Quantifying the role of soil microbial activity in cover crops on newly established fruit trees in the Western Cape

Thesis (MScAgric)--Stellenbosch University, 2022.ENGLISH ABSTRACT: Limited research is available on the role of cover crops in newly established deciduous fruit orchards in South Africa. Thus, three field studies were conducted to evaluate the contribution of cover crops towards soil health/fertility. The effect of cover crop combinations on soil characteristics (mineral composition, soil moisture, soil compaction, water holding capacity, microbial activity and tree performance) and the contribution towards weed suppression and biomass production for mulching in a perennial orchard were quantified during two and three consecutive seasons in two locations. In addition, soil microbial activity and diversity were quantified with three locally available methods (Molecular fingerprinting, CO 2-burst test and gas chromatography). A two-year field study (Trial 1 and 2) was conducted in Stellenbosch, (33°56'51.27" S 18°52'19.29" E) and a three-year field study (Trial 3), in Elgin (34°09’16.83’ S, 19°02’28.01” E), Western Cape. Trial 1 (Multi species: forage rye, radish and white mustard) and trial 2 (Single specie: forage rye) comprised four treatments each: i) Control (natural vegetation), ii) Cover crop with no fertilizer application (MNFA; SNFA), iii) Cover crop with a single organic fertilizer application (MSFA; SSFA) and iv), Cover crop with a double organic fertilizer application (MDFA; SDFA). Trial 3 consisted of five annual winter cover crop treatments: i) Phacelia (PC) ii) Forage rye & vetch (FRVC) iii) Forage radish & white mustard (FRWMC), iv) Forage barley & peas (FBPC) v), Forage rye (FRC). Cover crops provided a significantly higher above- and below ground biomass compared to natural vegetation in trial 1 and 2, however, biomass differed amongst cover crop treatments. Biomass production differed significantly between cover crop treatments in trial 3. FRC performed the best, followed by FRVC and FRWMC. Crop species were the primary factor in plant performance and other factors contributed less towards biomass. However, weed suppression efficiency was affected by cover crop biomass and species. In trial 3, Forage rye, in combination or monoculture, performed the best in terms of consistency, biomass for mulching purposes, root length and weed suppression. Cover crops did not affect tree performance at this stage but should be monitored in future when tree roots will reach the work row. Cover crops directly contributed towards stimulation of the microbial community and conserved soil moisture compared to the control (natural vegetation) during spring. However, from our results it was evident that soil is an interlinking system and that it should be managed holistically in terms of soil physical, chemical and biological components. A long-term study should be initiated to evaluate the effect of using cover crops to improve soil health (Soil physical, chemical and biological interrelationship) and how improving the soil biological component will contribute to an alternative system such as farming with nature, minimizing external inputs, building soil structure and providing nutrient dense food.AFRIKAANSE OPSOMMING: Daar is beperkte inligting beskikbaar oor die implimentering van dekgewasse in nuut gevestigde vrugte-boorde in Suid-Afrika. Drie veld studies is uitgevoer om die bydra van dekgewasse op grond-gesondheid en -vrugbaarheid te evalueer. Die effek van dekgewas kombinasies op grond einskappe (minerale-samestelling, grondvog, grond- kompaksie, waterhouvermoë, mikrobiese aktiwiteit) en boom prestasie en die bydra tot onkruid-onderdrukking en biomassa produksie van ‘n deklaag in ‘n meerjarige boord is gekwantifiseer gedurende twee en drie opeenvolgende seisoene, in twee verskillende geografiese gebiede. Additioneel is grond-mikrobe aktiwiteit en - diversiviteit gekwantifiseer met drie plaaslik beskikbare metodes (molekulêre vingerafdruk, ‘CO 2-burst’ en gas-chromatografie). ‘n Twee-jaar veld-proef (Proef 1 en 2) is uitgevoer in Stellenbosch (33°56'51.27"S 18°52'19.29"E) en ‘n drie-jaar veld proef (Proef 3), in Elgin (34°09’16.83”S, 19°02’28.01” E), Wes-kaap. Proef 1 (Multi-spesies: voerrog, radys en wit mosterd) en proef 2 (Enkel spesies: voerrog) het beide uit vier behandelinge bestaan: i) Kontrole (onkruid), ii) Dekgewas met geen kunsmis (MNFA; SNFA), iii) Dekgewas met ‘n enkele organiese kunsmis toediening (MSFA; SSFA) en iv), Dekgewas met ‘n dubbele organiese kunsmis toediening (MDFA; SDFA). Proef 3 het uit vyf eenjarige dekgewasse bestaan: i) Phacelia (PC), ii) Voerrog & wieke (FRVC), iii) Radys & wit mosterd (FRWMC), iv) Gars & ertjies (FBPC) en v), Voerrog (FRC). Dekgewasse het ‘n betekenisvolle hoër bo- en ondergrondse biomassa gelewer in vergelyking met die kontrole in proef 1 en 2, maar die biomassa het wel verskil tussen die dekgewas-behandelings. Biomassa produksie het betekenisvol verskil tussen die dekgewas-behandelings in proef 3. FRC het die beste gevaar, gevolg deur FRVC en FRWMC. Plant spesies was die primêre faktor wat plant- prestasie bepaal het en enige additionele faktore het minder bygedra tot biomassa produksie. Dekgewasse het ‘n positiewe effek op onkruid-onderdrukking getoon en kan as alternatief gebruik word. Die effektiwiteit van onkruid-onderdrukking is beïnvloed deur dekgewas-biomassa, asook die spesies. In proef 3, het voerrog, in kombinasie met ander dekgewasse of as monokultuur het die beste gevaar in terme van konsekwentheid, biomassa vir deklaag doeleindes, wortel-lengte en onkruid- onderdrukking. Dekgewasse het geen effek op boom prestasie gehad nie, maar dit moet gemonitor word oor tyd. Dekgewasse het ‘n direkte bydra gehad tot die stimulasie van mikrobiese-populasies en grondvog beter bewaar as die kontrole gedurende die lente. Dit was duidelik uit ons resultate dat grond ‘n dinamiese sisteem is en dat dit holisties bestuur moet word in terme van grond fisiese, chemiese en biologiese komponente. ‘n Lang-termyn studie is nodig om die gebruik van dekgewasse om grond-gesondheid (grond fisiese, chemiese en biologiese verhouding) te verbeter te evalueer en te bepaal hoe verbetering van die biologiese komponent sal bydra tot ‘n alternatiewe sisteem soos natuur boerdery, minimalisering van toediening van eksterne produkte, die verbetering van grond-struktuur en voorsiening van voedingsdigte voedsel.Master