Quantitatively ranking the influencing factors of ammonia volatilization from paddy soils by grey relational entropy

The increasing risk of ammonia volatilization in farmland from the recovery product of magnesium-modified biochar after nitrogen and phosphorus adsorption

Many studies have shown that magnesium modified biochar (MgBC) can recover nutrients from wastewater and be applied as an excellent slow-release fertilizer in farmland. However, the recovery products (NP-loaden MgBC), represented by struvite or magnesium phosphate, have a high degree of self-alkalinity, which may significantly increase the ammonia (NH3) volatilization in farmland. In this study, the optimal adsorption parameters, self-alkaline regulation process and co-adsorption mechanism of MgBC for ammonium ion (NH4+) and phosphate ion (PO43-) were studied through batch experiments. A field experiment was conducted with three treatments, including local conventional fertilization (N1B0) and the application of 5 t·ha-1 or 10 t·ha-1 NP-loaden MgBC in combination with local conventional fertilization (N1B1 and N1B2, respectively), to determine the impact of NP-loaden MgBC on NH3 volatilization, surface water c(NH4+-N) and pH. The results indicated that the maximum NH4+ and PO43- synergistic recovery of MgBC under the optimal adsorption parameters (dosage of 0.6 g·L-1; initial NH4+ and PO43- concentrations of 120 and 60 mg·L-1 and pH of 8) were 59.96 and 98.60 mg·g-1, respectively. Self-regulating alkaline MgBC maintained pH suitable for struvite, and precipitation mechanism controlled the adsorption. The presence of NP-loaden MgBC raised the pH levels in surface water during the basal fertilization stage and increased c(NH4+-N) in surface water during the topdressing stages. This, in turn, led to a significant increase in NH3 volatilization loss during the entire rice-growing period, with N1B1 and N1B2 experiencing a 23.87 % and 48.91 % increase respectively, compared to N1B0. Therefore, it is imperative to take into account the adverse impact of NP-laden MgBC on NH3 loss in paddy fields when considering its application in future field studies.

Improving the efficacy of different life-form macrophytes in phytoremediation of artificial eutrophic water by combined planting

Phytoremediation of the eutrophic water bodies by using various macrophytes has long been considered effective and economical. However, the understanding of combining macrophytes to maximize efficacy in the restoration is still limited. In this study, three different life-form macrophytes were employed to explore the optimal plant combination of eutrophic water purification, including Pontederia cordata L. (E: emergent), Pistia stratiotes L. (F: floating), and Hydrilla verticillata (L. f.) Royle (S: submerged). The effects on water quality, removal of the excess nutrients (TN, NH₃-N, NO₃-N, and TP) in the water, along with the growth response and the nutrient accumulation of the macrophytes were investigated both individually and in combination. The phytoremediation of every single macrophyte was significantly improved by combined planting and increasing the diversity of the combination led to better enhancements. In general, the treatment with macrophytes in three life forms (EFS) not only resulted in the highest removal rates of the TN, NH₃-N, NO₃-N, and TP (40.89, 33.50, 46.81, and 43.55%, respectively) but also decreased the turbidity and increased the dissolved oxygen more effectively and efficiently. Furthermore, EFS mitigated the environmental stress of plants and promoted the accumulation of TN and TP in them, especially the emergent macrophyte P. cordata. The combinations with macrophyte in two life forms (EF, ES, and FS) also exhibited unique strengths: the removal efficacy of TN (39.25%) and TP (46.16%) in FS, and NO₃-N in EF (48.54%) and ES (49.90%) were also at the forefront; the biomass and nutrient content of the submerged macrophyte H. verticillata in ES were the highest. Moreover, a strong correlation between the eutrophic factors and the plant physiological indexes was observed. These findings highlighted the role of combined planting in phytoremediation and provided a valuable reference for the development of ecological restoration for eutrophic ecosystems.

Investigation of Rice Yields and Critical N Losses from Paddy Soil under Different N Fertilization Rates with Iron Application

The application of iron powder stimulated the growth of iron-reducing bacteria as a respiratory substrate and enhanced their nitrogen (N)-fixing activity in flooded paddy soils. High N fertilization (urea) in the flooded paddy soils has caused adverse environmental impacts such as ammonia (NH₃) volatilization, nitrous oxide (N₂O) emissions, and nitrate (NO₃⁻) leaching. This study aims to investigate the effects of N fertilization rates in combination with an iron amendment on rice yields and N losses from flooded paddy fields. We performed a 2-year field plot experiment with traditional rice–wheat rotation in China’s Yangtze River Delta. The investigation consisted of seven treatments, including 100%, 80%, 60%, and 0% of the conventional N (urea and commercial organic manure) fertilization rate, and 80%, 60%, and 0% of the conventional N with the iron powder (≥99% purity) amendment. The rice yields decreased with a reduction in the conventional N fertilization rate, whereas they were comparable after the iron application under the 80% and 60% conventional N rate. The critical N losses, including NH₃ volatilization, N₂O emissions, and NO₃⁻ and NH₄⁺ leaching, generally decreased with a reduction in the conventional N fertilization rate. These N losses were significantly greater after the iron amendment compared with the non-amended treatments under the 80% and 60% conventional N fertilization rate in the first rice-growing season. However, it was comparable between the iron-amended and the non-amended treatments in the second season. Furthermore, NO₃⁻ leaching was the most significant N loss throughout the two rice seasons, followed by NH₃ volatilization. The iron amendment significantly increased soil Fe²⁺ content compared with the non-amended treatments irrespective of N fertilization, suggesting the reduction of amended iron by iron-reducing bacteria and their simultaneous N fixation. A combination of the iron application with 60–80% of the conventional N fertilization rate could maintain rice yields similar to the conventional N fertilization rate while reducing the critical N losses in the flooded paddy field tested in this study. Our study leads to the establishment of novel and practical rice cultivation, which is a step towards the development of green agriculture.