Solar radiation is an important contributing factor to decomposition in drylands. Research suggests that in the presence of water, previously irradiated plant litter experiences greater microbial decay than litter which was not exposed to radiation. It is unclear how exactly radiation alters litter to allow this photopriming of microbial decomposition. However, the relationship to water suggests that radiation may make litter more water-soluble, and therefore more accessible to decomposers once water enters the system. I tested the hypothesis that the abiotic impact of solar radiation on grass litter would (1) increase the production of dissolved organic carbon (DOC) when litter is subsequently extracted with water, and (2) produce DOC that stimulates more microbial activity compared to unexposed litter. Dried senesced grass litter from three species, Bromus diandrus, Avena fatua, and Hordeum murinum, were placed in sealed bags and subjected to abiotic decomposition in either an outdoor experiment or an indoor experiment. Treated litter was then soaked in water, and the extract was analyzed to determine the dissolved organic carbon concentration and its bioavailability. Exposure to radiation resulted in more DOC for all species in both the indoor and outdoor experiments, suggesting that solar radiation does enhance solubility of grass litter. During a microbial incubation, I observed a significant increase in CO2 production and a marginally significant increase in DOC consumption for samples exposed to more radiation in the indoor experiment. However, as a fraction of initial DOC available, radiation reduced these measures of microbial activity. This indicates that photodegradation produces compounds which are relatively difficult for microbes to decompose, but photodegradation can still stimulate microbial activity by increasing the total amount of available dissolved carbon. Taken together, these results suggest a possible mechanism for observed increases in mass loss due to photopriming: litter carbon is made more soluble by radiation, and is mobilized in the presence of water, allowing for increased microbial decomposition. This insight into decomposition mechanisms could aid in developing more mechanistic models of carbon cycling that include photopriming.

Wetland carbon sequestration is offset by carbon dioxide (CO2) and methane (CH4) emissions for which the magnitudes remain coarsely constrained. To better understand the spatial and temporal variations of gaseous carbon fluxes from marsh soils in a Mediterranean climate, I collected air and soil samples over the course of 10 months at Carpinteria Salt Marsh Reserve (CSMR) located in the County of Santa Barbara, California. The CSMR consists of four zones characterized by differences in elevation, tidal regime, soil properties, and vegetation. Twelve static chambers were deployed among two lower marsh zones, a mudflat, and a marsh-upland transition zone for fortnightly flux measurements from September 2015 to May 2016. In August 2015 and June 2016, soil cores up to 50 cm deep were extracted near the chambers, segmented by depth, and analyzed for soil moisture, bulk density, particle size distribution, electrical conductivity, pH, organic/inorganic carbon, and total nitrogen content. Averaged over the 9-month study period, the marsh-upland transition zone had the highest CO2 fluxes at 5.3 ± 0.7 g CO2 m-2 d-1, followed closely by the lower marsh zones (3.8 ± 0.6 g CO2 m-2 d-1 and 2.8 ± 0.7 g CO2 m-2 d-1), which were one order of magnitude higher than the CO2 fluxes from the mudflat (0.4 ± 0.1 g CO2 m-2 d-1). The CO2 fluxes varied significantly on a seasonal scale but were not consistently correlated with environmental variables measured. The CH4 fluxes had no clear seasonal patterns, but overall CH4 flux rates from the lower marsh zones (2.2 ± 1.5 mg CH4 m-2 d-1 and 1.9 ± 0.2 mg CH4 m-2 d-1) surpassed those from the mudflat (0.2 ± 0.06 mg CH4 m-2 d-1) by an order of magnitude, and the marsh-upland transition zone was a net methane sink (-0.07 ± 0.1 mg CH4 m-2 d-1). The CH4 fluxes correlated well with most soil properties by zone. Our results show that soil gaseous carbon fluxes from a coastal salt marsh vary by salt marsh zone.

Coastal wetlands are being lost at an alarming rate and thus wetland restoration is an important way to recruit coastal wetlands as an ally in our fight against climate change. To better understand and maximize the effectiveness of wetland restoration, it is important to evaluate the success of these restoration projects. In this study, we measured sediment accumulation and soil carbon density in North Campus Open Space, a recently restored wetland near the campus of UC Santa Barbara. The objectives of this study were to 1) assess rates of sediment accumulation since restoration, 2) evaluate spatial heterogeneity in sediment accumulation and soil carbon density, and 3) use this information to better understand the mechanisms that define the relationship between sediment accumulation and surface soil carbon storage to better predict carbon accumulation. Sediment accumulation rate averaged 2.16 mm/year, with a range of 0.4 mm/yr to 5.4 mm/yr. Sediment accumulation, surface soil carbon density, and subsurface carbon density all varied significantly by plot location. The spatial dynamics of vertical accretion were primarily driven by mineral rather than organic material, whereas surface soil carbon was primarily driven by aboveground plant biomass. Sediment accumulation was greatest downstream, whereas soil carbon accumulation was greatest upstream at the freshwater stream inputs. Some areas showed a negative relationship between sediment accumulation and soil carbon density, with high sediment accumulation associated with low surface soil carbon density. Although wetlands are touted to mitigate sea level rise and climate change through sediment accretion and carbon sequestration, these results suggest that sediment accumulation and carbon accumulation do not necessarily co-occur. The results imply that sediment accumulation alone cannot reliably predict carbon accumulation.

Application of organic amendments (OAs) has the potential to act as a greenhouse gas (GHG) emission mitigation strategy and increase soil carbon (C) storage in row crop agriculture. However, OA application may stimulate an increase in emissions of nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2). To determine how compost application rate affects both GHG emissions and crop yield, a field study was conducted at an organic vegetable farm in Santa Barbara County (SBC), California. Additionally, a targeted survey of compost-using farmers (n = 14) in SBC and adjacent counties was conducted to understand OA application practices of compost users in the region. In our field experiment, farm managers were asked to apply their normal, high compost (HC) treatment of 18.2 Mg ha-1 and a low compost (LC) treatment of 9.1 Mg ha-1 immediately prior to seeding of carrots (Daucus carota subsp. sativus). A commercial organic fertilizer (OF) was applied to beds of both HC and LC treatments 43 days after compost application (DAC) at the rate of 672 kg ha-1. N2O, CO2, and CH4 emissions from static flux chambers, along with soil nitrate (NO3-), soil ammonium (NH4+), gravimetric soil moisture, and soil temperature in beds and furrows of each treatment, were measured approximately once weekly for the entire 95 day carrot growing season. Soil total C, total N, bulk density, organic matter, pH, plant tissue C and N, and crop yield were also measured during the study. Seasonal N2O and CO2 emissions were higher in HC than LC in beds (p < 0.001 N2O, p = 0.035 CO2); seasonal net uptake of CH4 was not different in beds (p = 0.901), but was significantly higher in LC than HC in furrows (p = 0.027), though CH4 fluxes were often highly variable. Significantly higher emissions of N2O and CO2 in HC beds than LC beds were measured only after OF application (except CO2 5 DAC). Crop yield was higher in HC (2.82±0.21 Mg ha-1) than LC (2.17±0.06 Mg ha-1). Yield-scaled global warming potential (GWP), an indicator of agronomic efficiency, was not significantly different between treatments. CO2 was not included in GWP due to its biogenic origin and an inability to accurately account for net soil C loss during this short time period. A higher amount of biochemically available organic matter and soil moisture in HC than LC may have led to higher decomposition and denitrification rates following OF application, suggesting that C storage benefits of OA application can be reduced by fertilizer addition. Because GWP per unit crop yield was not different between LC and HC, HC may better concurrently address concerns regarding atmospheric GHG emissions and global food demand. Our survey results indicate that average compost application rate in and around SBC (12.45±6.54 Mg ha-1) was lower than HC in our study. While farmers understood the benefits of compost for long-term soil fertility and C storage, economic limitations to increasing compost application rate exist within this region of major crop production. To determine whether OA application affects decomposition of native soil organic matter for more accurate GWP calculation, future studies would benefit from including a control treatment and identifying origins of CO2-C emissions, potentially through use of isotopic labeling. New insights about the effects of OA application rate on GHG emissions can aid in assessment of OA use in agricultural C sequestration or GHG mitigation policies.

The Montecito debris flows of 9 January 2018 deposited sediment along many portions of the Santa Barbara coast, including Carpinteria Salt Marsh Reserve. Since disturbances have the potential to impact the ecosystem services and functions that wetlands provide, an understanding of how the ecosystem at Carpinteria Salt Marsh Reserve responded to this disturbance is important to its ongoing management. However, a lack of opportunity to collect field data around the time of disturbance makes this task difficult to complete by field methods. To address this gap, Sentinel-2 imagery from four dates (November 2017, January 2018, November 2018, November 2020) was used to calculate landcover fractions, normalized difference vegetation index (NDVI), and modified anthocyanin reflectance index (mARI) and used in tandem with random forest classification to produce maps of landcover before, during, and after the debris flow. Post-classification change detection was then performed on the classified maps to track changes in landcover through time. Results from the random forest classification showed that NDVI and green vegetation fractions were the most important variables in classifying landcover, though this varied on a date-by-date basis; error matrices indicated that the model had high accuracy with values of 0.994, 0.920, 0.956. and 0.963 for the respective dates. Change detection shows a pattern of returning to pre-debris flow vegetation extent in the marsh. While total vegetated area experienced little change (0.12% increase), there was a change in the extent of vegetation type with high marsh vegetation shifting to mid marsh vegetation in regions near where increases in bare soil landcover occurred. These results show that disturbance due to debris flow can cause vegetation changes which may affect ecosystem function, such as decrease primary productivity and marsh resilience to further disturbance, that will need to be taken into consideration when managing depositional event prone wetlands.

In my dissertation research, I investigated how coastal fog influences the water relations and distribution of Bishop pine (Pinus muricata D. Don), a drought sensitive species restricted to the fog belt of coastal California and offshore islands. I will discuss three related projects motivated by the following research questions: 1) Can the current spatial pattern of Bishop pine mortality on Santa Cruz Island be explained by important environmental and biological controls on plant available water? 2) How do summertime fog water inputs affect the water status of Bishop pines? Do adult and sapling trees respond to fog differently? and 3) What is the relative importance of fog-drip and fog immersion to the physiological function of Bishop pine saplings? I addressed these questions by using a variety of approaches ranging from remote sensing techniques to field-based plant physiology. The outcomes of these studies provide evidence that coastal fog is an essential element to augmenting plant available water during the dry season and that its occurrence supports the southern extent of its range on Santa Cruz Island. Moreover, this work advances our ability to make mechanistically-based predictions of how foggy coastal forests may respond to a warmer, drier climate.