Agriculture Reference
In-Depth Information
small at ~5% of total SOM. It includes some of the interaggregate fraction (i.e., not
contained within soil aggregates) of plant residues and a portion of the soil biotic
biomass, and has mean residence times (MRTs) of months to years. The slow pool,
with MRTs ranging from months to decades, accounts for ~40% of total SOM and
is a major source of soil nutrients that change with long-term management. Slow
pool dynamics are controlled by aggregation and the association of microbial prod-
ucts with the calcium (Ca) and sesquioxide minerals as well as some silt and clay
components. The SOM within soil aggregates (intraaggregate fraction) has both
young and old constituents, with MRTs of decades. The oldest SOM is associated
with silt and clay and is defined as the resistant pool; it is measured as the SOM
that remains after acid hydrolysis. This third pool has the longest MRTs (century
to millennia) and is best measured by carbon dating (Leavitt et al. 1996, Paul et al.
1997a).
Because SOM is a heterogeneous material including components with a wide
range of turnover in response to abiotic and biotic controls, we currently cannot
directly isolate and analyze SOM fractions based on their turnover dynamics. For
example, there appears to be some old C (possibly charcoal) associated with the
actively decomposing, recently added plant particulate material. On the other hand,
old fractions such as the clays (Haile-Mariam et al. 2008) have some recently
absorbed young microbial products. Old, resistant, non-hydrolyzable C is also
known to contain recent plant-derived lignin that is not soluble in acid, even though
on average the pool is very old (Paul et al. 2006).
Soils of the Kellogg Biological Station Long-term Ecological Research Site
(KBS LTER) are ideal for studying how organisms, climate, parent material, land-
scape, and management influence SOM composition, dynamics, and ecosystem
functioning. KBS soils developed in a moderately humid, temperate climate on
glacial outwash over a period of ~18,000 years. They are moderately fertile Typic
Hapludalfs (NRCS 1999) developed in an environment with a mean annual tem-
perature of 9.0°C and annual precipitation of ~1,000 mm, under a northern, mixed
hardwood forest with grassland openings attributed to fires promoted by native
inhabitants ~700 c.e. (Robertson and Hamilton 2015, Chapter 1 in this volume). In
the late nineteenth and early twentieth centuries, these soils were further disturbed
by widespread deforestation and cultivation.
The soils underlying the KBS LTER consist of two main series: (1) Oshtemo:
coarse-loamy, mixed, mesic, Typic Hapludalfs and (2) Kalamazoo: fine-loamy,
mixed, mesic Typic Hapludalfs (Austin 1979, Mokma and Doolittle 1993, Crum
and Collins 1995). The 0- to 20-cm surface layer has a texture of 39% sand, 43%
silt, and 18% clay. This grades to 87% sand at a depth of 50-100 cm with 5% silt
and 8% clay. Surface horizons can contain a small amount of inorganic C arising
from the application of agricultural lime (mainly calcium and magnesium carbon-
ates); in deeper horizons the presence of inorganic C reflects the calcareous origin
of parent materials (Hamilton et al. 2007). Soil organic matter in surface horizons
ranges from <10 to more than 30 g C kg
−1
depending on landscape position, vegeta-
tion, and management history (Syswerda et al. 2011).
The organic matter content of soils reflects the balance between photosyn-
thesis and decomposition, the two major driving factors in the global C cycle.
