Soil ecology and agroecosystem studies - Microbial Ecology in Sustainable Agroecosystems - page 12

Comparing desert, prairie, and rain forest sites in North and South America, Fierer et al.
(2007) found minimal taxonomic overlap between sites, with the archaeal and fungal com-
munities less even than the bacterial communities. Comparing bacterial communities in
ultisols of the southern Piedmont and the southern Appalachian Mountains, Upchurch
et al. (2008) noted that bacterial diversity (16S rRNA) was greater in conventional tillage
(CT) and no-tillage (NT) agroecosystems at Horseshoe Bend near Athens, Georgia, versus
forested sites nearby and at Coweeta LTER (Long Term Ecological Research), 160 km to the
north of it. The ultimate causal factors are yet unknown but may be linked to the greater
diversity of primary producers (crops and weeds) in the agroecosystem than in the forested
sites. In similar sites in the southeastern Piedmont of the United States, Jangid et al. (2008)
observed similar trends in agricultural ecosystems compared with nearby forested sites.
One of our agroecosystem studies in the U.S. southern coastal plain of Georgia used a
series of five commercial no-tillage cotton ( Gossypium hirsutum L.) field plots that followed
a chronosequence of 4-25 y since conventional tillage. These sites are loamy to fine-loamy
thermic, kaolinitic Kanhapludults and Paleaquults, with pH of 3.7-4. During the first 8 y
of tillage, there was some increase in organismal abundance, but only the two older fields
(8-25 y) had accumulated abundance and species richness that approached that of undis-
turbed sites. Total microbial biomass changed little over the years and tended to follow the
marked seasonal changes in total organic matter in spring and fall seasons ( Figure 1.4 ; Adl
et al., 2006). Microbial diversity, as measured by fatty acid methyl ester (FAME) analysis,
showed some shifts from bacterial to fungal dominance, with bacterial/fungal ratio high-
est in site 0 and lowest in sites 1 and 5 ( Figure 1.5a ; Simmons and Coleman, 2008). Levels
of 16:1ω5c, an arbuscular mycorrhizal fungal indicator, were all low, but were lowest in
the conventionally tilled field ( Figure 1.5b ; Simmons and Coleman, 2008). In contrast, the
changes in soil faunal species richness were more pronounced across the chronosequence.

5

40

4

20

3

C-Spring
C-Fall
C/N Spring
C/N Fall

2 0

1

-20

0

0 0 4-5 8-9 25-26 90-91

Years in no-till

Figure 1.4 Soil organic matter in a chronosequence of cotton fields (0-25 y) and a nearby pine forest
(90 y) in sandy soils in Coffee County, southern Georgia, United States. Note decrease in autumn
in 4- to 25-y fields (gray bars). (From Adl, M.S., D.C. Coleman, and F. Reed. 2006. Slow recovery of
biodiversity in sandy loam soils of Georgia after 25 years of no-tillage management. Agriculture,
Ecosystems and Environment 114:323-334. With permission from Elsevier.)