Agriculture Reference
In-Depth Information
When afforestation includes pines (
Pinus
spp.), soil calcium (Ca) content appears
to support SOM accrual in a manner not found under deciduous species (Paul et al.
2003). Addition of Ca to soils following afforestation with pines, and with the
addition of pine litter, increased the amount of stabilized SOM and decreased its
decomposition rate, confirming the importance of Ca in regulating litter decompo-
sition and SOM dynamics (Brewer 2004). Long-term stabilization of SOM through
increased Ca on soils planted to pines could improve site fertility on this and other
former agricultural sites on similar soil types. Calcium forms bridges between the
SOM and the soil matrix such that its interaction with silt and clay-sized particles
stabilizes and increases SOM levels (Baldock and Nelson 2000).
Pools and Fluxes That Control Soil Organic Matter Dynamics
A number of studies have provided insights into the mechanisms for SOM accrual
in agricultural soils, including: (1) physical protection of partially decomposed
plant residues found in inter- and intraaggregate fractions of SOM; (2) biochemi-
cal protection of SOM due to a molecular structure that is difficult for microbes
to metabolize and is therefore recalcitrant; (3) chemical protection due to SOM
associations with silt and clay; and (4) differences in soil biota that affect SOM
sequestration and turnover.
Whether the plant residues and their associated biota are free or protected
within soil aggregates controls their short-term breakdown (Fortuna et al. 2003).
The incorporation of plant and microbial-derived C into aggregates protects plant
residues (De Gryze et al. 2004), associated microbial C (Smith and Paul 1990),
and microbial products (Paul and Clark 1996) for periods of weeks to decades
depending on soil conditions (e.g., clay content and water availability) and man-
agement (e.g., tillage and crop rotation). Changes in land use that alter patterns of
soil disturbance and the quantity and quality of plant residues will influence aggre-
gate formation and stabilization, resulting in long-term effects on SOM dynamics
(Willson et al. 2001).
KBS represents a unique environment in which to study soil aggregation because
of the diversity of managed and unmanaged ecosystems that occur in close proxim-
ity on a common sand- and silt-dominated soil. There are few comparable places
where relationships between aggregation and SOM dynamics can be studied in
replicated systems that represent a range of management and chemical inputs. Most
studies of soil structural processes and organic-mineral interactions have examined
soils with high clay content. Differences in SOM across the seven MCSE systems
at the LTER main site, developed since its inception in 1989, are closely associ-
ated with changes in soil aggregates, particularly the distribution of SOC in differ-
ent aggregate size fractions, as indicated by an index of aggregation known as the
mean weight-diameter (Fig. 5.4). The Biologically Based and No-till systems have
more aggregates in the >2000-µm size class. Strong relationships between aggrega-
tion and SOM content indicate that physical protection is a key factor controlling
short-term SOM dynamics in KBS soils (Grandy and Robertson 2007).
Density, particle size, and incubation-based measurements have been used at
KBS to identify fractions enriched in SOM and sensitive to changes in management
