Fertilizer production and application to global agricultural land, along with incomplete nutrient removal during human excreta treatment, exerts a considerable strain on global biogeochemical cycles. There is increasing interest in the safe recovery of nutrients in human excreta for reuse in agriculture, often termed Ecological Sanitation (EcoSan). Often employed in EcoSan systems is the source-separation of urine and feces, which allows for distinct treatment processes that may reduce greenhouse gas emissions and water use compared to traditional sewerage, while conferring high nutrient recovery efficiencies. EcoSan may also provide a local source of plant-available nutrients to smallholder farmers, who are a foundational, yet underserved, part of the global food system. While EcoSan is gaining traction in terms of both research and operation, there is limited information on the biogeochemical outcomes of EcoSan end-use product fertilizer application. Particularly, soil and plant nutrient dynamics following the application of urine and urine-enriched biochar is understudied. Additionally, agronomic research on the application of feces-derived compost on agricultural land near EcoSan systems that produce such compost, is lacking. In this dissertation, we address these research gaps to further the field of EcoSan. In the first project, we investigated urine-enriched biochar as a nitrogenous fertilizer for tomato growth. We prepared urine-enriched biochar with three types of biochar combined with human urine stored in three realistic conditions. We found that the < 500-μm biochar particle size fraction retained significantly more nitrogen (N) than larger particles across biochars, and that urine-N in fresh urine had higher sorption affinities for > 500-μm biochar particles compared to urea-hydrolyzed urines. We also showed that urine-N applied alone is more immediately plant-available than urine-N sorbed to biochar. In our second project, we investigated the agronomic relevance of a suite of EcoSan fertilizers by assessing nitrogen (N) and phosphorus (P) mineralization of urine, urine-enriched biochar, and feces-derived compost in a 90-day amended soil incubation. We showed that urine applied alone is an excellent source of immediately plant-available N, while urine-enriched biochar application supplied approximately half of the N applied. We observed that feces-derived compost application stimulated substantial mining of native soil-P and led to a moderate, slow release of plant-available N. In the third project, we investigated the effect of repeated feces-derived compost application on sorghum production and soil health indicators over two consecutive cropping cycles in an agroecosystem in northern Haiti. We found that feces-derived compost, particularly when applied at 150% of the sorghum N demand at the start of the growing season, led to significant increases in bioavailable soil macro- and micronutrients by the second cycle of management. One application of feces-derived compost resulted in decreased soil bulk density. Moderate increases in soil carbon (C) and N in the topsoil were observed for compost applied at 150% of the N demand by the end of the second cropping cycle. We also provide a chapter reflecting on the unique challenges and opportunities inherent to EcoSan research. This dissertation shows that human excreta is an important, largely untapped resource that can provide nutrients to agricultural soils and improve their health. EcoSan is a powerful tool that can help ameliorate the negative impact of human activity on global biogeochemical cycles, which is urgent to preserve our Earth system.

Dairy manure is one of the largest emitters of greenhouse gases (GHG) and air pollutants from agriculture. In California, the nation's leader in dairy production, dairy manure accounts for 25% of total methane (CH4) emissions and contributes to poor air quality in agricultural regions. Recent policies such as the Global Methane Pledge and California's SB 1383 will require large reductions in CH4 emissions from dairies. While anaerobic digesters can mitigate emissions from liquid manure, very few management strategies exist to reduce emissions from solid manure. Recently, biochar-composting has been proposed as a strategy to cut emissions during the composting of organic materials. However, limited information exists on whether biochar could reduce emissions during the composting of dairy manure. Here, we investigated the potential of dairy manure biochar-composting to reduce both GHG and air pollutant emissions from dairies. In the first project, we measured GHG emissions during the full-scale composting of dairy manure with or without biochar, and we found that biochar-composting reduced CH4 emissions by 84%. Using life-cycle assessment (LCA), we also estimated the net climate impact of managing solid dairy manure through biochar-composting, composting, or stockpiling, and we found that biochar-composting was a climate-negative dairy manure management strategy. In the second project, we investigated whether biochar-composting could also reduce the emission of air pollutants from dairy manure. We found that biochar-composting cut the emission of hydrogen sulfide, volatile organic compounds, and nitrogen oxides by 70%, 61% and 67%, respectively. Using an integrated assessment model, we estimated that this reduction in air pollution would reduce health-related social costs by $66,000 annually per dairy farm. In the third project, we conducted a laboratory composting experiment to test the effect of biochar feedstock (walnut shells, almond shells, and almond clippings) and application rate (5% and 20%, by mass) on composting emissions. While there was no difference in emissions between biochar type, we found that high application rates increased CH4 emissions and reduced N2O emissions. We ascribe this pollution swapping to an increase in compost aggregation with biochar application, as aggregates likely acted as anoxic microsites for methanogenesis and complete denitrification. We recommend using lower biochar application rates, which reduced N2O emissions without increasing CH4 emissions in our study. This dissertation shows that biochar-composting of solid dairy manure is an untapped natural climate solution with a high potential to help meet climate goals and improve the health of rural, disadvantaged communities by reducing air pollution.

Ecosystem management practices that sequester carbon (C) may play an important role in mitigating climate change. Grasslands managed for livestock (e.g., rangelands) constitute the largest land-use area globally. Critical components of the long-term sustainability of rangelands are the maintenance of net primary production (NPP) and soil organic carbon (C) pools. However, overgrazing, plant invasions, and climate change have led to significant C losses from many rangeland ecosystems. Thus, management practices may have considerable potential to restore or increase grassland C storage and help mitigate climate change. Practices that promote C sequestration may have valuable co-benefits, including increased forage production and improved soil water holding capacity. Despite the potential for C sequestration through management interventions, the question remains largely unexplored in grassland ecosystems.

I used a combination of laboratory experiments, field manipulations, and modeling simulations to examine the effects of rangeland management practices on C sequestration and greenhouse gas emissions. The specific goals of this research were to 1) assess the immediate and carry-over effects of management practices on the net C balance and greenhouse gas emissions in grasslands amended with compost, 2) measure changes to soil C and N stocks following amendment, 3) investigate the long-term fate of compost C and net climate change mitigation potential, and 4) explore the extent of tradeoffs between C sequestration strategies and vegetation characteristics.

In the first chapter, I conducted a three-year field manipulation replicated within and across valley and coastal grassland sites to determine the effects of a single application of composted organic matter amendment on net ecosystem C balance. Amendments increased C losses through soil respiration, and estimates of net C storage were sensitive to models of respiration partitioning of autotrophic and heterotrophic components. Over the three-year study, amendments increased C inputs by stimulating net primary production by 2.1 ± 0.8 at the coastal grassland and 4.7 ± 0.7 Mg C ha-1 at the valley grassland. Carbon gains through above- and belowground NPP significantly outweighed C losses, with the exception of a sandy textured soil at the coastal grasslands. Treatment effects persisted over the course of the study. Net ecosystem C storage increased by 25 to 70 % over three years, not including direct C inputs from the amendment.

The purpose of chapter two was to further investigate changes to rangeland soil C and N stocks three years after a one-time application of composted organic material. Increases in bulk soil C, though often difficult to detect over short timeframes, were significant at the valley grassland study site. Physical fractionation of soil revealed greater amounts of C and N in the free and occluded light fractions by 3.31 ± 1.64 and 3.11 ± 1.08 Mg C/ha in the valley and coastal grassland, respectively. Analysis of the chemical composition of soil fractions by diffuse reflectance infrared Fourier transform (DRIFT) showed chemical protection and inclusion of compost C into the light fractions. The combination of physical and chemical analyses suggests that the newly incorporated C was physically protected and less available for decomposition.

In the third chapter, I employed the ecosystem biogeochemical model, DAYCENT, to investigate the short (10 yr), medium (30 yr), and long-term (100 yr) climate change mitigation potential of compost amendments to grasslands. Climate change mitigation potential was estimated as the balance of total ecosystem C sequestration minus soil greenhouse gas emissions and indirect emissions of N2O via nitrate leaching. The model was parameterized using site-specific characteristics and validated with data from the three-year field manipulation. Model simulations included variations in the applications rate and C:N ratio of the composted material. Above- and belowground NPP and soil C pools increased under all amendment scenarios. The greatest increase of soil C occurred in the slow pool. Ecosystem C sequestration rates were highest under low C:N scenarios, but these scenarios also resulted in greater N2O fluxes. Single or short-term applications of compost resulted in positive climate change mitigation potential over 10 and 30-year time frames, despite slight offsets from increased greenhouse gas emissions.

Finally, chapter four examined important tradeoffs between rangeland C sequestration activities and vegetation characteristics. I measured aboveground biomass, plant N content, vegetation communities, and the abundance of noxious weed species for four years following single management events of compost amendment, keyling plowing, and a combination of amendment and plowing. During the first year, plant N content and aboveground biomass was significantly higher in the amended plots and lower in the plowed plots. In the amended plots, forage quantity and quality increases were sustained over the four-year study. During spring grazing events, cows consumed more forage from amended plots without adversely increasing grazing impacts on residual biomass. Plant communities at both grasslands were relatively resistant to management events, however there were short-term declines in the abundance of a noxious annual grass at the valley grassland and increases in a noxious forb at the coastal grassland.

Grassland management practices, such as the application of composted organic matter, have considerable potential to mitigate climate change while improving plant production, soil fertility, and diverting organic wastes from landfills. This research illustrates the potential for grassland management to sequester while explicitly considering impacts on greenhouse gas emissions, plant production, and vegetation communities over multiple time frames. Overall, my dissertation contributes toward a better understanding of the role of ecosystem management interventions in climate change mitigation.

Loss of organic matter (OM) from soil through degradation can lead to decline in soil structure, depletion of plant nutrients, enhancing mineralization of organic compounds and contribute to climate change through the release of greenhouse gas emissions. To address this, it is important to identify and expand our current understanding of the management practices that ensure that the functions and services of soils in agricultural lands are maintained for the long-term. Sustainable agriculture aims to meet society’s needs for food, feed, and fiber, while protecting natural resources and is primarily guided by the quality of and aims to maintain or improve health of soils. A management practice that directly benefits soil quality, through the enhancement of soil organic matter (SOM), is the use of organic matter amendments (OMA). OMAs are a group of organic materials that are rich in nutrients that can be recycled back to the soil and can have positive effects for improving soil conditions and properties, and are proposed as a climate change mitigation strategy. Through extensive field, laboratory studies, and advanced analytical techniques, I show the potential benefits of recycling organic matter amendments into agricultural soils in California. Three projects were established and make up the three chapters of my dissertation: the first project focused on how the application of two different types of composts in almond orchards influenced soil fertility and nutrient cycling. Main findings from this chapter show that the applications of these OMA were heavily influenced by soil texture. After two years of OMA application there was an improvement in soil fertility. The second chapter focused on how long-term application of biosolids in agricultural soils influences carbon content, specifically focusing on the importance of accounting for deep soil carbon in order to determine a soils climate change mitigation potential in three sites in northern California. Main findings show that application rate of biosolids is not a determining factor when it comes to carbon accounting, but management practices play a bigger role. Additionally, not accounting for deep soil carbon leads to an underestimation of carbon sequestration in biosolid amended soils. Chapter 3 builds off Chapter 2, and investigates the stability of this C and N by determining what pools in the soil they are in. These different pools will provide insight on how vulnerable these reservoirs are to management induced changes which can contribute to C and N losses. Overall findings show that long-term amended soils contribute increase carbon content associated to unprotected pools in surface soils where it is easier for microbes to mineralize. Carbon content in deeper soils show that the carbon is associated to physically protected and mineral associated pools, indicating this carbon to be more persistent. Overall, these findings show that site conditions, measurement methodology, and management practices influence the potential of OMA to be beneficial to soil fertility and climate beneficial.

This thesis explores the diverse aspects of biochar production, its economic assessment, and its importance in mitigating climate change while supporting sustainable regional economic growth. Biochar, a carbonaceous biomass product, is gaining increasing prominence as an effective solution for curbing greenhouse gas emissions stemming from conventional agricultural practices. This research adopts a comprehensive approach that covers various dimensions of biochar. First, it emphasizes the necessity of shifting from the environmentally harmful practice of crop residue open burning to the more sustainable avenue of biochar production. Through a techno-economic analysis, the study reveals the range of production costs under uncertainty. Results from the first study show a probable range of biochar production costs between $448.78 and $1,846.96 (USD) Mg−1, with a 90% probability that costs will range between $571 and $1,455 Mg−1. A sensitivity analysis shows that production costs are most responsive to biochar production rates. Second, showing the significance of regional economic assessments for biochar projects. Recognizing the lack of a dedicated industry classification for biochar, innovative methodologies are employed to estimate the direct, indirect, and induced economic impacts of biochar production in Central Valley, California. Results suggest that depending on the biochar price and conversion rates, biochar would create between 16.56 to 17.69 new full- and part-time jobs per year that would contribute between $1.2 to $5.75 million per year to labor income. Biochar production would add to the Gross Domestic Product (GDP) about $106,295 ($5.2 million) per year with a conversion rate of %15 (%35) and a biochar price of $280 ($2,512) per metric ton. Similarly, biochar’s impacts on gross output would be positive, regardless of the biochar conversion rate and price, which suggests the need for more investment in the sector. We find that all regions would benefit in terms of employment, labor compensation, value addition, and gross output though Madera County would have the least economic returns. Meanwhile, Fresno County with the most biomass would have the most economic impacts suggesting that policy should be directed at encouraging biomass production and marketing in areas with the most biomass. Third, highlighting the critical importance of selecting the appropriate discount rate when evaluating biochar projects, particularly with a focus on climate change mitigation potentials. Two novel environmental-economic discounting models are used, one rooted in a modified Ramsey formula and the other in the Consumption Capital Asset Pricing Model. The first model yields a discount rate of 1.7%, while the second model suggests a declining rate of 5.96%. We recommend incorporating both rates in the biochar cost-benefit analysis and conducting a sensitivity analysis for a more comprehensive assessment. In summary, this thesis addresses biochar economics from different perspectives, namely, its economic evaluation, its vital role in addressing climate change, and its regional impact. The outcomes confirm the feasibility of transitioning towards a circular bioeconomy, highlight the regional economic benefits associated with biochar production and propose a nuanced discounting framework to ensure precise project evaluation. This research contributes to the development of sustainable practices, economic growth, and the simultaneous resolution of pressing environmental concerns.