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The addition of fresh substrates can alter the decomposition of the native soil organic matter, referred to as the priming effect (PE). It is a crucial process within the cycling of soil organic carbon (C) and believed to be regulated by nitrogen (N) input. However, the direction and magnitude of this N effect on the PE are complex due to the involvement of various factors following N addition into the soil. This review synthesizes the key factors driving the responses of the PE to N addition from the perspective of C-substrate quantity and quality, N addition rates and forms, soil properties including organic C stability, N availability, electrical conductivity, pH and pH buffer capacity. The temporal change in the N effect on the PE is also discussed. In studies observing a suppressive effect of N addition on the PE, the role of N addition in directly suppressing microbial community metabolism (e.g., osmotic stress and low pH) has been largely ignored. We propose the application of multi-omics techniques to examine the relationship between microbial functional traits and soil C cycling through the PE and its response to N addition, as well as the application of spatial omics and imaging techniques in exploring the connection between in-situ soil C distribution and the PE and its response to N addition. Such studies can potentially elucidate how elevated atmospheric CO2 may affect the PE and thus soil C stock via increasing plant C input and regulating nutrients sources for plant uptake under N inputs. [ABSTRACT FROM AUTHOR]

Soil provides multiple, diverse functions, with these underpinning both planetary and human health. For planetary health, soil contributes to multiple critical processes, including through biomass production, by regulating the carbon pool, providing a habitat for 25% of global biodiversity, cycling the nutrients upon which terrestrial systems depend, and cycling water. Soil also underpins human health; humans use soil to provide 98.8% of our food and sustain our nutrition, regulate pathogens, and supply medicines. However, humans have tended to focus on soil almost solely for producing biomass (food, fiber, and energy) through intensive agriculture, and this narrow focus now causes rapid soil degradation, including through loss of soil organic matter, erosion, and salinization. This degradation directly harms planetary health and reduces the ability of soil to support health of future human generations. We argue that a healthy soil is a soil that is multifunctional and is capable of underpinning human and planetary health. Using this definition, a broad conceptual framework is provided for quantifying soil health, with such an approach enabling a shift in the way that we think about, plan, and manage systems to ensure ongoing planetary and human health. [ABSTRACT FROM AUTHOR]

Building and protecting soil organic carbon (SOC) are critical to agricultural productivity, soil health, and climate change mitigation. We aim to understand how mechanisms at the organo-mineral interfaces influence SOC persistence in three contrasting soils (Luvisol, Vertisol, and Calcisol) under long-term free air CO 2 enrichment conditions. A continuous wheat-field pea-canola rotation was maintained. For the first time, we provided evidence to a novel notion that persistent SOC is molecularly simple even under elevated CO 2 conditions. We found that the elevated CO 2 condition did not change the total SOC content or C forms compared with the soils under ambient CO 2 as identified by synchrotron-based soft X-ray analyses. Furthermore, synchrotron-based infrared microspectroscopy confirmed a two-dimensional microscale distribution of similar and less diverse C forms in intact microaggregates under long-term elevated CO 2 conditions. Strong correlations between the distribution of C forms and O-H groups of clays can explain the steady state of the total SOC content. However, the correlations between C forms and clay minerals were weakened in the coarse-textured Calcisol under long-term elevated CO 2 . Our findings suggested that we should emphasize identifying management practices that increase the physical protection of SOC instead of increasing complexity of C. Such information is valuable in developing more accurate C prediction models under elevated CO 2 conditions and shift our thinking in developing management practices for maintaining and building SOC for better soil fertility and future environmental sustainability.