title: Effects of different land management practices on soil nitrogen cycling and microbial composition in riparian zone soils
Creators: name: Omidvar, Negar
Publisher: name: Griffith University
Year: 2021
Subjects: subject: Riparian zone
subject: land management practices
subject: Revegetation
subject: Weed control methods
subject: Glyphosate
subject: Organic-based herbicides
subject: Mulch
subject: Wood-based Biochar
subject: Soil N pools
subject: Soil microbial community structure

description: Riparian zones lie at the interface between aquatic and terrestrial ecosystems and usually have fertile soils, which play a crucial role in maintaining ecosystem productivity and biodiversity. Riparian zones are currently subject to vegetation loss and degradation due to human activities, intensification of agricultural practices, weed invasion, and global climate change. Therefore, there is an urgent need to revegetate and/or maintain the current vegetation of riparian zones. Soil nitrogen (N) is one of the most important growth-limiting nutrients and plays a crucial role in plant growth and productivity. Various management practices such as revegetation establishment, weed control methods, fertiliser applications, and/or organic amendments are practiced to sustain vegetation in riparian zones. However, management practices can alter soil N cycling. It is essential to understand the effect of different land management practices on soil N transformations, to maintain the long-term functional stability of riparian zones ecosystems. Revegetation of degraded riparian zones is considered one of the most effective measures in riparian management, which may influence soil N cycling. To date, no systematic literature review (meta-analysis) exists to assess the effects of different vegetation types and age on soil N pools regarding the revegetation of riparian zones (Chapter 2). Vegetation types along the riparian zones can be both native tree species and/or horticultural trees. The success of riparian revegetation projects with tree species currently depends on weed control to reduce the non-target vegetation competing over nutrients. I, hence, investigated the effects of chemical weed control on N cycling using glyphosate (commonly used herbicide) and organic-based herbicides including the registered products BioWeedTM, Pelargonic acid (Slasher®), and horticultural vinegar (acetic acid) compared with mulching. Additionally, there is a lack of information on how different herbicides could affect soil N dynamics and microbial community structure even after the application was ceased for at least one year (Chapter 3). Horticultural crops established within riparian zones are also one of the major land-use practices requiring large quantities of fertiliser, particularly N, and herbicide inputs. Glyphosate is commonly applied in horticultural lands and glyphosate interactions with organic amendments (e.g., biochar) remain uncertain. I explored the long-term interactions between biochar-amended soil and repeated application of glyphosate even after the application has ceased for over two years on soil microbial communities and soil N transformations are not well understood (Chapter 4). Glyphosate application is likely to be resumed even after stopping for a period of time and usually, very short-term N pools in response to glyphosate application in the presence of soil organic amendments like biochar are overlooked. Short-term N pools may have implications for long-term N management and hence it is important to understand short-term N pools with respect to treatments (Chapter 5). Therefore, my thesis specifically aimed to: (1) Examine the influence of riparian zone revegetation with different revegetation types and age on soil N pools by conducting a quantitative literature review (meta-analysis) (Chapter 2). (2) Assess and compare the effects of different organic-based herbicides (BioWeedTM, Slasher®, and acetic acid) and the commonly used herbicide glyphosate with mulch, on soil N dynamics and microbial community structure at two riparian revegetation sites up to two years repeated application of herbicides followed by one year after stopping herbicides applications (in total three years since the revegetation establishment) (Chapter 3). (3) Investigate the impacts of long-term repeated application of glyphosate (12 years) interacted with biochar (for three years) on soil N cycling, more specifically abundance of nitrifying populations such as the ammonia-oxidizing bacteria and archaea (AOB and AOA) as well as the overall soil microbial diversity and community structure (both fungi and bacteria) after two years following the cessation of herbicide applications (Chapter 4). (4) Explore how the interaction of biochar and glyphosate would immediately affect soil N transformations in short-term (in a 5-day laboratory incubation) using the 15N pool dilution techniques (Chapter 5) In Chapter 2: A meta-analysis was undertaken using a categorical mixed effect model to synthesise the results from 52 published articles. This study aimed to explore how different revegetation types (woodland, shrubland, and grassland) and revegetation age (&lt; 3 years, 3-10 years, 10-20 years, 20-40 years, or &gt; 40 years) would affect soil N pools. This meta-analysis revealed that revegetation of riparian zones significantly increased soil total nitrogen (TN) particularly in woodlands, which was associated with the presence of N fixing species and high litter inputs. Soil TN increased in revegetation ages between 10 and 40 years following revegetation, which might be associated with the increased soil organic carbon (SOC) inputs within those ages following establishment. Of the revegetation types considered in this study, NO3--N concentration in soil followed the order of grassland &lt; shrubland &lt; woodland, suggesting that woodland might be more efficient in soil NO3--N retention than grassland. However, revegetation significantly decreased soil moisture by 7.6% compared with the corresponding control, which might be associated with the selection of exotic species as dominant vegetation in riparian zone revegetation. This study provides insight into the influence of different revegetation types and age on soil N pools and soil moisture. This study also highlights the importance of revegetating riparian zones to increase soil TN, particularly in woodlands. Therefore, in Chapter 3, I used two newly established revegetation sites at (Kandanga and Pinbarren sites), planted with native wood species to investigate how different weed control methods including organic-based herbicides (BioWeedTM, Slasher®, and acetic acid) and the commonly used herbicide, glyphosate with mulch would affect soil N dynamics and microbial community structure over three years following revegetation establishment(two years repeated application of herbicides followed by one year after stopping herbicides applications). Soil samples were collected three times following revegetation establishment at months 2, 14, and 26 at the Kandanga site and at months 10, 22, and 34 at the Pinbarren site. The last sampling at the Pinbarren site (at month 34) occurred one year after the last herbicide was applied. In this study, I found that soil microbial biomass carbon (MBC) was significantly higher in response to mulch compared with glyphosate and organic-based herbicide treatments at months 26 at the Kandanga site and months 10 at the Pinbarren site following the revegetation establishment. However, soil MBC at month 34 at the Pinbarren site was only significantly higher in the mulch treatment than glyphosate and BioWeedTM. The level of MBC in glyphosate and BioWeedTM was also lower than the acceptable threshold at month 34 at the Pinbarren site. I also found that soil nitrate (NO3--N) was significantly higher in the soil treated with mulch than only glyphosate treatment at months 22 and 34 after revegetation at the Pinbarren site. Higher soil NO3--N in mulch compared with that of glyphosate could be partly explained by decreased denitrifying bacteria (Candidatus solibacter and Candidatus koribacter). However, there were no significant differences in soil NO3--N between mulch and other organic-based herbicides despite the fact that C. solibacter and C. koribacter were still lower under mulch than those of other herbicides. My study suggested that the application of mulch in the riparian revegetation projects would be beneficial for soil microbial functionality, particularly soil MBC as compared with [glyphosate and other organic-based herbicides. Additionally, this study suggested that some herbicides like glyphosate and BioWeedTM may have long-lasting effects on soil 125 microbial biomass even if they do not necessarily change microbial diversity. Hence, the long-term use of glyphosate and BioWeedTM needs to be considered with caution. Some woodlands within riparian zones are also horticultural sites where intensive weed control management practices are applied (Chapter 4). Therefore, an established horticultural site (Macadamia orchard) located in a riparian zone, with a history of repeated application of glyphosate (up to four times per annum for 12 years) was chosen to understand the impacts of repeated application of glyphosate interacted with biochar on soil N cycling. Wood-based biochar was first applied at this site in 2012 (64 months before my sampling time) at two different rates of 10 dry t ha-1 (B10) and 30 dry t ha-1 (B30) and the glyphosate had not been applied for two years before my sampling. Therefore, biochar and glyphosate applications coincided for three years before glyphosate was ceased. The results showed that soil TN was significantly higher in the glyphosate applied areas compared with those of no glyphosate, which may suggest that glyphosate may have stimulated N transformations in this study through increasing soil carbon (C) substrate resulting from the degradation of glyphosate and dieback of weeds. Similarly, no effect of glyphosate on soil microbial diversity and community structure was observed two years after glyphosate application had been ceased. However, glyphosate-treated soil had significantly higher AOB abundance than no glyphosate areas, which might be potentially associated with increased total carbon (TC). Conversely, biochar did not affect soil TN in this study, potentially due to the low TN content of the biochar used in this study (wood-based biochar), but had also significantly higher AOB abundance than those of no biochar plots, potentially through stimulated N cycling. In summary, neither glyphosate nor biochar impacted soil microbial diversity or community structure, but both increased AOB abundance with possible long-term implications on soil N cycling even after two years since the last glyphosate was applied. In Chapter 5, I then investigated the short-term N pools in response to the immediate interaction of glyphosate in an amended soil with biochar. I conducted an incubation study (5-day) to understand the short-term and immediate effects of glyphosate interacted with biochar on soil N transformations (ammonification, nitrification, and N mineralisation) using the 15N pool dilution techniques. Soil samples were collected from the same horticultural site used in Chapter 4. My 5-day incubation study showed that δ15N of NO3--N was not affected by any of the factors assessed including glyphosate, biochar, nd moisture content, nor were their interactions significant. However, this study showed that both nitrification and N mineralisation were stimulated by biochar. My results also showed that application of glyphosate significantly decreased δ15N of NH4+-N in soil following 3- and 5-day incubation, indicating that N mineralisation occurred, which could be related to stimulation of soil microbes due to the addition of C as a source for soil microbes resulting from the decomposition of glyphosate. In summary, my work highlighted the importance of revegetating riparian zones to increase soil N retention particularly using woodland species and showed that glyphosate and organic-based herbicides may not necessarily alter soil microbial diversity but there were long-lasting effects on soil microbial biomass observed about one year after the application being terminated. In the horticultural site, there was also an indication of both glyphosate and biochar having affected soil microbial abundance. This long-lasting effect of herbicides on soil N cycling may have implications for the long-term sustainability of riparian zones. Therefore, my work suggested that the application of mulch should be an alternative for chemical weed control when possible.