Agriculture Reference
In-Depth Information
key ecosystems processes. Here, SOM is broadly defined as nonliving organic material
within soils, regardless of form or origin (e.g., detritus, dead roots, corpses of soil biota,
traditional SOM). The balance in the activities of one pathway relative to the other is gov-
erned by the quality of SOM, with low C:N (<30) substrates favoring the bacterial pathway
and high C:N (>30) substrates favoring the fungal pathway. This model allows for changes
in the relative activities of the different pathways with natural seasonal variation in the
phenology of plants and season succession in plant types, as well as abrupt or gradual
changes that result from different land-use practices and increases in atmospheric CO
2
.
We discuss some of the last in the section that follows.
of concern to land managers
Viewing the agroecosystem holistically, it is essential to consider the many activities of the
soil micro-, meso-, and macrofauna. The roles of the larger soil invertebrates (e.g., earth-
worms) in burrowing, soil transport, and mixing are extensive and can involve up to sev-
eral tons per hectare annually of more readily assimilable organic materials (Lavelle et
al., 2001). Thus, a combination of conservation tillage and enhanced residue quality pays
considerable dividends for ecosystem health.
The previous history of a given agroecosystem can have strong effects on the extent of
soil food web complexity. In a 1-y field station trial in Davis, California (Alfisol), including
no tillage and continuous cropping, no tillage and fallow, standard tillage and continu-
ous cropping, and standard tillage and fallow, significant enhancements in SOM storage
were measured in the no tillage and continuous cropping in the top 0- to 5-cm layer, prin-
cipally due to greater amounts of fungal biomass in the microbial biomass carbon. The
soil food web, as denoted by the nematode diversity, was not changed in any of the treat-
ments, probably due to the elimination of higher trophic-level nematodes during previous
decades of cultivation (Minoshima et al., 2007).
A recent comparative study of decomposition rates in replicated conventional till, no-
till, and old-field agricultural sites in southern Michigan, United States, found that, after
one growing season, litter decomposition under conventional till was 20% greater than
in old-field communities (Wickings et al., 2010). In contrast, decomposition rates in no till
were not significantly different from those in either of the other two treatments. Wickings
et al. (2010) suggested that agricultural intensification can increase litter decomposition
rates, alter decomposer communities, and influence litter chemistry in ways that could
have significant and long-term effects on SOM dynamics. In the southern Australian agri-
cultural region, continuous cropping systems caused significant changes in the rate of
decomposition of crop residues and composition of microbial and faunal communities
compared to that under short-term (1- to 4-y) pasture-crop or fallow-crop rotation systems
(Pankhurst et al., 1995; Gupta et al., 2008; Wang and Dalal, 2006). Such changes had a nega-
tive effect on short-term nutrient turnover and soilborne biological constraints to agricul-
tural production, which in turn had an impact on long-term SOM dynamics and overall
ecosystem health. An increased frequency of pastures in rotation generally increased soil
organic C levels, whereas higher frequency of fallows caused losses in soil biota popula-
tions and amounts of labile pools of SOM (Grace et al., 1998).
The diversity of microbial (archaea, bacteria, fungi, and viruses) communities in soils
has only recently been evaluated in terms of small-subunit ribosomal RNA (rRNA) genes.
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