The effects of herbicides on soil chemical and soil microbial properties of riparian fynbos ecosystems

Cogill, Liam Sean (2019-03)

Thesis (MScConsEcol)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Woody invasive alien plants (IAP’s) have various impacts on natural ecosystems in the Western Cape Province of South Africa. These impacts include the absorbance of large quantities of water, increased veld-fire risk, the endangerment of local indigenous flora as well as increased homogeneity of various riparian fynbos habitats. To combat the invasion of IAP’s, the WfW programme has been established. Woody invasive species belonging to genera such as Eucalyptus, Acacia, and Populus are treated with specialized herbicides to eradicate the trees and to ultimately prevent further encroachment in demarcated areas. The herbicides used have proven to be effective at controlling the targeted IAP’s and is considered an agricultural or natural remedy for rehabilitating fynbos habitats. Although effective, these herbicides could have unforeseen detrimental impacts on non-target plant species as well as on the local soil environment. Many researchers have found that herbicides and other agrochemicals can affect both biotic and abiotic properties of the soil. Additionally, other researchers have shown that herbicides can in fact stimulate the soil environment and moreover induce no harm. There is an increased pressure from woody IAPs on riparian fynbos and other habitat types, which calls for increased usage of herbicides to control and effectively manage the encroaching weeds. It is, therefore, imperative that the potentially detrimental or beneficial effects of these herbicides on riparian fynbos are determined. This study therefore aimed to assess the potential impacts that conventional herbicides may have on the soil environment of riparian fynbos habitats in the Western Cape. The herbicides tested were those registered for treating A. mearnsii and E. camaldulensis which are woody invasive plants encroaching rather frequently in riparian fynbos areas. Two different herbicide products namely, Springbok 360 SL and Garlon 480 EC, were tested in stands of A. mearnsii, whilst one product namely, Plenum 160 ME, was tested in E. camaldulensis stands. Impacts on the soil environment were determined by studying the change in various soil chemical characteristics (such as soil pH, electrical conductivity, total soil nitrogen, nitrogen mineralization, soil available phosphorus, and phosphatase activity). Additionally, soil microbial activity was studied by evaluating carbon substrate metabolism (via microtiter eco-plates). Microbial community structure was also studied, and this included evaluating the change in measures of species richness and evenness (OTU’s and Shannonindex). The herbicides were applied at different concentrations [i.e. one tenth of the field rate (10 % FR), one times the field rate (100 % FR), ten times the field rate (1000 % FR), and untreated control (0 % FR)] in both in situ and ex situ experiments which ran for a duration of six weeks. When either of the herbicides were applied, soil pH levels decreased significantly (KruskalWallis ANOVA: p < 0.05) in both in situ and ex situ experiments. The pH generally dropped with 0.5 units or more and it appeared that the herbicides all had a slight, yet consistent acidifying effect on the soil. Additionally, soil available nitrogen increased significantly when Springbok 360 SL (glyphosate) treatment took place (Kruskal Wallis ANOVA: p = 0.02), where soil nitrogen levels increased from the tens to over 100 µg.g-1 . The acidity and nitrogen changes in the soil expressed significant changes with herbicide treatment, but other soil chemical characteristics studied showed little to no considerable impacts. An example includes the study of soil available phosphorus which revealed no changes when testing either herbicide treatment, even at the higher concentration dosages [In situ: Soil available P remained between 8 and 16 ppm, One Way ANOVA = p > 0.87; Ex situ: Soil available P remained between 6 and 14 ppm, One Way ANOVA = p > 0.53]. Similarly, soil microbial community structure seemed unaffected where both microbial carbon substrate utilization and soil microbial community structure remained stable. Soil microbial activity and community metabolic capacity in soils that were treated with herbicide showed no significant differences when compared to untreated soils. The rates of metabolism remained within a range of 1 and 4 absorbance at hour 168 of incubation. Furthermore, the microbial species richness and evenness showed variability (standard deviation values as high as 18) yet remained stable according to OTU numbers and Shannon index (One Way ANOVA: Springbok 360 SL treated, p = 0.76; Garlon 480 EC treated, p = 0.19; Plenum 160 ME treated, p = 0.06). An ANOSIM further presented great degrees of similarity of microbial groups before and after herbicide treatment irrespective of the concentration applied (R statistic values ranging between 0.05 and 0.15 showing for very little amounts of dissimilarity). The deduction from these results were that no detrimental or inhibitory effects occurred with treatment of either herbicide within a six-week period. As mentioned above, the herbicides tested showed an acidifying effect on fynbos soils yet lack any other detrimental impacts on the soil environment when regarding the other various aspects studied. This study revealed that these herbicides have minor impacts on riparian fynbos soil, but the fact remains that even only to a minor extent, there are consistent and significant effects (i.e. when considering pH). There is a research gap when regarding our understanding of how herbicides can affect soils and the greater ecosystem. Herbicide operators, farmers and nature-reserve managers should be cautious when selecting chemicals to apply in situ. If herbicide clearing is done and the soils are rendered significantly more acidic it could hinder long term rehabilitation or planting schemes as well as inhibit the soils ability to fulfill ecosystem functions. The extent of these herbicide impacts is determined by the type of herbicide used, the concentration thereof, as well as the assemblages present in the soil at the time of treatment. It is therefore important to further research herbicides in a variety of conditions including different application methods, different in situ and ex situ conditions, and with a broad spectrum of analyses. This is required to achieve a better understanding of how herbicides and other agrochemicals can be used in a manner that is more environmentally compatible with intricate fynbos ecosystems.

AFRIKAANSE OPSOMMING: Houtagtige indringer uitheemse plante (IAP’s) het verskeie impakte op natuurlike ekosisteme in die Wes-Kaapse Provinsie van Suid Afrika. Hierdie impakte sluit in die absorpsie van groot hoeveelhede water, verhoogde veldbrandrisiko, die bedreiging van plaaslike inheemse flora sowel as ‘n toename in homogeniteit van verskeie fynbos rivieroewer habitatte. Die WfW program was gestig om hierdie indringing te beveg. Houtagtige indringer spesies wat aan genera soos Eucalyptus, Acacia and Populus behoort word behandel met gespesialiseerde onkruiddoders om die bome te vernietig, en om uiteindelik verdere indringing in afgebakende areas te voorkom. Onkruiddoders wat gebruik is, is effektief om die teiken IAP’s te beheer en word oorweeg as ‘n landbou- of natuurlike oplossing vir die rehabilitasie van fynbos habitatte. Alhoewel die onkruiddoders effektief is, kan daar onvoorspelde nadelige impakte op nie-teiken plant spesies wees asook op die plaaslike grond omgewing. Navorsers het gevind dat onkruiddoders en ander landbouchemikalieë beide biotiese en abiotiese eienskappe van die grond kan affekteer. Daarbenewens, het ander navorsers gewys dat onkruiddoders die grond omgewing kan stimuleer en dus geen skade veroorsaak nie. Die verhoogde druk van houtagtige IAP’s op rivieroewers fynbos, het tot gevolg ‘n toenemende gebruik van onkruiddoders. Dit is daarom noodsaaklik dat potensiële nadelige of voordelige effekte van die onkruiddoders bestudeer word. Hierdie studie was daarom gemik om die potensiële impakte van algemene onkruiddoders op die grond omgewing van rivieroewer fynbos habitatte in die Wes-Kaap te evalueer. Die onkruiddoders wat getoets was, is geregistreer om die houtagtige IAP’s A. mearnsii en E. camaldulensis dood te maak, wat algemeen voorkom in rivieroewers fynbos areas. Twee produkte naamlik, Springbok 360 SL en Garlon 480 EC was getoets in aanplantings van A. mearnsii, terwyl Plenum 160 ME getoets was in aanplantings van E. camaldulensis. Die impak op die grond omgewing was bepaal deur die studie van die verandering in verskeie grond chemiese eienskappe (soos grond pH, elektriese geleidingsvermoë, totale grond stikstof, stikstof mineralisasie, grond beskikbare fosfor, en fosfatase aktiwitiet). Daarbenewens, grond mikrobiese aktiwiteit was bestudeer deur die evaluering van koolstof substraat metabolisme (via ‘microtiter eco-plates’). Mikrobiese gemeenskap struktuur was ook bestudeer, en dit sluit in die evaluering van die verandering in spesies rykheid en eweredigheid (OTU’s en Shannonindeks). Die onkruiddoders was toegedien teen verskillende konsentrasies [d.w.s een tiende van die veld dosis (10 % FR), een keer die veld dosis (100 % FR), tien keer die veld dosis (1000 % FR) en onbehandelde beheer (0 % FR)] in beide in situ en ex situ eksperimente wat oor ‘n tydperk van ses weke geskied het. Wanneer enige van die bogenoemde onkruiddoders toegedien was, het die grond pH vlakke beduidend afgeneem (Kruskal-Wallis ANOVA: p < 0.05) in beide in situ en ex situ eksperimente. Die pH het gemiddeld met 0.5 eenhede gedaal en dit kom voor dat die onkruiddoders ‘n effense, maar konstante versurende effek op die grond gehad het. Daarbenewens het grond beskikbare stikstof beduidend toegeneem wanneer Springbok 360 SL (glifosaat) behandeling plaas gevind het (Kruskal Wallis ANOVA: p = 0.02) waar grond N vlakke van die tiene tot oor 100 µg.g-1 gestyg het. Die waargeneemde suurheid en stikstof veranderinge in die grond het nie veel kommer gewek nie, omdat die ander grond chemiese eienskappe wat bestudeer was min tot geen beduidende impak vir enige van die onkruiddoders wat getoets was, selfs met hoër konsentrasie behandelings [In situ: One Way ANOVA = p > 0.87; Ex situ: One Way ANOVA = p > 0.53], gewys het nie. Soortgelyk kom dit voor dat die grond mikrobiese gemeenskap struktuur ongeaffekteer gebly het waar beide mikrobiese koolstof substraat benutting en grond mikrobiese gemeenskap struktuur stabiel was. Grond mikrobiese aktiwiteit en gemeenskap metaboliese kapasiteit in grond wat behandel was met onkruiddoder het geen beduidende verskille getoon wanneer dit met onbehandelde grond vergelyk was nie. Die tempo van metabolisme het binne ‘n absorbansie van 1 en 4 by uur 168 van inkubasie gebly. Verder het die mikrobiese spesies rykheid en eweredigheid veranderlikheid gewys (standard afwyking waardes so hoog soos 18), maar het stabiel gebly volgens die OTU getalle en Shannon-indeks (One Way ANOVA: Springbok 360 SL behandeling, p = 0.76; Garlon 480 EC behandeling, p = 0.19; Plenum 160 ME behandeling, p = 0.06). ‘n ANOSIM het verder ‘n hoë graad van ooreenkoms tussen mikrobiese groepe voor en na onkruiddoder behandeling gewys, ongeag die konsentrasie toegedien (R statistiek waardes wissel tussen 0.05 en 0.15 toon vir baie min mate van dissimariliteit). Op grond van bogenoemde resultate was die afleiding gemaak dat geen kommerwekkende of inhiberende effekte plaasgevind het, met behandeling van enige van die onkruiddoders binne ‘n ses-week tydperk nie. Soos genoem, het die onkruiddoders wat getoets was, gewys dat daar ‘n versurende effek op fynbos grond was, maar gebrek aan enige ander kommerwekkende impakte op die grond omgewing met verwysing na die ander verskeie aspekte wat bestudeer was. Hierdie studie het aan die lig gebring dat hierdie onkruiddoders klein impakte op oewerfynbosgrond het, maar die feit bly dat selfs in 'n mindere mate konsekwente en betekenisvolle effekte (d.w.s. by die oorweging van pH). Daar is 'n navorsingsgaping wat betref ons begrip van hoe onkruiddoders gronde en die groter ekosisteem kan beïnvloed. Onkruiddoderoperateurs, boere en natuurreservaatbestuurders moet versigtig wees wanneer chemikalieë gekies word om in situ toe te pas. As onkruiddoderopruiming gedoen word en die gronde beduidend meer suur gemaak word, kan dit langtermyn rehabilitasie of plantskemas belemmer, asook om die grond se vermoë om ekosisteemfunksies te vervul, te inhibeer. Die omvang van hierdie onkruiddoder-impak word bepaal deur die tipe onkruiddoder wat gebruik word, die konsentrasie daarvan, sowel as die samestellings wat in die grond teenwoordig is tydens behandeling. Dit is daarom belangrik dat verdere navorsing die toetsing van verskillende onkruiddoders in ‘n verskeidenheid kondisies insluitend verskillende toepassingsmetodes, verskillende in situ en ex situ voorwaardes, en met 'n breë spektrum van analise. Dit is ‘n vereiste om ‘n beter begrip te kry oor hoe onkruiddoders en ander landbouchemikalieë gebruik kan word op so ‘n wyse wat meer omgewingsvreindelik is met ingewikkelde fynbos ekosisteme.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105616
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