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Natural resources and energy
Special
Celebrating the International
Year of Soils:
Exploring some soil biological indicators
S
oils are of central importance for delivering ecosystem
services, such as food production and climate mitigation.
These services depend heavily on carbon (C) sequestration
and nutrient cycling, processes that are governed by soil
biota or biology. Soil biology is the study of microbial and faunal
activity and ecology in soil. However, soil biology is one of the most
unexplored frontiers associated with understanding the dynamics of
soil resources and their subsequent health or quality. There is grow-
ing recognition for the need to develop sensitive indicators of soil
health and biology that reflect the effects of land management on
soil and assist land managers in promoting regenerative conserva-
tion agriculture (CA).
Producers intuitively recognised the importance of healthy soils and
have used qualitative terms (i.e. colour, taste, touch and smell) to
describe soil condition and performance for crop production since
the dawn of agriculture about 10 000 years ago. At the beginning
of the 20th century, qualitative “life measuring” descriptions, how-
ever primitive they were, gradually became replaced by analytical
procedures to assess and evaluate soil almost exclusively from the
perspective of inorganic nutrients (chemistry) and crop yield.
In this article, a few soil biological indicators are discussed, which
are part of a growing pool of analytical options gaining ground in
measuring short term changes in soil biology or health inflicted by
changes in land management. They are discussed in relation to ac-
tual field measurements done for different cropping practices evalu-
ated in the Ottosdal CA project, which is funded by the Maize Trust
(MT), implemented by the Ottosdal No-till Club and co-ordinated by
Grain SA’s CA Programme. These cropping treatments (bare soil,
monocrop, rotations, mixtures and natural grassland or veld) include
cash crops (maize, soybean and sunflower), as well as cover crops
(e.g. Dolichos lablab, grain sorghum, babala, black oats and mixtures).
Soil microbial functional diversity
Functional diversity (the diversity of species behaviours) is an indica-
tion of the biological status of soil microbial populations in soil, since
it relates to the activities of soil micro-organisms contributing to soil
biological processes. Various soil microbial activities fulfil critical
functions in the decomposition of organic matter and biogeochemi-
cal cycling of nutrients such as carbon, nitrogen and phosphorus.
The Biolog
®
system is used to determine functional diversity of mi-
crobial communities. From these results, carbon source utilisation
profiles (CSUP), i.e. the functional
diversity
of soil microbial popula-
tions, were determined by using the amount and equitability of car-
bon substrates utilised as indicators of
richness
(Shannon-Weaver
index or the number of active microbial species present) and
even-
ness
(Evenness index or microbial abun-
dance), respectively.
Figure 1
indicates differences in microbial
populations’ functional diversity between
the different crop practices or treatments
at Ottosdal. Three main groups are distin-
guished by the results: Blue circle enclosing
bare soil and veld treatments; green circle
enclosing treatments with maize; and red
circle enclosing treatments with lablab.
Significant differences exist between the
amount and types of carbon sources uti-
lised by soil microbial populations present
in bare soil and veld (natural grassland),
lablab, grain sorghum on babala and babala
on maize treatments. The number of active
microbial species present in veld and bare
soil, maize monoculture and maize-soybean
crop treatments were low compared to the
rest (
Graph 1
). Microbial abundance was
significantly higher in lablab on sunhennop
than in the bare soil and veld treatments
(
Graph 2
). Microbial communities present
in the latter treatments are thus less diverse
and dominated by specific microbial spe-
cies, implying a state of imbalance.
HENDRIK SMITH,
CA facilitator: Grain SA,
WILLIE PRETORIUS,
Crop Systems and Radicle Soil Health Laboratory,
JOHAN HABIG,
researcher: ARC-Plant Protection Research Institute,
GERRIE TRYTSMAN,
researcher: ARC-Animal Production Institute and
LIESL WIESE,
researcher: ARC-Institute for Soil, Climate and Water
54
November 2015
Figure 1: Ordination plot illustrating the differences in average CSUP between crop treatments.