Not all soil carbon is created equal: Labile and stable pools under nitrogen input.

Anthropogenic activities have raised nitrogen (N) input worldwide with profound implications for soil carbon (C) cycling in ecosystems. The specific impacts of N input on soil organic matter (SOM) pools differing in microbial availability remain debatable. For the first time, we used a much-improved approach by effectively combining the 13C natural abundance in SOM with 21 years of C3-C4 vegetation conversion and long-term incubation. This allows to distinguish the impact of N input on SOM pools with various turnover times. We found that N input reduced the mineralization of all SOM pools, with labile pools having greater sensitivity to N than stable ones. The suppression in SOM mineralization was notably higher in the very labile pool (18%-52%) than the labile and stable (11%-47%) and the very stable pool (3%-21%) compared to that in the unfertilized control soil. The very labile C pool made a strong contribution (up to 60%) to total CO2 release and also contributed to 74%-96% of suppressed CO2 with N input. This suppression of SOM mineralization by N was initially attributed to the decreased microbial biomass and soil functions. Over the long-term, the shift in bacterial community toward Proteobacteria and reduction in functional genes for labile C degradation were the primary drivers. In conclusion, the higher the availability of the SOM pools, the stronger the suppression of their mineralization by N input. Labile SOM pools are highly sensitive to N availability and may hold a greater potential for C sequestration under N input at global scale.

Short-term continuous monocropping reduces peanut yield mainly via altering soil enzyme activity and fungal community.

Continuous monocropping can lead to soil sickness and increase of soil-borne disease, which finally reduces crop yield. Microorganisms benefit plants by increasing nutrient availability, participating in auxin synthesis, and defending against pathogens. However, little is known about the influence of short-term successive peanuts cropping on soil properties, enzyme activities, its yield, plant-associated microbes, and their potential correlations in peanut production. Here, we examined the community structure, composition, network structure and function of microbes in the rhizosphere and bulk soils under different monocropping years. Moreover, we assessed the impact of changes in the soil micro-environment and associated soil microbes on peanut yield. Our results showed that increase of monocropping year significantly decreased most soil properties, enzyme activities and peanut yield (p < 0.05). Principal co-ordinates analysis (PCoA) and analysis of similarities (ANOSIM) indicated that monocropping year significantly influenced the fungal community structure in the rhizosphere and bulk soils (p < 0.01), while had no effect on the bacterial community. With the increase of continuous monocropping year, peanut selectively decreased (e.g., Candidatus_Entotheonella, Bacillus and Bryobacter) or increased (e.g., Nitrospira, Nocardioides, Ensifer, Gaiella, and Novosphingobium) the abundance of some beneficial bacterial genera in the rhizosphere. Continuous monocropping significantly increased the abundance of plant pathogens (e.g., Plectosphaerella, Colletotrichum, Lectera, Gibberella, Metarhizium, and Microdochium) in the rhizosphere and negatively affected the balance of fungal community. Besides, these species were correlated negatively with L-leucine aminopeptidase (LAP) activity. Network co-occurrence analysis showed that continuous monocropping simplified the interaction network of bacteria and fungi. Random forest and partial least squares path modeling (PLS-PM) analysis further showed that fungal community, pathogen abundance, soil pH, and LAP activity negatively affected peanut yield. In conclusion, short-term continuous monocropping decreased LAP activity and increased potential fungal pathogens abundance, leading to reduction of peanut yield.

Soybean inclusion reduces soil organic matter mineralization despite increasing its temperature sensitivity.

Legume-based cropping increased the diversity of residues and rhizodeposition input into the soil, thus affecting soil organic matter (SOM) stabilization. Despite this, a comprehensive understanding of the mechanisms governing SOM mineralization and its temperature sensitivity across bulk soil and aggregate scales concerning legume inclusion remains incomplete. Here, a 6-year field experiment was conducted to investigate the effects of three cropping systems (i.e., winter wheat/summer maize, winter wheat/summer maize-soybean, and nature fallow) on SOM mineralization, its temperature sensitivity, and the main drivers in both topsoil (0-20 cm) and subsoil (20-40 cm). Soybean inclusion decreased the SOM mineralization by 17%-24%, while concurrently increasing the majority of soil biochemical properties, such as carbon (C) acquisition enzyme activities (5%-22%) and microbial biomass C (5%-9%), within the topsoil regardless of temperature. This is attributed to the increased substrate availability (e.g., dissolved organic C) facilitating microbial utilization, thus devoting less energy to mining nutrients under diversified cropping. In addition, SOM mineralization was lower within macroaggregates (∼12%), largely driven by substrate availability irrespective of aggregate sizes. In contrast, diversified cropping amplified the Q10 of SOM mineralization in mesoaggregates (+6%) and microaggregates (+5%) rather than in macroaggregates. This underscores the pivotal role of mesoaggregates and microaggregates in dominating the Q10 of SOM mineralization under soybean-based cropping. In conclusion, legume-based cropping diminishes soil organic matter mineralization despite increasing its temperature sensitivity, which proposes a potential strategy for C-neutral agriculture and climate warming mitigation.

Spatiotemporal dynamic of rice production and its carbon footprint in Hainan, China: Implications for food security and environmental sustainability.

Rice (Oryza sativa L.) feeds more than half of the global population and faces the critical issues related to food security and environmental sustainability. This study analyzed double rice production data from 2010 to 2020 to assess its spatiotemporal dynamic in food production and carbon (C) footprint in Hainan province, China. The results revealed a 29.5% reduction in rice planting area, leading to a significantly decreased rice self-sufficiency rate from 38% to 33% from 2010 to 2020. During this period, the carbon footprint per unit area (CFa) for early, late, and double rice showed a fluctuating upward trend ranging from 8.1 to 8.4, 8.9 to 9.2, and 17.0 to 17.4 t CO2-eq ha-1, respectively. The total greenhouse gas (GHG) emissions of rice production decreased to around 2 million t CO2-eq, primarily due to reduced planting area. The C sequestration initially increased before decreasing to 1.2 million t C in 2020 at a temporal scale. Spatially, the northeast and southwest regions exhibited ∼70% of the total GHG emissions and ∼80% of C sequestration. The regional C footprint per unit yield displayed less favorable outcomes, with some areas (e.g., Wenchang and Haikou) experiencing emission hotspots in recent years. Higher yield and smaller CFa for Lingao and Tunchang were observed compared to the average between 2010 and 2020. This study provides insights into the spatiotemporal dynamics of double rice production and GHG emissions in Hainan, offering a scientific reference for regional food security and environmental sustainability.