Citizen science has advanced science for hundreds of years, contributed to many peer-reviewed articles, and informed land management decisions and policies across the United States. Over the last 10 years, citizen science has grown immensely in the United States and many other countries. Here, we show how citizen science is a powerful tool for tackling many of the challenges faced in the field of conservation biology. We describe the two interwoven paths by which citizen science can improve conservation efforts, natural resource management, and environmental protection. The first path includes building scientific knowledge, while the other path involves informing policy and encouraging public action. We explore how citizen science is currently used and describe the investments needed to create a citizen science program. We find that:1. Citizen science already contributes substantially to many domains of science, including conservation, natural resource, and environmental science. Citizen science informs natural resource management, environmental protection, and policymaking and fosters public input and engagement. 2. Many types of projects can benefit from citizen science, but one must be careful to match the needs for science and public involvement with the right type of citizen science project and the right method of public participation. 3. Citizen science is a rigorous process of scientific discovery, indistinguishable from conventional science apart from the participation of volunteers. When properly designed, carried out, and evaluated, citizen science can provide sound science, efficiently generate high-quality data, and help solve problems.

U.S. land management can contribute 1.2 Pg CO2e year−1 of greenhouse gas mitigation, 76% of which costs USD 50 Mg CO2e−1 or less.

Mitigating climate change requires clean energy and removing atmospheric carbon. Building soil carbon is an appealing way to increase carbon sinks and reduce emissions due to the associated benefits to agriculture. However, practical implementation of soil carbon climate strategies lag behind the potential, partly because we lack clarity around the magnitude of opportunity and how to capitalize on it. Here we quantify the role of soil carbon in natural (landbased) climate solutions (NCS), and review some of the project design mechanisms available to tap into the potential. We show that soil carbon represents 25% of the 23.8 GtCO2eyr-1 NCS potential of which 40% is protection of existing soil carbon and 60% is rebuilding depleted stocks. Soil carbon comprises 9% of the mitigation potential of forests, 72% for wetlands, and 47% for agriculture and grasslands. Soil carbon is important to land-based efforts to prevent carbon emissions, remove atmospheric carbon dioxide and deliver ecosystem services in addition to climate mitigation. Protecting and restoring soil organic matter delivers many benefits to people and nature 1,2. Globally, soils hold three times more carbon than the atmosphere 3 , and the role of soil organic matter as a regulator of climate has been recognized by scientists for decades 4. Recent work has highlighted the historical loss of carbon from this pool 3 , and the threat of future accelerated loss under warming scenarios 4,5. Soil organic carbon as a natural climate solution (NCS) thus has a role both through restoring a carbon sink and protecting against further CO 2 emissions in response to predicted land use change and climate change. This dual role for soil in the global carbon budget suggests climate benefits can be achieved through strategies that both conserve existing soil organic carbon stocks (avoid loss), and restore stocks in carbon-depleted soils 6. There are important additional benefits. Protecting and increasing soil carbon storage can (i) protect or increase soil fertility, (ii) maintain or increase resilience to climate change, (iii) reduce soil erosion, and where implemented through conservation of natural ecosystems iv) reduce habitat conversion, all in line with the United Nations Sustainable Development Goals (SDG's) 7 , the goals of the United Nationals Framework Convention on Climate Change (UNFCCC) and the United Nations Convention on Combating Desertification (UNCCD). As such, soil carbon is promoted as a common denominator amongst a variety of global and national initiatives 7. Although recent academic comment and perspective pieces point the way towards accelerated action on soils 8,9 , there remains much uncertainty around actionable pathways for achieving the global opportunity. Here we examine the scientific and policy context surrounding soil carbon projects, to aid prioritization and decision making.

Biodiversity loss is proceeding at an unprecedented rate, yet we lack a thorough understanding of the consequences of losing diversity at different scales. While species diversity is known to impact community and ecosystem processes, genotypic diversity is assumed to have relatively smaller effects. Nonetheless, a few recent studies suggest that genotypic diversity may have quantitatively similar ecological consequences compared to species diversity. Here we show that increasing either genotypic diversity of common evening primrose (Oenothera biennis) or species diversity of old-field plant species resulted in nearly equivalent increases (approximately 17%) in aboveground primary production. The predominant mechanism explaining this effect, niche complementarity, was similar for each type of diversity. Arthropod species richness also increased with both types of plant diversity, but the mechanisms leading to this effect differed: abundance-driven accumulation of arthropod species was important in plant genotypic polycultures, whereas resource specialization was important in plant species polycultures. Thus, similar increases in primary productivity differentially impacted higher trophic levels in response to each type of plant diversity. These results highlight important ecological similarities and differences between genotypic and species diversity and suggest that genotypic diversity may play a larger role in community and ecosystem processes than previously realized.