Wilder rangelands as a natural climate opportunity: Linking climate action to biodiversity conservation and social transformation.

Rangelands face threats from climate and land-use change, including inappropriate climate change mitigation initiatives such as tree planting in grassy ecosystems. The marginalization and impoverishment of rangeland communities and their indigenous knowledge systems, and the loss of biodiversity and ecosystem services, are additional major challenges. To address these issues, we propose the wilder rangelands integrated framework, co-developed by South African and European scientists from diverse disciplines, as an opportunity to address the climate, livelihood, and biodiversity challenges in the world's rangelands. More specifically, we present a Theory of Change to guide the design, monitoring, and evaluation of wilder rangelands. Through this, we aim to promote rangeland restoration, where local communities collaborate with regional and international actors to co-create new rangeland use models that simultaneously mitigate the impacts of climate change, restore biodiversity, and improve both ecosystem functioning and livelihoods.

Mycorrhizal mycelium as a global carbon pool.

For more than 400 million years, mycorrhizal fungi and plants have formed partnerships that are crucial to the emergence and functioning of global ecosystems. The importance of these symbiotic fungi for plant nutrition is well established. However, the role of mycorrhizal fungi in transporting carbon into soil systems on a global scale remains under-explored. This is surprising given that ∼75% of terrestrial carbon is stored belowground and mycorrhizal fungi are stationed at a key entry point of carbon into soil food webs. Here, we analyze nearly 200 datasets to provide the first global quantitative estimates of carbon allocation from plants to the mycelium of mycorrhizal fungi. We estimate that global plant communities allocate 3.93 Gt CO2e per year to arbuscular mycorrhizal fungi, 9.07 Gt CO2e per year to ectomycorrhizal fungi, and 0.12 Gt CO2e per year to ericoid mycorrhizal fungi. Based on this estimate, 13.12 Gt of CO2e fixed by terrestrial plants is, at least temporarily, allocated to the underground mycelium of mycorrhizal fungi per year, equating to ∼36% of current annual CO2 emissions from fossil fuels. We explore the mechanisms by which mycorrhizal fungi affect soil carbon pools and identify approaches to increase our understanding of global carbon fluxes via plant-fungal pathways. Our estimates, although based on the best available evidence, are imperfect and should be interpreted with caution. Nonetheless, our estimations are conservative, and we argue that this work confirms the significant contribution made by mycorrhizal associations to global carbon dynamics. Our findings should motivate their inclusion both within global climate and carbon cycling models, and within conservation policy and practice.

Mapping soil organic carbon stocks and trends with satellite-driven high resolution maps over South Africa.

Estimation and monitoring of soil organic carbon (SOC) stocks is important for maintaining soil productivity and meeting climate change mitigation targets. Current global SOC maps do not provide enough detail for landscape-scale decision making, and do not allow for tracking carbon sequestration or loss over time. Using an optical satellite-driven machine learning workflow, we mapped SOC stocks (topsoil; 0 to 30 cm) under natural vegetation (86% of land area) over South Africa at 30 m spatial resolution between 1984 and 2019. We estimate a total topsoil SOC stock of 5.6 Pg C with a median SOC density of 6 kg C m-2 (IQR: interquartile range 2.9 kg C m-2). Over 35 years, predicted SOC underwent a net increase of 0.3% (relative to long-term mean) with the greatest net increases (1.7%) and decreases (-0.6%) occurring in the Grassland and Nama Karoo biomes, respectively. At the landscape scale, SOC changes of up to 25% were evident in some locations, as evidenced from fence-line contrasts, and were likely due to local management effects (e.g. woody encroachment associated with increased SOC and overgrazing associated with decreased SOC). Our SOC mapping approach exhibited lower uncertainty (R2 = 0.64; RMSE = 2.5 kg C m-2) and less bias compared to previous low-resolution (250-1000 m) national SOC mapping efforts (average R2 = 0.24; RMSE = 3.7 kg C m-2). Our trend map remains an estimate, pending repeated measures of soil samples in the same location (time-series); a global priority for tracking SOC changes. While high resolution SOC maps can inform land management decisions aimed at climate mitigation (natural climate solutions), potential increases in SOC are likely limited by local climate and soils. It is also important that climate mitigation efforts such as planting trees balance trade-offs between carbon, biodiversity and overall ecosystem function.

The importance of nutritional regulation of plant water flux.

Transpiration is generally considered a wasteful but unavoidable consequence of photosynthesis, occurring because water is lost when stomata open for CO(2) uptake. Additionally, transpiration has been ascribed the functions of cooling leaves, driving root to shoot xylem transport and mass flow of nutrients through the soil to the rhizosphere. As a consequence of the link between nutrient mass flow and transpiration, nutrient availability, particularly that of NO(3)(-), partially regulates plant water flux. Nutrient regulation of transpiration may function through the concerted regulation of: (1) root hydraulic conductance through control of aquaporins by NO(3)(-), (2) shoot stomatal conductance (g(s)) through NO production, and (3) pH and phytohormone regulation of g(s). These mechanisms result in biphasic responses of water flux to NO(3)(-) availability. The consequent trade-off between water and nutrient flux has important implications for understanding plant distributions, for production of water use-efficient crops and for understanding the consequences of global-change-linked CO(2) suppression of transpiration for plant nutrient acquisition.

Cluster roots of Leucadendron laureolum (Proteaceae) and Lupinus albus (Fabaceae) take up glycine intact: an adaptive strategy to low mineral nitrogen in soils?

BACKGROUND AND AIMS: South African soils are not only low in phosphorus (P) but most nitrogen (N) is in organic form, and soil amino acid concentrations can reach 2.6 g kg(-1) soil. The Proteaceae (a main component of the South African Fynbos vegetation) and some Fabaceae produce cluster roots in response to low soil phosphorus. The ability of these roots to acquire the amino acid glycine (Gly) was assessed. METHODS: Uptake of organic N as 13C-15N-Gly was determined in cluster roots and non-cluster roots of Leucadendron laureolum (Proteaceae) and Lupinus albus (Fabaceae) in hydroponic culture, taking account of respiratory loss of 13CO2. KEY RESULTS: Both plant species acquired doubly labelled (intact) Gly, and respiratory losses of 13CO2 were small. Lupin (but not leucadendron) acquired more intact Gly when cluster roots were supplied with 13C-15N-Gly than when non-cluster roots were supplied. After treatment with labelled Gly (13C : 15N ratio = 1), lupin cluster roots had a 13C : 15N ratio of about 0.85 compared with 0.59 in labelled non-cluster roots. Rates of uptake of label from Gly did not differ between cluster and non-cluster roots of either species. The ratio of C : N and 13C : 15N in the plant increased in the order: labelled roots < rest of the root < shoot in both species, owing to an increasing proportion of 13C translocation. CONCLUSIONS: Cluster roots of lupin specifically acquired more intact Gly than non-cluster roots, whereas Gly uptake by the cluster and non-cluster roots of leucadendron was comparable. The uptake capacities of cluster roots are discussed in relation to spatial and morphological characteristics in the natural environment.