Agriculture Reference
In-Depth Information
Decomposition transforms plant residues into microbial products that can become
stabilized in the soil matrix, thereby maintaining 5-10% of the plant residue C
as SOM (Follett et al. 1997). As the recipients of decomposition processes, soils
represent a valuable storehouse of information on past vegetation, climate, and dis-
turbance. They are also a major source of nutrients, especially nitrogen (N), and
provide physical structure and moisture retention. Interactions of SOM within the
soil matrix—particularly of silts and clays, but also aggregation with sand parti-
cles—determine the rooting environment for plants and both the storage and move-
ment of soil moisture essential for plant growth.
The major factors controlling SOM dynamics are: (1) the quality of the incom-
ing substrates, (2) the role of the soil biota and especially the microorganisms,
(3) physical protection such as in aggregation, (4) interaction with the soil matrix
such as the silts and clays as well as Ca and sesquioxides, and (5) the chemical
nature of the SOM itself. These factors interact and are best studied together.
However, because soil takes so long to form and because SOM is such a small
fraction of bulk soils, examination of these factors must involve long-term studies
on well-characterized sites. Sites such as those in the LTER Network can supply the
continuity and replication required to investigate SOM transformations and dynam-
ics. In this chapter, we synthesize studies from the KBS LTER that focus on SOM
dynamics in agricultural soils. We begin by examining soils of the Main Cropping
System Experiment (MCSE, Table 5.1) and discuss the effect of landscape, vegeta-
tion, and agricultural management on SOM dynamics. We then discuss the bio-
chemical controls on SOM dynamics, particularly as characterized by examining
the size and turnover rates of important SOM constituents. And we end with an
examination of the role of C inputs and microbial activities on the fate of C in
agricultural soils.
Effect of Landscape, Vegetation, and Management on Soil
Organic Matter Dynamics
Soils at the KBS LTER are prone to periodic drought, with some historical erosion,
and thus have lower SOM content compared to some other nonglaciated cultivated
soils in the Great Lakes region (Table 5.2). The Deciduous Forest system of the
KBS LTER MCSE (Table 5.1) is representative of the native soil of the region (see
Robertson and Hamilton 2015, Chapter 1 in this volume, for a description). The
C in the SOM (hereafter referred to as soil organic carbon, SOC) of surface soils
(0-20 cm; Table 5.2) has a MRT of 422 years, as determined by carbon dating
(Paul et al. 2001a). This increases to 1712 years at 50- to 100-cm depth. Using acid
hydrolysis to identify the old, resistant SOC (Paul et al. 2006, Plante et al. 2006),
which is called non-hydrolyzable C (NHC), Paul et al. (2001a) found that 56% of
the total SOC in the surface layer is NHC and is 977 years old (Table 5.2). Deeper
in the forest soil profile (50- to 100-cm layer), the NHC accounts for only 23% of
the SOC but is 4406 years old.
Such soil characterizations are most informative when examined in comparisons.
Unifying concepts such as the content and structure of SOM and the role of the soil
