Effects of C/N ratio on N2O emissions and nitrogen functional genes during vegetable waste composting.

Nitrous oxide (N2O) generation during composting not only leads to losses of nitrogen (N) but also reduces the agronomic values and environmental benefits of composting. This study aimed to investigate the effect of the C/N ratio on N2O emissions and its underlying mechanisms at the genetic level during the composting of vegetable waste. The experiment was set up with three treatments, including low C/N treatment (LT, C/N = 18), middle C/N treatment (MT, C/N = 30), and high C/N treatment (HT, C/N = 50). The results showed that N2O emission was mainly concentrated in the cooling and maturation periods, and the cumulative N2O emissions decreased as the C/N ratio increased. Specifically, the cumulative N2O emission was 57,401 mg in LT, significantly higher than 2155 mg in MT and 1353 mg in HT. Lowering the C/N ratio led to increasing TN, NH4+-N, and NO3--N contents throughout the composting process. All detected nitrification-related gene abundances in LT continued to increase during composting, significantly surpassing those in MT during the cooling period. By contrast, in HT, there was a slight increase in the abundance of detected nitrification-related genes but a significant decrease in the abundance of narG, napA, and norB genes in the thermophilic and cooling periods. The structural equation model revealed that hao and nosZ genes were vital in N2O emissions. In conclusion, increasing the C/N ratio effectively contributed to N2O reduction during vegetable waste composting.

Straw Addition Enhances Crop Yield, Soil Aggregation, and Soil Microorganisms in a 14-Year Wheat-Rice Rotation System in Central China.

Straw return utilizes waste resources to reduce the use of chemical fertilizers worldwide. However, information is still lacking on the relative impact of straw return on soil fertility, the nutrient composition of different soil aggregates, and soil microbial communities. Therefore, this study aimed to understand the effects of different management practices on the crop yield, soil fertility, and soil community composition in a 14-year wheat-rice rotation system. The treatments included a control (without fertilizer and straw addition), chemical fertilization (NPK), straw return without fertilizer (S), and straw addition with chemical fertilizer (NPKS). The results showed that NPKS improved the wheat and rice yield by 185.12% and 88.02%, respectively, compared to the CK treatment. Additionally, compared to the CK treatment, the N, P, and K contents of the wheat stem were increased by 39.02%, 125%, and 20.23% under the NPKS treatment. Compared to the CK treatment, SOM, TN, TP, AN, AP, AK, CEC, AFe, AMn, ACu, and AZn were increased by 49.12%, 32.62%, 35.06%, 22.89%, 129.36%, 48.34%, 13.40%, 133.95%, 58.98%, 18.26% and 33.33% under the NPKS treatment, respectively. Moreover, straw addition promoted the creation and stabilization of macro-aggregates in crop soils. The relative abundance of macro-aggregates (0.25-2 mm) increased from 37.49% to 52.97%. Straw addition was associated with a higher proportion of aromatic and carbonyl carbon groups in the soil, which, in turn, promoted the formation of macro-aggregates. Redundancy analysis showed that straw return significantly increased the microbial community diversity. These findings demonstrate that straw addition together with chemical fertilizer could increase the crop yield by improving soil fertility, soil aggregate stability, and the diversity of fungi.

14 year applications of chemical fertilizers and crop straw effects on soil labile organic carbon fractions, enzyme activities and microbial community in rice-wheat rotation of middle China.

Traditional fertilization management can damage soil structure and lead to severe soil erosion. The practice of crop straw returning to the field reduces the negative impact of straw burning and improves soil quality. We investigated the effects of these agricultural practices on soil organic carbon components, enzyme activities, and soil microorganisms over 14 years of field experiments. Specifically, we studied four management strategies: no fertilizer or crop straw returning (CK), traditional chemical fertilization (NPK), crop straw returning (S), and crop straw returning with chemical fertilizer (NPKS). We found NPKS treatments significantly (P < 0.05) increased the dissolved organic carbon (DOC), microbial biomass carbon (MBC), particulate organic carbon (POC) and readily oxidized organic carbon (ROC) concentrations by 79.32 %, 82.16 %, 92.46 %, and 104.32 % relative to CK. Furthermore, under NPKS, the activities of soil enzymes related C, N, and P (α-glucosidase (αG), ß-glucosidase (ßG), cellulase (CBH), xylanase (ßX), acetyl ß-glucosaminidase (NAG), leucine aminopeptidase (LAP), and acid phosphate (AP)) were increased by 54.66 %, 113.26 %, 76.73 %, 52.41 %, 45.74 %, 56.69 %, and 68.92 % relative to CK, respectively. Redundancy analysis and structural equation modelling showed that straw returning had positive effects on soil microbial community diversity and richness, and also improved microbial activity which is favorable in the degradation of soil carbon. Furthermore, we found that soil fungi were more sensitive than bacteria to changes in soil carbon composition and enzyme activities following straw returning. These results suggest that straw returning combined with chemical fertilizer can be an effective strategy to improve soil labile organic carbon components, enzyme activities, and ecological function of microorganisms.

Sulfate-accelerated photochemical oxidation of arsenopyrite in acidic systems under oxic conditions: Formation and function of schwertmannite.

The weathering of arsenopyrite is closely related to the generation of acid mine drainage (AMD) and arsenic (As) pollution. Solar radiation can accelerate arsenopyrite oxidation, but little is known about the further effect of SO42- on the photochemical process. Here, the photooxidation of arsenopyrite was investigated in the presence of SO42- in simulated AMD environments, and the effects of SO42- concentration, pH and dissolved oxygen on arsenopyrite oxidation were studied as well. SO42- could accelerate the photooxidation of arsenopyrite and As(III) through complexation between nascent schwertmannite and As(III). Fe(II) released from arsenopyrite was oxidized to form schwertmannite in the presence of SO42-, and the photooxidation of arsenopyrite occurred through the ligand-to-metal charge-transfer process in schwertmannite-As(III) complex along with the formation of reactive oxygen species in the presence of O2. The photooxidation rate of arsenopyrite first rose and then fell with increasing SO42- concentration. In the pH range of 2.0-4.0, the photooxidation rate of arsenopyrite progressively increased in the presence of SO42-. This study reveals how SO42- promotes the photooxidation of arsenopyrite and As release in the AMD environment, and improves the understanding of the transformation and migration of As in mining areas.

Cadmium accumulation, availability, and rice uptake in soils receiving long-term applications of chemical fertilizers and crop straw return.

Fertilization and straw return have been widely adopted to maintain soil fertility and increase crop yields, but their long-term impacts on the accumulation and availability of cadmium (Cd) in paddy soils are still unconfirmed. Therefore, this study was undertaken in central China to investigate the accumulation, availability, and subsequent uptake of Cd by rice (Oryza sativa L.) in two adjacent field trials (P1 and P2, lasting for 10 and 12 years, respectively) under long-term straw return or in combination with chemical fertilizers. Obvious Cd accumulation, probably due to the notable Cd input from irrigation and traffic exhaust in the bulk soil (0-20 cm) of P1, was observed. The bulk soil of P2 received homogeneous straw return and chemical fertilizers, as did that of P1; however, the P2 soil almost showed Cd balance. Long-term straw return increased the portion of soil DTPA-extractable Cd to the total pool for both sites, but only P1 showed significant differences when compared to the controls. However, the highest Cd concentrations and the maximum bioconcentration factors in rice straw and grain were obtained using solo application of chemical fertilizers at both sites. Continuous additional applications of crop straw, in contrast, resulted in slightly decreased Cd uptake in rice straw, but not in grain. These findings demonstrate that neither long-term straw return nor fertilization leads directly to notable Cd accumulation, but that the promotion effects of long-term chemical fertilizer applications on Cd uptake in rice need more attention.