HARNESSING THE POWER OF SORGHUM-SUDANGRASS HYBRIDS TO TRANSFORM SOIL HEALTH IN ANNUAL ROW CROP SYSTEMS: A NARRATIVE FROM NEMATODES AND BEYOND.

Abstract

Sorghum and sorghum-sudangrass hybrids (SSgH, Sorghum bicolor) are fast-growing, carbon-rich, drought-tolerant cover crops known for their soil building capability and biofumigation property. However, practices to terminate the cover crops to maximize soil health enhancement effects often compromise the biofumigation potential of the cover crop. The overall goal of this dissertation was to explore approaches to maximize SSgH ability to improve soil health while suppressing soilborne pathogens (Meloidogyne incognita, Rotylenchulus reniformis, Fusarium commune). It was hypothesized that SSgH performance would vary by SSgH variety, and their soil health improvement ability would also differ by initial soil health conditions. This dissertation focused on the use of nematode community analysis and soil microbial profiles estimated by phospholipid fatty acid (PLFA) analysis as soil health indicators to understand the changes in soil food web structure over multiple cropping cycles of a SSgH and eggplant (Solanum melongena) rotation. This dissertation is organized into five chapters. Chapter one provides a comprehensive literature review on the use of SSgH cover cropping and the nematode community as indicators of soil health. Chapter one also addresses the challenge of maximizing the soil health benefits of SSgH while ensuring that their biofumigation capabilities are not compromised. Chapter two documents the first attempt to terminate several varieties of SSgH cover crops in a no-till system. Practicing SSgH cover cropping in a no-till system only improved soil moisture and increased the abundance of omnivorous nematodes (indicating improvement in the stability of the nematode communities) without suppressing plant-parasitic nematodes (PPNs) after an eggplant crop. This result was not surprising as it is anticipated that biofumigant from SSgH (hydrolysis of dhurrin to hydrogen cyanide, HCN) is highly volatile. Thus, it is important to incorporate SSgH residues into the soil. Chapter three adopts an alternative approach to evaluate the performance of SSgH cover crops in strip- and low-till systems. Strip-till method minimizes soil disturbance while capturing some of the HCN in the soil. To examine SSgH cover crops in two soil health conditions, a total of 4 SSgH-eggplant field trials were conducted at the Poamoho Experiment Station, University of Hawaii. Two of these trials were conducted successionally at Location 1 previously fallowed for more than 5 years (nutrient depleted and disturbed), whereas the other two were conducted successionally at a 5-year no-till cover crop site (Location 2, nutrient enriched but disturbed). In all field trials, 7 SSgH varieties were grown for 3 months, terminated using a flail mower followed by handheld tilling of 20-cm wide, 10-cm deep strips. Two successional trials were conducted at each location. As hypothesized, the performance of SSgH in a low-till system was found to be location-specific. A rapid response of nematode communities was observed in nutrient-depleted and disturbed soil (Location 1), but a more subtle or gradual response to SSgH was found in the already biologically active soil (Location 2). Nonetheless, low-till SSgH cover cropping improved soil health and managed plant-parasitic nematodes in both locations. This field study also demonstrated that comprehensive soil health improvements required more than one cropping cycle of SSgH-cash crop rotation to achieve top-down regulation of plant-parasitic nematodes. Once the soil became more enriched and structured, a multivariate analysis showed that eggplant yield was negatively related to the abundance of root-knot nematodes and root gall index on eggplant, but positively related to volumetric soil moisture, volumetric aggregate stability, water infiltration, SSgH biomass, enrichment index, structure index, and Solvita microbial respiration. These results supported the hypothesis that promoting soil health by SSgH strip-till cover cropping would lead to the suppression of plant-parasitic nematodes. Chapter four examines factors affecting the SSgH biofumigation effect, including SSgH varieties, age of SSgH, environmental stress, and mulching vs soil incorporation. Initially, dhurrin concentration from 2-month-old tissues of 7 varieties of SSgH was analyzed using high-performance liquid chromatography (HPLC) and was determined to be highest in ‘NX-D-61’ and ‘Latte’. This HPLC assay confirmed differential concentrations of dhurrin among SSgH varieties. Regression analysis between dhurrin content and the number of R. reniformis infecting cowpea roots revealed a strong negative relationship (r² = 0.69; p = 0.02). Greenhouse and field trials were then conducted to examine the age effect of SSgH on biofumigation against Meloidogyne incognita and Rotylenchulus reniformis. The shoot biomass of several SSgH varieties was harvested at 1, 2, and 3 months of growth and used as soil amendments compared to an unamended control. Two greenhouse bioassays showed that ‘NX-D-61’ and ‘Latte’ amendments were most suppressive against the female development of M. incognita on mustard green (Brassica juncea) regardless of SSgH age, although other varieties showed a decrease in biofumigation effect against M. incognita as SSgH matured. When tested against R. reniformis, tissue amendment of all varieties of SSgH suppressed R. reniformis development in cowpea (Vigna unguiculata) roots However, discrepancies were observed in the age effect of SSgH against R. reniformis between trials. In Trial I, 2-month-old amendments had the greatest suppression, whereas in Trial II, 1-month-old amendments were more suppressive to R. reniformis. This led to a notion of the environmental effect on the dhurrin content. The 2-month-old tissues of SSgH in Trial I were collected from the driest month, making 2-month-old tissues more potent than the 1-month-old tissues, consistent with the literature that suggested dryer conditions led to higher dhurrin content in SSgH. To further examine the biofumigation effect of SSgH on soil-borne fungal pathogens, 2-month-old ‘NX-D-61’ tissue was amended into or mulched on field soil infested with Fusarium commune and planted with mustard green in greenhouse pot trials. Soil incorporation was more effective than surface mulching of ‘NX-D-61’ in reducing the disease severity of F. commune on mustard green in two greenhouse trials. The effectiveness of ‘NX-D-61’ biofumigation in the field against F. commune was examined in two trials, but only one of these trials suppressed F. commune where the cover crop was terminated at 1-month old and following two successive plantings of SSgH-mustard green rotation. Chapter five presents the conclusion of the dissertation. Improvements in soil health through SSgH cover cropping depend on both the SSgH varieties and the initial soil health conditions. High-dhurrin SSgH varieties can generate effective biofumigants when soil-incorporated at 1- or 2-month old against soil-borne pathogens, M. incognita, R. reniformis, and F. commune, but the effect might be better if continuous strip- and low-till practice of SSgH-crop rotation for at least 2 times in the same location. This dissertation clarified factors that affect SSgH biofumigation against soil-borne pathogens, especially plant-parasitic nematodes, and demonstrated that the benefits of SSgH do not have to compromise each other. In fact, better soil health improvement by SSgH could lead to better suppression of plant-parasitic nematodes beyond that achieved by SSgH biofumigation alone.