Diversifying crop rotations with temporary grasslands - Université de ...

productivity on the rotation scale. Moreover, the inclusion of a nitrogen-fixing legume crops
contributed to increase the soil fertility.
The regulation of pests, weeds and diseases is the second reason why farmers use crop
rotations instead of monocultures. The basic hypothesis is that each crop creates specific
living conditions (ecological niches) determined both by characteristics of the crops (such as
plant morphology, physiology and growth dynamics determining plant cover, microclimate
and soil characteristics) and by the associated crop management practices (such as the types
and dates of soil tillage, crop sowing, pesticide and fertilizer applications, irrigation and
harvesting) (Doucet et al., 1999). Crop plants and the associated management techniques act
thus as biotic and abiotic ‘filters’ that determine the assembly of plant communities (Booth
and Swanton, 2002). Therefore, some adapted weed species or ecotypes may develop and
reproduce in a given crop situation, while many others cannot successfully terminate their life
cycles. If the same or similar crop types are grown during several consecutive years on the
same field (‘monoculture’), the living conditions will be similar in every year. Such constant
and predictable selection pressures could thus favour some adapted species, whose
populations may thus steadily increase and reduce the yield or quality of the crop. At the same
time, all other species are likely to decline or disappear reducing biodiversity. In contrast,
alternating different crop types and different associated management practices on the same
field would provide different selection pressures in every year, hence (i) avoiding continuous
population increases of single adapted species but also (ii) increasing the species diversity.
This would correspond to the ‘diversity begets diversity’-hypothesis introduced by Whittaker
(cited in Palmer and Maurer, 1997). Crop rotation may thus also be favourable to biodiversity,
which would be a third function.
The importance of crop rotations decreased after World War II. Its main agronomic functions
could be replaced by mineral fertilizers and synthetic herbicides, fungicides and pesticides and
farming machinery (Peoples et al., 1995; Stoate et al., 2001). Other reasons probably included
the globalization of agricultural markets leading to a specialization of farms and regions to
some kind of products and public subsidies favouring some crops types more than others
(Liebman et al., 2008). Today, conventional cropping systems use rather simple rotations or
loose successions of 2-3 annual cash crops (or even monocultures) of the economically most
profitable crops. Such short and simple crop rotations may provide only limited benefits for
weed management and biodiversity and these benefits may be only visible in low-input
systems (Barberi et al., 1997; Smith and Gross, 2006). The (re-)diversification of crop
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