Cotton farming and N cycling : adaptation to climate change

Abstract

Climate models have projected the atmospheric carbon dioxide (CO2) concentration will double and a 1.1-6.4oC rise in global average temperature by the end of 21st century. Simultaneously, extreme weather events including flooding and drought have been predicted to increase in frequency and intensity. Such changes are expected to have profound effects on agriculture. Soil nitrogen (N) cycling, particularly nitrification and denitrification plays an important role in the availability of N in soils for plant uptake, and hence changes in these processes due to global change may considerably influence crop productivity. These N processes are microbially driven and little is known about the role of soil microorganisms in regulating the process rates and how they respond to environmental disturbances. Therefore, my study aimed to elucidate the responses of soil nitrification to climate change and extreme weather events, and subsequent consequences for crop yields, by using cotton as a model system. Additionally, the legacy effects of extreme weather events and the impacts of added N-fertilizer on soil N, C processes and microbial communities and the subsequent consequences for crop productivity were examined. First effects of waterlogging on soil nitrification and nitrifying community in cotton farming were examined. This study was a field-based experiment conducted at the Australian Cotton Research Institute (ACRI) in Narrabri, NSW. Waterlogging events simulated by running furrow irrigation for 120 hours were applied at the early and late flowering stages, respectively. Waterlogging had strong effects on soil moisture, pH, potential nitrification rate (PNR) and soil nitrate (NO3-) concentration. The abundance of ammonia-oxidizing bacteria (AOB) decreased approximately 10-fold whereas that of ammonia-oxidizing archaea (AOA) decreased about 2-fold after waterlogging. Shifts in AOB and AOA community structures were also observed after waterlogging. Significant correlations between both AOB and AOA communities and PNR were observed; however, AOB was more strongly correlated to PNR than AOA. Significant linear negative correlations between soil moisture and ammonia-oxidizing communities and PNR were also obtained. These results indicate that waterlogging impacted on soil physicochemical properties, resulting in changes in ammonia-oxidizing communities and nitrification activity. In the second chapter, the effects of elevated temperature (+1.1oC) alone and elevated temperature in combination with elevated CO2 (550 ppm) on soil nitrification and nitrifying communities in cotton farming were investigated using field-based environmentally-controlled chambers. This study was conducted at ACRI in Narrabri, NSW. Elevated temperature did not affect soil PNR and AOB community abundance and structure. The AOA community responded significantly to elevated temperatures by increasing its abundance and shifting their community structure. Combined elevated CO2 and temperature significantly increased both AOB and AOA abundance, and resulted in shifts in AOB and AOA community structures. Both AOB and AOA communities were significantly correlated with PNR, although AOA exhibited a weaker relationship with PNR than AOB. Effects of climate factors on soil nitrification and nitrifying community depended on the stage of cotton growth since treatment effects were only observed when cotton reached the early flowering stage. Thirdly, the responses of soil N processes (nitrification, denitrification, and N mineralization), functional microbial communities, crop growth and productivity to different N fertilizer regimes (0, 100, 200 and 300 kg N/ha) after exposure to waterlogging and prolonged-drought were investigated by conducting a glass-house experiment at Western Sydney University (WSU). Prolonged-drought prior to cotton planting established a strong legacy effect on soil N processes, ammonia-oxidizing communities, nosZ-containing community, plant growth and productivity. N fertilizer application up to 300 kg N/ha could not counteract the legacy effect of prolonged drought on soils and plants although N supply improved soil fertility. These results suggest that the depleted functional microbial communities may take a long time to recover after drought. Waterlogging prior to planting had a legacy effect on soil NO3- content and the nosZ-containing community. The legacy effect of waterlogging on soil NO3- was diminished completely by N addition. Despite an increase in nosZ gene abundance due to waterlogging before sowing, soil N availability and crop growth and productivity was not impacted. However, N loss from the plant-soil system can be significant if further waterlogging occurs, thereby potentially affecting crop yields. Finally, the responses of the soil bacterial community and microbial respiration to legacy effects of waterlogging and prolonged-drought, and N fertilizer addition were also examined. This experiment was carried out to examine links between N and C cycling in farming systems. Prolonged-drought prior to planting generated a strong legacy effect on soil bacterial abundance, diversity, and composition, and microbial respiration rates. N fertilizer supply increased soil bacterial abundance and diversity, and altered bacterial community composition. However, N fertilizer application up to 300 kg N/ha could not counteract the legacy effects of prolonged-drought on the soil bacterial community. Additionally, different bacterial phyla responded differently to the legacy effects of prolonged-drought and N supply. In contrast to prolonged-drought, waterlogging did not establish a legacy effect on the soil bacterial community and microbial respiration. This suggests that the soil bacterial community might be resistant to waterlogging or recover completely upon water stress. N fertilizer supply inhibited soil microbial respiration by inhibiting C-degraded enzyme activities. Some weak but significant correlations between soil total bacterial community and microbial respiration were observed; however, addition of N fertilizer weakened these relationships further. Overall, my study provides novel evidence of soil N cycling responses to climate change and extreme weather events in cotton farming systems. My study is the first to demonstrate a legacy effect from extreme weather events and external N supply on soil N and C processes, and subsequent consequences on crop productivity. Data obtained in this study will support the development of robust predictive models and adaptation strategies to sustain crop yields under future climatic conditions via effective N management.

Date of Award	2017
Original language	English