The nitrogen cycle and ecohydrology of seasonally dry grasslands

Abstract

This thesis addresses the coupling of hydrologic and biogeochemical processes and, specifically, the organization of ecosystem traits with the water, carbon, and nitrogen cycles. Observations from a factorial irrigation-fertilization experiment in a seasonally dry annual grassland are combined with a simple ecosystem model to identify relationships between vegetation, nitrogen availability, and hydrology. Assuming primary productivity is water-limited, data analysis indicates that soil moisture and canopy conductance are insensitive to nitrogen supply, owing to a trade-off between canopy density and leaf conductance that maximizes efficient use of available water. That is, fertilization-induced increases in leaf area index are offset by reduced leaf area-based stomatal conductance. When primary productivity is assumed to be co-limited by water and nitrogen availability, total surface conductance is estimated to be insensitive to nitrogen supply, but added nitrogen increases the ratio of transpiration to evaporation. This coupled water-carbon-nitrogen model is then extended to predict ecosystem sensitivity across independently varied gradients of water and nitrogen supply rates. This analysis reveals two distinct regimes of plant-resource organization. In arid climates, rooting depths decrease with increasing aridity, while in humid climates, rooting depths increase with aridity. In all climates, rooting depths increase with increased nitrogen supply. Further, relative root-carbon allocation always increases with aridity and decreases with nitrogen supply. These resource use strategies result in an efficient use of available water in arid climates and efficient use of available nitrogen in humid climates. The associated ecosystem process rates indicate that nitrogen supply is an important determinant of surface water and carbon fluxes in humid climates, but only of carbon fluxes in arid climates.