Continental-scale controls on soil organic carbon across sub-Saharan Africa

Soil organic carbon (SOC) stabilization and destabilization has been studied intensively. Yet, the factors which control SOC content across scales remain unclear. Earlier studies demonstrated that soil texture and geochemistry strongly affect SOC content. However, those findings primarily rely on data from temperate regions where soil mineralogy, weathering status and climatic conditions generally differ from tropical and subtropical regions. We investigated soil properties and climate variables influencing SOC concentrations across sub-Saharan Africa. A total of 1601 samples were analyzed, collected from two depths (0–20 and 20–50 cm) from 17 countries as part of the Africa Soil Information Service project (AfSIS). The data set spans arid to humid climates and includes soils with a wide range of pH values, weathering status, soil texture, exchangeable cations, extractable metals and land cover types. The most important SOC predictors were identified by linear mixed-effects models, regression trees and random forest models. Our results indicate that geochemical properties, mainly oxalate-extractable metals (Al and Fe) and exchangeable Ca, are equally important compared to climatic variables (mean annual temperature and aridity index). Together, they explain approximately two-thirds of SOC variation across sub-Saharan Africa. Oxalate-extractable metals were most important in wet regions with acidic and highly weathered soils, whereas exchangeable Ca was more important in alkaline and less weathered soils in drier regions. In contrast, land cover and soil texture were not significant SOC predictors on this large scale. Our findings indicate that key factors controlling SOC across sub-Saharan Africa are broadly similar to those in temperate regions, despite differences in soil development history.

Determining sedimentary material sources in a Brazilian urban lake using Zn stable isotope compositions of bottom sediments: a preliminary study

Zinc isotopes ratios provide qualitative and quantitative information about contributions from different Zn sources in the environment. Here we present the results of a preliminary study to identify the main sources controlling Zn isotope signatures of sediments from Lake Paranoá (Brazil), an urban water reservoir. The Zn isotope ratios were measured by MC-ICP-MS and expressed as δ66/64Zn notation relative to the JMC 3-0749-L solution. The studied sediments showed a narrow isotopic range (+0.00 to +0.13 ‰; n = 10) similar to that reported for intensely weathered soils worldwide (from -0.4 to +0.2‰). The geoaccumulation index (Igeo) also suggests low or negligible influence of Zn anthropogenic source. This research provides a baseline assessment of the Zn isotopes signatures in lake sedimentary environments under the influence of these weathered soils.

Stem and soil nitrous oxide fluxes from rainforest and cacao agroforest on highly weathered soils in the Congo Basin

Although tree stems act as conduits for greenhouse gases (GHGs) produced in the soil, the magnitudes of tree contributions to total (soil + stem) nitrous oxide (N2O) emissions from tropical rainforests on heavily weathered soils remain unknown. Moreover, soil GHG fluxes are largely understudied in African rainforests, and the effects of land-use change on these gases are identified as an important research gap in the global GHG budget. In this study, we quantified the changes in stem and soil N2O fluxes with forest conversion to cacao agroforestry. Stem and soil N2O fluxes were measured monthly for a year (2017–2018) in four replicate plots per land use at three sites across central and southern Cameroon. Tree stems consistently emitted N2O throughout the measurement period and were positively correlated with soil N2O fluxes. 15N-isotope tracing from soil mineral N to stem-emitted 15N2O and correlations between temporal patterns of stem N2O emissions, soil–air N2O concentration, soil N2O emissions and vapour pressure deficit suggest that N2O emitted by the stems originated predominantly from N2O produced in the soil. Forest conversion to extensively managed, mature (>20 years old) cacao agroforestry had no effect on stem and soil N2O fluxes. The annual total N2O emissions were 1.55 ± 0.20 kg N ha−1 yr−1 from the forest and 1.15 ± 0.10 kg N ha−1 yr−1 from cacao agroforestry, with tree N2O emissions contributing 11 % to 38 % for forests and 8 % to 15 % for cacao agroforestry. These substantial contributions of tree stems to total N2O emissions highlight the importance of including tree-mediated fluxes in ecosystem GHG budgets. Taking into account that our study sites' biophysical characteristics represented two-thirds of the humid rainforests in the Congo Basin, we estimated a total N2O source strength for this region of 0.18 ± 0.05 Tg N2O-N yr−1.

The Shear Strength of Granite Weathered Soil Under Different Hydraulic Paths

At present, there is no clear understanding of the influence of differences in soil mineral composition, particle size grading, and hydraulic paths on the shear strength of unsaturated soil, and the related strength models are not applicable. The shear strength characteristics of different saturation specimens under different hydraulic paths were studied on two granite weathered soils. The experimental results show that the shear strength index of the prepared specimen is “arched” with the increase of saturation, and the dehydration specimen decreases linearly with the saturation. As considering the cementation of free oxides in soils and the interaction among soil particles at different saturations, it is assumed that there are three different contact modes among soil particles: direct contact, meniscus contact, and cement contact. The difference in contact modes will reflect the different laws of shear strength. A shear strength model capable of distinguishing between the capillary effect and the adsorptive effect was established. The model predicted and verified the shear strength data of granite weathered soil under different hydraulic paths well, and then theoretically explained the evolution law of the shear strength of granite weathering soil under the change of saturation.

Plants sustain the terrestrial silicon cycle during ecosystem retrogression

The biogeochemical silicon cycle influences global primary productivity and carbon cycling, yet changes in silicon sources and cycling during long-term development of terrestrial ecosystems remain poorly understood. Here, we show that terrestrial silicon cycling shifts from pedological to biological control during long-term ecosystem development along 2-million-year soil chronosequences in Western Australia. Silicon availability is determined by pedogenic silicon in young soils and recycling of plant-derived silicon in old soils as pedogenic pools become depleted. Unlike concentrations of major nutrients, which decline markedly in strongly weathered soils, foliar silicon concentrations increase continuously as soils age. Our findings show that the retention of silicon by plants during ecosystem retrogression sustains its terrestrial cycling, suggesting important plant benefits associated with this element in nutrient-poor environments.

Enhancing Cation Exchange Capacity of Weathered Soils Using Biochar: Feedstock, Pyrolysis Conditions and Addition Rate

The addition of alkaline and high-cation exchange capacity (CEC) biochars is a suitable strategy to increase the CEC of weathered soils. The aim of this study was to evaluate the effect of biochar from different feedstocks and pyrolysis temperatures on the CEC of two contrasting Oxisols. Biochars produced from chicken manure (CM), eucalyptus sawdust (ES), coffee husk (CH) and sugarcane bagasse (SB),plus a control (without biochar), at 350, 450, and 750 °C were mixed with the soils at 2; 5; 10 and 20% (w/w) and incubated for 9 months. Feedstock, pyrolysis temperature and addition rate of biochar were key factors controlling the alteration of soil CEC. The CH biochar pyrolyzed at 350 °C was the most effective matrix at increasing soil CEC. In a rate-dependent way, ES and SB biochars increased C contents of both soils without improving soil CEC. The efficiency of high-ash biochars in enhancing soil CEC in both Oxisols was limited by the alkalization caused by high rates of CH and CM biochars. The increase in CEC is soil-dependent and modulated by high-ash biochar CEC and application rate, as well as by the original soil CEC.