Sulfur (S) is essential for rice growth and plays a pivotal role in soil pollution remediation. Wheat straw (W) amendment and sulfate (SO42-) fertilization are common agricultural practices in rice cultivation, yet their combined effects on S transformation, including organic sulfur (OS), available sulfate (AS), and reduced inorganic sulfur (RIS), in waterlogged paddy soils remain poorly understood. We conducted a 162-day incubation experiment with different W levels (0 %, 0.1 %, 0.5 %, and 1.0 %, w/w) and S rates (0 and 30 mg center dot sulfur center dot kg-1, as SO42-) in flooded soil. The results demonstrated that both W and S applications alone enhanced RIS formation, and their coapplication exhibited synergistic effects. Compared to the control, the co-application of W and S increased the proportion of RIS in total S by 76 % (90 %), 70 % (54 %), and 94 % (65 %) with 0.1 %, 0.5 %, and 1.0 % W at the early (middle) stages of incubation, respectively. The increase of RIS was attributed to the enhanced reduction of iron oxides and SO42-, mediated by reducing bacteria (especially Fe reducing bacteria) under low pe + pH. Besides, W addition increased AS levels during the early stage irrespective of S application, especially at 1 % W, due to organic sulfate mineralization. In addition, the influence of W and S applications on S transformation diminished over time. These findings suggest that the co-application of W and S under waterlogged soil could optimize sulfate bioavailability and RIS formation, which could meet the demand of rice sulfur nutrition and contribute to soil pollution remediation.

The impacts of biochar application on the carbon (C) and nitrogen (N) cycles in soil profiles in vegetable fields have rarely been reported. A three-year field experiment (CK, control; BC, biochar; N, nitrogen fertilizer; BCN, biochar and nitrogen fertilizer) was conducted in fluvo-aquic soil with a wild cabbage-Chinese cabbage rotation to investigate biochar effects on soil organic carbon (SOC) sequestration, N retention, and nitrate (NO3-) leaching in the soil profile (topsoil, 0-20 cm; subsoil, 20-50 cm; third-layer soil, 50-100 cm). The results showed that the biochar-induced increase in topsoil SOC under N fertilization was greater for Chinese cabbage season than for wild cabbage season. Excluding biochar-N, biochar application caused an extra increase in topsoil total nitrogen (TN) under N fertilization. Biochar decreased the NO3--N concentration of third-layer soil solution, particularly under N fertilization, indicating that biochar reduced the potential source of NO3- leaching. Under N fertilization, biochar reduced the abundances of carbon-degrading bacteria (Streptomyces, Bacillus, Mycobacterium, and Sphingomonas) and genes (sga, xylA, lig, and pgu) in the topsoil, as well as the abundances of ammonia-oxidizing bacteria (Nitrosospira and Nitrosomonas) and gene (amoA) in the third-layer soil. However, biochar increased the abundance of ammonia-assimilating bacteria (Rhodococcus) and gene (glnA) in the topsoil. Biochar inhibited the microbial function associated with C degradation by affecting soil TN, thus enhancing topsoil SOC sequestration. Biochar promoted the soil microbial function related to ammonia assimilation by affecting dissolved organic carbon (DOC), subsequently enhancing ammonia assimilation. Moreover, biochar initially inhibited ammonia oxidation in the third-layer soil by increasing soil DOC, subsequently affecting nitrite oxidation and ultimately reducing soil NO3- leaching. Biochar significantly increased the N use efficiency of vegetables in the last two years. This study provides insights into biochar effects on the changes in soil microbial function, which promoted SOC sequestration, enhanced N retention and mitigated nitrate leaching in a vegetable rotation field.

Nitrate (NO-3) and nitrite (NO-2) leaching threatens groundwater quality. Soil C:N ratio, i.e., the ratio of soil organic carbon to total nitrogen, affects mineralization, nitrification, and denitrification; however, its mechanism for driving soil NO-3 and NO-2 accumulation and leaching remains unclear. Here, a field investigation in a fluvo-aquic soil and a soil column experiment were performed to explore the relationships between soil C:N ratio and soil NO-3 and NO-2 leaching in three soil layers (0–20, 20–40, and 40–60 cm) under heavy rainfall (rainfall rate > 25 mm d–1). The field investigation results showed that both soil NO-3-N and NO-2-N contents decreased exponentially (P < 0.001) with increasing soil C:N ratio in each soil layer. Furthermore, negative exponential relationships (P < 0.001) were found between soil C:N ratio and both NO-3-N and NO-2-N concentrations in soil solution in each soil layer under heavy rainfall. The soil column divided into three layers was leached with simulated heavy rainfall; the results confirmed negative exponential relationships (P < 0.05) between soil C:N ratio and both NO-3-N and NO-2-N concentrations in the leachate from each soil layer. A total of 18 soil samples obtained from three depths at six field sites during the rainy season were used to elucidate the microbial mechanisms induced by soil C:N ratio using high-throughput sequencing and real-time polymerase chain reaction. High abundances of ammonifying bacteria (Flavobacterium, Bacillu, and Pseudomonas), ammonia-oxidizing bacteria (Nitrosospira), and nirS/K gene were observed when soil C:N was low, concomitant with low abundances of NO-2-oxidizing bacteria (Nitrospira) and narG gene. Partial least squares path modeling showed that the high NO-3 and NO-2 levels at low soil C:N ratio might be attributed to the inhibition of NO-3 reduction (i.e., low narG gene) and NO-2 oxidation (i.e., low Nitrospira) and thus the accumulation of soil NO-3 and NO-2, respectively. Therefore, the leaching of NO-2 and NO-3 in low C:N soils requires more attention during the rainy season.

Background Sulfur (S) has been widely used to alleviate cadmium (Cd) toxicity and control Cd accumulation in rice under waterlogging conditions. However, the results are contradictory, and the reasons remain unclear. This could be because most studies rarely simultaneously monitor the processes of S-induced soil Cd bioavailability and Cd accumulation in rice throughout its growth period. A pot experiment was conducted to investigate the influence of two sulfur levels (0, and 30 mg S kg −1 ) on Cd concentration and translocation in rice at three growth stages (booting, filling and maturity) under waterlogging conditions. Paddy soil deficient in S but contaminated with Cd (10.16 mg Cd kg −1 ) was used for the pot experiment. Results S application increased concentrations of Cd in grain at the filling stage partially because S induced the promotion of Cd transfer from roots to stems, leaves, and grains, and S induced the accumulation and fixation of Cd in iron plaques at the filling stage. However, the application of S significantly reduced Cd concentrations in brown rice at the maturity stage, which could be attributed to three aspects, as described below. First, S supply reduced the availability of Cd in soil and iron plaque on the root surface by reducing dissolved Cd in soil pore water and transferring Cd from iron plaque on the root surface to roots. Second, S supply inhibited the transfer of Cd in other tissues to brown rice based on Cd transfer factors from roots, stems, leaves, and husks to brown rice, which were obviously lower with S supply than without S supply at the maturity stage. Third, S induced the dilution of Cd in brown rice because the application of S significantly increased brown rice biomass by 215%. Conclusions A S-induced decline in Cd accumulation in brown rice was related to S-regulated Cd transfer among rice plants, S-induced promotion of rice growth and a decrease in Cd bioavailability in S-deficient but Cd-contaminated paddy soil under waterlogging conditions. This study provides valuable information for growing rice in low-S and Cd-contaminated paddy soil and reducing the risk of Cd in rice to humans. Graphical abstract