Agriculture Reference
In-Depth Information
fungal/bacterial ratios—were associated with differences in chemistry, indicating
that the activity and structure of the decomposer community can influence chemi-
cal changes during decomposition (Wickings et al. 2011). Results demonstrate that
efforts to predict long-term SOM turnover and stabilization dynamics may therefore
need to consider the influence of different decomposer communities on changes in
litter chemistry during decomposition.
Plante et al. (2009) used pyrolysis-molecular beam mass spectrometry to iden-
tify carbohydrates, N compounds, lignin derivatives, aliphatics, and sterols in SOM.
They found similar overall constituents for forested and prairie soils. Grandy et al.
(2007, 2008) also found similar chemical patterns in the silt and clay fractions from
different ecosystems, supporting the hypothesis of Fierer et al. (2009) that all soils
share similar overall characteristics that are a function of the biota and their physi-
ological constraints. Much of the N resistant to acid hydrolysis was identified as
amino N, indicating the protected proteinaceous nature of the resistant N. This sup-
ports the conclusions of Di Costy et al. (2003) who, by using
15
N-enriched clover
additions and nuclear magnetic resonance analysis, found that the majority of N in
soil is proteinaceous in nature.
The Role of Carbon Inputs and Microbial Activity in Soil Organic
Matter Dynamics
The Fate of Plant Carbon Inputs
The amount and quality of SOM are dependent on the amount and type of
plant inputs as well as their microbial turnover before stabilization. One of the
best ways to examine these factors is the use of carbon isotope tracers. In pre-
vious sections, we noted the importance of the Poplar system for understand-
ing both sequestration of SOM and aggregate formation. The amount of C that
is provided as litter and from the roots is important in these transformations.
Horwath (1993) and Horwath et al. (1994) used
14
C to observe the distribution
of photosynthesis-derived C in the poplar plant, its movement belowground,
and decomposition by the microbial biomass (Table 5.7). Trees were uniformly
labeled in July or September by exposing the foliage to
14
CO
2
for a single day and
the movement of
14
C was traced through the plant-soil system by sampling trees
14 and 372 days after labeling.
Roots that accounted for 18.4% of total-tree C accounted for 9.8% of the
14
C
label (Table 5.7). Some of the photosynthate was stored in coarse roots and moved
to fine roots the following spring. During the 3-week labeling period, belowground
respiration accounted for 7.7% of the label taken up by photosynthesis. Microbial
biomass C, accounting for 1.5% of the soil C, received 0.4% of the
14
C, showing
significant turnover during the labeling period. The 0.3% of the label present in the
biomass after 328 days shows a slow turnover of root and microbial C during the
period after labeling, with some transfer to the soil C component. Labeled litter,
added to the soil in a separate experiment, lost 67% of its
14
C content during the first
year, and 73% over a 2-year period (Horwath 1993; Horwath et al. 1994).
