Grazing management for soil carbon in Australia: A review

Arbuscular mycorrhizal fungi inoculation and biochar application enhance soil carbon and productivity in wheat and barley

Influencing the global carbon cycle via modification to the terrestrial soil carbon pool has been suggested as one solution to help mitigate climate change. Cropping systems cover a vast expanse of earth's surface and represent a major carbon exchange point. Investigating management practices and biotechnologies capable of influencing soil carbon in cropping systems is thus a valuable endeavour, as even modest interventions have the capacity to increase carbon stocks and improve soil fertility and plant production. Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs forming mutually beneficial relationships with a wide array of symbiotic partners. Increasingly, AMF are being investigated for their potential to enhance agricultural productivity through inoculation of soil and seeds with living propagules or spores. Beyond their positive influence on plant growth and resilience, AMF may have some capacity to influence the global carbon cycle through several conceptually recognised yet poorly understood mechanisms, warranting further exploration. Here, we evaluate the potential of AMF as an inoculant to promote soil carbon sequestration in wheat and barley under greenhouse conditions. We assess the growth response of these crops and explore interactive effects of AMF with several organic amendments. Both wheat and barley exhibited a strong mycorrhizal growth response, with inoculation significantly increasing biomass (root and shoot dry weight) and productivity (head dry weight), especially under low nutrient conditions. Effects of AMF on soil carbon cycling were assessed through soil respiration, total carbon (TC) content, and easily extractable organic carbon. Inoculation significantly increased soil TC concentration in both the unamended control and the biochar-amended wheat treatments. We reveal evidence for a biochar + AMF carbon stabilisation pathway, whereby biochar may act to stabilise new fungal derived carbon inputs while reducing soil respiration. We discuss these results in the context of carbon credit generation and climate change mitigation potential.

Soil quality under rotational and conventional grazing in Mediterranean areas at desertification risk

Rotational grazing (RG) could be a valid alternative to continuous grazing (CG) in Mediterranean extensive pastures to fight land degradation. This study aimed to compare soil quality under RG and CG management, in paired RG-CG Portuguese pasture areas under strong aridity stress, with RG sites converted from CG management in 2018. Soils were sampled in 2022, at 10 cm depth, over 71 ha of RG and 37 ha of CG pastures, subdivided in 16 and 10 sampling plots, respectively. In each plot, five soil samples were taken to provide one composite sample. Physico-chemical and microbial indicators of soil quality were measured immediately after. Principal Component Analysis (PCA) and Redundancy analysis (RDA) showed a clear separation between RG and CG sites with significantly higher level of soil organic carbon (27%-67%), total nitrogen (67%-77%), cation exchange capacity (9%-36%), and, at a minor extent, water holding capacity (6%-17%), in the RG plots respect to CG plots. No significant difference was found for soil bulk density, microbial and fungal biomass and microbial diversity between sites with different management, although the latter was positively correlated with CG sites in the RDA analysis. Soil organic carbon was significantly correlated to the most relevant physico-chemical parameters for nutrient cycling and water balance and to microbial fungal biomass and N-mineralization, confirming the central role of soil organic carbon for soil health. Results support RG management as an effective choice to improve soil quality in areas under desertification risk.

Compositional and functional analysis of the bacterial community of Mediterranean Leptosols under livestock grazing

The composition and functioning of soil bacterial communities, as well as their responses to multiple perturbations, are not well understood in the terrestrial ecosystems. Our study focuses on the bacterial community of erosive and poorly developed soils (Haplic Leptosols) in Mediterranean rangelands of Extremadura (W Spain) with different grazing intensities. Leptosols from similar natural conditions were selected and sampled at two depths to determine the soil properties as well as the structure and activity of bacterial communities. As grazing intensified, the soil C and N content increased, as did the number and diversity of bacteria, mainly of fast-growing lineages. Aridibacter, Acidobacteria Gp6 and Gp10, Gemmatimonas, and Segetibacter increased their abundance along the grazing-intensity gradient. Firmicutes such as Romboutsia and Turicibacter from livestock microbiome also increased. In functional terms, the KEGG pathways enriched in the soils with moderate and high grazing intensity were ABC transporters, DNA repair and recombination proteins, the two-component system, and the degradation of xenobiotics. All of these proved to be related to stronger cell division and response mechanisms to environmental stressors such as drought, warming, toxic substances, and nutrient deprivation. Consequently, the bacterial community was affected by grazing, but appeared to adapt and counteract the effects of a high grazing intensity. Therefore, a clearly detrimental effect of grazing was not detected in the bacterial community of the soils studied.

Ruminating on soil carbon: Applying current understanding to inform grazing management

Among options for atmospheric CO2 removal, sequestering soil organic carbon (SOC) via improved grazing management is a rare opportunity because it is scalable across millions of globally grazed acres, low cost, and has high technical potential. Decades of scientific research on grazing and SOC has failed to form a cohesive understanding of how grazing management affects SOC stocks and their distribution between particulate (POM) and mineral-associated organic matter (MAOM)-characterized by different formation and stabilization pathways-across different climatic contexts. As we increasingly look to grazing management for SOC sequestration on grazinglands to bolster our climate change mitigation efforts, we need a clear and collective understanding of grazing management's impact on pathways of SOC change to inform on-the-ground management decisions. We set out to review the effects of grazing management on SOC through a unified plant ecophysiology and soil biogeochemistry conceptual framework, where elements such as productivity, input quality, soil mineral capacity, and climate variables such as aridity co-govern SOC accumulation and distribution into POM and MAOM. To maximize applicability to grazingland managers, we discuss how common management levers that drive overall grazing pattern, including timing, intensity, duration, and frequency can be used to optimize mechanistic pathways of SOC sequestration. We discuss important research needs and measurement challenges, and highlight how our conceptual framework can inform more robust research with greater applicability for maximizing the use of grazing management to sequester SOC.