Decomposition of harvest residue needles on peatlands drained for forestry – Implications for nutrient and heavy metal dynamics

Abstract

In the boreal and temperate zones about 15 million hectares of peatlands and wetlands have been drained for forestry purposes and a large number of these forests are now approaching their commercial thinning or regeneration age. One of the major concerns raised in connection with an increased harvesting of drained peatland forests is the deterioration of the downstream water quality due to an enhanced nutrient transport. Harvest residues left on the site are a potential high source of nutrients to recipient water bodies and both increased N and P exports have been reported after conventional stem-only clear-cutting. We studied the decomposition of Picea abies and Pinus sylvestris harvest residue needles at two clear-cut areas and two uncut forested areas on drained peatlands at two locations in southern Finland. Our results indicated that P is easily released from harvest residue needles. After the first three growing seasons, spruce and pine needles had lost approximately 31% and 47% of their initial P contents, respectively. There was no clear gain or loss of nitrogen. Most of the heavy metals accumulated in the needles as the decomposition process proceeded, especially at the more southern study location near the heavily industrialized Helsinki capital area with high atmospheric deposition. The conclusion is that harvest residue needles are not a likely source of the increased N export that has been observed to occur from peat soils soon after clear-cutting, but that P release from harvest residues may be a cause for the reported high P losses.

Introduction

Since the early 20th century, drainage of waterlogged peatlands has been part of the normal forestry practices in, for instance, Fennoscandia, the Baltic countries, the British Isles, and some parts of Russia (Paavilainen and Päivänen, 1995). A large number of these forests are approaching their commercial thinning or regeneration age and the rate of forest harvesting on drained peatlands will undergo a rapid increase in the near future. One of the major concerns raised in connection with an increased harvesting of drained peatland forests is the deterioration of the downstream water quality due to an enhanced nutrient export.

The export of nutrients after forest harvesting is generally higher from peatland dominated catchments than for mineral soil forests (Lundin, 1998, Ahtiainen and Huttunen, 1999, Cummins and Farrell, 2003, Nieminen, 2003, Nieminen, 2004, Rodgers et al., 2010). The reason for higher nitrogen export from peatland dominated catchments may be because the nitrogen reserves in organic soils are much larger. Similarly, the export of phosphorus is more from drained peatlands than for mineral soils, most likely because of the very low phosphate adsorption capacity of most peat soils (Kaila, 1959, Cuttle, 1983, Nieminen and Jarva, 1996).

Harvest residues left on site after conventional stem-only clear-cutting are a potentially high source of nutrients to watercourses (Rodgers et al., 2010). For instance, Norway spruce harvest residues may contain 25–31 kg ha⁻¹ of P and 245–320 kg ha⁻¹ of N (Hyvönen et al., 2000). Rodgers et al. (2010) found that the water extractable phosphorus contents in the soil were significantly higher below harvest residue material than in harvest residue-free areas. In a short-term perspective, the most important source of N and P are the needles and twigs as the larger harvest residue components may release nutrients for more than a few decades (Hyvönen et al., 2000). A litter bag study conducted by Palviainen et al. (2004b) on a mineral soil forest in eastern Finland showed that conifer needles had a potential of releasing about 10 kg of P ha⁻¹ during 3 years.

Decomposition of litter and harvest residues is a complex phenomenon influenced by the activity and nutrient demand of heterotrophic decomposers. The activity is regulated by environmental conditions such as soil temperature, nutrient availability and moisture conditions (Gosz et al., 1973, Prescott, 2005, Laiho, 2006). There are a number of nutrient release studies on conifer needle litter (e.g. Nilsson, 1972, Berg and Staaf, 1980, Rustad and Cronan, 1988, Laskowski and Berg, 1993, Rustad, 1994, Laskowski et al., 1995, Vesterdal, 1999, Lehto et al., 2010) and harvest residues (e.g. Lundmark-Thelin and Johansson, 1997, Palviainen et al., 2004a, Palviainen et al., 2004b) from mineral soil forests, whereas the knowledge on the element release dynamics from litter and especially from harvest residues on drained peatlands is scarce. As the environmental conditions for peat soils are different than for mineral soil sites, the nutrient release pattern from harvest residues and litter may also be different. However, Coulson and Butterfield (1978) and Moore et al. (2005) have reported that there were no consistent differences in mass loss rates of several litter materials between upland and nearby peatland sites. However, in the same study Moore et al. (2005) found that Douglas-fir needles decomposed significantly faster in peatlands than in upland sites.

A common assumption is that decomposition is faster in clear-cuts than in undisturbed forests. However, decomposition rates for clear-cut sites have been reported to be faster, slower and similar compared to uncut control sites, most probably depending on the extent of harvest induced change in soil moisture and regional microclimate (e.g. Yin et al., 1989, Hendrickson et al., 1985, Palviainen et al., 2004b, Palviainen, 2005). The effect of clear-cutting on harvest residue decomposition depends largely on the extent of change in the fungal and microbial activity and the processes influencing their activity (Lundmark-Thelin and Johansson, 1997, Prescott, 2005). On peatlands, clear-cuttings cause a significant rise in the water table level and increase peat temperature (Huttunen et al., 2003).

In this study, we investigated the mass loss and the dynamics of the nutrients and metals in decomposing Norway spruce and Scots pine harvest residue needles on a clear-cut and an un-cut forested peatland of corresponding site types at two locations in southern Finland, at Ruotsinkylä and Vesijako. Although the chemical and physical conditions on peatlands are different than on mineral soils, we expect the general trends in nutrient and heavy metal dynamics to be similar as reported earlier for mineral soil sites. Thus, we expect that P is released from the harvest residue needles in the early phases of decomposition whereas N is mostly immobilized (Palviainen et al., 2004b). The base cations largely present in the needle cell solution, e.g. potassium and magnesium, are also expected to be released rapidly (e.g. Rustad and Cronan, 1988, Laskowski et al., 1995, Palviainen et al., 2004b), whereas Ca, mainly present in needle cell wall structures, is expected to be released slower (Palviainen et al., 2004a). Knowledge of Al and heavy metal dynamics during the decomposing processes of harvest residues on drained peatlands is scarce, but based on earlier studies with needle litter or harvest residue needles on mineral soils (Laskowski and Berg, 1993, Rustad and Cronan, 1988, Laskowski et al., 1995, Palviainen et al., 2004a), the metals should accumulate or immobilize during the first few years of decomposition. The Ruotsinkylä experimental site is situated in a heavily industrialized area near the Helsinki capital region where the heavy metal deposition is significantly higher than in the rural Vesijako area (Poikolainen et al., 2004). It has been shown that in heavily polluted areas, needle litters after losing their protective epiderm and cuticule act like mosses and lichens by absorbing heavy metals from the air (Tyler, 1972). Therefore, we also anticipate that the accumulation of heavy metals during needle decomposition is larger at Ruotsinkylä than in the Vesijako area.