Climate change is a major global threat affecting food security and sustainability. Land use systems involving trees have the potential to positively impact climate change by reducing atmospheric carbon dioxide (CO₂) and providing long-term carbon (C) storage. This review evaluated the C sequestration potential of two major land use systems of the United States (US) involving trees, forests and agroforests, which can also provide other ecosystem services.The estimated total forest C stock on forest land in the US in 1990 was 50,913 Tg and another 1885 Tg remained in harvested wood and discarded wood products. From 1990 to 1995, total C stock rose by 2%, and from 2000 to 2005, it rose by 1.7%. The US forests collectively lose (flux) about 200 Mg C y^-1 from disturbance and harvesting. Currently, about 12% of the conterminous US forest land is at high or very high risk of wildfire. Annually, insects and diseases could transfer ~ 21 Tg of live aboveground biomass to litter and woody debris pools. A scenario that targets an afforestation policy for rural landowners in the eastern US and a reforestation policy targeting understocked federal forest lands in the western would improve US annual sequestration compared to the baseline of 323 Tg CO₂ eq yr^-1 in 2015 to 469 Tg CO₂ eq yr^-1 in 2050.Agroforestry offers greater potential to increase C sequestration of predominantly agriculture-dominated landscapes than monocrop agriculture by storing C in above- and belowground biomass, soil, and living and dead organisms and further extending the duration of C in soils. The estimated total C sequestration of current alley cropping (211,938 ha), riparian buffers (640,732 ha), silvopasture (34 Mha), and windbreak (2.37 Mha) practices is 219 Tg C yr^-1. The total C sequestration would be 240 Tg C yr^-1 with 5% of the US cropland converted to alley cropping (3.7 Tg yr^-1), 15-m wide riparian buffers on both sides of 5% of the total stream length (4.75 Tg yr^-1), 34 Mha converted to silvopasture (207 Tg yr^-1), and windbreaks on 5% (7.45 Mha) of the cropland (25 Tg yr^-1). Despite many limitations including uncertainty of land areas under agroforestry, lack of standardized estimation protocols, and lack of accountability on various C stocks (source-sink services, detritus C, insect/pest damages, etc.), we believe these new accrual rates and the land areas under each practice are much more realistic as new information became available over the last decade.The total C sequestration by forests (776) and agroforests (219) is 995 Tg yr^-1 and represents approximately 15% of the US CO₂ emissions. This review highlights the importance of sustainable management of forests and integration of agroforestry on agricultural lands to mitigate climate challenges further while meeting society’s need for food and a healthy environment.

National-scale carbon footprints of livestock production are commonly computed from a set of production system characteristics that serve as inputs for greenhouse gas (GHG) emission models. We evaluated the feasibility of using such equations at a finer scale to derive a simple farm-scale indicator of emission intensity (milk yield per head). Using probabilistic simulations, we quantified the impact of input variable uncertainty on emission estimates for smallholder dairy farms in Kenya. We simulated emissions for farm-scale scenarios generated from a survey of 414 households and published or expert-estimated uncertainty bounds. We simulated the impacts of five interventions: changing breeds, retiring unproductive males, keeping fewer replacement males, feeding forage supplements, and balancing animal diets. Impacts were assessed against a true counterfactual and against a more realistic scenario affected by random effects. We estimated errors incurred in classifying farms into adopters and non-adopters of the innovations based on changes in milk yield per animal. Given the current uncertainty, such classification would either miss a large percentage of adopters or misclassify many non-adopters as adopters. As a critical uncertainty, we identified the milk yield of dairy cows. Added precision on this metric reduced but did not eliminate classification errors. We remain cautiously optimistic about using milk yield per head to proxy emission intensity, but its effective use will require further reduction of critical uncertainties. Replacing generic recommendations of parameter uncertainties with context-specific error estimates might lead to a more efficient quantification of the carbon footprint of milk production on smallholder farms.

Biochar, a carbonaceous solid material obtained from the pyrolysis of biomass, has received considerable research attention because of its unique properties and potential to improve crop yields and soil carbon (C) sequestration while reducing environmental degradation and carbon footprints (CF). This paper summarizes the available results on several aspects of biochar research from numerous studies despite their short-term nature. The studies have shown that (1) biochar from the same source added at a given rate to different soils could have different effects, particularly on phosphorus (P) release/retention, based on the respective soil properties; (2) the elemental composition of a feedstock (the biomass source used for biochar production) is not an indication of plant-nutrient availability; (3) pyrolysis temperature has a significant influence on the properties of the biochar, but the optimal temperature depends on the desired qualities of the product such as P release, cation exchange capacity, and surface area; and (4) the risk of nutrient loss during biochar application depends on the nutrient release potential of the biochar as well as the nutrient retention properties of the soil. Some evidence from nature suggests that biochar can hold C in soils for thousands of years, but the mechanisms involved are not fully understood. In general, the available results on the effect of biochar application on field crops have been variable and site-specific so that general conclusions cannot be drawn on their applicability to a wide spectrum of situations and systems. A number of researchable priorities were identified, including CF under biochar. Similarly, although the land application of biochar to decrease CF sounds like a promising proposition, rigorous long-term studies under farm settings are required before recommending it for large-scale adoption.