title: Short- and long-term impacts of harvest residue management, biochar addition and elevated CO2 on C and N pools in soil-plant systems
Creators: name: Zhang, Yaling
Publisher: name: Griffith University
Year: 2017
Subjects: subject: Harvest residue management
subject: Soil-plant systems
subject: Plant responses

description: The growing public interest in the environment is associated with nature conservation embracing preservation, restoration and enhancement of the natural environment. Intensive forest management activities and surface mining, due to the growing demand for wood products and raw resources from mining, are known to cause considerable soil disturbance and loss of chemical, biological and physical fertility. Retention of harvest residues during the inter-rotation period and vegetation regeneration emerge as the most promising methods for maintaining soil organic matter (SOM) in forest plantations and mine rehabilitation, respectively. Numerous studies show that retaining residues on-site increased soil carbon (C) and nitrogen (N) pools, which could be ascribed to the inputs of labile C and N released from the residues into soil, while the impacts are relatively ephemeral since these labile C and N are easily decomposed. However, the long-term impacts are not well known. Since soil total N usually does not follow the same pattern as soil total C in the later period because residue N is apparently immobilized within the residues following an initial release of the most labile N fraction. Hence, it is highly likely that harvest residues may decrease N leaching and thus, improve N retention in a relatively longer term compared to the soil C pools. Moreover, the C and N released from the harvest residues may become more recalcitrant and decrease in quantity as the progress of residue decomposition. Hence, the long-term harvest residue retention probably would select and activate different bacterial groups, with the ability to utilize recalcitrant C and N and survive under oligotrophic conditions, which would be quite different from those of short-term studies. Surface mining usually involves destruction of the existing vegetation and causes extreme conditions at mine sites, including deficiency of water and essential nutrients, extreme pH, coarse texture and compacted structure. Re-vegetation has emerged as the most promising method for mine rehabilitation. However, the extreme conditions in mine spoil, especially the deficiency of N, have presented decisive barrier for initial vegetation establishment. Due to the large amounts of N needed for plant growth, it would be too expensive to solve N deficiency via N fertilizer application. Additionally, N is quickly leached in mine spoil due to its lack of clay minerals and organic matter. Therefore, building N capital is a priority for mine spoil rehabilitation. Biochar application has the potential to increase plant growth and soil N retention. However, most previous studies have focused on forest and agricultural soils, and very few studies have paid attention to mine spoils. Hence, in this study we assessed the potential of using biochar in mine rehabilitation in terms of improving plant growth and soil N capital in mine spoils. Moreover, in the context of steadily rising atmospheric CO2, it is necessary to evaluate whether elevated CO2 would influence the potential of biochar application in mine rehabilitation. Hence, the objectives of this study were addressed in each chapter: (1) to i o investigate the long-term effects of harvest residue management on soil C and N pools in forest plantations (Chapter 3); (2) to examineexamineexamineexamine examine the long-term effects of harvest residue management on soil bacterial community composition in forest plantations (Chapter 4); (3) to investigate plant responses to biochar addition in mine spoils under elevated CO2 (Chapter 5); and (4) to assess the potential of biochar addition in mine rehabilitation in terms of building soil N capital under elevated CO2 concentrations (Chapter 6). Three experiments were carried out to achieve these aims, with Experiments 1 and 2 designed for the first two objectives, while Experiment 3 for the remaining two. Experiment 1 was established in 1996, located in the exotic pine plantations of southeast Australia. This experiment involved even distribution of harvest residues and, included three residue treatments: (1) residue removal (R0); (2) residue retained (R1); and (3) double quantities of residue retained (R2). Soil sampling was carried out in March 2015, 19 years after the establishment of the experiment. Soil samples were taken in depths of 0-10, 10-20 and 20-30 cm. Water and hot water-extractable organic C and total N (WEOC, HWEOC, WETN, HWETN), total C (TC), total N (TN), as well as δ13C and δ15N were measured to study the impact of long-term harvest residue retention on both soil labile and total C and N pools (Chapter 3). Nuclear magnetic resonance (NMR) spectroscopy analysis was used to determine the residue retention effects on chemical functional group composition of SOM (Chapter 4). The influence on soil bacterial community was also investigated via high throughput sequencing technology in this experiment (Chapter 4). Experiment 2 was established in 1998, located in the hoop pine plantations of southeast Australia. The residue materials were placed into two windrows in this experiment and, each of the windrows contained 522 t ha-1 of dry matter, and was approximately 2.5 m in width and spaced 15 m apart from each other. Soil samples were taken from 11 positions (A-J, M) along three transects across the two windrows, respectively from 0-5, 5-10 and 10-20 cm depths in March 2015. The sample analysis was the same as in Experiment 1 without NMR or high throughput sequencing analysis. Hence, this experiment was designed only for examination of the long-term harvest residues on soil C and N pools (Chapter 3). Experiment 3 was conducted continuously for two years at the University of Western Sydney (Sydney, NSW, Australia) from February 2013 to February 2015. The experiment was set-up according to a split-plot design. In this design, we considered the CO2 flow Chambers as whole plot, while the plant species and biochar treatments were considered as sub-treatments. The experiment consisted of four environmentally controlled CO2 flow chambers, with two chambers CO2 exposed to elevated CO2 at 700 μL L-1, and the remaining two at 400 μL L-1. Each chamber contained all levels of the four biochar treatments: without biochar addition in mine spoil, with the application of biochar at 8% rate by weight, produced at three pyrolytic temperatures (650, 750 and 850 °C, respectively). Mine spoil was sampled from Glencore Mt. Owen coal mine rehabilitation site, Australia. The plant species included a tree species (Eucalyptus crebra), N-fixing shrubs (Acacia floribunda and Allocasuarina littoralis), C3 and C4 grasses (Austrostipa ramossissima, Austrodanthonia tenuior, Dichelachne micrantha and Themeda australis) and sedges rushes (Isolepis nodosa, Lomandra longifolia). There were 9 replicates for each treatment. Among the nine plant species, A. ramossissima, D. micrantha and I. nodosa were selected for the objective 3 to study plant responses to biochar addition in mine spoil under elevated CO2 concentrations. We measured leaf, root and total biomass of the three species and also, examined the C and N allocation at the whole-plant level to elucidate the physiological implications for the plant growth responses to biochar addition (Chapter 5). We selected the soil samples under all the 9 plant species except for I. nodosa for the objective 4 of assessing the potential of biochar addition in mine rehabilitation in terms of building soil N capital under elevated CO2 concentrations. Soil analysis was the same as Experiment 2 (Chapter 6). The main results are listed as follows: (1) In Chapter 3, the results of Experiment 1 showed that the retention of harvest residue increased N retention, as indicated by the significant increase in soil HWETN at depth of 10-20 cm. This was probably due to the decreased N leaching with residue retention as suggested by the significant decreases of soil δ15N (P &lt; 0.001). Similarly, the retention of windrowed-harvest residues of Experiment 2 showed that the position M just beneath the residues had the significantly highest soil labile N (WETN and HWETN) and total N, whereas only the highest soil labile C (WEOC and HWEOC) appeared under position M. Hence, our study strongly suggested that harvest residues retention improved N pools in a relatively longer term compared to soil C pools probably via decreasing N leaching. However, the positive effects on soil C and N pools of the windrowed harvest residue retention were only limited under the position M and thus, only the plants close to position M could benefit from the residue retention. (2) In Chapter 4, the long-term residue retention of Experiment 1 significantly increased the relative abundances of Actinobacteria (P = 0.016) and Spartobacteria (P &lt; 0.001), whereas decreased soil bacterial diversity as indicated by the Shannon index (P = 0.025). Actinobacteria and Spartobacteria are well known for consuming more recalcitrant C and N compounds and thriving under oligotrophic conditions. The results are different from the previous short-term studies, which found significant increases in the bacterial groups responsive to labile C and N and increase in soil bacterial diversity. Hence, our study probably suggested that since the residue C and N became more recalcitrant and decreased in quantity as the progress of residue decomposition, long-term harvest residue retention probably would favour the oligotrophic bacterial groups specialized in the degradation of more recalcitrant C and N compounds. This conclusion was further supported by the positive relationships of alkyl C with Actinobacteria and Spartobacteria, since the alkyl C region was assigned to recalcitrant long chain aliphatic structures. Moreover, the favouring of these specific bacterial groups is highly likely to be the reason for the decreased soil bacterial diversity with long-term residue retention. (3) In Chapter 5, our data showed that biochar addition significantly decreased leaf total N and δ15N and the decreases were more pronounced than those of root samples. These responses are commonly seen under elevated CO2, via which plants could achieve optimal N distribution and thus, higher N-use-efficiency. Hence, our data suggested that plants with the higher potential to improve N-use-efficiency would benefit most from biochar application in the N-deficient mine spoil. This was consistent with our observation of species-specific growth responses of the three plant species to biochar application. Our data also showed that more pronounced effects of biochar addition on improving plant biomass appeared under ambient CO2 than those under elevated CO2 concentration. Hence, it would remain a strong possibility that elevated CO2 decreased the ability of biochar addition in improving plant growth. (4) In Chapter 6, soil total N concentrations significantly increased following biochar application across all species selected except for T. australis, with E. crebra having the highest soil total N (0.197%, 0.198% and 0.212% for biochar produced at 650, 750 and 850 °C, respectively). Hence, our results suggest that biochar addition showed the potential in mine rehabilitation in terms of improving soil N pool, especially with E. crebra. While elevated CO2 only exerted negligible influences on soil properties under the grasses and the N-fixing shrubs, CO2 enrichment significantly increased soil WEOC and HWEOC, and decreased total C under E. crebra. These results reflected that the nutrients, particularly N, were not met in the N-depleted mine spoil, hence, it would be more difficulty to rehabilitate mine spoils in future with the rising atmospheric CO2 concentration. In conclusion, long-term harvest residue retention still positively affected soil C and N pools and, exerted influence on soil bacterial community composition. Hence, harvest residue retention could act as a promising method for maintaining C and especially N pools of harvest site, particularly for the sandy soil in the exotic pine plantations with high potential of N leaching. The results of Experiment 3 showed that biochar application had a promising prospect for mine rehabilitation in terms of improving plant growth and building soil N capital, but the effects varied with different plant species. However, elevated CO2 would probably deprive the potential of biochar application in mine rehabilitation.