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4The impact on soil health of long-term no-tillage (NT) and cover cropping (CC) practices, alone 5 and in combination, was measured and compared with standard tillage (ST) with and without 6 cover crops (NO) in irrigated row crops after 15 years of management in the San Joaquin Valley 7 (SJV) CA, USA. Soil aggregation, rates of water infiltration, content of carbon, nitrogen, water 8 extractable organic carbon (WEOC) and organic nitrogen (WEON), residue cover, and 9 biological activity were all increased by NT and CC practices relative to STNO. However, 10 effects varied by depth with NT increasing soil bulk density by 12% in the 0 -15 cm depth and 11 10% in the 15 -30 cm depth. Higher levels of WEOC were found in the CC surface (0 -5cm) 12 depth in both spring and fall samplings in 2014. Surface layer (0 -15 cm) WEON was higher in 13 the CC systems for both samplings. Tillage did not affect WEON in the spring, but WEON was 14 increased in the NT surface soil layer in the fall. Sampling depth, CC, and tillage affected 1-15 day soil respiration and a soil health index assessment, however the effects were seasonal, with 16 higher levels found in the fall sampling than in the spring. Both respiration and the soil health 17 index were increased by CC with higher levels found in the 0 -5 cm depth than in the 5 -15 and 18 15 -30 cm depths. Results indicated that adoption of NT and CC in arid, irrigated cropping 19 systems could benefit soil health by improving chemical, physical, and biological indicators of 20 soil functions while maintaining similar crop yields as the ST system. 21 . 22 23 Keywords 24

sufficiently to survive the winter. Cover crop residue can modify the conditions under which weeds germinate Cover crops often reduce density and biomass of annual weeds in or regrow in the spring. Such effects could be due to no-till cropping systems. However, cover crops that over-winter also changes in soil temperature, increase in soil moisture, have the potential to reduce crop yield. Currently, there is an interest in annual medics (Medicago spp.) and other annual legumes that release of allelopathic chemicals, and physical impediwinter-kill for use as cover crops in midwestern grain cropping systems. ments to weed seedlings (Facelli and Pickett, 1991; Teas-A 2-yr study was conducted at East Lansing and the Kellogg Biological dale, 1996; Teasdale and Mohler, 1993). Station, Michigan, to investigate the influence of annual legume cover Many legume species that are used as cover crops in crops on weed populations. Two annual medic species [burr medic (M. no-till corn production are winter annuals or short-lived polymorpha cv. Santiago) and barrel medic (M. truncatula Gaertn. cv. perennials. In northern regions of the USA, over-win-Mogul)], berseem clover (Trifolium alexandrinum L. cv. Bigbee), tering species are normally established in the summer and medium red clover (Trifolium pratense L.) were no-till seeded or fall and accumulate most of their biomass when they as cover crops into winter wheat (Triticum aestivum L.) stubble in

The adoption of conservation tillage systems has been challenged by concerns potential weed species shifts. A 9-yr study from 1988 to 1996 was conducted Delhi, Ontario, on a loamy sand soil to evaluate the effect of tillage systems (conventional [CT] and no-till [NT]), cover cropSecale cereale, and nitrogen (N) rate (0, 50, 100, 125, 150, and 200 kg N ha−1) on monocropZea maysL. (corn) yield and changes in the composition of the weed flora. CT consisted of spring moldboard plowing followed by cultivation with a tooth cultivator. Weed counts were taken in the last 3 yr of the study (1994, 1995, and 1996) prior to postemergence herbicide application and then again 2 to 3 wk after herbicide treatment. Composition or the weed flora was analyzed by canonical discriminant analysis (CDA). The relationship between weed density and tillage system was not consistent. Weed species composition differed between CT and NT systems.Chenopodium albumandAmaranthus retroflexuswere associated with CT andDigitaria sanguinaliswith NT. N rate and cover crop did not affect weed density or species composition. Proper management of weeds with herbicides appeared to minimize any long-term effect on the weed flora resulting from varying N rates.Zea maysyields did not differ between CT and NT systems but were greater in both systems with a cover crop at the higher N rates. Disturbance caused by tillage was more important than N rate and cover crop as a mechanism influencing composition of the weed flora.

The development of integrated weed management strategies requires knowledge of mechanisms that influence compositional changes in weed flora. A 9‐year study was initiated in 1988 at Delhi, Canada, on a loamy sand soil to evaluate the effect of tillage systems [conventional (CT) and no‐till (NT)] and cover crops (only in NT) on weed density, species composition and associations, and crop yield in a winter wheat (Triticum aestivum L.)/bean/winter wheat rotation. Three bean types: soyabean (Glycine max L. Merr.), white bean (Phaseolus vulgaris L.) and kidney bean (P. vulgaris L.) were included. The NT system included variations: rye (Secale cereale L.) or maize (Zea mays L.) cover crop, volunteer wheat disked after harvest and wheat stubble. Data were collected in 1994, 1995 and 1996. Tillage systems, cover crops and crop type had differential effects on weed densities, species composition and associations. Weed densities were not affected by tillage or cover crops in wheat but, in the beans, densities were greater in the CT than in the NT systems. Various associations of weed species with tillage system, cover crop and crop type were observed. Crop yields were not affected by tillage type or cover crop, except that soyabean yields were highest in plots with cover crops.