Agriculture Reference
In-Depth Information
Table 15.3). Organic management relied on compost and cover crops for N supply.
These C-rich nutrient sources may have temporarily immobilized inorganic N and
reduced N loss. However, immobilization of N can also reduce crop yields, depend-
ing on the competitiveness of plant roots for N, and how fast N is turned over during
microbial assimilation (Drinkwater and Snapp 2007).
Pest Suppression
The effects of crop rotation and management system (Integrated Conventional vs.
Organic) on weed suppression services in the LFL experiment were investigated
from 2001 to 2004 (Smith and Gross 2006a). Over that time, and similar to what
has been observed in the MCSE (Davis et al. 2005; Gross et al. 2015, Chapter
7 in this volume), weed biomass was more than 10 times higher in the Organic
compared to the Integrated Conventional system. In addition to total weed bio-
mass, the composition of the weed community also differed between the two
management systems, with smooth crabgrass ( Digitaria ischaemum [Schreb.]
Schreb. ex Muhl.) and Carolina horsenettle ( Solanum carolinense L.) dominating
the Integrated Conventional system, and common ragweed ( Ambrosia artemisi-
ifolia L.) and common lambsquarters ( Chenopodium album L.) dominating the
Organic system.
In contrast to the overriding influence of management system, crop rotation
per se did not affect weed biomass. Crop rotation did, however, interact with
management system to affect the interannual variability of the composition and
structure of the weed community. Compared to conventionally managed crops
and the Organic continuous corn system, weed community composition in the
organic rotation was significantly more variable from one year to the next (Smith
and Gross 2006a). This result is likely due, in part, to greater annual changes
in the composition of weed species added to the seed bank (Smith and Gross
2006b), and suggests that an important regulating ecosystem service that crop
rotation and diversification provide is to reduce the likelihood of developing a
consistent weed community that is resistant to other weed management practices
applied to the cropping system.
Ecosystem Service Trade-offs
As compared to the Integrated Conventional system, reduced inputs in the Organic
system resulted in both yield reduction and enhanced ecosystem services in the
form of soil C accretion and lower nitrate leaching losses (Table 15.3). To explicitly
explore this trade-off, yield reductions in the Organic two-species and six-species
systems were calculated relative to yield in the Integrated Conventional two-species
system—chosen as a baseline system that followed all recommended integrated
practices. We simultaneously evaluated soil C gains and the extent of nitrate leached
in the Organic vs. Integrated Conventional systems over a 6-year period. Finally,
the yield reduction was expressed in relation to soil C gained and nitrate leaching
reduced (Table 15.3).
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