A synthesis of the impacts of ditch network maintenance on the quantity and quality of runoff from drained boreal peatland forests

Abstract

Drained peatlands are an important source of forest biomass in boreal regions and ditch network maintenance (DNM) operations may be needed to restore the drainage functions of ditches. By reviewing the available literature, as well as utilizing an existing hydrological model and analyzing the characteristics of eroded sediments, we assessed the impacts of DNM on runoff and exports of suspended solids (SS), dissolved organic carbon (DOC), nitrogen (N), and phosphorus (P). In general, DNM had minor impact on runoff and dissolved N and P, and it decreased rather than increased DOC exports. To increase the understanding of the hydrochemical impacts of DNM, future research should focus on the characteristics of SS and particulate nutrient exports. A major gap in knowledge is also the very limited regional representativeness of the available studies. High erosion risk in the ditches reaching the mineral soil below peat should be acknowledged when planning mitigation measures.

Keywords: DOC; Nitrogen; Phosphorus; Runoff; Suspended solids; Water quality.

Fig. 1

Annual runoff scenarios for drained Sphagnum (SP) and Carex (CP) peat-dominated peatland areas with varying tree stand stem volumes during the first year after DNM as simulated by the FEMMA model (Koivusalo et al. 2008). The weather input data are as in 2009 in southern (60°27′N; 24°57′E) and northern (64°43′N; 26°0′E) Finland. Pre-DNM and post-DNM ditch depths are 25 and 100 cm, respectively, and ditch spacing is 40 m. The bulk density and hydraulic conductivity are 0.075 g cm⁻³ and 2.12 cm h⁻¹ in SP and 0.156 g cm⁻³ and 0.58 cm h⁻¹ in CP, respectively

Fig. 2

Mean concentrations of suspended solids ± SE (mg l⁻¹) in ditch outflow water from ditch networks with varying soil characteristics 1–2 years before (year 0) and up to 20 years after DNM. An asterisk indicates statistically (p < 0.05) significant difference from the pre-DNM period according to Tukey’s HSD test. Source: Joensuu (2013). Classification of mineral soils in the ditch profiles into different texture classes as in Tamminen and Mälkönen (1999). Fine, medium, and coarse sizes of mineral soil particles refer to the grain sizes of < 0.063 mm, 0.063–0.630 mm, and > 0.630 mm, respectively

Fig. 3

DOC (Joensuu et al. ; Nieminen et al. 2010) and TOC (Åström et al. 2001a, b) concentrations (mg l⁻¹) in ditch outflow from forestry-drained peatlands 1–3 years before (year 0) and after DNM in the different data sets from Finland. Source: Palviainen and Finér (2013)

Fig. 4

Mean concentrations of DOC ± SE (mg l⁻¹) in ditch outflow from drained peatlands with differing post-DNM ditch soil characteristics 1–2 years before (year 0) and up to 20 years after DNM. For further explanation, see Fig. 2

Fig. 5

Mean concentrations of total dissolved N (TDN), total dissolved P (TDP), dissolved ammonium-nitrogen (NH₄-N), and dissolved nitrate-nitrogen (NO₃-N) 1–2 years before (year 0) and up to > 10 years after DNM in 40 catchment areas. Values marked with the same letter are not statistically different (p < 0.05) according to Tukey’s HSD test. Redrawn from Joensuu et al. (2006)

Fig. 6

Relationship between total C (%) and N (%) contents, and C (%) and P (%) contents in the deposited sediments collected from 12 sedimentation ponds in drained peatland forests in Finland