Ecology. Bad news for soil carbon sequestration?

Inclusive Indian Central Himalayan soil carbon estimates underscores significant inorganic carbon contribution and temporal dynamics: Implications for carbon sequestration

Soil carbon estimates in the Indian Himalayan region-a global climate change hotspot-primarily rely on the lossy wet oxidation method and predominantly focus on soil organic carbon (SOC), neglecting the soil inorganic carbon (SIC) component. Sensitive and holistic soil carbon estimates are crucial for effective policy planning. By incorporating eight major Central Himalayan forest types along a 3000 m elevational gradient, we report that the acidic Himalayan soil (surface soil pH: 4.74-6.84) of the selected forest types hold up to 31% of the total soil carbon stock as SIC stock. Using soil carbon and soil organic matter assays based on elemental analyzer and the loss-on-ignition method, we established that these Himalayan soils store less than 50% of SOC in SOM, challenging the use of universal factors in the region. The amount of SOC in SOM also showed temporal variability. The machine learning Random Forest algorithm highlighted the influence of SOM and climate variables in regulating the distribution of SOC, microbial biomass carbon, and key carbon cycling soil enzymes. The vertical distribution of SOC was more uniform than that of SIC. We found higher activity of soil carbon-cycling enzymes (dehydrogenase, beta-glucosidase, and phenol oxidase) in the forest types. Sensitive and higher soil carbon estimates substantiate a lower microbial quotient (0.17-1.23 %) than the regional trend. Notably, we explained how seasonal and temporal changes in soil carbon estimations hinder a constant positive soil carbon flux. Meanwhile, the mean surface SOC flux (4.63 Mg C ha-1 yr-1) and SIC flux (1.68 Mg C ha-1 yr-1) indicate that the Himalayan soils have significant potential for carbon sequestration. In conclusion, our research indicates substantial soil organic and inorganic carbon storage in major Central Himalayan forest types, with negative anthropogenic activities posing a clear and present threat to the soil carbon stocks.

Dynamic changes in soil organic carbon induced by long-term compost application under a wheat-maize double cropping system in North China

Soil organic carbon (SOC) plays a vital role in improving soil quality and alleviating global warming. Understanding the dynamic changes in SOC is crucial for its accumulation induced by compost application in agroecosystem. In this study, soil samples were collected from three treatments: high-rate bio-compost (BioMh), low-rate bio-compost (BioMl), and control (CK, no fertilization) during 2002-2020 in a wheat-maize double cropping system in North China. The soils were separated into three functional fractions, i.e., coarse particle organic matter (cPOM, >250 μm), microaggregates (μAgg, 53-250 μm) and mineral-associated organic matter (MAOM, < 53 μm), and the associated SOC contents were determined. During 1993-2002, SOC contents in bulk soil significantly increased with the duration in the BioMh and BioMl plots. However, there was no significant correlation between SOC content and duration during 2002-2020. These results suggested that compost application positively improved SOC sequestration, while the duration of SOC sequestration (i.e., the longevity of increased SOC with time) under compost inputs maintained only 9 years. Moreover, there was a significant increase in mean annual SOC contents in bulk soil with compost application rate during 2002-2020, indicating that carbon saturation did not occur. Additionally, the SOC contents in the cPOM fraction increased with time (p < 0.01), but the corresponding μAgg and MAOM associated SOC was insignificant (p > 0.05). The MAOM fraction exhibited no additional carbon accumulation with expanding compost application, confirming a hierarchical carbon saturation in these fractions. We concluded that soils under wheat-maize double cropping system in North China have greater potential to sequester C through additional compost inputs, despite showing hierarchical saturation behavior in the non-protected coarse particulate fraction.

The potential of arbuscular mycorrhizal fungi in C cycling: a review

Arbuscular mycorrhizal fungi (AMF) contribute predominantly to soil organic matter by creating a sink demand for plant C and distributing to below-ground hyphal biomass. The extra-radical hyphae along with glomalin-related soil protein significantly influence the soil carbon dynamics through their larger extent and turnover period need to discuss. The role of AMF is largely overlooked in terrestrial C cycling and climate change models despite their greater involvement in net primary productivity augmentation and further accumulation of this additional photosynthetic fixed C in the soil. However, this buffering mechanism against elevated CO₂ condition to sequester extra C by AMF can be described only after considering their potential interaction with other microbes and associated mineral nutrients such as nitrogen cycling. In this article, we try to review the potential of AMF in C sequestration paving the way towards a better understanding of possible AMF mechanism by which C balance between biosphere and atmosphere can be moved forward in more positive direction.

Emergent Properties of Microbial Activity in Heterogeneous Soil Microenvironments: Different Research Approaches Are Slowly Converging, Yet Major Challenges Remain

Over the last 60 years, soil microbiologists have accumulated a wealth of experimental data showing that the bulk, macroscopic parameters (e.g., granulometry, pH, soil organic matter, and biomass contents) commonly used to characterize soils provide insufficient information to describe quantitatively the activity of soil microorganisms and some of its outcomes, like the emission of greenhouse gasses. Clearly, new, more appropriate macroscopic parameters are needed, which reflect better the spatial heterogeneity of soils at the microscale (i.e., the pore scale) that is commensurate with the habitat of many microorganisms. For a long time, spectroscopic and microscopic tools were lacking to quantify processes at that scale, but major technological advances over the last 15 years have made suitable equipment available to researchers. In this context, the objective of the present article is to review progress achieved to date in the significant research program that has ensued. This program can be rationalized as a sequence of steps, namely the quantification and modeling of the physical-, (bio)chemical-, and microbiological properties of soils, the integration of these different perspectives into a unified theory, its upscaling to the macroscopic scale, and, eventually, the development of new approaches to measure macroscopic soil characteristics. At this stage, significant progress has been achieved on the physical front, and to a lesser extent on the (bio)chemical one as well, both in terms of experiments and modeling. With regard to the microbial aspects, although a lot of work has been devoted to the modeling of bacterial and fungal activity in soils at the pore scale, the appropriateness of model assumptions cannot be readily assessed because of the scarcity of relevant experimental data. For significant progress to be made, it is crucial to make sure that research on the microbial components of soil systems does not keep lagging behind the work on the physical and (bio)chemical characteristics. Concerning the subsequent steps in the program, very little integration of the various disciplinary perspectives has occurred so far, and, as a result, researchers have not yet been able to tackle the scaling up to the macroscopic level. Many challenges, some of them daunting, remain on the path ahead. Fortunately, a number of these challenges may be resolved by brand new measuring equipment that will become commercially available in the very near future.

Ecosystem Carbon Storage in Alpine Grassland on the Qinghai Plateau

The alpine grassland ecosystem can sequester a large quantity of carbon, yet its significance remains controversial owing to large uncertainties in the relative contributions of climate factors and grazing intensity. In this study we surveyed 115 sites to measure ecosystem carbon storage (both biomass and soil) in alpine grassland over the Qinghai Plateau during the peak growing season in 2011 and 2012. Our results revealed three key findings. (1) Total biomass carbon density ranged from 0.04 for alpine steppe to 2.80 kg C m-2 for alpine meadow. Median soil organic carbon (SOC) density was estimated to be 16.43 kg C m-2 in alpine grassland. Total ecosystem carbon density varied across sites and grassland types, from 1.95 to 28.56 kg C m-2. (2) Based on the median estimate, the total carbon storage of alpine grassland on the Qinghai Plateau was 5.14 Pg, of which 94% (4.85 Pg) was soil organic carbon. (3) Overall, we found that ecosystem carbon density was affected by both climate and grazing, but to different extents. Temperature and precipitation interaction significantly affected AGB carbon density in winter pasture, BGB carbon density in alpine meadow, and SOC density in alpine steppe. On the other hand, grazing intensity affected AGB carbon density in summer pasture, SOC density in alpine meadow and ecosystem carbon density in alpine grassland. Our results indicate that grazing intensity was the primary contributing factor controlling carbon storage at the sites tested and should be the primary consideration when accurately estimating the carbon storage in alpine grassland.