The amount of carbon residing in world soils at any moment is greater than all the carbon in the atmosphere and biosphere (living things) combined. In fact, among the fast cycling global carbon reservoirs, soil is exceeded in quantitative importance only by the oceans. The large amount of carbon in soils makes understanding what controls the rates at which carbon is metabolized in soils very important for understanding global carbon cycling.
Carbon enters soil primarily through decaying plant material that either falls onto the soil as leaf litter or is incorporated through dying roots. Most plant-material carbon is metabolized by soil organisms and released back to the atmosphere as CO2, but some fraction of that carbon remains in the soil for tens to hundreds of years. With this in mind, ESD’s Peter Nico and Markus Kleber of Oregon State University, along with other scientists (see below), have recently been employing the unique capabilities of DOE-LBNL’s Advanced Light Source (ALS) to probe the nanoscale life of soil carbon.
For many years, the prevailing view of soil carbon was that certain complex organic structures, such as aromatic rings, were harder for microorganisms to metabolize than other organic structures, and therefore remained in the soil longer and accounted for the “old” carbon in soils. To test this assumption, the team has used the ALS’ scanning transmission X-ray microscope (STXM) beamlines 11.0.2 and 5.3.2. These specialized instruments use the high intensity X-rays from the ALS to probe the molecular structure of carbon at a spatial scale of tens of nanometers. Using this capability, the team is able to probe the molecular structure of carbon that is closely associated with the surface of soil minerals.
It has been shown by 14C dating that carbon close to mineral surfaces is on average much older than carbon found in bulk soil. This analysis showed that this older carbon was not particularly high in aromatic groups. These results feed into a growing body of evidence supporting the newly prevailing view that the molecular structure of soil carbon is not the dominant factor in controlling soil residence time—and that other factors such as association with mineral surfaces are more important.
To better study these other types of processes, the LBNL team has partnered with Jennifer Pett-Ridge’s group at LLNL, and French researchers Delphine Derrien and Laurent Remusat, to couple the capabilities of STXM with that of nano-secondary ion mass spectrometry (Nano-SIMS). Nano-SIMS is a mass spectrometry technique that measures the isotopic composition of materials at a spatial resolution similar to STXM. Measuring isotopic composition is very useful for studying carbon transformations, because isotopic labels can be placed similarly to how radio tracking tags are placed on animals. By using organic matter enriched in stable isotopes of C and N, it is possible to following the physical and chemical transformations of a particular group of atoms through real or model soil systems.
In the example shown below in Figure 1, yeast and bacterial cells were grown under conditions so that they became enriched in the 13C isotope of carbon (most carbon is 12C) and the 15N isotope of N (most nitrogen is 14N). Figure 1a shows an SEM image of the labeled organic material after it was mixed with a common soil mineral, to form a simple synthetic organic-mineral microaggregate of the type potentially important for stabilizing carbon in soils. Figure 1b shows the distribution in carbon chemical form as determined by STXM. The carbon was grouped into three broad chemical types: protein (red), amino-sugar (blue), and phospholipids (green), which are characterized by the spectra shown in Figure 1e. It can be seen from the colors on the image that the phospholipid material is mostly associated with the minerals, while the protein appears to be grouped into structures that resemble intact microbial cells. Lastly, the large feature in the center that is high in phospholipids and amino-sugar resembles a largely intact yeast nucleus. Panels C and D of Figure 1 are taken from the Nano-SIMS analysis showing that the different structure have different isotopic compositions as well as chemical forms, with the bacterial cells being enriched in 13C and the yeast nucleus in 15N.
Figure 1. Coupled SEM, STXM/NEXAFS, and NanoSIMS analysis of a micromineral aggregate showing differential isotopic enrichment of different structures and the corresponding chemical composition.
While this is a simple example used to perfect the coupling of the two techniques, this approach can now be used in more complex model systems and real soils, allowing us to gain a better understanding of the previously secret life of soil carbon.
This work was performed under the auspices of the U.S. Department of Energy at Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, using funding from the Laboratory Directed Research and Development (LDRD) program.