The blade of the cultivator passes through the soil at 2 to 6 inches deep, which may explain why greater soil compaction was measured there. Cultivations often also occurred when the soil was still moist following an irrigation, which may have contributed to the development of compacted layers over time.Moisture. Average, volumetric soil moisture levels at the 6- to 42-inch depth increased after the first rain events of the season, such as in winter 2002-2003 . Soil moisture declined most rapidly with ‘Merced’ rye in the middles during periods without rainfall each year , presumably due to its greater early-season growth and greater potential evapotranspiration, compared to the ‘Trios 102’ triticale. Soil moisture levels were similar between the bare and ‘Trios 102’ triticale treatments until May for all years. During the irrigation season, average soil moisture levels at the 6- to 42-inch depths were higher in rows than middles. Soil moisture in the rows and middles steadily declined during the irrigation season for all treatments during all years . Moisture levels declined most in middles with ‘Trios 102’ triticale cover during each irrigation season, presumably due to the later growth of this cover crop . In addition, the row soil-moisture levels also declined the most adjacent to ‘Trios 102’ triticale for the 2003 and 2004 irrigation seasons , vertical grow rack system but not during the 2005 irrigation season . Runoff. Total precipitation at the field trial was 7.4 inches during the 2002-2003 winter, 7.6 inches during the 2003-2004 winter and 9.9 inches during the 2004-2005 winter.
A majority of the runoff was collected during Decemberand January for the 2002-2003 and 2004-2005 winters, and February for the 2003-2004 winter. Cumulative runoff collected from individual plots during the three winters ranged from 0.02% to 3% of seasonal rainfall. Runoff was usually collected during rain events greater than 1 inch per day. Runoff was highest during the second and third years of the trial. During three consecutive winters, runoff was significantly lower in the covercrop treatments . ‘Trios 102’ triticale and ‘Merced’ rye had significantly less runoff than the bare treatment . Suspended sediment and turbidity were also significantly lower in runoff collected from the cover-crop treatments than in bare middles during winter 2004, but nutrient levels were similar among all treatments .Vines. Weed control and cover treatments did not have any significant effect on the nutritional status of the grape vines as measured by nutrient levels of the leaf petiole tissues, as determined by ANOVA. Although the nutrient levels by year were significantly different, the interactions of weed control-by-cover and weed control-bycover-by-year were not significant . Weed control and cover treatment also had no significant effect on blade nutrient content with the exception of boron and phosphate content. Vines adjacent to cover crops had significantly lower boron and phosphate levels in the leaf blade tissue than vines adjacent to bare row middles.
As with the petioles, there was an absence of significance between the interaction of weed control-by-cover and weed control-by-cover-by-year for all nutrients analyzed .Soil cores indicated that most of the vine roots at this site were located under the vine row and few of the roots extended out to the row middles. This root distribution probably occurred because irrigation water was applied under the vines, and low rainfall at the site does not facilitate root growth into row middles. Thus, the lower nutrient levels in vines near cover crops may have been accentuated by irrigation effects that reduced vine root exploration of the soil to a narrow band under the vines. Since cover-crop roots probably grew into this zone there may have been competition between vines and cover crops for some nutrients. Soil. Cultivated rows had significantly lower levels of nitrate-nitrogen . Although the nutrient levels by year were significantly different, there was an absence of significance between the interaction of weed control-by-cover and weed control-by-cover-by-year . The differences observed in nitrate-nitrogen in the cultivation treatment may be due to the impact of loosening soil on water movement and leaching. Weed control treatments had occasional impacts on soil mineral nutrition in the middles, but results were inconsistent from year to year .
The most significant impacts of the vineyard floor treatments were of the cover-crop treatments on soil parameters in the middles. Soil organic matter in cover-cropped middles was higher than in bare middles each year . Cover crops affected key soil nutrients in the middles; for instance, cover crops greatly reduced nitrate-nitrogen , and to a lesser extent, extractable phosphorus , which may be beneficial in reducing loss of these nutrients in runoff during winter storms, but which also may have reduced the phosphorus content in the vines. In addition, cover crops in the middles also significantly reduced soil boron , extractable sodium and pH , and increased chloride and zinc when compared to bare soil.Soil microbial biomass. Microbial biomass varied as a result of both the cover-crop and weed control treatments. In both the middles and vine rows, microbial biomass was higher in rye cover-crop plots compared to bare plots . These results confirm earlier observations by Ingels et al. that microbial biomass carbon was higher in cover-cropped middles compared to bare middles. In the vine rows, microbial biomass was greater in plots adjacent to rye cover-cropped plots compared to bare plots. The effect of cover crops grown in the middles on soil in the vine rows may be due to cover-crop roots or tops extending into the vine rows and their subsequent decomposition, providing a food source for soil microbes. Microbial biomass varied between the weed treatments in the vine rows but not middles . In the vine rows, microbial biomass was significantly higher in the cultivation plots compared to the pre-emergence weed control plots . The most likely explanation is the incorporation of greater amounts of weed-derived carbon into the surface soil of the cultivated plots. Mycorrhizae. AMF can benefit grapevines by improving the nutritional 35 30 25 20 15 10 5 0 Cultivation Post-emergence Pre-emergence herbicide herbicide Bare ground Merced rye Trios 102 status of the plant and producing a highly branched root system. We quantified AMF reproductive structures in grapevine roots to determine if the weed control treatments in the rows and/or cover-crop treatments in the middles had significant effects on mycorrhizal colonization from 2003 through 2005. Based on ANOVA, the effects of weed control on colonization were not consistent among covercrop treatments . Grapevines adjacent to ‘Merced’ rye had higher colonization compared to those adjacent to ‘Trios 102’ triticale or bare ground, in both the cultivation and pre-emergence treatments . In contrast, grapevines in the postemergence treatment had the lowest colonization when adjacent to ‘Merced’ rye. These findings were consistent in each study year, based on the absence of significant main or interactive effects of time . It is possible that low colonization of grapevines in the post-emergence-by-‘Merced’ rye treatment is associated with this treatment’s weed community. Indeed, weed species vary in their ability to host AMF , so their presence or absence may affect mycorrhizal colonization of grapevines. Indeed, reports on the influence of plant community composition on AMF suggest that plant diversity has a strong effect on AMF diversity , and this may affect the colonization of individual plant species.All yield, fruit quality and vine growth parameters varied by year, and this was the only significant effect for these parameters, vertical farming racks with the exception of berry weight and titratable acidity . No differences in crop yield or fruit composition were observed from 2001 to 2005 due to weed control treatments . Cover-crop treatments also had no significant effect on yield or fruit composition, although in 2001 and 2004, there was a reduction in berry size in the ‘Trios 102’ triticale treatment. Weed control treatments also had no effect on vine growth , based on shoot counts and pruning weights taken at dormancy. Cover-crop treatments had no significant effect on vine growth when averaged over 5 years, although in 2001 and 2005 the ‘Trios 102’ triticale treatment significantly reduced pruning weights. The trend for lower pruning weights may be related to the greater decline in soil moisture in the middles where this cover crop was used.
It appears that vine growth, yield and grape quality are more significantly affected by annual precipitation than by vineyard floor management practices.In low rainfall areas the choice of cover crop is critical because of its effect on available soil moisture. We observed that late-maturing ‘Trios 102’ used more soil moisture during the vine growing season; if irrigation water does not compensate for water used by the cover crop, reduced vine growth and yield losses may result. The clear benefits of cover crops were increased organic matter in the middles and reduced sediment loss. Microbial biomass was increased in cover-cropped middles and there were indications that this effect extended to under the vines. Although there were no negative impacts of weed control treatments on vine productivity, we observed increased compaction over time from the use of cultivation. This study indicated that the choice of weed control strategy and cover crop must be carefully considered to maximize the benefits and minimize negative impacts of the practices. The benefits of cover crops are concentrated in the middles,and future research should focus on evaluating practices that improve the quality of soil under the vines.Understanding the drivers of plant community composition is increasingly critical as many ecosystems face novel species interactions due to invasion and global change. Plant-soil feedbacks can impact plant neighbor interactions and drive legacy effects of former plant communities on current community composition . Plants can change soil physical, chemical and/or biological properties, which can impact plant growth or fitness, and thus the trajectory of the community . A large body of scholarship demonstrates the existence of PSFs but there are still key gaps in our understanding of how important they are, relative to other drivers of plant community composition, and thus, many have called for investigating the role of PSFs in long-term field settings that incorporate competition at the community level . Recent studies demonstrate that plant-soil feedbacks may be dependent on the length of the experiment, and often differ in field settings compared to greenhouse conditions . Short conditioning phases may over-emphasize the role of microbial communities and nitrogen cycling and miss the impacts of longer-term changes to soil organic matter and water holding capacity . Similarly, short feedback phases can fail to capture the impact of annual variation in environmental conditions or how feedbacks can develop over time . Further, in variable field environments, it may be important to not only consider surface soil, but also plant impacts on subsurface soil. Similarly, it may be important to go beyond a focus on above ground biomass as an indicator of feedback, instead assessing multiple traits across multiple life stages . There is particular interest in the scientific and management communities in understanding how plant-soil feedbacks may mediate interactions between invasive and native plant species. PSF studies have shown that native plants tend to have negative feedbacks, often through accumulation of pathogens decreasing native plant performance. This negative feedback likely drives plant co-existence and diversity . In contrast, invasive plants can shift microbial community composition and physical and chemical properties to benefit themselves , sometimes to the detriment of native growth . This suggests that successful restoration of natives into invaded sites may require an initial step of restoration of soil conditions. Both inter- and intra- specific plant competition influence the overall importance of PSFs in natural communities. Direct competitive interactions among plants are known to interact with or be additive to plant-soil feedbacks . For example, feedbacks impacting a weaker competitor’s performance may be overwhelmed by competition from a stronger neighbor or feedbacks negatively impacting a stronger competitor may allow co-existence of the weaker species . By assessing feedbacks on plants growing in a community instead of neighbor-less plant individuals, we can better evaluate their relevance in a natural setting .We use multiple plant performance variables to assess the importance of PSFs in California grasslands with a long-term field experiment using mixed communities composed of native and/or exotic species. These grasslands are heavily invaded with exotic annual grasses and forbs, with the loss of natives occurring more than 300 years ago .