Linkages between soil enzyme activities and critical ecosystem processes in grasslands

Abstract

Soils are important life-supporting systems to human society and their long-term sustainability is high on both the scientific and political agenda. However soil functioning remains poorly understood because of the difficulty in studying complex biogeochemical interactions, which mostly occur belowground. This PhD study aims to improve the understanding of plant-soil-microbial interactions in grassland ecosystems. First, it was addressed how key microbial functions (i.e. extracellular enzyme activities) might respond to common management practices (e.g. nutrient fertilization) and to land use change (i.e. conversion from arable to grassland). Second potential linkages between enzyme activities and critical ecosystem processes such assoil carbon (C) and nitrogen (N) sequestration were investigated.

Overall this PhD project demonstrates that changes in both present and past grassland management significantly affect the activity of soil extracellular enzymes and this in turn influences soil C and N content. One of the key findings of this study is related to the existence of a potential ‘enzyme link’ between long-term inorganic N fertilization and the C content of organo-mineral soils. It was found that the activity of the C-acquiring enzyme β-1,4-glucosidase (BG, necessary for cellulose hydrolysis) significantly increased under chronic N fertilization and this in turn was associated with greater soil C sequestration. This work is the first to propose the hypothesis that the ‘enzyme link’ occurs because (a) BG activity is stimulated by increased microbial C demand relative to N under chronic fertilization, and (b) increased BG activity causes more C from roots and from microbial metabolites to accumulate and stabilize into organo-mineral C fractions.

This study also shows how the activity of C- and N-acquiring enzymes is positively related to soil C and N content across grassland soils with different pH values and management histories but not in intensively managed grasslands where greater soil compaction negatively affected enzyme activities.

Finally, the findings support the existence of a historical (legacy) effect on soil biogeochemistry whereby former cultivated grasslands have higher soil C and N content and significantly different microbial enzyme activities than permanent grassland. Altogether, the research findings suggest that any combination of management practices that could influence enzyme activities will have far reaching implications for the functioning and long-term sustainability of grassland soils.

Date of Award	Apr 2016
Original language	English
Sponsors	Department of Employment and Learning, Northern Ireland
Supervisor	Dario Fornara (Supervisor), Geoffrey Mc Mullan (Supervisor) & Nigel Ternan (Supervisor)