Carbon and nitrogen stocks in Norwegian forest soils — the importance of soil formation, climate, and vegetation type for organic matter accumulation

Abstract

Relationships between soil C and N stocks and soil formation, climate, and vegetation were investigated in a gridded database connected to the National Forest Inventory in Norway. For mineral soil orders, C and N stocks were estimated to be 11.1–19.3 kg C·m−2 and 0.41–0.78 kg N·m−2, respectively, declining in the following order: Gleysols > Podzols > Brunisols > Regosols. Organic peat-type soils stored, on average, 31.3 kg C·m−2 and 1.10 kg N·m−2, whereas shallow Organic folisols stored, on average, 10.2 kg C·m−2 and 0.34 kg N·m−2. For Norway’s 120 000 km2 of forest, the total of soil C stocks was estimated to be 1.83 Gt C, with a 95% CI of 1.71–1.95 Gt C. Podzolic soils comprise the largest soil group and store approximately 50% of the forest soil C. Sixty percent of the soil C stock in Podzolic soils was stored in the mineral soil, increasing with temperature and precipitation. Poorly drained soil types store approximately 47% of the total forest soil C in Norway. Soils with water saturation have large C stocks mainly in the forest floor, suggesting that they are more susceptible to forest management and environmental change. Soil C stocks under pine and spruce forests were similar, although pine forests had larger C stocks in the forest floor, while spruce forests had the highest C stocks in the mineral soil compartment. C stocks in the forest floor increase from dry to moist ground vegetation, while ground vegetation nutrient classes reflect better the C and N stocks in the mineral soil.

Underestimation of boreal forest soil carbon stocks related to soil classification and drainage

“…Due to the distribution of the forest area, most soil data are located in the southeastern parts of the country compared with the central mountain regions, the north, and along the west coast. Soil profiles were systematically placed and described according to standard procedures, and soil samples were collected by soil horizon in the soil profile, as well as in auger samples from across the plot (see details on soil sampling and chemical analyses elsewhere in this issue; Strand et al 2016). Registrations included general area data, topography, landform, parent material, and drainage.…”

“…We applied the C stock calculation approach as described in Strand et al (2016). The C-to-N ratio (C:N) was calculated for the organic horizons or, if missing, the surface A horizon only.…”

“…Bulk density was estimated from a pedotransfer function (Baritz et al 2010) or from separate data (de Wit and Kvindesland 1999;Strand et al 2016). Fifteen percent of the profiles had no information on the exact thickness of the deepest horizon; for these horizons, we set a thickness of 0.2 m, less if this caused the total profile depth to exceed 1 m. For mineral soil horizons missing information on stone (6% of horizons) and gravel (15% of horizons) contents, we estimated these fractions by using either the average of the above and below horizons or the median of these fractions from soils of the same parent material (Strand et al 2016). Error estimates in Strand et al (2016) suggest that the largest errors were found for the hydromorphic soils (1.0-6.2 kg C·m −2 ), less for the self-drained mineral soils (0.7-3.2 kg C·m −2 ).…”

“…Using the earlier version of this model , it was suggested that for regions with high precipitation (P) and (or) acid bedrock, the simulated soil C stocks of only 40%-50% of observed stocks as found in regions of Switzerland (Thürig et al 2005) and in Norway (de Wit et al 2006) were due to the lack of representation of the effects of water saturation on soil aeration and the transport and stabilization of organic matter in the soil pedon over time. Norwegian mineral forest soils contain, on average, 13-14 kg C·m −2 to the depth of the soil profile (de Wit and Kvindesland 1999;Strand et al 2016), nearly twice the amount found in boreal forest mineral soils of Sweden and Finland (Rantakari et al 2012;Stendahl et al 2010). In Scandinavia, soil C stocks are found to increase with mean annual precipitation (MAP) and mean annual temperature (MAT) for well-drained soils (Callesen et al 2003) and with increasingly poor drainage (Olsson et al 2009).…”

“…In this study, we evaluate the Yasso07 model's climate dependency in relatively extreme climatic situations with the aim to describe the limits of applicability in climate and drainage gradients. The steep and irregular topography found in Norway combined with many shallow soils and a wet and cool climate promotes a high frequency of hydromorphic soils (32% based on soil survey; Strand et al 2016). This, in addition to a predominant soil process of podsolization on acid soils in regions of high precipitation, will challenge a soil model in which effects of anoxic conditions and transport and stabilization of organic matter on the accumulation of C are not specifically addressed.…”