Amorphous Fe substrate enhances nitrogen and phosphorus removal in sulfur autotrophic process

The autotrophic denitrification of coupled sulfur and natural iron ore can remove nitrogen and phosphorus from wastewater with low C/N ratios. However, the low solubility of crystalline Fe limits its bioavailability and P absorption capacity. This study investigated the effects of amorphous Fe in drinking water treatment residue (DWTR) and crystalline Fe in red mud (RM) on nitrogen and phosphorus removal during sulfur autotrophic processes. Two types of S-Fe cross-linked filler particles with three-dimensional mesh structures were obtained by combining sulfur with the DWTR/RM using the hydrogel encapsulation method. Two fixed-bed reactors, sulfur-DWTR autotrophic denitrification (SDAD) and sulfur-RM autotrophic denitrification (SRAD), were constructed and stably operated for 236 d Under a 5-8-h hydraulic retention time, the average NO3--N, TN, and phosphate removal rates of SDAD and SRAD were 99.04 %, 96.29 %, 94.03 % (SDAD) and 97.33 %, 69.97 %, 82.26 % (SRAD), respectively. It is important to note that fermentative iron-reducing bacteria, specifically Clostridium_sensu_stricto_1, were present in SDAD at an abundance of 58.17 %, but were absent from SRAD. The presence of these bacteria facilitated the reduction of Fe (III) to Fe (II), which led to the complete denitrification of the S-Fe (II) co-electron donor to produce Fe (III), completing the iron cycle in the system. This study proposes an enhancement method for sulfur autotrophic denitrification using an amorphous Fe substrate, providing a new option for the efficient treatment of low-C/N wastewater.

Accumulation of organic compounds in paddy soils after biochar application is controlled by iron hydroxides

Soil acidity is one of the vital factors that influence organic matter transformation and accumulation. Long-term studies on the mechanisms of biochar's effects on soil organic matter (SOM) accumulation dependent on pH values are lacking. A four-year column experiment was conducted without and with biochar application (11.3 Mg ha^-1 crop^-1) in acid (pH = 5.24) and alkaline (pH = 8.22) soils under paddy rice/wheat annual rotation. To explore organic matter accumulation mechanisms, SOM pools were extracted (physical-chemical fractionation) and their chemical structures were analyzed using advanced solid-state ¹³C nuclear magnetic resonance (¹³C NMR) techniques. Biochar increased the proportion of aromatic carbon (C) in all SOM pools, which led to an increased C content in two soils. The elevated pH after biochar application (∆pH = 1.03) increased Fe (III) oxidation and precipitation, and therefore, stimulated amorphous Fe content in 53-μm pool in the acid soil. This change increased the interaction between organic compounds and Fe (hydr)oxide, which impeded bacteria access to substrates, and in turn, promoted SOM accumulation in the acid soil. Conversely, low Fe (hydr)oxide availability resulted in the decomposition of the labile substrates (di-O-alkyl C, NCH, and OCH) in mobile humic acids via microbial respiration, thereby lowering the effect of SOM sequestration in the alkaline soil. Our study revealed that organic matter accumulation after biochar amendment is not solely dependent on the chemical recalcitrance of biochar, but also is controlled by the transformation of Fe (hydr)oxide in SOM pools.

Long-term impact of manuring and fertilization on enrichment, stability and quality of organic carbon in Inceptisol under two potato-based cropping systems

Soil organic matter (SOM) stability is a prerequisite for long-term C sequestration. The long-term effect of manuring and fertilization on stability of SOM in rice-potato-wheat (R-P-W) and maize-potato-onion (M-P-O) cropping systems was studied in an Inceptisol of semi-arid subtropical India. Soil samples were collected (0-15 and 15-30cm soil depths) from the following treatments: control, 100% NPK-Fertilizer, 100% N-Vermicompost (VC), 50% NPK-Fertilizer+50%N-Vermicompost (VC), 100%NPK-Fertilizer+crop residue (CR), 100% N-VC+CR. The stability of SOM was studied by correlating the ammonium oxalate extractable Fe, Al and Si with stable soil organic C (SOC). Application of 100% N-VC and 100% N-VC+CR increased the SOC stock by 66% and 32%, respectively over 100% NPK in R-P-W cropping system, while in M-P-O system, the above treatments increased the SOC stock by 28% and 12%. As compared to 100%NPK, the integrated use of 50%NPK+50%N-VC increased the SOC stock by 58% and 35% in R-P-W and M-P-O system, respectively. The extractable Fe, Al and Si significantly correlated with stable OC and the multiple regression model developed between these parameters could well predict the stable OC in soil. Complete or partial substitution of chemical fertilizer N with VC could be a promising nutrient management strategy for enhancing the SOC stock in Inceptisol under potato-based cropping systems of semi-arid sub-tropical India. However, the integrated use of 50%NPK+50%N-VC could be promising due to lowest yield reductions and moderate SOC sequestration potential. The enrichment (47%), as well as quality (81%) of SOC was higher in R-P-W system than in M-P-O system. The organic treatments like 100%NPK+CR and 100%N-VC+CR showed higher stability of SOC in R-P-W (31% and 26%) than in M-P-O cropping system. The stability of SOC could well be predicted by extractable amorphous and poorly crystalline Fe, Al and Si and SOC in both the cropping systems.