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Global temperatures are expected to increase by 1.1 to 6.4°C over the next century and, over the same period, nitrogen inputs to terrestrial ecosystems are expected to increase as a result of increased crop fertilisation and atmospheric nitrogen deposition. Both of these global change drivers are expected to affect net carbon balance by increasing both gross primary production and ecosystem respiration, yet the balance between these processes, and the potential interactive effects of the drivers, require quantification. The ability to accurately predict the effects of warming and nitrogen addition on all components of terrestrial carbon balance will be critical in determining the likely positive feedback to rising atmospheric CO₂ from terrestrial ecosystems. Tussock grasslands are a widespread and important carbon store within New Zealand and are representative of temperate grasslands worldwide. This thesis addresses the question: Will tussock grasslands act as a positive feedback to rising atmospheric CO₂ concentration in response to soil warming and nitrogen addition? Using a combination of controlled-environment and field-scale studies of tussock grassland, net ecosystem carbon exchange was partitioned into gross primary production, ecosystem respiration and the autotrophic and heterotrophic components of soil respiration. Soil respiration in the field increased by 41% in response to a 3°C soil warming treatment and by 12% in response to a 50 kg N ha⁻¹ y⁻¹ nitrogen addition treatment. Only warming resulted in enhanced heterotrophic decomposition of soil organic matter (37% increase). However, a controlled-environment study indicated that caution must be used when interpreting temperature responses of heterotrophic respiration from root-free soils, as priming effects were shown to decrease the sensitivity of heterotrophic respiration to temperature. Measurements of net ecosystem exchange in the field showed that warming-enhanced heterotrophic respiration lead to a significant 49 g m⁻² reduction in net ecosystem carbon uptake. Neither nitrogen addition nor combined warming and nitrogen addition treatment resulted in significant changes in net ecosystem carbon balance. These results suggest that tussock grasslands will act as a positive feedback to rising atmospheric CO₂ concentration. However, increased nitrogen deposition will serve as a potential mitigating factor for climate driven feedbacks.

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Northern hardwood forests are expected to experience an increase in mean annual air temperatures, and a decrease in winter snowpack and greater frequency of soil freeze/thaw cycles (FTCs) by the end of the century. As a result of these anticipated changes, northern hardwood forests in the northeastern U.S. will also have warmer soil temperatures in the growing season and colder soils in winter. Prior studies show that warmer soils in the growing season increase net primary productivity (NPP) and C storage as a result of increased soil net N mineralization, while increases in soil freezing in winter reduces plant uptake of N and C as a result of root damage. However, the combined effects of warmer soils in the growing season and increased soil freeze/thaw cycles in winter on tree litter mass and chemistry are unknown. We report here results from the Climate Change Across Seasons Experiment (CCASE) at Hubbard Brook Experimental Forest in New Hampshire, USA to characterize the response of leaf litter mass and chemistry to growing season warming combined with soil freeze–thaw cycles in winter. Across the years 2014-2017, litterfall mass and chemistry (%C, %N, C:N) were not significantly affected by changes in soil temperature; however, there was a trend of higher total litterfall mass and litter N mass from plots where soils were warmed in the growing season, but this increase disappeared with the addition of FTCs in winter. These results indicate that while rates of NPP and the total mass of N could be increased with rising soil temperatures over the next century in northern hardwood forests, the combination of warmer soils in the growing season and colder soils in winter may ultimate have little to no impact on litter mass or chemistry. We conclude that considering the combined effects of climate changes in the growing season and in winter is vital for the accurate determination of the response of litterfall mass and chemistry in northern hardwood forests.

Climate and agricultural practice interact to influence both crop production and soil microbes in agroecosystems. Here, we carried out a unique experiment in Central Germany to simultaneously investigate the effects of climates (ambient climate vs. future climate expected in 50–70 years), agricultural practices (conventional vs. organic farming), and their interaction on arbuscular mycorrhizal fungi (AMF) inside wheat (Triticum aestivum L.) roots. AMF communities were characterized using Illumina sequencing of 18S rRNA gene amplicons. We showed that climatic conditions and agricultural practices significantly altered total AMF community composition. Conventional farming significantly affected the AMF community and caused a decline in AMF richness. Factors shaping AMF community composition and richness at family level differed greatly among Glomeraceae, Gigasporaceae and Diversisporaceae. An interactive impact of climate and agricultural practices was detected in the community composition of Diversisporaceae. Organic farming mitigated the negative effect of future climate and promoted total AMF and Gigasporaceae richness. AMF richness was significantly linked with nutrient content of wheat grains under both agricultural practices.

Phenotypic plasticity plays an important role in organisms’ adaptability to environmental change such as global warming caused by greenhouse-gas emissions. One plastic response to increased temperatures is for organisms to shift their phenology. It is of great concern that the phenologies of interacting species, such as plants and pollinators, may be shifting at different rates, causing temporal mismatches, which for plants can lead to unsuccessful reproduction. The “reproductive assurance hypothesis” states that plants capable of self-pollination should be under high selection to employ this as their main reproductive strategy in the event of pollinator scarcity to ensure reproduction, and consequently invest less in attracting pollinators. This study examines how soil warming in the Hengill geothermal area in Iceland affects the flowering phenology, reproductive strategy and investment in attractiveness to pollinators in the self-compatible herb Cerastium fontanum (Caryophyllaceae), when grown in a common garden in Stockholm, Sweden. Previous research showed that C. fontanum from warmed soils flowered earlier in situ than plants from colder soils, and later when grown in a common environment. In this study, C. fontanum plants collected along a temperature gradient followed the same counter-gradient pattern, where plants from warmer soils flowered later than plants from colder soils. Soil temperature at site of origin positively affected flower number but had no effect on flower size, seed production from autogamous self-pollination or visitation rate. Based on my findings it does not appear that C. fontanum, despite having an earlier flowering phenology in situ, is under any selection to alter its reproductive strategy or investment in attractiveness to pollinators when grown in a common temperature, and therefore it seems unlikely that plants are experiencing a temporal mismatch with insect pollinators. However, it would be worthwhile to conduct a similar experiment in Iceland to better understand how an earlier flowering affects pollination systems.