The Remediation Strategy and Mechanism of Combined Passivation and Foliar Inhibition for Safe Rice Production in Red Paddy Soil Contaminated With Heavy Metals

To solve the problem of Cd in rice over the food-safe standard, the present field study was carried out to explore the combined passivators (a mixture of quicklime (Q), polyacrylamide (A), sepiolite (S)) and Si-/Se- containing foliar inhibitors (Si or Se) at low (1) and high (2) application rates were added into the red paddy soil. After harvest the rice, the soil samples were collected to examine the soil properties, bacterial community, and the availability of heavy metals (Cd, Pb, and Cu) in soil. The rice samples were obtained to investigate the accumulation of heavy metals by rice. All of the treatments increased the soil's pH, but the treatments excluding QSe2 decreased the total P and soil organic matter (SOM), which was favourable for the immobilisation of heavy metals in red paddy soil by decreasing 14.29%-42.86% of available Cd, 10.18%-63.17% of available Pb, and 6.95%-36.81% of available Cu. With the increasing application rates, QAS significantly decreased the heavy metals available because of the enhanced immobilisation, while QSi and QSe significantly increased the heavy metals available because of the inhibited plant uptake. After remediation, QA1, QSi2, and QSe2 most effectively decreased the uptake Cd by rice in the present red paddy soil to solve the problem of Cd exceeding the threshold value according to the National Food Safety Standard of China (GB2762-2017). Additionally, the treatments, with the exception of Q1, QA1, QSi1, and QSi2, did not dramatically change the community structure of bacteria at the genus level in soil. Considering the safety and stability of soil, QSe2 was the primary recommendation for remediating Cd-contaminated red paddy soil.

The effects of the depth of fertilization on losses of nitrogen and phosphorus and soil fertility in the red paddy soil of China

Nitrogen (N) and phosphorus (P) losses from agroecosystems are dominant nonpoint pollution. To minimize the losses of N and P, the optimal depth of fertilization was explored using a soil column study with the red paddy soil as the research objects. The losses of N and P were measured under five depths of fertilization (0, 5, 7.5, 10, and 12.5 cm) as well as no fertilization. The results showed that ammonia volatilization was significantly decreased with increasing fertilization depth within 0–10 cm, and there was no significant difference among the 10 cm, 12.5 cm, and no-fertilization treatments. Comparing with surface fertilization (0 cm), N and P losses by runoff could be reduced by 30.7–67.1% and 96.9–98.7% respectively by fertilization at 5–12.5 cm. In addition, deep fertilization (5–12.5 cm) did not increase N and P losses by leaching at the depth of 40 cm. Total N and P contents in the tillage layer of soil were increased by 5.1 to 22.8% and by −1.0 to 7.5%, respectively. Fertilization at 10cm depth has the potential to minimal environmental impact in the red paddy soil of south China, at this depth, NH3 volatilization was reduced by 95.1%, and N and P losses by runoff were reduced by 62.0% and 98.4%, respectively, compared with surface fertilization.

Long-term nitrogen fertilization shaped the nifH, nirK and nosZ gene community patterns in red paddy soil in south China。

To understand the diversities of diazotrophs and denitrifiers in red paddy soil under long-term fertilization conditions, nifH, nirK and nosZ libraries were constructed based on the PCR-RFLP method. nirK gene diversity proved to be lower than that of nosZ and nifH, and nirK and nosZ genes were more sensitive to different fertilization treatments than those with the nifH gene. Diverse microbes including the α-, β-, γ- and δ- subclasses of the Proteobacteria dominated the three libraries. Long-term addition of urea with straw-mulching and azophoska increased the abundance of non-symbiotic diazotrophs, which indicated that non-symbiotic diazotrophs were responsible for the majority of the nitrogen-fixing ability in paddy soil. In addition, a potential link between nifH and nosZ was found due to the existence of nitrogen fixers, such as Bradyrhizobium and Ralstonia in the nosZ library. The main chemical factors affecting the three genes were identified, pH was the most important factor of nifH community, and nirK genes were more affected by pH and organic matter, available potassium and carbon to nitrogen ratio significantly influenced the community structure of the nosZ genes.