Agriculture Reference
In-Depth Information
Table 5.7. Distribution of total carbon (C) in the MCSE Poplar system and a
radiocarbon tracer (
14
C) 14 and 328 days after a 1-day labeling event.
a
Component
Total C
14
C Distribution (%)
(g)
(% of total)
After 14 days
After 371 days
Leaves + litter
448
37.0
24.0
0.2
Stems + branches
520
44.0
48.0
26.0
Roots <0.5-mm dia.
52
4.4
1.7
1.3
Roots >0.5-mm dia.
166
14.0
8.1
8.4
Root + soil respiration
7.7
Microbial biomass
103
1.5
0.4
0.3
Soluble C
75
Soil C
6788
1.2
1.4
a
Distribution of
14
C expressed as % of recovered label. See Table 5.1 for a description of the MCSE Poplar system.
Source:
Horwath (1993) and Horwath et al. (1994).
The uptake of
14
CO
2
by photosynthesis and its later distribution in the plant-soil
system in a laboratory-grown sorghum crop showed results quite similar to those
found for poplars in the field (Calderón 1997). Fifty percent of the
14
C remained in
the aboveground biomass, 30% in the roots, and 6% was transferred to the soil in a
24-day period. Belowground respiration accounted for 12% and shoot respiration
for 5% of the label in plants with symbiotic, mycorrhizal fungi. Non-mycorrhizal
plants had more aboveground allocation and less allocation to the roots and soil
respiration. The mycorrhizal plants did not quite compensate for the needs of their
microbial partners by increased photosynthesis, indicating that the fungi can act as
both symbionts and parasites (Calderón et al. 2011, 2012).
The Role of Microbial Biomass and Composition
Microbial biomass is an important component of SOM and, in particular, the active
fraction that is so important to soil fertility (Paul et al. 1999a, b). Microbes also pro-
vide the enzymes required for decomposition and the microbial products that inter-
act with the soil matrix to stabilize SOM (Paul and Clark 1996). Microbial biomass
in KBS soils was measured from 1993 to 1996 (Horwath and Paul 1994, Horwath
et al. 1996). The bacterial and fungal biomass of the Deciduous Forest soils con-
stituted, on average, 107 mg C kg
−1
soil and 179 mg C kg
−1
soil, respectively. In
MCSE agronomic systems, the biomass of the bacteria and fungi was nearly equal
at 85-89 mg C kg
−1
soil. The microbial biomass represented 1.2-1.8% of the total
soil organic C, and MCSE systems with greater SOM accumulation (No-till and
Biologically Based) had higher percentages of microbial biomass.
These values are similar to the 1.5% of soil C measured as microbial bio-
mass in the Poplar system (Table 5.7). Fungal and bacterial biomass in this sys-
tem were similar during the first few years of growth, but this changed as Poplar
