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Purpose: Paddy soils may play an essential role in global greenhouse gas mitigation and food security; however, ecological indices of soil organic carbon (C), total nitrogen (N), and total phosphorus (P), and their environmental controls in paddy soils at regional scale, are not fully understood. The aim is to quantify the stoichiometry ratio of C, N, and P, and its correlation with environmental factors in subtropical paddy soils. Materials and methods: A first-hand regional-scale field survey of subtropical paddy soil profiles was conducted in the Hunan Province of central China. The survey included 110 paddy sites and 587 soil horizons. Pearson correlation analysis was employed to investigate the correlation of soil C:N:P ratio with altitude, parent material, soil horizon, soil group, and soil physiochemical properties. Results and discussion: The depth-weighted average value of C:N, C:P, N:P, and C:N:P ratios in subtropical paddy soil profiles was 12.6, 49, 3.9, and 38:3:1, respectively. Soil organic C content was positively related with total N content, and the average C:N ratio did not vary distinctly with parent materials or soil groups. In contrast, there was no relationship neither between soil organic C and total P nor between total N and total P, and the mean values of both C:P and N:P ratios varied significantly with parent materials, soil groups, and soil horizons. Correlation analysis showed that C:N ratio was only correlated with altitude and soil bulk density, whereas C:P and N:P ratios were correlated not only with altitude and soil bulk density but also with soil texture (sand, clay, silt), and total iron and its dithionite-citrate-bicarbonate (DCB)–extractable fraction. Conclusions: Our study suggests a strong coupling effect between element C and N, but noncoupling effect between C and P and between N and P in subtropical paddy soils. The ratios of C:P and N:P are inconstant and more sensitive to environmental factors than C:N ratio. [ABSTRACT FROM AUTHOR]

Organic fertilizers may contain cadmium (Cd) and the transformation of organic materials in soil also has a role in soil-plant Cd distribution, both of which lead to Cd accumulation in plant edible parts. However, the advisability of applying organic fertilizer to remediate soils that are moderately and slightly contaminated with Cd has not been clarified. In this study, we investigated the impacts of an organic amendment (chicken manure) on the Cd concentration in rice grains (Cd R ) and the soil chemical properties over a five year period (10 rice seasons) within a slightly contaminated paddy soil in Hunan Province, subtropical China. We found that the Cd R was reduced by 28%-56% as a result of the organic amendment. The within-year reduction in Cd R was higher in late rice (43%-56%, averaging 51%) than in early rice (28%-45%, averaging 38%); however, the inter-annual reduction in Cd R was fairly stable (40%-49%), which suggests that chicken manure amendment has a long-term and persistent remediation potential. The concentrations of DTPA-extractable Cd and exchangeable plus water-soluble Cd fractions in soil were reduced, whereas soil pH and the concentrations of soil organic C and its labile fractions increased. These results indicate a lower apparent phytoavailability of Cd in soil following organic amendment. A two-variable empirical model using DTPA-Cd extracted from the soil at the full heading stage of rice and a climatic factor (total precipitation during the rice growing season) showed great potential in effectively predicting Cd R . Our study suggests that Cd phytoavailability in soil (indexed by DTPA-extractable and exchangeable Cd) and climatic factors (such as temperature and precipitation) may control inter-annual reductions in Cd R following organic amendment in slightly contaminated paddy soils., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)