Trade-offs between fertilizer-N availability and Cd pollution potential under crop straw incorporation by 15 N stable isotopes in rice.

Application of crop residues and chemical nitrogen (N) fertilizer is a conventional practice for achieving high yield in a rice system. However, the fallacious combination of N fertilizers with crop straw not only significantly reduces the N use efficiencies (NUEs) but also leads to serious environmental problems. The present study employed five treatments including no N fertilization and no straw incorporation (ck), N fertilization incorporation only (S0), N fertilization with 40% straw (S40), N fertilization with 60% straw (S60), and N fertilization with 100% straw (S100) to improve N use efficiency as well as reduced Cd distribution in rice. The crop yields were largely enhanced by fertilization ranging from 13 to 52% over the straw addition treatments. Compared with ck, N fertilizer input significantly decreased soil pH, while DOC contents were raised in response to straw amendment, reaching the highest in S60 and S100 treatments, respectively. Moreover, straw addition substantially impacted the Cd accumulation and altered the bacterial community structure. The soil NH4+-N concentration under S0 performed the maximum in yellow soil, while the minimum in black soil compared to straw-incorporated pots. In addition, the soil NO3--N concentration in straw-incorporated plots tended to be higher than that in straw-removed plots in both soils, indicating that crop straw triggering the N mineralization was associated with native soil N condition. Furthermore, the NUE increased with 15 N uptake in the plant, and the residual 15 N in soil was increased by 26.8% with straw addition across four straw application rates. Overall, our study highlights the trade-offs between straw incorporation with N fertilizer in eliminating potential Cd toxicity, increasing fertilizer-N use efficiencies and help to provide a feasible agricultural management.

Combined cadmium and fluorine inhibit lettuce growth through reducing root elongation, photosynthesis, and nutrient absorption.

Cadmium (Cd) and fluorine (F) often coexist in environment and are toxic to organisms; however, their combined effects on plants are still not well documented. In this study, the co-effects of Cd and F on germination, biomass, photosynthesis, and nutrients uptake of lettuce were carried out in hydroponic culture. The results showed that the seed germination and seedling biomass decreased with an increase in Cd and F supplementation. The root morphology verified these effects as excess combined Cd and F diminished the root tips and surface area of lettuce, while single Cd and F inhibited the growth by decreasing root length and average diameter, respectively. These effects were also consistence with a reduction in photosynthesis which was mainly regulated by reducing the quantum yield of PS II, electron transport activity, stomatal conductance, intercellular CO2 concentration, and transpiration rate in response to the pollutants. Moreover, when lettuce exposed to Cd and F stress, the accumulation of several essential elements in shoot decreased. In a sum, the synergistic negative effects of Cd and F on the seed germination and seedling growth of lettuce were observed, and these might be owed to nutrient absorption and translocation in the plant. These findings aid in understanding the harmful effects and specific mechanisms of action of Cd and F on plants.

Cadmium accumulation in rice straws and derived biochars as affected by metal exposure, soil types and rice genotypes.

Straw residues, as one of the conservation farming practices, are being strongly encouraged in China, including some cadmium-polluted areas. Nowadays, a portion of this plant residue is promoted to be removed for reducing excess metal(loid) in the soil and to be used for bioenergy production. Nevertheless, the possible influences of contaminated straw or the burial of its derived biochars on Cd accumulation in soil and data based on health risk assessment associated with different status and extent of soil contamination were relatively unknown. Thus it is important to provide a more systematical understanding of contaminated straw burial at specific contamination zones, which may provide useful guidance for straw utilization. In this study, we harvested two genotypes of rice straw from 6 contaminated levels among three soil types to comprehensively study the total Cd contents in straws and its derived biochars and correlate the sets of straw characteristics and Cd contents in three different contamination zones. The total Cd concentration in straws grew at a steady rate relatively with increasing soil Cd contamination levels, compared to those in biochars which performed more fluctuate due to the strong burning. According to correlational analyses, three-way ANOVA showed that the moisture, ash, volatile and fixed carbon were all significantly affected by straw CdTotal contents (p < 0.001). Such relationships were attributed to guide straw removal portions for gasification. Meanwhile, there was a significant correlation between straw Cd concentration and soil types (p < 0.001), confirming that it might be worth determining soil remediation by straw removal according to site-specific farmland conditions. This work will help to assess efforts toward predicting Cd concentration in the paddy soils related to kinds of contamination status and would also give useful guidance to make sustainable management strategies for crop straws in polluted regions.Novelty statement This work provided data on how much rice straw is needed to remove to ensure the minimal amount to control soil contamination and reduce costs according to site-specific conditions and soil Cd contamination status. It also explains the correlations between straw characteristics related to bioenergy use and soil conditions which would give guidance to balance using crop straw for increased bioenergy production and the need to also protect, preserve, and enhance soil resources.

Effects of straw return with N fertilizer reduction on crop yield, plant diseases and pests and potential heavy metal risk in a Chinese rice paddy: A field study of 2 consecutive wheat-rice cycles.

Understanding the comprehensive effect on crop production and quality, soil acidification, and Cd accumulation and distribution for wheat-rice rotation under N fertilization and continuous straw return is important for proper contaminated agricultural soil management. A 2-year paddy field experiment was conducted to study the effects of above factors change in the Zhejiang province, China. Fertilization treatments included: conventional N fertilizer application (N3), 20% reduction of N application (N2), 40% reduction of N application (N1), combined with three portions of straw incorporation: all straws retention (N3), half of the straws into the fields (S2), 20% straws retention(S1). The N1 treatments significantly decreased crop yields compared to N2 and N3 treatments. Except for C2-wheat, soil pH generally decreased with increasing N fertilizer input in the order of N1>N2>N3, regardless of how many straws was amended. Moreover, we found that straw addition plus N fertilization had a intersystem impacts on Cd accumulation, distribution and availability. Although total Cd had different trends among 4 experimental seasons, when the N reduced 20% applied, the DTPA-Cd contents were lowest among 3 out of four experimental seasons, except for that of C2-wheat, where N2 treatments ranked the second lower contents. For most seasons, Cd contents in straws were higher than soils and lowest in grains, and S2N2 treatment performed an intermediate value among all treatments. Furthermore, our study demonstrated that S2 or N2 treatments or S2N2 reduced the potential risk of plant diseases and pests with lower disease index, disease cluster rate. Notably, the relative outbreak of pests was remarkably suppressed under S2 treatments, especially S2N2. Thus, these findings demonstrated that in wheat-rice rotation reducing 20% N fertilization with 50% straw returning may be a win-win practice in this region for the equilibrium between agricultural productivity, quality and low Cd polluted risk.

Phytoavailability, translocation and soil thresholds derivation of cadmium for food safety through soil-wheat (Triticum aestivum L.) system.

Cadmium (Cd) pollution in cultivated soils has posed a great risk to human health through the soil-plant-human pathway. Therefore, it is important to derive soil thresholds for the low-Cd accumulating genotype of wheat (Triticum aestivum L.) to promote its application in agricultural production on Cd-contaminated sites. Here, a pot experiment was performed to explore the transfer characteristics of Cd in two contrasting wheat genotypes at three different soils and the effect of soil parameters together with soil safety Cd thresholds derivation. Generally, grain Cd highly accumulating wheat genotype (Zhenmai10, HT) showed higher Cd accumulation in grains than grain Cd weakly accumulating wheat genotype (Aikang58, LT). Stepwise multiple linear regression (SMLR) analysis (log-transformed Freundlich-type) indicated that Cd accumulation in wheat grains was strongly related to soil total Cd concentration and pH for both genotypes (R2 = 0.907*** for HT; R2 = 0.910*** for LT). Combining the simple regression model of soil-plant transfer system with the risk assessment method based on human health, soil total Cd thresholds for three soils were calculated with the values of 0.62, 0.82, and 0.62 mg kg-1 in LT genotype and 0.31, 0.77, and 0.49 mg kg-1 in HT genotype. Therefore, we suggested that when deducing soil thresholds, the ability of wheat genotypes to accumulate Cd and soil properties should be considered because of the large differences in soil thresholds between different genotypes and types of soils. We believe our results will promote the application of low-Cd wheat genotypes to agricultural production, thereby ensuring the safety of their products.

Spatial variation and fractionation of fluoride in tobacco-planted soils and leaf fluoride concentration in tobacco in Bijie City, Southwest China.

Fluorosis is a chronic systemic disease induced by excessive intake of fluoride (F-). Fluoride in water and foods has been widely concerned, while limited reports focused on F- in soils and tobacco leaves which could transfer to human body. In the present study, we mainly focused on the distribution of F- in tobacco-planted soils and tobacco leaves in Bijie City, Southwest China. Soil total F- concentration ranged from 443.7 to 5,979 mg kg-1. The level of F- extracted by water (FH2O) and KCl solution (FKCl) ranged from 0.58 to 25.55 mg kg-1 and from 0.67 to 21.35 mg kg-1, respectively; hence, FH2O could be used to indicate the bioavailability of soil F- in the study area. The sequential extraction of F- show that the residual and exchangeable F- was the highest (97.44-99.73% of the total F-) and lowest (less than 0.25%) fractions of collected soil samples, respectively. According to the distribution of total and soluble F- in the soil profiles at the depth of 0-100 cm, soils were polluted mainly at the 0-40 cm layer. The soluble F- content in rhizosphere soils were higher than that in bulk soils, and tobacco leaves accumulated F- ranged from 16.73 to 111.3 mg kg-1 which was affected by soil pH and Ca content. Tobacco leaves F- level was related to the maturity of the leaves, with the F-content of medium leaves being higher than that of top leaves. More attention should be paid to tobacco with high F- content since F- pollution may transfer to human body via tobacco smoking.

Responses of soil bacterial community and Cd phytoextraction to a Sedum alfredii-oilseed rape (Brassica napus L. and Brassica juncea L.) intercropping system.

Soil pollution with heavy metals has become a common problem in agricultural ecosystems and poses a threat to food safety and human health. Intercropping is now considered a promising alternative to address this issue. However, our understandings about the influences of intercropping systems on rhizosphere microbiota composition and their association with plant performance are still limited. In this study, rhizobox microcosm experiments were conducted to investigate the influence of cropping regimes (i.e. monoculture and intercropping) on the rhizosphere bacterial microbiota and their linkages with the phytoextraction of cadmium (Cd) by Zhongyouza 19 (Brassica napus L.), Xikou Huazi (Brassica juncea L.) and Sedum alfredii using 16S rRNA gene sequencing. Cadmium accumulation in shoots of B. napus and B. juncea grown under intercropping were enhanced by 370% and 27.8% respectively, as compared to monoculture. Soil compartmentation as a major determinant explained 57.6% of the rhizosphere bacterial microbiota variation, whereas plant species and cropping regime accounted for 26.4% of the variation. The overall abundance of the taxa was Proteobacteria, Acidobacteria, Bacteroidetes, Chloroflexi, Verrucomicrobia, and Actinobacteria. Intercropping significantly enriched amplicon sequence variants (ASVs) abundance belonging to Actinobacteria, Bacilli, Deltaproteobacteria while depleting that of Acidobacteria in rhizosphere. Intercropping with S. alfredii influenced more on microbial composition of B. napus rhizosphere. The change in rhizosphere bacterial communities was related to metal availability, soil properties, and plant parameters. The enriched families of Pedosphaeraceae, Ruminococcaceae, Chitinophagaceae, Gemmatimonadaceae, Nitrosomonadaceae, and Parachlamydiaceae were positively correlated with metal concentration in plants. These results indicate that S. alfredii and oilseed rape intercropping could be a promising approach for enhancing the remediation of Cd contaminated soil. Understanding the complex plant-microbe-metal interactions of intercropping system could facilitate the development of remediation strategy for phytoremediation of contaminated soils and sustainable agricultural production.