Soil carbon

Soil is one of the core habitats of macro-ecological biodiversity, including microorganisms (e.g., bacteria), micro- (e.g., Nematoda), meso- (e.g., Collembola), and macrofauna (e.g., Oligochaeta). This high biodiversity plays critical roles in driving multiple ecosystem functions and services, including climate regulation, nutrient cycling, and food production.
Coastal plains’ soils demonstrate the promising potential of the application of regenerative farming principles to not only restore degraded biodiversity, recycle nutrients, improve farm profitability, and reduce chemical inputs, but also to sequester atmospheric C and simultaneously help reduce the effect of global climate change while creating healthy soils.
The effects of N (Nitrogen), P (Phosphorus), and combined N and P addition on soil microbial carbon use efficiency (CUE) (the ratio between Carbon (C) allocated to growth and C taken up by microorganisms) from a total of six grassland soils from South Africa, USA, and UK. N addition significantly reduced microbial respiration and C uptake in the topsoil. Taken together, N, P, and NP addition did not influence microbial CUE and biomass turnover time.
SOC losses were highly correlated to the type of LUC and social variables, while SOC gains correlated most strongly with years since LUC and the biophysical variables. The analyses confirm that one of the biggest drivers of SOC loss is conversion to agroindustrial scale cropping, whereas with regard to the recuperation of SOC after LUC, the factor “time since conversion” emerged as the most important predictive variable, which must be better integrated in respective policy expectations.
Soil microbes produce chemically diverse, stable soil organic matter (SOM). SOM accumulation is driven by distinct microbial communities more so than clay mineralogy. Microbial-derived SOM accumulation is greatest in soils with higher fungal abundances and more efficient microbial biomass production.
Microbial necromass can make up more than half of soil organic carbon. Hence, we suggest next‐generation field management requires promoting microbial biomass formation and necromass preservation to maintain healthy soils, ecosystems, and climate. Our analyses have important implications for improving current climate and carbon models.
Root inputs are approximately five times more likely than an equivalent mass of aboveground litter to be stabilized as SOM.
There is a clear relationship between soil aggregate size classes, microbial community composition, and biogeochemical effects on cycling of soil organic carbon.
Increasing C below-ground inputs to soil can be achieved through deep-rooting perennials.
Living root inputs are 2–13 times more efficient than litter inputs in forming both slow‐cycling, mineral‐associated SOC as well as fast‐cycling, particulate organic C.
Deep SOC is not intrinsically “stable” and is vulnerable to potentially rapid decomposition when the environmental conditions under which it accumulated change (microbial controls, root biomass and surface area, organic matter, or soil properties (e.g.temperature and moisture).
Subsurface C has a significant role in C cycling and storage dynamics. A significant part (39% – 73%) of subsurface SOC was found to be associated with C pools that turn over on time scales of decades or less. Dissolved Organic Carbon transport appeared to be dominant in distributing the added C to the deeper soil layers, making the SOC content profile deeper than that of the root litter (C) input.
SOC stocks increased when land-use changed from less to more complex systems.
Plant diversity increases rhizosphere carbon inputs into the microbial community resulting in both increased microbial activity and carbon storage.
SOC content and stock are on average 5 and 8% higher in species mixtures than in monocultures. Species‐mixture effects are consistent across forest, grassland, and cropland systems and are independent of background climates.
Increasing C belowground inputs to soil can be achieved through cropping deep rooting crop varieties.
Soil acidity impacts microbial mediation of carbon added and retained in soil. "Microbial carbon use efficiency (CUE) was studied in 970 agricultural soils. CUE was highly correlated with soil pH and exchangeable Al content. A critical transition point in microbial CUE occurred at pH 5.5.
Other soil factors showed little, or no, relationship with CUE. A decline in CUE at low pH is due to greater use of C to overcome acid stress.”
Mammal diversity is positively correlated to carbon concentration in the soil.
Presence of dingoes is linked to healthier soils, because they suppress the numbers of kangaroos that graze on the vegetation.
SOC stock in the 200–500 cm layer was 7.62 kg m−2, accounting for 44% of the total carbon in the 0–500 cm soil profile. A large amount of organic carbon is stored in deep soil, indicating that a better understanding of the reserves and cycles of deep soil carbon is a critical factor in the effective management of terrestrial ecosystems.
Australia’s “Direct Action” climate change policy relies on purchasing greenhouse gas abatement from projects undertaking approved abatement activities. Management of soil organic carbon (SOC) in agricultural soils is an approved activity, based on the expectation that land use change can deliver significant changes in SOC. However, there are concerns that climate, topography and soil texture will limit changes in SOC stocks. This work analyses data from 1482 sites surveyed across the major agricultural regions of Eastern Australia to determine the relative importance of land use vs. other drivers of SOC. Variation in land use explained only 1.4% of the total variation in SOC, with aridity and soil texture the main regulators of SOC stock under different land uses. Results suggest the greatest potential for increasing SOC stocks in Eastern Australian agricultural regions lies in converting from cropping to pasture on heavy textured soils in the humid regions.
Our results suggest that improved soil nutrient and grazing management of permanent pasture can lead to an increase of 500–700 kg C/ha.year where the initial SOC concentrations are well below steady-state concentrations that could be expected after long periods of improved management. No difference was found between perennial pasture and annual pasture to the depth measured (0–0.3 m). Our results suggest that pasture holds the key to maintaining, and even increasing, SOC under crop/pasture in this environment.