Climatic, temporal, and biotic controls on soil biogeochemistry in volcanic soils [electronic resource]
Keywords: Environmental Earth System Science

Soils are one of the life support systems upon which human civilization is built. They provide myriad ecosystem services including water purification, food production, and climate regulation. Soil properties and development are ultimately controlled by five independent factors—climate, biology, time, topography, and parent material—and interactions between these factors. By investigating soil properties along the axes of these factors, we understand how soils will respond to changes. In this dissertation I investigate the climatic, biological, and temporal controls on soil development in volcanic soils of the Hawaiian Islands. I focus on a water balance gradient and invasive population of the nitrogen fixing Ulex Europaeus on soils developed from Mauna Kea Volcano parent material that is approximately 20 ky old, a previously understudied tropical soil age. Studying soils developed from the 20 ky parent material allows me to examine a climatic gradient that contains the transition from kinetic to supply limitation on plant nutrients in surface soils; whereas other climate gradients in Hawai'i cover soils that are constrained to either the kinetic or supply limitation side of this transition. It is also a young endmember on an age gradient of soils derived from fine-grained volcanic ash in Hawai'i. In Chapter 2, I focus on identifying climatic controls on soil properties in soils developed on intermediate aged substrate. I use a novel statistical approach to identify abrupt, non-linear shifts in soil properties across the water balance gradient. I then compare these results to patterns in soils developed on older and younger substrates. I demonstrate that for soils developed on intermediate aged substrate, there are three distinct soil development domains related to water balance. Comparing across substrate age, the transition between domains occurs in drier regions as substrate age increases. In Chapter 3, I focus on the soil fertility response to invasion by the nitrogen fixing shrub U. europaeus and subsequent restoration efforts. I extract the invasion and restoration effects from the background environmental variations identified in Chapter 2. In this chapter I show that on the spatial scale of the U. europaeus invasion, climate does not have a significant effect on most soil properties; that U. europaeus acidifies soils and decreases soil fertility with respect to calcium; and although a nitrogen fixer, U. europaeus has no significant effect on soil nitrogen content. The restoration efforts restored soil fertility to pre-invasion levels, highlighting that the effects of the invasion are reversible. Continuing to build the work done in Chapter 2, Chapter 4 is an investigation of iron and aluminum dynamics in soil across the water balance gradient. Iron and aluminum solid phases make up a substantial fraction of the solids found in these soils, and consequently they play a strong role in nutrient cycling and carbon storage. I find that across the water balance gradient neither iron nor aluminum is lost from these soils in an appreciable amount, though there are significant shifts in the form and distribution of both elements throughout the soil profile. Furthermore, carbon storage is largely associated with aluminum in the drier sites, and increasingly associated with iron as water balance increases. I find that water balance drives substantial diversity in soil types over the 20 ky of soil development. The results show that the studied soils vary from shallow, poorly weathered parent material with little carbon accumulation in the driest sites, to deep carbon rich, heavily weathered soils in the wettest sites on the water balance gradient. Additionally, the invasive species effects indicate the relative importance of biological vs. climatic controls is scale dependent.