Temporal trends of carbon stocks in agricultural soils

Background and Objective

The amount of  soil organic matter (SOM) positively affects nutrient retention and storage as well as the water-holding capacity in soil and supports a good soil structure, which in turn may reduce soil erosion. Hence, SOM is highly relevant in terms of sustainable agricultural use of soils. Depending on site and management, soils can act as a carbon (C) source or sink and thereby influence the release of greenhouse gases, especially CO2. It is assumed that long-term croplands are in a steady state, i.e. they are neither C source nor sink. Evaluation of a data set of the long-term soil monitoring sites in Lower Saxony (years 1994-2014) within a Master Thesis revealed an average of significantly decreasing C contents in croplands (only mineral soils), yet with large differences between soil types. Nonetheless, these results show that the currently assumed steady state situation for croplands in Germany may not be applicable for all mineral soils and soil sites.

In this project, data sets from grasslands and croplands of the long-term soil monitoring sites of the federal states are collectively evaluated. Reasons for the observed temporal trends of the C stocks will be evaluated based on analyses of temporally resolved yield and management data as well as further explaining variables.

Aims of the project are

• Analysis and evaluation of the development of organic carbon stocks in the soil monitoring sites under long-term cropland or grassland
• Analysis and assessment of driving variables for the temporal trends of organic C in agricultural soils. Which sites show changes in C stocks and which do not?
• Acquisition and evaluation of uncertainties resulting from repeated soil sampling via different staff on long-term monitored soils. What are potential sources of error and how can these be minimized?
• Development of a profile of requirements and a strategy for the evolution of data harmonization on long-term monitored soil sites.

Approach

1. Inventory and evaluation of temporal trends of organic C stocks in tthe agriculturally used soils of the long-term monitoring sites in Germany.

The representativity of the existing data will be evaluated in order to assess trends and reasons for the temporal development of soil C contents. The nationally consistent evaluation of the data takes place with agreement of the UBA and in consultation with the federal states.

2. Analysis of driving variables of temproral trends of organic C in order to gain better understanding of regional differences.

Using the evaluated data set, regional differences of C contents and their development will be worked out and evaluated with regard to climate and management factors. How do climate, groundwater level and site management influence temporal trends of soil C stocks?

3. Development of a strategy for the repeated sampling of the sites investigated in the Agriculatural Soil Inventory based  on existing data with repeated samplings from the long-term soil monitoring sites.

Based on existing data from the long-term soil monitoring sites of the different federal states, the variability within a collected data set will be evaluated and the minimum detectable difference (MDD) will be determined. Using the knowledge gained here, a strategy for the planned repeated sampling of the Agricultural Soil Inventory sites will be developed.