Agriculture Reference
In-Depth Information
In general, cultivations are verified as organic after 3 years of organic management. But
what happens to the soil microbial community during this transition period? To what extent
the characteristics of soil microbial communities in the newest organic cultivations are
intermediate between those of conventional and the oldest organic ones? Can we talk for a
gradual improvement of soil quality with the duration of organic cultivation? Although
several studies examined differences in soil properties between transitional and established
organic systems (Scow et al. 1994, Drinkwater et al. 1995), the existence of a number of
confounding factors makes interpretations difficult. Some answers to these questions are
given by the studies of Martini et al. (2004) and Monokrousos et al. (2006). Martini et al.
(2004) examined the microbial community in established organic plots (> 5 yrs) and in
transitional ones (< 1 yr). Conventional plots were also included. Based on FAME analysis,
the authors showed differences between microbial communities from conventional and
organic systems reflecting differences in communities composition or in abundances.
Distinction was also revealed when the functional diversity was taken into account. The
hypothesis that the microbial communities from transitional plots exhibit intermediate
characteristics between conventional and organics systems was not supported by the data of
Martini et al. (2004). In the study of Monokrousos et al. (2006) the duration of organic
cultivation varied from 2 to 6 yrs (2, 3, 5 and 6 years). The composition of microbial
community, in terms of bacterial and fungal contribution, was unaffected by the age of
organic management but was differentiated between organic and conventional plots. The
activities of almost all enzymes related to N and P-cycle were higher in plots with
intermediate duration of organic cultivation and lowest in the conventional one. The data
regarding microbial structure and activity supports the suggestion of Martini et al. (2004).
Changes in microbial communities are not gradual from conventional to transitional and then
to organic systems. This gradual response characterized only the communities of
microbivores nematodes (bacterial and hyphal-feeding nematodes; Tsiafouli et al. in press).
On the contrary, when the functional diversity was examined in phaseolus organic
cultivations of different age (2, 5, 8, 10 and 14 yrs), the analysis revealed a gradual evolution
from young organic plots with lower diversity and abundance to more diverse and with higher
abundances older plots (unpublished data). In this study, the bacterial community in
conventional plots seemed to be less functionally diversified and more stable over time.
Evidently, in order to detect differences in soil microbial community between newest and
older organic areas more than 6 yrs of cultivation are necessary.
Apart from organic farming, the agricultural soils of Mediterranean areas are under the
influence of salinization. Soil salinization arises from the improper use of salt-rich waters in
agriculture (Szabolcs 1998). The main influence of saline water in soil is the dispersion of
clay particles with consequent changes in soil physicochemical properties. The modification
of soil properties is expected to induce changes in bacterial communities. To test this
hypothesis all aspects of bacterial diversity were measured and compared to those in an
unaffected agriculture system; an organic one (Grecchio et al. 2004). Depression of microbial
size and activity and modification of phenotypic and genetic diversity were recorded in salt-
irrigated soils. It was concluded that different long-term irrigation systems exert a strong
influence over the development of distinct microbial communities similar to the influence
exerted by the plant type or the rhizosphere's products.
The effects of winter-cover cropping vs winter fallow practices were examined by
Schutter et al. (2001). Relevant effects were compared to changes induced by season and soil
