What are the sources of soil fertility

Our most important carbon store: How the soil, as the thin skin of the earth, influences global material cycles and the climate

Research Report 2011 - Max Planck Institute for Biogeochemistry

Shrink, Marion; Trumbore, Susan
Department of Biogeochemical Processes
Soils are the largest terrestrial reservoir (“sink”) for carbon and at the same time one of the most important natural sources of CO2 in the atmosphere. This means that soil organic matter is not only important for soil fertility, but also as a transshipment point for greenhouse gases for climate change. The Max Planck Institute for Biogeochemistry is investigating how sensitively the carbon fluxes in the soil react to environmental changes and how the interrelationships between vegetation, climate, soil organisms and soil properties affect carbon storage.

Importance of carbon in the soil

Organic matter in the soil consists of about half of carbon and is an important characteristic of soil fertility. Soils with a high content of organic matter can store more nutrients and water and give them to plants than soils with less organic matter. Their better soil structure ensures less leaching of nutrients and pollutants into the groundwater. Soil organic matter has long been an important research focus for the development of sustainable strategies in agriculture. Since organic decomposition processes in the soil are one of the most important natural sources of CO2 the role of soils in global carbon cycles has been intensively researched for around two decades. Because about 10 times the amount of CO escapes annually2 is released from soils into the atmosphere than when burning fossil fuels. This amount is subject to strong natural fluctuations, but is also influenced by land use and environmental changes. A much discussed question is currently whether global warming accelerates degradation processes in the soil and, as a result, an increased release of CO2 comes from the soil and thus leads to a positive feedback on the climate. In order to be able to predict how soils will react to changes in land use or environmental changes such as climate change, the processes that lead to the storage or mobilization of carbon in the soil must be understood and quantified.

Why is there so much carbon in the soil?

Plants are the most important source of soil carbon. Dead plant parts get into the soil above and below ground and are broken down and converted into soil carbon by soil organisms via complex food webs (Fig. 1). The breakdown of soil carbon to CO2, the mineralization, mainly take over the microorganisms. However, it is still not clear why part of the carbon in the soil is converted quickly while another part remains in the soil for decades to millennia [1]. For a long time it was assumed that special molecular structures of some carbon compounds are more difficult to break down than others, and that this leads to a selective accumulation of these compounds. Today there are two main factors responsible for the accumulation of carbon in the soil: (1) Some molecules are only accessible to degrading organisms and their enzymes to a limited extent, (2) a lack of nutrients or energy sources limits the growth of microorganisms [1] . Carbon compounds can be protected from degradation, for example, by binding to soil minerals. Analyzes at the MPI for Biogeochemistry have shown that soils in which a large part of the carbon is bound to minerals, less CO2 release. The mineral composition of the soil thus also plays a special role in carbon storage.

How much new carbon is left in the soil?

Determining the exact amount of organic compounds that enter the soil each year is a methodological challenge. There are currently no simple methods for direct measurement of the root mass that dies annually or for determining the carbon that also enters the soil through excretions from living roots (root exudates). Marking the plants with stable isotopes is an indirect way of tracking the path of carbon from the plant into the soil. For this purpose, plants are kept under an artificial atmosphere in which the ratio of stable isotopes 12C and 13C in all of CO2–The proportion of air has changed compared to natural conditions. Since the plants from this CO2 build up their biomass, it is possible to determine the new plant-borne proportion of the carbon found in the soil by measuring the isotope ratios. How quickly the isotope markings of the plants appear in different groups of soil organisms also allows conclusions to be drawn about which carbon sources they use. In an extensive field experiment (Quasom project), the institute is for the first time continuously marking plants under natural CO2-Concentrations to investigate the interrelationships between plants, soil and soil organisms (Fig. 2).

14C measurement to determine the age and retention times of carbon in the soil

With the help of accelerator mass spectrometry (AMS) at the MPI for Biogeochemistry, the natural part of the radioactive 14C-carbon isotopes measured. Is the 14C concentration in the biomass of the dead plants is known, then can be determined by the decay times of the 14C The mean age of the carbon in a soil sample can be determined. In addition, you can use a 14C signal, which was generated by the aboveground atomic bomb tests in the 1960s. This resulted in a doubling of the 14C concentration in atmospheric CO2. This enrichment came via the CO2-Fixation of the plants in the soil, where it can still be detected today [2]. A comparison of the 14C concentrations in soil samples at a site that were taken before and after the atomic bomb tests (archived samples) show how quickly and to what extent the old carbon in soil samples was replaced by new ones. Using this 14At the MPI for Biogeochemistry, the residence time of carbon at various locations is determined using mathematical models.

Overall, the mean carbon age increases significantly with the depth of the soil. This is primarily due to the carbon bound to minerals, which in the subsoil is significantly older than the free, unbound carbon. The change in the mineral-bound carbon age with depth seems to be dependent on the amount and mobility of the dissolved organic carbon in the soil. Further experiments should show under what circumstances the old minerally bound carbon can be mobilized and broken down again.

Soil and Climate Change

It is assumed that with climate change at higher temperatures, the conversion processes in the soil also accelerate, so that there is a positive feedback between global warming and further CO2-Release would come from the ground. However, plant growth and the decomposition capacity of soil organisms depend not only on the temperature, but also on the availability of water and nutrients. The community of organisms and their metabolic activity will also adapt to the changed environmental conditions. In natural systems, therefore, simple reactions of the conversion processes in the soil to temperature changes are not to be expected and forecasts are difficult. The MPI for Biogeochemistry examines the dependence of CO in field and laboratory experiments2-Release from the soil on the temperature, soil moisture, soil properties as well as on the decomposer community and its activity. Mathematical models for simulating the processes in the soil are tested with the results and further developed.

Importance of land use for soil carbon

Through the choice of plants, fertilization and tillage, humans influence both the carbon input into the soil and the living conditions of the microorganisms and thus the proportion of carbon that is mineralized. In particular, changes in land use such as the conversion of forests and meadows to fields or the drainage and use of wetlands lead to increased CO2Emissions and a reduction in soil carbon. As part of two large German biodiversity experiments (Biodiversity Exploratorien and The Jena Experiment), the institute is investigating how different forms of management and species communities in forest and grassland locations affect carbon storage in the soil. In addition to the influence of vegetation and fertilization, the role of soil organisms for conversion processes in the soil is also determined.

Evidence of changes in soil carbon content

Current research results show that forests, meadows and pastures in Europe are currently carbon sinks, whereas arable sites are weak sources [3]. These results are based on measurements of the gas exchange between ecosystems and the atmosphere as well as on model calculations. However, direct evidence of the changes in the soil reserves and in the biomass is still pending. The high spatial variability of the soil properties, which is not easily visible and measurable from the surface, makes direct investigations difficult. In order to still be able to detect the small changes in the large existing stocks, very large numbers of samples must be examined over a longer period of 10 or more years. In 2004, as part of the EU project CarboEurope, an extensive initial inventory of soil carbon contents was carried out at 12 European (FLUXNET) locations with different land uses (Fig. 3) [4]. These areas are currently being sampled bit by bit. The first results of the repeat inventories confirm the sink function of the forests [5].

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