Close to one third of Germany is forested. Forests are able to store significant quantities of carbon (C) in the biomass and in the soil. Coordinated by the Thünen Institute, the German National Forest Inventory (NFI) and the National Forest Soil Inventory (NFSI) have generated data to estimate the carbon storage capacity of forests. The second NFI started in 2002 and had been repeated in 2012. The reporting time for the NFSI was 1990 to 2006. Living forest biomass, deadwood, litter and soils up to a depth of 90 cm have stored 2500 t of carbon within the reporting time. Over all 224 t C ha^-1 in aboveground and belowground biomass, deadwood and soil are stored in forests. Specifically, 46% stored in above-ground and below-ground biomass, 1% in dead wood and 53% in the organic layer together with soil up to 90 cm. Carbon stocks in mineral soils up to 30 cm mineral soil increase about 0.4 t C ha^-1 yr^-1 stocks between the inventories while the carbon pool in the organic layers declined slightly. In the living biomass carbon stocks increased about 1.0 t C ha^-1 yr^-1. In Germany, approximately 58 mill. tonnes of CO₂ were sequestered in 2012 (NIR 2017).

The phosphorus nutrition status of European forests has decreased significantly in recent decades. For a deeper understanding of complementarity and competition in terms of P acquisition in temperate forests, we have analyzed α-diversity, organic layer and mineral soil P, P nutrition status, and different concepts of P use efficiency (PUE) in Fagus sylvatica L. (European beech) and Picea abies (L.) H. Karst. (Norway spruce). Using a subset of the Second National Soil Survey in Germany, we correlated available data on P in the organic layer and soil with α-diversity indices for beech and spruce forests overall and for individual vegetation layers (tree, shrub, herb, and moss layers). Moreover, we investigated α-diversity feedbacks on P nutrition status and PUE of both tree species. The overall diversity of both forest ecosystems was largely positively related to P content in the organic layer and soil, but there were differences among the vegetation layers. Diversity in the tree layer of both forest ecosystems was negatively related to the organic layer and soil P. By contrast, shrub diversity showed no correlation to P, while herb layer diversity was negatively related to P in the organic layer but positively to P in soil. A higher tree layer diversity was slightly related to increased P recycling efficiency (P_Plant/P_{organic layer}) in European beech and P uptake efficiency (P_Plant/P_soil) in Norway spruce. The diversity in the herb layer was negatively related to P recycling and uptake efficiency in European beech and slightly related to P uptake efficiency in Norway spruce. In spruce forests, overall and herb species richness led to significantly improved tree nutrition status. Our results confirm significant, non-universal relationships between P and diversity in temperate forests with variations among forest ecosystems, vegetation layers, and P in the organic layer or soil. In particular, tree species diversity may enhance complementarity and hence also P nutrition of dominant forest trees through higher PUE, whereas moss and herb layers seemed to show competitive relationships among each other in nutrient cycling.

The present report is based on the National Forest Soil Inventory (NFSI) as well as on case studies from Intensive Forest Monitoring sites in Germany. The current as well as the temporal development of forest soil conditions and the major threats to forest ecosystems are discussed. Following the discussion on forest decline in 1980s, a national forest monitoring was established, consisting of the Forest Condition Survey, the Intensive Forest Monitoring as well as the NFSI. In 1984, the sites for the Forest Condition Survey were installed on a 16 × 16 km grid which is part of the UN/ECE ICP Forests Level I Monitoring as well as of the European Soil Monitoring Project (BioSoil). The NFSI plots were subsequently established based on the same grid with an 8 × 8 km raster yielding at about 1800 sampling sites. The first NFSI took place between 1987 and 1992. In 2006, the second NFSI was conducted simultaneously with the BioSoil programme. The NFSI focused on specific questions for forest soils in respect to nitrogen levels and sensitivity to inputs of nitrogen; to carbon storage and carbon stock changes; to background loading with heavy metals and organic trace elements; to the interplay of soil characteristics and forest nutrition, crown condition and vegetation; to the risk due to negative changes such as nutrient removal or soil acidification; and to effects in terms of soil chemistry and nutritional status measures to stabilize forest ecosystems. To fulfil these objectives, the NFSI included the sampling of chemical and physical soil parameters, the determination of foliar element concentrations, the assessment of the composition and diversity of understory vegetation as well as an inventory of the tree stands including dead wood. A comprehensive concept of quality control and quality assurance for field assessments, laboratories, data management and evaluations has been established to ensure data harmonization and a high quality of statistical evaluations.

Key message: A Soil Monitoring Law to improve soil health across all land uses has been proposed by the European Commission. As forests soils have different chemical and physical properties as well as biogeochemical dynamics compared to agricultural land, they also face different challenges in maintaining and restoring soil health. Examples are soil acidification, eutrophication by atmospheric deposition, responses to climate change, and loss of biodiversity. Therefore, we propose forest soil specific health descriptors and thresholds based on experience and knowledge from existing long-term monitoring programs.