Effects of different potassium fertilizers on cadmium uptake by three crops.

Cadmium contamination of agricultural soils has aroused worldwide concern because of the threats posed to human health through accumulation in food chains. A greenhouse pot experiment was conducted with in situ Cd-contaminated soil to study the influence of different potassium fertilizers (KCl, K2SO4, and KNO3) on Cd accumulation in rice, wheat, and pak choi as well as the NH4NO3-extractable Cd (NEX-Cd) content in soils. In our study, rice and wheat biomass increased in the presence of K fertilizers, whereas pak choi biomass remained stable. Moreover, our experiment demonstrated that Cl- increased Cd uptake by crops more effectively than SO42- or NO3-. The KCl treatments increased the Cd content of all three crops; as the KCl dose was increased, the Cd content of rice grains, wheat grains, and pak choi shoots increased by 10.8-192.8%, 17.1-67.7%, and 15.1-40.4%, respectively. The KNO3 treatment also increased the Cd content of all three crops; however, the K2SO4 treatment only slightly increased the Cd content of wheat and pak choi and greatly decreased the Cd content of rice. In addition, both of the NEX-Cd content of wheat soil and pak choi soil were much higher than that of rice paddy soil. The KCl treatment resulted in a significant increase in the NEX-Cd content of rice paddy soil, but there were no significant differences in the NEX-Cd content of wheat soil or pak choi soil, regardless of which types or doses of K fertilizers were supplied. Based on these results, when K fertilizers are applied to Cd-contaminated soils, both types and doses should be carefully considered to mitigate Cd accumulation in crops, especially the edible part.

Accumulation, subcellular distribution, and oxidative stress of cadmium in Brassica chinensis supplied with selenite and selenate at different growth stages.

Despite not being an essential element for plants, Se has been proved to reduce Cd accumulation and Cd-induced oxidative stress, although the underlying mechanisms are not fully understood. A pak choi hydroponic experiment was conducted to investigate the effects of Se on Cd accumulation, subcellular distribution, and Cd-induced oxidative stress at different growth stages. The results showed that on day 19 after germination, Cd content was significantly reduced by 32% by selenite, but was increased by 15% by selenate. Accordingly, selenite improved cell-wall Cd sequestration by 20%, whereas selenate caused enhanced translocation of Cd from the root to the shoot. However, the effects of selenite on the reduction in Cd accumulation and distribution in pak choi seedlings were completely dismissed on day 40. Nevertheless, both forms of Se enhanced antioxidative defense, as they both inhibited the accumulation of H2O2 and malondialdehyde. On day 19, ascorbate peroxidase and glutathione reductase activities were increased by more than 50% by selenite; additionally, superoxide dismutase, catalase, and peroxidase activities increased by up to 86%, 63%, and 24%, respectively, in the presence of selenite, when compared to Cd treatment alone. Activities of most of the antioxidants remained significantly unaffected by both forms of Se on day 40. Consequently, selenite and selenate affected Cd accumulation in pak choi seedlings by altering Cd subcellular distribution and by enhancing antioxidative defense, but such effects depended on the Se forms applied and the growth stage as well.

Effect of selenium on the uptake kinetics and accumulation of and oxidative stress induced by cadmium in Brassica chinensis.

Pak choi can readily accumulate cadmium (Cd) into its edible parts; this can pose a threat to human health. Although not essential for higher plants, selenium (Se) can be favorable for plant growth and antioxidative defense under heavy metal stress conditions. A pak choi hydroponic experiment was conducted to investigate the effect of two forms of Se on the Cd uptake kinetics and accumulation and oxidative stress. The results showed that selenite and selenate remarkably enhanced Cd uptake kinetics in pak choi. The maximum Cd uptake rate increased by more than 100% after treatment with 5 µM of selenite and selenate, and it further increased after treatment with 20 µM of both Se forms. The effects of Se on Cd content depended on the Se form, exposure time, and Cd dosage. Selenite reduced the Cd content in shoots by 41% after 3 days of treatment with 10 µM Cd, whereas selenate increased this rate by 89%. Both forms of Se decreased Cd content in the shoots by 40% after 7 days of treatment with 10 µM Cd, but they increased the Cd content by approximately 30% after treatment with 50 µM Cd. Se enhanced Cd-induced oxidative stress in pak choi. Malondialdehyde (MDA) generation was promoted by more than 33% by selenite and selenate treatments in combination with 10 µM Cd, and it was further enhanced by 106% and 185% at 50 µM Cd, respectively. Selenite also increased the H2O2 content at both Cd doses, but selenate did not have significant effects on H2O2 production. The effects of Se on antioxidative enzyme activity also depended on the dose of Cd. Selenite and selenate inhibited catalase activity by 11% and 29%, respectively, at 10 µM Cd, and by 13% and 42%, respectively, at 50 µM Cd. Moreover, both forms of Se increased superoxide dismutase activity after treatment with 10 µM Cd but inhibited its activity at 50 µM Cd. Therefore, Se exhibits dual effects on Cd accumulation and oxidative stress in pak choi and might cause further stress when combined with higher doses of Cd.

Optimizing the nitrogen application rate for maize and wheat based on yield and environment on the Northern China Plain.

Optimizing the nitrogen (N) application rate can increase crop yield while reducing the environmental risks. However, the optimal N rates vary substantially when different targets such as maximum yield or maximum economic benefit are considered. Taking the wheat-maize rotation cropping system on the North China Plain as a case study, we quantified the variation of N application rates when targeting constraints on yield, economic performance, N uptake and N utilization, by conducting field experiments between 2011 and 2013. Results showed that the optimal N application rate was highest when targeting N uptake (240kgha-1 for maize, and 326kgha-1 for wheat), followed by crop yield (208kgha-1 for maize, and 277kgha-1 for wheat) and economic income (191kgha-1 for maize, and 253kgha-1 for wheat). If environmental costs were considered, the optimal N application rates were further reduced by 20-30% compared to those when targeting maximum economic income. However, the optimal N rate, with environmental cost included, may result in soil nutrient mining under maize, and an extra input of 43kgNha-1 was needed to make the soil N balanced and maintain soil fertility in the long term. To obtain a win-win situation for both yield and environment, the optimal N rate should be controlled at 179kgha-1 for maize, which could achieve above 99.5% of maximum yield and have a favorable N balance, and at 202kgha-1 for wheat to achieve 97.4% of maximum yield, which was about 20kgNha-1 higher than that when N surplus was nil. Although these optimal N rates vary on spatial and temporal scales, they are still effective for the North China Plain where 32% of China's total maize and 45% of China's total wheat are produced. More experiments are still needed to determine the optimal N application rates in other regions. Use of these different optimal N rates would contribute to improving the sustainability of agricultural development in China.