Drylands under pressure

: how variation in rainfall influence ecosystem properties and biogeochemical cycling

Abstract

Drylands are widespread and valuable ecosystems, present on every inhabited continent. They are characterized by unique climate that ranges from hyper-arid to dry subhumid and support important land-use including grassland, pasture, cropland and forestry. Thus, drylands contribute to essential ecosystem services, including food production, biodiversity, and climate regulation. However, being primarily water limited, drylands are sensitive to observed and projected changes in rainfall amounts, frequency of events and magnitude, which threaten their ability to provide ecosystem services in the future. The effects of rainfall variation on biogeochemistry of soil systems associated with changes in microbial and plant health, however, remain uncertain. Therefore, in this thesis, I (i) evaluated how variation in rainfall and soil water content affects plant health for different land-uses and climate across a time series; (ii) investigated effects of reduced rainfall on microbial biomass and activity and associated changes in carbon (C), nitrogen (N) and phosphorus (P) cycling; (iii) provide insight into plant C allocation and shifts in soil C and nutrient cycling under reduced soil water availability using stable isotope labelling (13C and 15N); and (iv) investigated how drought moderates the effects of extreme rainfall events on nutrient losses via leaching and subsequent shifts in soil C and nutrient availability. I conclude that variation in rainfall patterns, and associated soil water availability, can have large effects on plants performance across short time frames (1-3 months). Moreover, the impact will be moderated by climatic conditions given stronger correlations in dryer areas most likely due to the well-documented water pulse effects in drylands. Hence, short-term changes in vegetation can be predicted reasonably well in drylands using current technologies, with potential utility to land managers. Changes in vegetation performance caused by variation in rainfall availability and soil moisture will inevitably have detrimental impact on drylands nutrient cycling and microbial community, significantly contributing to reduction in available nutrient pools and enzymatic activity. While microbial biomass showed limited responses to drought, rainfall strongly moderated microbial activity, with greater investment in production of enzymes associated with less available nutrients. Moreover, the observed lack of response from microbial biomass suggests a disconnect between growth and microbial activity or that much of the biomass was dormant, particularly in the reduced rainfall plots. Drought can also moderate and change microbe-plant relationships with strong dependence on the nutrient reservoir. Nutrient rich environments can promote cooperation and benefit both plants and microbes under reduced water conditions while in environments with lower nutrient pools positive plant-microbe interactions can be limited, even under drought stress. Drought can greatly moderate the soil nitrogen nutrient pool, making it more susceptible to N loss during flood events. This loss could be irreversible causing detrimental long-term effects on dryland ecosystems.

Date of Award	2023
Original language	English
Awarding Institution	Western Sydney University
Supervisor	Uffe Nielsen (Supervisor)