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Background and aims: Liming is widely known as a cost-efficient agricultural practice to reduce cadmium (Cd) concentration in rice (Oryza sativa L.) grain (CdR) in the short term. However, the effect of long-term continuous liming on CdR-reduction in the context of other environmental controls remains unknown. Methods: We quantified the temporal stability of CdR-reduction in response to biannually continuous liming and its correlations with dynamic soil properties and weather conditions based on a 7-year field experiment in a subtropical double-cropping rice ecosystem in a moderately Cd-contaminated acidic soil. Results: The CdR value declined by 51–90% with liming without compromising the rice yield. The degree of CdR-reduction was similar between early and late rice seasons. However, the between-year CdR-reduction was variable, remaining relatively stable in the first three years and fluctuating in the last four years. Soil pH and Olsen-P concentration at the ripening stage rose significantly, while the concentrations of diethylenetriaminepentaacetic acid (DTPA)-extractable Cd, Mn, and Zn in soil remained unaffected. The soil pH did not further increase after the second year of liming. The soil pH and Olsen-P concentration were positively correlated with cumulative liming, but the CdR and CdR-reduction exhibited no correlations with cumulative liming. The CdR was correlated negatively with precipitation but positively with temperature and sunshine duration. Conclusions: The effect of 7-year continuous liming on CdR-reduction was temporally unstable and had no significant cumulative effect on CdR. The between-year CdR-reduction was not controlled by accumulative liming but primarily by dynamic soil properties and weather conditions. [ABSTRACT FROM AUTHOR]

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]