Soil microorganisms are important for the stabilization and preservation of a good soil structure. Management practices can affect the diversity and population of microorganisms, which could beneficially change soil properties and promote a more sustainable dryland system. This study was established near Pontotoc, MS on Atwood silt loam to evaluate the impacts of cover crops and fertilizer sources on selected biologically related soil properties in a no-tillage, dryland soybean system. Soil analyses included total carbon and nitrogen, permanganate oxidizable carbon (POXC), easily extractable glomalin-related soil protein (EE-GRSP), water stable aggregate (WSA) and soil pH. Cover crop biomass and percent groundcover, soybean leaf area index (LAI), plant height, and yield were also determined. Results indicated that fertilizer source did have an impact on total nitrogen, EE-GRSP and soybean yield. A positive impact on soil properties is expected with the use of cover crops if studied for a longer period of time.

Wine vineyards in humid environments like the mid-Atlantic United States are characterized by vines that develop too much vegetative growth for optimum quality wine production. Cover crops, rootstocks and rootzone restriction were evaluated for their effect on vegetative and reproductive growth on Cabernet Sauvignon. Treatments were arranged in a strip-split-split plot arrangement with under-trellis cover crops (UTCC) compared to row-middle only cover crop combined with 1-m weed-free strips in the vine row as main plots. Rootstocks riparia Gloire, 420-A, and 101-14 were sub-plots, while sub-sub-plots comprised two treatments: vines were either planted in root-restrictive (RR), fabric bags (0.016 m3) at vineyard establishment (2006), or were planted without root restriction. All three factors were effective in suppressing vegetative development as measured by rate and extent of shoot growth, lateral shoot development, trunk circumference, and dormant pruning weights. Canopies of vines with UTCC and RR had reduced leaf layer values by approximately 21% and 23% compared to conventional controls. The principal effect of the UTCC and the RR treatments was a sustained reduction in stem (xylem) water potential. UTCC and RR caused significant 7 and 10% reductions in berry weight, compared to their conventional controls. Berry weights of vines grafted to riparia were greater than those of vines grafted to other rootstocks. Wine made from UTCC and RR treatments increased red wine color compared to herbicide UTGC and NRR, respectively. This study identified treatments that improve vine balance while simultaneously improving grape composition and potential wine quality. / Master of Science

The response of upland cotton (Gossypium hirsutum L.) to legume and small grain cover crop establishment, in-season nitrogen (N) rate, and fertilizer N placement was investigated in two experiments located in coastal plain Virginia and North Carolina. The first experiment examined 1) soil compaction and cotton yield response to strip-tillage compared to no-tillage with a precision planted tillage radish and 2) the influence of legume mix, rye, and legume mix/rye combination cover crops with four in-season nitrogen (N) rates applied to cotton on cover crop biomass, cover crop nutrient uptake, soil compaction, soil N cycling, petiole nitrate-N (NO3-N) during the first week of bloom, cotton lint yield, and fiber quality parameters over two years. Legume mix cover crops resulted in greater N uptake, soil NO3-N during the growing season, and lint yields compared to LMR, rye, and fallow treatments over both study years. Soil compaction and lint yields were not significantly different between strip-tilled and no-till with tillage radish treatments in either year. Relative lint yields after LM were maximized at 93% relative yield with 110 kg N ha-1 applied in-season while relative lint yields for cotton following LM with 0 kg N ha-1 applied reached 75%, measuring at least 9% higher than cotton following other cover crop treatments. The second experiment investigated the effect of five N rates (0, 45, 90, 135, and 180 kg N ha-1) and three placement methods (broadcast, surface banded, and injected) on lint yield, petiole nitrate-N (NO3-N), lint percent turnout, and fiber quality parameters. Nitrogen rate and placement had a significant effect on lint yield but only N rate affected petiole NO3-N concentration. It was estimated that injecting fertilizer N requires an N rate of 133 kg N ha-1 to achieve 95% relative yield while surface banded fertilizer N required a rate of 128 kg N ha-1 to produce 90% relative yield. A critical petiole NO3-N concentration threshold of 5,600 mg NO3-N kg-1 was calculated to reach 92% relative yield. Other agronomic management practices such as cover crop termination timing, cover crop species blends, and number of fertilizer N applications are of interest in order to develop better recommendations and promote conservation agricultural practices in coastal plain Virginia and North Carolina. / Master of Science / Upland cotton (Gossypium hirsutum L.) response to diverse species cover crop mixes, conservation tillage method, fertilizer N rate, and fertilizer N placement at side-dress was measured in two field studies conducted on the coastal plain soil in Virginia and North Carolina from 2016-2018. The objectives of the following research were to 1) examine the influence of two conservation tillage practices and four cover crop mixes on cover crop biomass production, soil compaction, cover crop nutrient uptake, soil N cycling, petiole nitrate (NO3-N) and cotton lint yield and 2) measure cotton performance in response to five N rate and three placement application methods. Legume mix (LM) cover crops contained more N in biomass, resulting in higher soil NO3-N during the growing season and higher lint yields at harvest compared to a legume mix and rye combination (LMR), rye, and fallow treatments. Soil compaction and lint yield were not significantly different between strip-tilled and no-till/tillage radish treatments in either year. Nitrogen rate and placement had a significant effect on lint yield but only N rate affected petiole NO3-N concentration. Injection of fertilizer N required an N rate of 133 kg N ha1 to achieve 95% relative yield while surface banded fertilizer N required a rate of 128 kg N ha-1 to produce 90% relative yield. A critical petiole NO3-N concentration threshold of 5,600 mg NO3-N kg-1 was also calculated to reach 92% relative yield. Future application of these results can include investigation of optimal N source for Virginia cotton production, best N placement method for cotton grown in high residue systems, and an economic analysis to determine optimum agronomic management for Virginia coastal plain cotton production.

To understand the impact of N management on harvestable cover crop systems, seven research trials compared: 1) standard intensive management (SIM) (both fall and spring N application), 2) No fall N, a single spring N application, and 3) Cover N (no N application) effects on winter barley (Hordeum vulgare L.) plant biomass (PB), plant N uptake (PNU), grain yield, residual soil nitrate (RSN), and ammonium (RSA). In general, at winter dormancy, SIM resulted in increased PB and PNU but not RSN or RSA. At cover crop termination; SIM and the No fall N practices increased PNU, and at harvesting stage; they produced higher grain yields than the Cover N practice with little significant effect on RSN or RSA values, under normal climatic conditions. While overall yields for the No fall N treatment were lower (8%) than SIM yields, partial net return was similar due to decreased fertilizer input. Nitrogen use efficiency (NUE) of soft red winter wheat (SRWW) can be improved by characterizing genotypes for NUE using canopy spectral reflectance [(CSR), a cheap, rapid and non-destructive remote sensing tool]. The other objectives of this study were to evaluate the predictive potential of vegetative reflection indices for wheat nitrogen use efficiency (NUE) by genotype and the appropriate stages of CSR sensing. An elite panel of 281 regionally developed SRWW genotypes was screened under low and normal N regimes in two crop seasons for grain yield, N uptake, nitrogen use efficiency for yield (NUEY) and nitrogen use efficiency for protein (NUEP). The best models incorporating CSR data at wheat heading explained a significant proportion of total variation in grain yield, N uptake, NUEY and NUEP. Based on the best linear unbiased predictor values, genotypes were ranked and grouped into quartiles and the most efficient and responsive genotypes were identified. A significant proportion of the genotypes with high NUEY under high N conditions also had high NUEY under N stress; however, this was not the case for NUEP. Similarly, a significant proportion of genotypes with high NUEY also had high NUEP under both normal and low N conditions. / Ph. D.

Research examining cover crop-corn systems focuses on corn planted into chemically desiccated cover crop residue using no-tillage practices. Two field studies including rye (Secale cereale L.), hairy vetch (Vicia villosa Roth), and rye+hairy vetch cover crops were established to determine the influence of cover crop management on N availability to corn (Zea mays L.). The presidedress soil nitrate test (PSNT) and the Nitrate Quick Test were utilized to determine their viability for making N fertilizer recommendations in cover cropcorn systems. Soil N concentrations peaked in the majority of cover crop treatments at the corn four to five leaf stage. Delaying cover crop desiccation from early flowering of rye (early) to early flowering of vetch (late) caused an increase in biomass accumulation in all cover crop treatments, but an increase in N accumulation only in vetch. Vetch provided more plant available N, less soil moisture conservation, and higher corn yields than cover crops including rye. Early desiccation enhanced plant available N, soil moisture, and corn yield. Mowing cover crop residue enhanced plant available N, soil moisture, and corn yield. The PSNT was a viable method of assessing corn N sufficiency in cover crop systems, but sampling to a depth of 15 cm was adequate. The Nitrate Quick Test was accurate in determining soil nitrate concentrations, but accuracy decreased as soil nitrate decreased. / Master of Science

Complete vineyard floor cover cropping and root pruning (RP) were evaluated for their ability to regulate excessive vegetative growth and improve berry and wine composition of ‘Cabernet Sauvignon’ (Vitis vinifera L.). Treatments were: tall fescue (Festuca arundinacea Shreb.) ‘KY-31’ and ‘Elite II’, hard fescue (Festuca ovina L.) ‘Aurora Gold’, perennial ryegrass (Lolium perenne L.), orchardgrass (Dactylis glomerata L.), and an under-trellis herbicide strip combined with KY-31fescue interrows. Compared to herbicide strip/non-root pruned (NRP), Elite II fescue reduced vine pruning weight (kg/vine) 28%, individual cane weight (g) 20%, and canopy leaf layer number 25%. KY-31 fescue/RP lowered vine pruning weights 29% compared to an 8% reduction in pruning weights of vines grown in herbicide strip/NRP plots from 2005 to 2010. KY-31 fescue produced the greatest biomass and stand density. With the exception of a yield reduction in vines grown with KY-31 fescue in 2006, cover crops minimally decreased grape yield. Yearly climatic variation had a greater effect on berry weight and composition (pH, TSS, TA) than did treatments. Limited treatment differences detected in chemical compounds by gas chromatography–mass spectrometry (GC-MS) analysis in wines made from treatment vines in 2010 were correlated to descriptive sensory terms. Cover crop water use, as evapotranspiration, determined by mini-lysimeter (ML), ranged from 3.28 mm/d for KY-31 fescue to 1.52 mm/d for herbicide-treated plots. In 2008, root biomass of vines grown on KY-31 fescue/RP was increased at the 60 to 80 and 80 to 100 cm soil depths compared to root biomass of KY-31 fescue/NRP vines at those depths. Cover crops minimally impacted vine water potential (ΨPD, Ψmd, Ψstem) and grapevine nitrogen levels relative to the herbicide strip, indicating that the grasses were not overly competitive with grapevines. Root pruning and complete vineyard floor cover crops favorably reduced grapevine vegetative growth, although treatment effects diminished over time, possibly in response to redistribution of grapevines’ roots and climatic variation at the site. / Ph. D.

The diverse cropping system of eastern Virginia's coastal plain offers limited opportunity to establish winter annual cover crops (WCC) for nitrogen (N) scavenging. The winter fallow niche after double-crop or full-season soybean (Glycine max L. Merr.) encompasses the majority of acres left fallow. Our objective was to evaluate interseeded WCC N scavenging performance following soybean and N supplying capacity to subsequent corn (Zea mays L.). Field studies were conducted at four different locations in each of the two study years. The experimental design was split plot with cereal rye, hairy vetch, and RV mix WCC as main plots and ten fertilizer nitrogen (FN) rates in a factorial arrangement (0 and 45 kg FN ha-1 as starter; and 0, 45, 90, 135, and 180 kg FN ha-1 at sidedress) to corn as subplots. The highest N uptake for cereal rye at winter dormancy was 18 kg N ha-1, but the average was 6-7 kg N ha-1. At WCC termination average N uptake for cereal rye was 35 and 40 kg N ha-1 in 2013 and 2014, respectively. Average biomass dry matter (DM) at WCC termination for cereal rye, cereal rye + hairy vetch mix (RV mix), and hairy vetch was 2356, 2000, and 1864 kg ha-1 in 2013; and 2055, 2701, and 692 kg ha-1 in 2014, respectively. Average cereal rye N uptake was 35 kg N ha-1 in 2013 and 40 kg N ha-1 in 2014. Significant differences for residual soil nitrogen were most apparent for soil nitrate (NO3-N) at lower depths (15-30 and 30-60 cm) during WCC termination and in the upper 0-15 cm during corn growth stage (GS) V4 of both years. Corn grain yield plateau following hairy vetch WCC was 0.7 and 0.6 Mg ha-1 higher than when following cereal rye WCC at zero and 45 kg ha-1 starter FN, respectively. Average agronomic optimum FN rates (AONR) were 26 and 9 kg ha-1 lower following hairy vetch than cereal rye WCC at zero and 45 kg ha-1 starter FN, respectively. Estimated hairy vetch FN reductions by FN replacement and AONR difference methods were 48 and 18 kg FN ha-1 in plots receiving zero starter FN; and 58 and -43 kg FN ha-1 in plots receiving 45 kg ha-1 starter FN. In-season soil N tests did not offer adequate information in order to predict sidedress FN reductions. These findings suggest that cereal rye and RV mix have the potential to scavenge and conserve residual soil N and hairy vetch is more than capable to supply PAN to subsequent corn when interseeded into soybean. / Master of Science

The objectives of this research were: 1) to quantify the amount of supplemental nitrogen (N) to maximize organic broccoli (Brassica olearcea var. italica) on transition soils, 2) to evaluate the ability of leguminous cover crops lablab (Dolichos lablab L.), soybean (Glycine max L.), sunn hemp (Crotalria juncea L.) and a sunn hemp and cowpea mixture (Vigna sinensis Endl.) to supply N and suppress weeds and, 3) to compare the effect on N availability and broccoli yield potential of incorporating cover crops with conventional tillage (CT) or mulching cover crops with no-tillage (NT) practices. Broccoli was grown during the third year of organic transition in the spring and fall of 2006 at the Kentland Agricultural Research Farm in Blacksburg, VA. Supplemental N significantly increased broccoli yield up until 112 kg ha-1 with a quadratic correlation with leaf N. The NT treatment yielded no difference during the spring, but in the fall CT surpassed NT. On the other hand, N uptake, measured by leaf N, under NT conditions increased with supplemental N, which suggests NT has equivalent yield potential as CT when N is not limiting. Yields from leguminous residues did not differ, even though quality and quantity of cover crop biomass did. This suggests that N availability from cover crop legumes may be impacted other ecological process such as soil microbial activity. Also, cover crop residues differed in their ability to suppress weeds. The results from this study give organic growers in transition tools to maximize productivity and sustainability. / Master of Science

M.S.

Anaerobic soil disinfestation (ASD) is a promising alternative to chemical fumigation to control soil-borne plant pathogens and weeds. This research focused on evaluating several locally available carbon sources for ASD on weed control, evaluating the performance of brewers' spent grain (a promising carbon source) under field conditions, and evaluating whether yeast addition enhanced the effectiveness of ASD treatments. A series of greenhouse trials were conducted at the Southern Piedmont AREC (Agricultural Research and Extension Center). The greenhouse trials were conducted in PVC tubes, 20 cm tall and 15 cm in diameter. The first set of trials evaluated ASD conducted over 21-day periods of ASD using locally available carbon sources. The carbon sources included brewer`s spent grain, buckwheat (Fagopyrum esculentum), cowpea (Vigna unguiculata), paper mulch, peanut (Arachis hypogaea) shells, rice bran, sorghum-sudangrass (Sorghum drummondii), and waste coffee grounds applied at 4 mg of C/g of soil. The targeted weed species included common chickweed (Stellaria media (L.) Vill.), redroot pigweed (Amaranthus retroflexus L.), white clover (Trifolium repens L.), and yellow nutsedge (Cyperus esculentus L.). All ASD treatments significantly reduced weed viability compared to the non-treated control. The yeast amendments enhanced weed control over ASD without yeast. The second set of greenhouse trials was focused on ASD using brewer`s spent grain, and on evaluating ASD at the half and one-third carbon dose rates. The target pests were the same weed species in the first set of trials, and Pythium irregulare was added as an additional target pest. This set of trials indicated yeast enhanced addition the effect of BSG in ASD on both weeds and P. irregulare, indicating the potential to reduce carbon input necessary for effective ASD. A follow-up, two seasons, open-field trial conducted over two growing seasons at the Hampton Roads AREC focused on understanding the effects of ASD on weed density and strawberry fruit yield and fruit quality in annual hill strawberry production. The treatments included ASD at standard or half carbon dose rates, with or without yeast. Fumigation (80% chloropicrin + 20% 1,3-dichloropropene) and non-treated plots were used as control groups. Weed suppression with ASD was consistent for most of the broadleaf weed species, and total weed counts were significantly reduced compared to non-treated controls. Yield from ASD with yeast was higher than ASD without yeast and non-treated control in one growing season, while the increase in yield did not occur in another growing season. Yeast may have potentially enhanced the yield effects of ASD but lacked consistency. Yeast may have the potential to enhance ASD effectiveness. / Doctor of Philosophy / Strawberry is a high-value crop known for its brightly colored, sweet tasting, juicy and fleshy fruit that possesses a unique aroma. The southern region is the second large region of strawberry production in the United States. Strawberry is susceptible to soil-borne pests, including weeds and diseases. Preplant control of soil-borne diseases and weeds is important for strawberry production. Early season weeds can compete with newly transplanted strawberry plugs for nutrients, light, and other resources. However, currently, the limited options of pre-plant chemical fumigants and herbicides available in strawberry plasticulture make weed control a challenge in strawberry production. Anaerobic soil disinfestation (ASD) may be an effective alternative to preplant chemical fumigation. Anaerobic soil disinfestation involves three steps- applying carbon sources to the soil, covering the bed with black tarp, and watering the soil to maintain certain soil moisture to field capacity generally for 21 days. However, there are only a few studies on weed control using ASD in the southern region; locally available carbon sources also need to be evaluated. Thus, this study focused on evaluating several locally available carbon sources (cover crops, brewer`s spent grain, used coffee ground, paper mulch, peanut shell) for ASD to control troublesome weeds (common chickweed, redroot pigweed, white clover, yellow nutsedge). This study also explored a new method that involves mixing distiller's yeast with solid carbon sources in order to enhance the ASD weed control effect. Additionally, this study evaluated the effect of ASD using reduced carbon inputs, potentially reducing the total cost of ASD by reducing the carbon input. A series of greenhouse studies were conducted at the Southern Piedmont Agricultural Research and Extension Center (AREC), Blackstone, VA, with a follow-up field study done at the Hampton Roads AREC. The greenhouse trials evaluated carbon sources including brewer`s spent grain, buckwheat, cowpea, paper mulch, peanut shells, rice bran, sorghum-sudangrass, and waste coffee grounds. These greenhouse experiments were conducted in containers made from PVC tubes, and strawberry plants were not involved. The main objective of the greenhouse trial was to test the suppression of four troublesome weeds, including common chickweed, redroot pigweed, yellow nutsedge, and white clover. The most effective treatments in the greenhouse studies were further investigated in the field trial. The brewer`s spent grain was again used in the field trial, and treatments included ASD using a full or half dose of brewer's spent grain, with or without yeast. We evaluated the effects of these treatments on weed control, plant crop growth, and crop yields. Fruit quality factors, including fruit firmness, sweetness, and size, were also evaluated. In summary, all of the carbon sources evaluated provide similar weed control. Adding yeast showed potential to enhance the effect of ASD using brewer`s spent grain. Adding yeast also increased the effectiveness of the half-rate of the carbon source, showing the potential for effective pre-plant pest control for strawberry using ASD treatments with significantly reduced C dose rates.