Using Annual Summer Cover Crops To Manage Nitrogen Fixation And Weed Suppression In Agro-Ecosystems

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Cover crops perform multiple functions in agro-ecosystems, such as nitrogen (N) fixation, nutrient retention and weed suppression. However, there is often a trade-off between N fixation and weed suppression - legumes fix N but are not very weed suppressive, while non-legumes suppress weeds but do not fix N. Legume-based mixtures, consisting of species with spatially and/or temporally complementary traits, can be a strategy to effectively manage N fixation and weed suppression. The aim of this study was to: (1) evaluate the N fixation and weed suppression characteristics of eight legume and four non-legume, annual summer species that can be used in the Northeastern U.S.A, (2) evaluate the performance of legume-based mixtures, in terms of biomass production, N fixation and weed suppression, (3) evaluate the competitive ability of the different legume and non-legume species in mixtures by using a replacement series design and (4) evaluate different management strategies to effectively manage mixtures for N fixation and weed suppression. Experiments were conducted over two years, and mixtures were designed using a replacement series. In monoculture, Crimson Clover fixed the most N (111kgN.Ha-1 in 2008 and 71kgN.Ha1 in 2009) and it was the most weed suppressive legume (12g.m-2 in 2008 and 20g.m-2 in 2009). Cowpea fixed the lowest amount of N in both years (12 kg N.Ha-1 in 2008 and 13 kg N.Ha-1 in 2009), but accessed more soil N (50 kg N.Ha-1 in 2008 and 58 kg N.Ha-1) than any of the legumes and it was the least weed suppressive legume in 2009 (250g.m-2 in 2009). Regarding the non-legumes, Sorghum Sudan (938g.m-2 and 632g.m-2) had the greatest biomass production in monoculture at high and low seeding densities (Tukey's HSD, p less than 0.0001), while Buckwheat was the most weed suppressive (15 and 30g.m-2) species in monoculture (Tukey's HSD, p less than 0.0001). Buckwheat took up similar amounts of soil N than Sorghum Sudan, even though it had lower above ground biomass. In all the mixtures, except Buckwheat, the LER was generally greater than one. The legumes in all the mixtures, except in the Buckwheat mixture, relied slightly more on N fixation than in monoculture. For non-viny legumes, the total N fixed was significantly greater in monocultures than in all the mixtures (Tukey's HSD, p less than 0.05). For the viny legumes, total N fixed in mixtures with C-4 grass species was not significantly different from the monocultures, but in Buckwheat mixtures significantly less N than monoculture was fixed (Tukey's HSD, p less than 0.05). The weed suppressive capacity of the mixtures depended on the species involved, and there was no consistent improvement in mixture weed suppression compared to the monocultures (Tukey's HSD, p less than 0.05). The competitive ability of the non-legumes can be ranked as follows: Buckwheat to Sorghum Sudan to Japanese Millet to Flax. Within non-viny legumes, the Berseem Clover was more competitive than Crimson Clover, and within the viny species Cowpea was more competitive than Soybean Tyrone and Chickling Vetch. There was a functional trade-off between N fixation and weed suppression: mixtures that are effective at suppressing weeds (Buckwheat mixtures) also suppress legumes and legumes that are competitive (Cowpea) in mixture do not fix a lot of nitrogen. In mixtures containing species with complementary growth times (Clovers and Sorghum Sudan / Buckwheat), mowing the competitive non-legumes increased legume biomass five-fold in Buckwheat mixtures and two-fold in Sorghum Sudan mixtures, while weed suppression was maintained. Nitrogen fixation increased eight- to ten-fold in mowed Buckwheat mixtures and two-to four-fold in mowed Sorghum Sudan mixtures. Mowing competitive species in temporally complementary mixtures can avoid the trade-offs in N fixation and weed suppression.

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