Stocks of organic carbon in German agricultural soils—Key results of the first comprehensive inventory

Abstract

Background: There is considerable uncertainty about the actual size of the global soil organic carbon (SOC) pool and its spatial distribution due to insufficient and heterogeneous data coverage.Aims: We aimed to assess the size of the German agricultural SOC stock and develop a stratification approach that could be used in national greenhouse gas reporting.Methods: Soils from a total of 3104 sites, comprising 2234 croplands, 820 permanent grasslands and 50 sites with permanent crops (vineyards, orchards) were sampled in a grid of 8 × 8 km to a depth of 100 cm in fixed depth increments. In addition, a decade of management data was recorded in a questionnaire completed by farmers. Two different approaches were used to stratify cropland and grassland mineral soils and derive homogeneous groups: stratification via soil type (pedogenesis) and via SOC‐relevant soil properties.Results: A total of 146 soils were identified as organic soils, which stored by far the highest average SOC stock of 528 ± 201 Mg ha−1 in 0–100 cm depth. Of the mineral soils, croplands and permanent crops stored on average 61 ± 25 and 62 ± 25 Mg ha−1 in 0–30 cm (topsoil) and 35 ± 30 and 44 ± 28 Mg ha−1 in 30–100 cm (subsoil), while permanent grasslands stored significantly more SOC (88 ± 32 and 47 ± 50 Mg ha−1 in topsoil and subsoil). Overall, topsoils stored 67 ± 14% and subsoils 33 ± 14% of total SOC stocks. Soil C:N ratio, clay content and groundwater level were major factors that explained the spatial variability of SOC stocks in mineral soils. Accordingly, Podzols, Gleysols and Vertisols were found to have the highest SOC stocks.Conclusions: Stratification via soil properties yielded the most comparable cropland and grassland strata and is thus preferable for estimating land‐use change effects, e.g., for greenhouse gas inventories.In total, 2.5 Pg C are stored in the upper 100 cm of German agricultural soils, making them the largest organic carbon pool in terrestrial ecosystems of Germany. This bares a large responsibility for the agricultural sector and society as a whole to maintain and, if possible, enhance this pool.

Plot‐scale variability of organic carbon in temperate agricultural soils—Implications for soil monitoring#

“…The aim of this study was to develop a resampling design for the German Agricultural Soil Inventory. One of the specific features of this inventory was that a profile pit was dug and described in detail to a depth of 1 m. Soils were not only characterised by their soil physical and chemical properties, but a wealth of pedological data was also evaluated from the soil profile (Poeplau et al., 2020). In addition to the soil profile, eight soil cores in a circle of 20 m diameter around the profile pit were taken.…”

“…The first German Agricultural Soil Inventory was completed in 2018. At a total of 3104 sites comprising croplands, grasslands and permanent crops, one soil profile pit was opened and sampled at fixed depth increments to a depth of 100 cm (Poeplau et al., 2020). The aim of the present study was to assess the plot‐scale variability of cropland and grassland soils of differing properties to develop a resampling strategy for the abovementioned national inventory.…”

Achievable agricultural soil carbon sequestration across Europe from country‐specific estimates

“…The EA was taken from the national GHG inventory reports of each country submitted in 2020 (National Inventory Submissions 2020|UNFCCC). 2014) except for Switzerland (Leifeld et al, 2005) and Bavaria in Germany (Wiesmeier et al, 2020) and Baden Württemberg (Poeplau et al, 2020). Last column also indicates the language of the source.…”

Inventory and assessment of pH in cropland and grassland soils in Germany#

“…Soils were supposed to contain carbonates when the topsoil pH was considerably higher than the subsoil pH and when the soil pH was >6.2. TOC was determined by combustion at 550 • C and IC was analysed by subsequent combustion at 1000 • C with LECO RC612 (Jacobs et al, 2018;Poeplau et al, 2020). The factor of 1.72 was used to convert TOC to soil organic matter (SOM) (Eckelmann et al, 2006).…”

“…Based on the first national inventory of agricultural soils in Germany (Jacobs et al., 2018; Poeplau et al., 2020), this study analysed the current pH status of grassland and cropland soils in Germany. The analysis was based on an 8 × 8 km soil‐sampling grid across Germany.…”