The capacity for terrestrial ecosystems to sequester additional carbon (C) with rising CO₂ concentrations depends on soil nutrient availability^1,2. Previous evidence suggested that mature forests growing on phosphorus (P)-deprived soils had limited capacity to sequester extra biomass under elevated CO₂ (refs. ^3–6), but uncertainty about ecosystem P cycling and its CO₂ response represents a crucial bottleneck for mechanistic prediction of the land C sink under climate change⁷. Here, by compiling the first comprehensive P budget for a P-limited mature forest exposed to elevated CO₂, we show a high likelihood that P captured by soil microorganisms constrains ecosystem P recycling and availability for plant uptake. Trees used P efficiently, but microbial pre-emption of mineralized soil P seemed to limit the capacity of trees for increased P uptake and assimilation under elevated CO₂ and, therefore, their capacity to sequester extra C. Plant strategies to stimulate microbial P cycling and plant P uptake, such as increasing rhizosphere C release to soil, will probably be necessary for P-limited forests to increase C capture into new biomass. Our results identify the key mechanisms by which P availability limits CO₂ fertilization of tree growth and will guide the development of Earth system models to predict future long-term C storage.

Review Article Improving rice zinc biofortification success rates through genetic and crop management approaches in a changing environment Niluka Nakandalage1, Marc Nicolas2, Robert M Norton3, Naoki Hirotsu4, Paul J Milham5 and Saman Seneweera6* 1Faculty of Veterinary & Agricultural Sciences, University of Melbourne, Creswick, VIC 3363, Australia; Email: nnakandalage@student.unimelb.edu.au 2Faculty of Veterinary & Agricultural Sciences, University of Melbourne, Parkville, VIC3010, Australia; Email: marcen@unimelb.edu.au 3International Plant Nutrition Institute, 54 Florence St, Horsham, Victoria, Australia 4Faculty of Life Sciences, Toyo University, 1-1-1 Izumino, Itakura, Gunma 374-0193, Japan; Email: hirotsu@toyo.jp 5Hawkesbury Institute for the Environment, Western Sydney University, LB 1797 Penrith, NSW 2751, Australia; Email: p.milham@westernsydney.edu.au 6Centre for Crop Health, University of Southern Queensland, Toowoomba Qld 4350, Australia; Email: saman.seneweera@usq.edu.au * Correspondence: Saman Seneweera, Centre for Crop Health University of Southern Queensland Toowoomba Qld 4350 Australia Email: saman.seneweera@usq.edu.au Number of words: 5132 Number of figures: 02 Headings and sub headings of paper Abstract 1. Introduction 2. Zinc and human health on a global perspective 3. Rice as source of Zn 4. Key determinents of grain zinc concentration 4.1. Zn uptake and translocation in rice 4.2. Zn remobilization and loading into the grain 4.3. Phytosiderophores in zinc homeostasis 4.4.Plant available Zn in soil 4.5. Rising [CO2] and grain Zn 4.6. Fertilizers as a grain Zn determinant 4.7. Anti-nutrients and Zn bioavailability 5. Ways to overcome global dietary Zn deficiency 6. Conventional breeding as an effective tool for Zn biofortification 7. Conclusions 8. Future research focus Conflicts of Interest Authors and Contributors Acknowledgments References Though rice is the predominant source of energy and micronutrients for more than half of the world population, it does not provide enough zinc (Zn) to match human nutritional requirements. Moreover, climate change, particularly rising atmospheric carbon dioxide concentration, reduces the grain Zn concentration. Therefore, rice biofortification has been recognized as a key target to increase the grain Zn concentration to address global Zn malnutrition. Major bottlenecks for Zn biofortification in rice are identified as low Zn uptake, transport and loading into the grain; however, environmental and genetic contributions to grain Zn accumulation in rice have not been fully explored.

Over the years, cultivation using sustainable tillage practices has gained significant importance, but the impact of tillage on soil water infiltration is still a concern for landowners due to the possible effects on crop yield. This study investigates the impact of different tillage managements on the infiltration rate of sandy clay loam soil under a semiarid environment. Field experiments were conducted in Chott Mariem Sousse, Tunisia. The tillage practices consisted of three treatments, including a tine cultivator (TC, 16 cm), moldboard plows (MP, 36 cm) and no-tillage (NT). Three infiltration models, Kostiakov, Philip and Horton, were applied to adjust the observed data and evaluate the infiltration characteristics of the studied soils. Comparison criteria, including the coefficient of determination (R²), along with the root mean square error (RMSE) and mean absolute error (MAE), were used to investigate the best-fit model. The results showed that moldboard plowing enhanced soil infiltration capacity relative to tine cultivation and no-tillage treatments. The mean saturated hydraulic conductivity was highest under MP, while it was lowest in NT, with 33.4% and 34.1% reduction compared to TC and MP, respectively. Based on the obtained results, Philip’s model showed better results with observed infiltration due to a higher R² (0.981, 0.973 and 0.967), lower RMSE (3.36, 9.04 and 9.21) and lower MAE (1.46, 3.53 and 3.72) recorded, respectively, for NT, MP and TC. Horton’s model had a low regression coefficient between observed and predicted values. It was suggested that the Philip two-term model can adequately describe the infiltration process in the study area.

The importance of phosphorus (P) in regulating ecosystem responses to climate change has fostered P-cycle implementation in land surface models, but their CO ₂ effects predictions have not been evaluated against measurements. Here, we perform a data-driven model evaluation where simulations of eight widely used P-enabled models were confronted with observations from a long-term free-air CO ₂ enrichment experiment in a mature, P-limited Eucalyptus forest. We show that most models predicted the correct sign and magnitude of the CO ₂ effect on ecosystem carbon (C) sequestration, but they generally overestimated the effects on plant C uptake and growth. We identify leaf-to-canopy scaling of photosynthesis, plant tissue stoichiometry, plant belowground C allocation, and the subsequent consequences for plant-microbial interaction as key areas in which models of ecosystem C-P interaction can be improved. Together, this data-model intercomparison reveals data-driven insights into the performance and functionality of P-enabled models and adds to the existing evidence that the global CO ₂ -driven carbon sink is overestimated by models.

Excessively high concentrations of selenium (Se) in soil are toxic to crop plants, and inoculation with arbuscular mycorrhizal fungi (AMF) can reverse Se stress in maize (Zea mays L.). To investigate the underlying mechanisms, maize seedlings were treated with sodium selenate (5 mg Se[VI] kg^-1) and/or AMF (Funneliformis mosseae and Claroideoglomus etunicatum). Dual RNA sequencing in mycorrhiza and 16 S ribosomal DNA sequencing in soil were performed. The results showed that Se(VI) application alone decreased plant dry weight, but increased plant Se concentration, total Se content (mainly selenocysteine), and root superoxide content. Inoculation with either F. mosseae or C. etunicatum increased plant dry weight, decreased Se accumulation and selenocysteine proportion, enhanced root peroxidase activity, and alleviated oxidative stress in Se(VI)-treated plants. Inoculation also downregulated the expression of genes encoding Se transporters, assimilation enzymes, and cysteine-rich receptor-like kinases in Se(VI)-stressed plants, similar to plant-pathogen interaction and glutathione metabolism related genes. Conversely, genes encoding selenium-binding proteins and those related to phenylpropanoid biosynthesis were upregulated in inoculated plants under Se(VI) stress. Compared with Se(VI)-free plants, Se tolerance index, symbiotic feedback percentage on plant dry weight, and root colonization rate were all increased in inoculated plants under Se(VI) stress, corresponding to upregulated expression of 'key genes' in symbiosis. AMF inoculation increased bacterial diversity, decreased the relative abundances of selenobacteria related to plant Se absorption (e.g., Proteobacteria and Firmicutes), and improved bacterial network complexity in Se(VI)-stressed soils. We suggest that stress-mediated enhancement of mycorrhizal symbiosis contributed to plant Se(VI) tolerance, whereas AMF-mediated reshaping of soil bacterial community structure prevented excessive Se accumulation in maize.