Soil organic matter mobilization by re-compaction of old forest skid trails

https://doi.org/10.1016/j.ejsobi.2020.103173Get rights and content

Highlights

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    Re-compaction increased microbial biomass.

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    Re-compaction decreased SOC/total N and increased MBC/SOC ratios.

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    Re-compaction mobilized the extractable SOC fraction.

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    A part must be mineralizable directly after re-compaction.

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    At soil sampling, basal respiration was affected by re-compaction.

Abstract

Wood harvesting is restricted to a system of permanent skid trails in order to minimize the soil disturbance and damage. Therefore, it is not the compaction of previously undisturbed forest soil, but the re-compaction of already existing skid trails that is of practical relevance when investigating machinery-induced wood harvest effects on soil properties. This study investigated the effects of machinery-induced re-compaction on soil physical, chemical, and microbiological properties of an old skid trail in a spruce forest by wheeling 8 times with a maximum total load of 32.2 Mg, using a genuine old skid trail as a control. Re-compaction significantly increased the bulk density and reduced the porosity, whereas the soil organic carbon (SOC) and total nitrogen (N) contents were not significantly affected. However, re-compaction reduced the SOC/total N ratio by 10%, suggesting considerable SOC mineralization after re-compaction. K2SO4 extractable C contents were increased by 94% at 0–3 cm and 67% at 7–10 cm depth after re-compaction. This led to 20% and 90% increased microbial biomass C/soil organic C (MBC/SOC) ratios at 0–3 cm and at 7–10 cm depth, respectively. In contrast, the ergosterol/MBC ratio was significantly decreased by 10% at 0–3 cm and by about 30% at 7–10 cm depth by re-compaction, apparently due to the promotion of bacteria and mobilization of soil organic matter.

Introduction

Forests are important economic resources for many countries and there is an increasing demand for forest products, resulting in high environmental pressures. At the same time, there is a consensus that forests must be used sustainably. In Germany, for instance, the forest production area lost to skid trails ranges between 10 and 20%, adding up to a total of 1.5–2 million ha [1]. Nowadays, different machines like skidders, forwarders, and tractors are used for forest management. The weight of these forestry vehicles has been increasing constantly over the past three decades up to 45 Mg [2]. This affects soil mechanical stability and soil functions, such as carbon (C) storage, filter, and habitat function, in and next to skid trails [3]. This is especially a problem in heavily compacted soils with a silt loam texture and acidic soil pH, which will not regenerate in foreseeable time.

Soil compaction is one of the undesired consequences of using heavy equipment on forest sites, reducing tree growth [1,3] and most likely also affecting soil microbial properties to a largely unknown extent. Compaction has been found to reduce microbial biomass C (MBC) contents in the forest soil [4,5], which has been associated with negative changes in physical soil properties such as total porosity, pore size distribution as well as air and water conductivity [4]. In other studies, no significant impact of soil compaction on MBC contents was observed [6,7].

One reason for these conflicting results might be that the compaction events were not clearly defined, i.e. the number and weight of machine crossings were not exactly known. Also, the timespan between compaction event and sampling for soil microbiological analysis is often too short to be reflected in an MBC decline, as compaction usually does not kill soil microorganisms directly. Moreover, the selection of an appropriate control may introduce a bias into the interpretation of results, for example, the comparison of newly compacted and non-compacted soil differs from the comparison of a re-compacted old skid trail with a genuine old skid trail before re-compaction. In Germany and other European countries, wood harvesting is restricted to a system of permanent skid trails in order to minimize the soil disturbance and damage. Therefore, it is not the compaction of previously undisturbed forest soil, but the re-compaction of already existing skid trails that is of practical relevance when investigating machinery-induced wood harvest effects on soil properties. For this reason, a genuine old skid trail was used as a control for the current re-compaction treatment, unlike previous studies.

Different microorganisms may respond differently to compaction, for example, bacteria seem to be less sensitive than fungi [7]. This means that results based on phospholipid fatty acid [7,8] and DNA extraction [9], which both have a clear focus on bacteria, may not fully reflect compaction damage to soil fungi. The fungal cell-membrane component ergosterol might respond more sensitive to compaction stress. Ergosterol is the most specific biomass marker for Ascomycota and Basidiomycota in solid substrates, i.e. in fungal phyla dominating acidic forest soils [10].

The study site was located on an acidic silt loam under a spruce (Picea abies) stand in Central Germany. Skid trail re-compaction effects on soil physical, chemical, and microbiological properties were determined approximately 10 months after wheeling with a defined passing number and machine load. The underlying hypothesis was that re-compaction reduces microbial activity and biomass, specifically fungal biomass by lowering C availability and O2 supply.

Section snippets

Site, treatments, and sampling

The study site was a pure spruce stand (Picea abies L.) located near Holzerode (N 51°36′13 E 10°04‘37, Lower Saxony, Germany). The soil is classified as Gleyic Luvisol [11] with a silt loam texture. The field experiment was performed on an old skid trail in November 2016. The age of the skid trail was assessed to be approximately 40–50 years. Before the wheeling experiment, only skidders with a maximum wheel load of 3.9 Mg had been used for logging operations with an unknown frequency on the

Results

Re-compaction significantly increased the bulk density and reduced the porosity at both depths in comparison with the genuine old skid trail (Table 1). The soil pH was significantly lower at 0–3 cm than at 7–10 cm depth. Re-compaction led to a slight but significant lower soil pH at the re-compacted skid trail, leading to a significant compaction × depth interaction. Re-compaction did not affect the SOC and total N contents but led to a 10% reduction in the SOC/total N ratio. The SOC and total

Discussion

A striking feature of the current results is the increased MBC/SOC ratio in the re-compaction treatment. A small increase in the MBC/SOC ratio has already been observed, but not explicitly noted and explained [8]. The MBC/SOC ratio is an indicator for substrate availability to soil microorganisms [15], which is extraordinarily low in the current skid trails, even considering that the MBC/SOC ratios are generally low in acidic soils [15,16]. The current results suggest a strong mobilization of

Conclusions

In contrast to our hypothesis, re-compacting of an old skid trail significantly increased the MBC/SOC ratio, an indicator of C availability for soil microorganisms, in comparison with the genuine old skid trail at 0–3 cm and 7–10 cm depth. This increase in C availability by unknown SOC mobilization processes was accompanied by a strong increase in the K2SO4 extractable C fraction by about 80% at both depths. The absence of any effects on the basal respiration indicates that the actual K2SO4

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank the German Federal Ministry of Food and Agriculture (BMEL) for funding the “Bodmech” project with the funding code 22028715. We are grateful to Gabriele Dormann, Markus Hammer-Weis, Margit Rode and Pouria Sadeghianfar for providing skillful technical assistance.

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