title: Symbiotic nitrogen fixation and soil carbon and nitrogen pools in a suburban native forest of subtropical Australia
Creators: name: Taresh, Sabah
Publisher: name: Griffith University
Year: 2021
Subjects: subject: repeated prescribed burning
subject: biological nitrogen fixation (BNF)
subject: Toohey Forest
subject: subtropical Australia
subject: soil biological
subject: native forest ecosystem
subject: South-East Queensland

description: Prescribed burning is a tool used in Toohey Forest management systems as a suburban forest fuel reduction burning to mitigate major bushfire risks and has played an important role in the development of plant communities in Australia. Fire influences terrestrial ecosystems and processes, which include vegetation distribution and structure, carbon (C) and nitrogen (N) cycling and climate. The current study examined the effects of prescribed burning (9-14 years after burning) on C and N cycling. We hypothesized that prescribed burning would be able to increase the availability of N, which may benefit regenerating plants shortly after the fire. The prescribed burning would influence C and N pools as well as BNF, water use efficiency, and plant growth of understory acacia species in short time, but its long-term impact might be 0 minimal due to the ecosystem recovery and resilience in the suburban forest ecosystem of southeast Queensland, Australia. Forest litterfall and litter floor recovery would play important roles in the recovery process of soil C and N pools after 9-14 years. Soil C and N pools as well as forest litter floor would gradually be recovery after 9-14 years of prescribed burning in the suburban Toohey Forest ecosystem. Labile soil C and N pools could be affected 9-14 years after prescribed burning in the suburban Toohey Forest ecosystem of subtropical Australia. This study examined the effect of prescribed burning in the short- and medium-term on plant growth and eco-physiological responses of understory acacia species and symbiotic N fixation as well as soil C and N pools in the suburban forest ecosystem of subtropical Australia. This project sought to determine the impacts of repeated prescribed burning on biological nitrogen fixation (BNF) by understorey acacia species, examine the N dynamics and plant N concentrations in Toohey Forest, a suburban forest in subtropical Australia, at different time periods following prescribed burning (9-14 years after prescribed burning). The study examined how prescribed burning could alter the soil biological, physical and chemical properties, particularly N cycling and availability, soil C pools and dynamics as well as soil microbial composition, under specific fire regimes that are followed in a native forest ecosystem of southeast Queensland, Australia. However, the study explored also short-term responses of biochar amendment in typical Toohey Forest on BNF, water use efficiency, soil mineral N pools as well as tree growth of Acacia leiocalyx and Acacia disparimma 1, 2, 3, 6 and 9 months after three rates of biochar application (0, 5 and 10 t per ha) following about 12 months of prescribed burning at Site 7. The studies were undertaken within Toohey Forest which is located 10 km south of the Brisbane, in South-East Queensland, Australia (27°30′S, 135°E). This forest has been subjected to fuel reduction prescribed burning over the last 30 years. The fuel reduction prescribed burning is usually done on a cycle of 7-10 year, depending on seasonal conditions and fuel loads. The studies were conducted at five sites within Toohey Forest. The first site, S7-B4 (B4- means 4 years after prescribed burning) was last burnt in August 2017. Site 4-B6 (B6-means 6 years after prescribed burning) was last burnt in May 2014. Site 1-B9 (9 years after prescribed burning) “27°32'26.79 S” &amp; “153°02'39.81E”, was last burned in August 2011. Site 2-B12 (12 years after prescribed burning) “27°32'34.02S” &amp; “153°03'09.06E”, was last burnt in April 2008. Finally, Site 3-B14 (14 years after prescribed burning) “27°32'28.61S”&amp; “153°02'50.60E”, was last burnt in May 2006. The main aims of this study were to assess the long-term impacts of prescribed burning on BNF, water use efficiency (WUE), and plant rowth of understory acacia species as well as soil C and N pools in the Toohey Forest ecosystem. The objectives of this PhD project, addressed in each of Chapters 3, 4 , 5, and 6 were: (1) to examine the long-term effect of prescribed burning on biological nitrogen fixation (BNF) and water use efficiency as well as tree growth of understory acacia species in a native forest ecosystem of subtropical Australia, as revealed by 15N natural abundance method during the growing season (summer) and non-growing season (winter) (Chapter 3); (2) to evaluate the contribution of BNF by the understory acacia species to ecosystem N availability and cycling processes as well as ecosystem recovery in the Toohey Forest Ecosystem of subtropical Australia, in the context of fuel reduction prescribed burning and climate change (Chapter 3); (3) to examine whether biochar addition could reduce N loss and improve biological, physical and chemical properties of surface soils (Chapter 4); (4) to examine the short-term effect of biochar addition on BNF, WUE and tree growth of understory acacia species in the Toohey Forest of southeast Queensland, Australia (Chapter 4); (5) to quantify the recovery of forest litter floor and total C, total N, C and N isotope composition (δ13C and δ15N) in the 0-5, 5-10, and 10-20 cm soil at Site 1 (S1, 9 years after burning), Site 2 (S2, 12 years after burning) and Site 3 (S3, 14 years after burning) in the suburban Toohey Forest ecosystem of subtropical Australia (Chapter 5); (6) to examine the role of forest litter floor in the nutrient cycling and forest litter floor recovery in the soil C and N pools and their dynamics in the periods of 9-14 years after prescribed burning in the suburban Toohey Forest ecosystem (Chapter 5); (7) to quantify the effect of prescribed burning on soil labile C and N pools 9-14 years after prescribed burning in the Toohey Forest ecosystems of subtropical Australia (Chapter 6); and (8) to examine the ability of soil labile C and N pools at the three burned sites to recover 9-14 years after the prescribed burning in the Toohey Forest ecosystems (Chapter 6). In Chapter 3 (Study 1), there were no significant differences in foliar total C and δ13C between Acacia leiocalyx and Acacia disparimma at all the four sites after long term prescribed burning (6-14 years after burning). However, our result showed that foliar total N concentrations of A. leiocalyx was higher at sites of S1, S2, S3, and S4 than those of A. disparimma after 6-14 years of prescribed burning. There were significant differences in foliar δ15N between Acacia leiocalyx and Acacia disparimma at Site 1 (9 years after prescribed burning), and Site 3 (14 years after prescribed burning), while there were no significant differences in foliar δ15N between Acacia leiocalyx and Acacia disparimma at Site 2 (12 years after prescribed burning), and Site 4 (6 years after prescribed burning). However, our result showed that δ15N values of A. leiocalyx were significantly higher for August 2019 (winter-dry season) at sites of S1-B9 and S3- B14 after 9-14 years of prescribed burning respectively than those of A. disparimma in May 2019 (autumn) and August 2019 (winter) after 9 and 14 years of prescribed burning. Overall, at Site 4 (6 years after burning), BNF rates were in the range of 65-85% for Acacia leiocalyx and Acacia disparimma, while the corresponding BNF values were 40-55% at Site 1 (after 9 years of prescribed burning), 58-73% at Site 2 (after 12 years of prescribed burning), and 48-62% at Site 3 (after 14 years of prescribed burning). However, BNF of Acacia leiocalyx did not differ from that of Acacia disparimma at any of the study sites (S1, S2, S3, and S4). Meanwhile, we assessed the WUE of two acacia species growing at suburban Toohey Forest sites subjected to prescribed burns. We found that 6-14 years after prescribed burning, A. disparimma had higher WUE than that of A. leiocalyx for sites of S2, S3, and S4, but not at S1. In general, this study demonstrates that A. disparimma for all sites S1, S2, S3, and S4 had relatively higher WUE than A. leiocalyx in winter season sampling of August 2019. In Chapter 4 (Study 2), the study explored short-term responses of biochar amendment in typical Toohey Forest on BNF, water use efficiency, soil mineral N pools as well as tree growth of Acacia leiocalyx and Acacia disparimma 1, 2, 3, 6 and 9 months after three rates of biochar application (0, 5 and 10 t per ha) following about 12 months of prescribed burning at Site 7. There were significant differences in foliar total C and total N as well as foliar δ15N, δ13C and tree growth (tree height and diameter at ground level) of both Acacia leiocalyx and Acacia disparimma among the biochar application rates in the Toohey Forest, southeast Queensland, Australia, with the highest values in the biochar application at 5 t per ha compared with those of the control and the biochar rate at 10 t per ha, in the first 6 months. The present study indicated that the biochar treatment of 10 t ha-1 improved foliar total N, BNF, and δ15N of understory acacia species for all sampling months, but not at month 2 (July 2019). However, our study showed that the biochar applied at 5 t ha-1 contributed to increases in total C, δ13C and WUE, but did not differ from those of the 10t ha-1 in the Toohey Forest ecosystem of subtropical Australia. Foliar total N concentrations of A. leiocalyx was higher in the autumn of March 2020 after 9 months of biochar application, also that foliar total N concentrations were significantly higher (P &lt; 0.05) in the treatment of biochar applied at 10 t ha-1 in the autumn of March 2020 compared with those of the 5t ha-1 and control. This study demonstrated that A. disparimma had relative higher BNF in the winter sampling than those of A. leiocalyx, and also the current study showed that BNF was increased after two months of biochar applied at 5 t ha- 1, which did not differ than those of biochar applied at 10 t ha-1. The BNF ranged from 42.2 % to 80.7 % for A. leiocalyx and from 62.4 % to 93.4 % for A. disparimma for all foliar samples before and after biochar application during the autumn and winter of April and July 2019. There were positive relationships among δ13C, δ15N and TN “R2= 36.6% and R2= 13.3%”, P &lt; 0.05 at month 0 and 6 for autumn and spring of April and November 2019 respectively, but there was no significant difference at month 9 for autumn of March 2020. Meanwhile, there were positive relationships among δ13C, δ15N and TN “R2=15.2%, R2= 22.8% and R2= 19.9%”, P &lt; 0.05) at 1-2 months after biochar applied for winter of June and July 2019. There were no significant differences in plant growth for both tree height and diameter at ground level between of A. leiocalyx and A. disparimma at S7-B3. However, there were significant higher in plant growth for both tree height and diameter at ground level among biochar treatments at S7-B3 in the Toohey Forest. We conclude that different biochar application rates led to different effects on BNF and WUE due to the high variation in biochar properties, highlighting the role of BNF in improving plant WUE and subsequently tree growth after biochar was applied in the suburban Toohey Forest. In Chapter 5 (Study 3), this study aimed to determine the long term (9-14 years) impact of prescribed burning on the dynamics of litter floor quantity and quality, and soil C and N pools as well as their dynamics in the Toohey Forest Ecosystems. The S1-B9 (9 years after prescribed burning), S2-B12 (12 years after prescribed burning), and S3-B14 (14 years after prescribed burning), were sampled for litter floors and 0-20 cm soil profile in August 2019. Soil samples were collected at 0-5, 5-10 and 10-20 cm depths from each of the four plots at each of the three sites (S1, S2, and S3) for total C, total N, δ13C and δ15N 9-14 years following the prescribed burning to examine the long term impact of prescribed burning on soil C and N pools (total C, total N, δ13C and δ15N) and their dynamics. The litter floor samples were also collected from each plot of the three sites (S1, S2, and S3) for total C, total N, δ13C and δ15N 9-14 years after prescribed burning, which would be expected to be burnt in the next 1-2 years. The soil total C, total N, δ13C and δ15N at the 0-5, 5-10 and 10-20 cm soil depths at Site 1 after 9 years following the last prescribed burning were also compared with those of the corresponding properties shortly before the last prescribed burning in August 2011. Similarly, litter floor total C, total N, δ13C and δ15N as well as total C and N content at Site 1 after 9 years of prescribed burning would be compared with those of the corresponding properties in the Forest litter floor at Site 1 shortly before prescribed burning in August 2011 to quantify the recovery of C and N pools in the top 20 cm soil and litter floor 9 years after prescribed burning. Overall, after 9 years of prescribed burning at Site 1, soil total C and total N concentration at 0-5, 5-10 and 10-20 cm soil have recovered and indeed been higher than those of the corresponding soil depths shortly before the burning. Similarly, litter floor total C and N contents have also recovered after 9 years of the mild prescribed burning at Site 1, but the C:N ratio in the forest litter floor 9 years later is about 94, which is significantly higher than that of 68 sampled about 9 years ago, shortly before the prescribed burning. This highlights that the surface soil total C and total N concentrations have recovered and indeed been higher than those of the corresponding soil samples collected shortly before the prescribed burning about 9 years after prescribed burning. Similarly, the forest floor total C and N contents have also recovered about 9 years after prescribed burning at Site 1, but the C:N ratio of the forest litter floor 9 years later (96) is significantly higher than that of 68, highlighting that the forest litter floor would be more N limiting, with much less N losses or released into the soil via litter decomposition since litter floor δ15N is much less 9 years after prescribed burning at Site 1 than that collected shortly before prescribed burning, while forest litter floor δ13C is significantly higher 9 years later than that shortly before prescribed burning, suggesting that the vegetation / forest WUE would be higher, partly due to increasing water limitation under climate change and partly due to the suburban N deposition / N fertilization during the past 9 years. Soil total C and total N at Sites 2 and 3 have also been significantly higher 9 years later than those of soil samples collected about 9 years ago. Both soil δ13C and δ15N at Sites 1, 2 and 3 (9, 12 and 14 years after prescribed burning) have also been significantly lower 9 years later than those of soil samples collected 9 years ago, highlighting increasing N limitation and decreasing N availability as well as significant N deposition in the suburban forest ecosystems. In Chapter 6 (Study 4), this study aimed to examine the availability of soil labile C and N pools such as hot water extractable organic C (HWEOC) and hot water extractable total N (HWETN), water soluble organic C (WSOC) and water soluble total N (WSTN), as well as microbial biomass C (MBC) and microbial biomass N (MBN) at the three burned sites to recover 9-14 years after prescribed burning; and to quantify the long term effect of prescribed burning on soil labile C and N pools 9-14 years after the prescribed burning in the Toohey Forest ecosystems of subtropical Australia. The S1-B9 (9 years after prescribed burning), S2-B12 (12 years after prescribed burning), and S3-B14 (14 years after prescribed burning), were sampled for 0-20 cm soil profile in August 2019. Soil samples were collected at 0-5, 5-10 and 10-20 cm depths from each of the four plots at each of the three sites (S1, S2, and S3) for determining the soil labile C and N pools. Labile C and N pools such as HWEOC, HWETN, WSOC, WSTN, MBC and MBN are commonly used as sensitive indicators of soil quality and fertility and soil organic matter stability. The repeated measures analysis revealed that labile C and N pools were recovered after 9-14 years of prescribed burning for HWEOC and HWETN, but in regard to the time since fire for 9-14 years, the present study indicated that WSOC and WSTN were decreased for all soil depth highlighting that both WSOC and WSTN were not yet recovered. Meanwhile, the results have also been indicated that fire had significantly increased MBC for both depths of 5-10 and 10-20 cm at sites of S2 and S3 after 12-14 years of prescribed burning compared with those of samples collected in August 2019. The current study also indicated that MBN was significantly decreased after 9-14 years of prescribed burning for all soil depths. In conclusion, all soil labile C and N pools were recovered 9-14 years after prescribed burning, except for WSOC and WSTN which were not recovered for S1 9 years after prescribed burning. At S1-B9, while the biomass of the litter floor was recovered, the C:N ratio of 96 was much higher than that of 68 for litter floor about 9 years ago before the last prescribed burning. This would limit the decomposition of litter floor, which would be closely linked to the soil WSOC and WSTN that were not recovered either. The lower soil δ15N and higher δ13C in the litter floor highlights that both water and N are becoming increasingly limiting due the climate change and prescribed burning. Overall, our study concluded that soil total C and total N were fully recovered after 9-14 years of prescribed burning, as well as litter floor leaf and twigs total C and total N were fully recovered 12-14 years after prescribed burning in the suburban Toohey forest ecosystem of subtropical Australia. Litter floor leaf and twigs total C and total N were not fully recovered 9 years after the prescribed burning at S1, compared with those of corresponding samples collected shortly before the last burn in June 2011. Litter floor leaf and twigs δ13C 9, 12 and 14 years after prescribed burning at sites of S1, S2 and S3 were significantly higher compared with those of corresponding samples collected shortly before, 3 and 5 years after the prescribed burning. Meanwhile, soil HWEOC and HWETN were recovered after 9-14 years of prescribed burning. MBN of 0-5 and 5-10 cm soil at S1 did not recover even 9 years after the prescribed burning. However, WEOC and WETN did not recover even 9 years after prescribed burning at S1. Similar trends of WEOC and WETN were also noted for sites of S2 and S3 12-14 years after prescribed burning in Toohey Forest ecosystem of subtropical Australia.