organic matter in response to forest harvesting operations (Trasar-Cepeda et al. 2008). Soil enzyme activity is often involved in the decomposition and synthesis of soil organic matter, cycle and nutrient availability, as well as soil fertility and quality (Wang et al. 2012; Moghimian et al. 2017). Soil enzyme activity is significantly associated with microbial biomass, which is also very important for soil structure formation (Ludwig et al. 2015). Research has shown that vegetation regeneration imports considerable amounts of nutrients into the soil environment, which in turn may increase soil microbial respiration and stimulate enzyme activity (Cui et al. 2019). Likewise, Moghimian et al. (2017) showed that soil enzyme activity is dependent on changes in microbial activity such as the content of nitrogen microbial biomass, because most enzyme activity occurs at the highest level of nitrogen microbial biomass.
So far, several studies have been conducted to determine the effect of land use change, forest harvesting operations and vegetation composition (litter) on physical, chemical, biological, microbial and enzyme properties of soil with conflicting results (Ponder and Tadros 2002; Jordan et al. 2003; Tan et al. 2008; Jourgholami et al. 2018; Kooch et al. 2020; Nazari et al. 2021). Studies have shown that soil compaction has negative effects on physical properties such as total porosity, pore size distribution, as well as air and water conductivity, which leads to a decrease in the soil carbon microbial biomass content (Tan et al. 2008). However, in other studies, no significant effect of soil compaction was observed on the carbon microbial content (Ponder and Tadros 2002; Jordan et al. 2003). These contentious results may be due to the unclear definition of factors affecting soil compaction during and after skidding operations, considering the short time interval between compaction event and sampling to analyze soil characteristics despite the long-term effects of soil compaction (Nazari et al. 2021).
Therefore, the present study aimed to test the effectiveness of adding litter of different tree species (leaf fall during natural processes) as a natural and ecological method to improve soil microbial and enzyme properties of soils on skid trails exposed to different traffic intensities compared to the undisturbed area over a 20-year period after skidding operations. The hypotheses of the present study were: (1) addition of litter on the compaction-induced soil can restore soil physio-chemical, microbial and enzyme properties in skid trails compared to the undisturbed area, (2) litter of different trees can have significant difference in recovery level of soil properties.