Soil carbon sequestration by grasslands to mitigate climate change

Grasslands over a quarter of land free area and due to large carbon (C) stock they represent a key global C pool. Grassland soil C sequestration is an alternative method for reduction of atmospheric carbon dioxide (CO2) which can also deliver other benefits including greater ecosystem stability and increase of biodiversity resulting from improved land management in agroecosystems or through restoration of degraded land. Available options for increasing C stock include increasing biomass yield and related delivery of more plant organic matter into the soil, decreasing losses of organic C from the soil and increasing mean residence time of C in the soil. Surveying regenerating grasslands with different time period of extensive land management (land management history) and performing field experiments, this thesis sought to increase understanding of soil organic C storage in grasslands in order to improve knowledge for greater soil C sequestration. This involved detailed measurements of responses of soil aggregate fractions (coarse and fine silt, clay and POM (POM)) and associated organic carbon (OC) concentration in soils with different land management histories together with microbial community composition. Secondly, grassland field experiment was established where soil pH was modified and potential drivers of ecosystem C cycle were monitored together with ecosystem process rates including soil microbial community composition, soil and plant nutrient pools, plant growth and community composition, soil potential extracellular enzyme activities and ecosystem C fluxes. Further, fungicide was applied to part of the grassland field experiment in order to determine response of the system to a disturbance. Thirdly, field mesocosms experiment was established testing response of community C and nitrogen (N) cycles to interactions of plant species differing in their nutrient acquisition strategies as well as response of soil microbial community composition to these plant interactions. The findings indicated that coarse silt and fine silt fractions responded differently to land management history and greater C accumulation in coarse fraction which increased with time without significant intensive agricultural management was suggested to be related to POM presence and organic C associated with clay fraction. Soil pH was found to be a strong driver of soil processes and microbial and plant communities when increasing soil pH by lime application promoted substrate availability in the soil, which resulted in changes in microbial composition and a greater microbial activity resulting in increased mineral N availability, which further affected plant growth and composition and ecosystem C respiration. Changes in soil C were not observed in this relatively short term study. Application of fungicides affected N availability and ecosystem C fluxes after the application but these largely returned to control levels later in the season. Comparing system responses on different pH levels plots showed different impact of applied fungicides depending on soil pH level. Studying plant species interactions demonstrated different impact of plant species with opposite life strategies on soil N cycle and further suggested higher N use efficiency with plant species mixtures than for monocultures of these species. These findings are especially interesting because all the species were selected from family Poacea. Overall assessment of microbial community composition showed responses of bacterial and fungal communities to land management history and soil pH initiated nutrient availability differences. The thesis demonstrated that soil organic carbon (SOC) pool consists of different sub-pools with different dynamics as determined based on soil aggregate fractionation. Traits and life strategies approach for characterizing functional differences of different species, as demonstrated on plant species interactions, can be further used for understanding of microbial community diversity and its impact on soil processes and soil C storage, especially at smaller spatial scales such as individual aggregate scale.