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Purpose: The chemistry of soil organic matter (SOM) is fundamental for sustainable and climate-smart agroecosystems. However, the differences in SOM chemistry between the upland and paddy soils developing under the same climatic and edaphic conditions are unclear. Materials and methods: Py-GC/MS was applied to characterize the biochemical features of SOM in three physical size fractions: coarse particulate (> 0.25 mm, cPOM), fine particulate (0.053–0.25 mm, fPOM), and mineral-associated OM (< 0.053 mm, MAOM) of upland and paddy fields under long-term (> 30 years) mineral and manure fertilizations. Results and discussion: Paddy fields had higher contents of soil organic carbon (SOC) and total nitrogen (TN) mainly accumulated in MAOM fraction than uplands. These two soils had different molecular compositions of SOM: N-containing compounds including amino-N and heterocyclic-N compounds enriched in the uplands, whereas paddy had higher proportions of lipids and phenolics. The SOM composition was also dependent on particle size, especially in the uplands, where POM fractions had high contents of lignin and MAOM accumulated N-containing components. In contrast, POM in paddy accumulated polysaccharides, whereas MAOM was enriched with lipids. Particle size controlled the C oxidation state (Cox), and paddy soils had higher Cox than that of uplands, mainly in the MAOM fraction. Conclusions: The molecular composition SOM was primarily regulated by land-use type, following by fraction size and fertilization regime, while the Cox was controlled by fraction size. The Cox needs more attention to understand the direction of formation of SOM fractions. [ABSTRACT FROM AUTHOR]

Paddy fields serve as significant reservoirs of soil organic carbon (SOC) and their potential for terrestrial carbon (C) sequestration is closely associated with changes in SOC pools. However, there has been a dearth of comprehensive studies quantifying changes in SOC pools following extended periods of rice cultivation across a broad geographical scale. Using 104 rice paddy sampling sites that have been in continuous cultivation since the 1980s across China, we studied the changes in topsoil (0–20 cm) labile organic C (LOC I), semi‐labile organic C (LOC II), recalcitrant organic C (ROC), and total SOC. We found a substantial increase in both the content (48%) and density (39%) of total SOC within China's paddy fields between the 1980s to the 2010s. Intriguingly, the rate of increase in content and density of ROC exceeded that of LOC (I and II). Using a structural equation model, we revealed that changes in the content and density of total SOC were mainly driven by corresponding shifts in ROC, which are influenced both directly and indirectly by climatic and soil physicochemical factors; in particular temperature, precipitation, phosphorous (P) and clay content. We also showed that the δ13CLOC were greater than δ13CROC, independent of the rice cropping region, and that there was a significant positive correlation between δ13CSOC and δ13Cstraw. The δ13CLOC and δ13CSOC showed significantly negative correlation with soil total Si, suggesting that soil Si plays a part in the allocation of C into different SOC pools, and its turnover or stabilization. Our study underscores that the global C sequestration of the paddy fields mainly stems from the substantial increase in ROC pool. [ABSTRACT FROM AUTHOR]

Accumulation of microplastics (MPs) and cadmium (Cd) are ubiquitous in paddy soil. However, the combined effects of MPs and Cd on physiochemical and microbial mechanisms in soils and the attendant implications for greenhouse gas (GHG) emissions, remain largely unknown. Here, we evaluated the influence of polylactic acid (PLA) and polyethylene (PE) MPs on GHG emissions from Cd-contaminated paddy soil using a microcosm experiment under waterlogged and drained conditions. The results showed that PLA significantly increased CH 4 and N 2 O emission fluxes and hence the global warming potential (GWP) of waterlogged soil. Soils treated with MPs+Cd showed significantly reduced GWP compared to those treated only with MPs suggesting that, irrespective of attendant consequences, Cd could alleviate N 2 O emissions in the presence of MPs. Conversely, the presence of MPs in Cd-contaminated soils tended to alleviate the bioavailability of Cd. Based on a structural equation model analysis, both the MPs-derived dissolved organic matter and the soil bioavailable Cd affected indirectly on soil GHG emissions through their direct influencing on microbial abundance (e.g., Firmicutes , Nitrospirota bacteria). These findings provide new insights into the assessment of GHG emissions and soil/cereal security in response to MPs and Cd coexistence that behaved antagonistically with respect to adverse ecological effects in paddy systems. [Display omitted] • MP enrichment in paddy soil enhance GHG emissions but inhibit Cd bioavailability. • MPs lead to increase of dissolved organic matter and Firmicutes bacteria abundance. • PLA have high potential to adsorb and fix soil Cd2+ due to abundant oxygenous groups. • 90-day incubation makes PLA decomposing and loading more soil particles than PE. [ABSTRACT FROM AUTHOR]