Agriculture Reference
In-Depth Information
mycorrhizal associations switched from endomycorrhizal (AM) to a mixed
endomycorrhizal-ectomycorrhizal (AM-EM) symbiosis. Fungal biomass increased
with the addition of extra-radical mycelia associated with ectotrophic mycorrhiza.
When the trees were sampled in 1997, both AM and EM fungi were present. Kosola
et al. (2004) showed the differential response of these symbionts to environmental
conditions: N fertilization reduced AM colonization of roots from 16% to 14% and
increased EM colonization from 15% to 18%.
Bacterial communities are also affected by environmental conditions (Schmidt
and Waldron 2015, Chapter 6 in this volume) and can be influenced by SOM dif-
ferences associated with aggregates. Blackwood et al. (2006), for example, found
greater inter- than intraaggregate variability in bacterial community structure in
a comparison of soils from KBS and Wooster, Ohio. They found a higher number
of active bacteria (as indicated by a cell volume greater than 0.18 µm
3
) within
aggregates than elsewhere, indicating potentially higher microbial activity within
aggregates. The number of bacteria and the proportion of large, active cells were
not affected by the cropping system; the larger bacteria accounted for 30-50%
of the number of cells, but composed 85-90% of the biomass (Blackwood and
Paul 2003).
The fungi showed more microspatial variability than bacteria (Horwath et al.
1994, 1996). They were larger and more often associated with plant residues than
the bacteria. And whereas bacterial biomass varied by a factor of 50% during the
growing season, fungal biomass varied by a factor of 100%. Fungi, however, tend to
have cytoplasm-free cells, and thus the DNA content of soils may be 80% bacterial
even though the two populations may have similar biomass. Harris and Paul (1994)
used the incorporation of
3
H-thymidine to measure bacterial growth rates and found
that the bacteria of the Conventional system soils doubled every 160 days, while
those in soils of the Mown Grassland (never tilled) community, which have a higher
SOM content, doubled every 107 days.
High seasonal variation in bacterial and fungal biomass during the growing sea-
son (Horwath and Paul 1994, Horwath et al 1996) indicates that the microbial bio-
mass is a significant proportion of the active pool of SOM and changes in microbial
biomass might be used to manage SOM dynamics and N fertility (Fortuna et al.
2003). This could be especially important when combined with the catalytic effect
of rhizosphere microbiota on SOM decomposition. In the KBS Living Field Lab
experiment (Snapp et al. 2015, Chapter 15 in this volume), Sánchez et al. (2002,
2004) found that cover crops such as clover increased both intraaggregate SOM and
microbial biomass. A corn crop that followed soil N enrichment by a clover cover
crop mineralized 168 kg N ha
−1
from SOM pools, whereas wheat mineralized only
116 kg N ha
−1
, and bare soils with roots excluded mineralized 108 kg N ha
−1
from
SOM pools (Table 5.8).
A corn crop after a clover cover crop contained 168 kg N ha
−1
mineralized from
SOM pools, as compared to 108 kg N ha
−1
of mineralized N in microplots where
roots were excluded and 116 kg N ha
−1
of mineralized N in a wheat crop (Table 5.8).
Soil planted with corn mineralized more C than bare soil with roots excluded This
suggests that microbes stimulated by labile C from the corn rhizosphere specifically
degraded N compounds. The role that legumes and cover crops (Harris et al. 1994),
