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Organic fertilization, a critical agronomic practice for improving nutrient availability and crop growth, may increase the risk of phosphorus (P) loss. To investigate the effect of organic fertilization on P availability, crop growth and P loss, a paddy field experiment was conducted in the saline-alkaline paddy soil. We designed four treatments: CK (no fertilizer), NPK (mineral fertilizer), and NPKC1 and NPKC2 (mineral fertilizer with low and high levels of organic fertilizer, respectively); the amounts of N, P2O5, and K2O were equal in the NPK, NPKC1, and NPKC2 treatments. Compared with the NPK treatment, organic fertilization enhanced the content of total, organic, and available P in the topsoil. Moreover, organic fertilization significantly improved both the growth and P removal of crops. The content of oxalate Fe, Al, and dissolved organic carbon, the main carriers of P loss, in the soil was the highest in the NPKC2 treatment. The soil P saturation ratio (PSR) in the NPKC2 treatment was 0.25, exceeding the threshold PSR for P loss of 0.16. Therefore, organic fertilizer application could improve P availability and crop growth; however, high-level organic fertilization increases the risk of P loss in coastal saline-alkaline paddy soils. [ABSTRACT FROM AUTHOR]

Aims: Organic fertilizer addition plays a significant role for soil organic carbon (SOC) storage. The interaction between bacteria and clay minerals for the mechanism of SOC storage with organic fertilization was investigated in saline-alkaline paddy soil. Methods: A 9-year field experiment was arranged, which included: (a) CK, without fertilizer addition, (b) NPK, mineral N, P and K fertilizer addition, (c) NPKC1, and (d) NPKC2, mineral N, P and K fertilizer plus 450 and 900 kg C ha−1, respectively. Results: In comparison to only mineral fertilization, the content of humic acid-C and humin-C was significantly increased by 58.8%–70.6% and 46.9–53.1% with organic fertilizer addition, respectively. The 2:1 type clay minerals (vermiculite and illite) were also increased, which were positively correlated with SOC storage due to the formation of clay minerals-humus complexes. Meanwhile, the abundance of Geobacter and Anaeromyxobacter was increased with organic fertilization, which accelerated the transformation of clay minerals by promoting Fe reduction, and then increased SOC storage. While, compared with NPKC1 treatment, more Gammaproteobacteria and Anaerolineae, involved in the decomposition of SOC, were found in NPKC2 treatment. Relative to NPK treatment, SOC storage of NPKC1 and NPKC2 treatments increased by 23.3% and 29.8%, respectively, and there was no significant difference between the two treatments. Conclusions: Therefore, appropriate addition of organic fertilizer is a better fertilization practice to promote stability and storage of SOC, which provided an important contribution to elucidate the role of mineral-microbial interactions for storage of SOC in saline-alkaline paddy soil. [ABSTRACT FROM AUTHOR]

In order to indicate the effect of volatile fatty acids (VFAs) on the characteristics of feammox and dissimilatory iron reducing bacteria (DIRB) in paddy soils, different VFAs were selected with paddy soils for anaerobic cultivation. Five treatments were set up, respectively, only adding N and both adding N and C (formate + NH4+ (Fo-N), acetate + NH4+ (Ac-N), propionate + NH4+ (Pr-N), and butyrate + NH4+ (Bu-N)) treatments. The concentration of Fe(II), Fe(III), NH4+, and VFAs was assessed within 45 d, and the bacterial community was determined after cultivation. The oxidation rates of NH4+ were the highest in N treatment, while it was the lowest in Fo-N treatment. Under the four C treatments, the consumption of NH4+ and Fe(III) was the fastest in Pr-N treatment, which was consumed by 31.2% and 76.3%, respectively. Different VFAs selected for distinct DIRB. Compared with N treatment, Ac-N and Bu-N treatment increased the relative abundance of DIRB, such as Geobacter and Clostridia, which increased the consumption of VFAs during incubation. Overall, VFAs, especially formate, could promote Fe(III) reduction and compete with the feammox process for the electron acceptors to decrease the feammox reaction, and prohibited soil NH4+ loss. Therefore, VFAs, which was released from organic fertilizer, could reduce NH4+ loss in feammox process of saline-alkaline paddy soils. [ABSTRACT FROM AUTHOR]

Purpose: NO3−-N reduction is the main way of soil nitrogen (N) loss, especially in saline-alkaline soil, and NO3−-N-reducing Fe(II) oxidation (NRFO) is an important but less known process for NO3−-N reduction. An anoxic cultivation experiment was carried out on coastal saline-alkaline paddy soil to determine the effect of the external environment on NRFO process and microbial community composition. Methods: A laboratory cultivation experiment was studied to elucidate the effect of pH and acetate on NRFO in saline-alkaline soil. Nine different treatments were designed with two factors: (1) three pH levels: 7.5, 8.0, and 8.5; (2) three acetate concentrations: 15 (C1), 20 (C2), and 25 mM (C3). Concentration of Fe(II), Fe(III), NO3−-N, NO2−-N, and NH4+ and dynamic changes in microorganisms were assessed within 36 days. Results: The reduction of NO3−-N showed a positive correlationship with Fe(II) oxidation and Fe(III) formation. In addition to Fe(II), acetate also participated in NO3−-N reduction as an electron donor. The reduction rate of NO3−-N was the highest in pH 8.5 + C3 treatment, in which the contribution of Fe(II) was 46.10%. The loss of NO3−-N was the least in pH 7.5 + C2 treatment, and the contribution of Fe(II) was 37.03%. Illumina high-throughput sequencing showed that the phyla of Proteobacteria and Firmicutes were enriched in all treatments. The classes of Alphaproteobacteria, Bacilli, and Gammaproteobacteria were found to be the dominant in all treatments. Pseudomonas, a NO3−-N reduction bacteria, was dominant in pH 8.5 + C3 treatment, in which the relative abundance was 24%. Meanwhile, a Fe(II) oxidizing bacteria, Acidovorax, was dominant in the pH 8.5 + C1 treatment, and the relative abundance was 15%. Conclusion: In coastal saline-alkaline paddy field, high C input and high pH (pH 8.5 + C3 treatment) could increase NO3−-N reduction, which might be the important reasons for N loss. Therefore, proper application of organic fertilizer and N fertilizer should be paid attention to in actual production in the Yellow River Delta. [ABSTRACT FROM AUTHOR]

Purpose: The poor soil structure and low stability of organic carbon limit the sustainable development of agriculture. The effect of organic fertilizer application on soil aggregates, soil organic carbon (SOC) functional groups, Fe oxides, SOC stability, and storage was assessed in saline-alkaline paddy soil. Methods: A 7-year field experiment was designed with organic fertilization in saline-alkaline soil of Yellow River Delta including (i) CK (no fertilizer), (ii) NPK (255, 28, and 190 kg ha−1 year−1 N, P, and K mineral fertilizers, respectively), (iii) NPKC1 (NPK + 450 kg C ha−1 year−1), and (iv) NPKC2 (NPK + 900 kg C ha−1 year−1). Results: Compared with NPK treatment, the mean weight diameter (MWD) of soil aggregates in NPKC1 and NPKC2 treatments was increased by 27.8% and 41.1%, respectively. Soil amorphous Fe oxide (Feo) content was significantly increased with organic fertilizer addition, especially in small macro-aggregates. Meanwhile, aromatic-C was primarily distributed in small macro-aggregates, which could be retained in aggregate interfaces by forming aromatic-Fe complexes with soil Feo. Furthermore, the specific C mineralization rate (SCMR) in NPKC1 treatment was lower than that in other treatments, in which the lowest SCMR was found in small macro-aggregate. Compared with NPK treatment, SOC storage in NPKC1 and NPKC2 treatments was increased by 14.4% and 20.5%, respectively, which was due to the improvement of soil structure, organo-Fe complex formation, and reduction of soil C mineralization. Conclusions: The combined organic and mineral fertilizer application was an optimized management practice to improve the C stability and sequestration in coastal saline alkaline paddy soil. [ABSTRACT FROM AUTHOR]

Purpose: Phosphorus (P), a potential limiting nutrient in soil, is an indispensable nutrient for crop growth and development. Effects of organic fertilizer on P fractions and its adsorption–desorption in soil aggregates were studied through a 6-year experiment in coastal saline-alkaline paddy field, which provided a theoretical basis in improving the retention capacity of P in soil. Methods: Four treatments were designed: (1) CK (no fertilization), NPK (only NPK chemical fertilizer), NPKC1 (NPK plus 450 kg C hm−2) and NPKC2 (NPK plus 900 kg C hm−2). Soil aggregates distribution, P fractions and adsorption–desorption of P were assessed. Results: The application of organic fertilizer increased the concentrations of labile P (L-P), iron- and aluminum-bound P (Fe/Al-P), humic P (Hu-P), and residual P (Re-P) in soil aggregates, while the concentrations of calcium- and magnesium-bound P (Ca/Mg-P) and moderately organic P (Ml-Po) were decreased. L-P and Re-P were mainly enriched in large macroaggregates (LM, > 2.0 mm), Fe/Al-P, Hu-P, and Ml-Po were mainly enriched in microaggregates (MI, 0.053–0.25 mm). Small macroaggregates (SM, 0.25–2.0 mm) carried more Ca/Mg-P. Soil organic carbon (SOC) and exchangeable calcium (Ca) were the main factors affecting the distribution of P fractions in all soil aggregates, while in LM, the key factor was iron oxides (Feox), and in SM and silt + clay particles (SC, < 0.053 mm), exchangeable magnesium (Mg) was the main factor. The P retention capacity in soil aggregates was highest in NPKC1 treatment, while the leaching risk of P was higher in NKPC2 treatment. Redundancy analysis (RDA) showed that SOC, Feox, aluminum oxides (Alox), exchangeable Ca, and exchangeable Mg were positively correlated with the maximum adsorption capacity of P (Sm), while negatively correlated with the percentage of P (Prr). Multivariate linear regression analysis showed that Feox, Alox, and exchangeable Mg were the three important soil properties related to P adsorption–desorption in > 0.25-mm aggregates, while in < 0.25-mm aggregates, the main soil factors were SOC, exchangeable Ca, and exchangeable Mg. Conclusion: In the coastal saline-alkaline paddy field, moderate amount of organic fertilizer application (NPKC1) showed a better soil aggregate distribution and phosphorus (P) retention capacity, which provided a theoretical basis in improving soil fertility and reducing environmental pollution risk in the Yellow River Delta. [ABSTRACT FROM AUTHOR]

Methods: Soil was sampled (0–20 cm) from a 5-year Yellow River Delta paddy field experiment: no fertilizers (Control), mineral fertilizers (NPK), and NPK plus organic fertilizer (soybean and bean litter) applied at rates of 450 and 900 kg C ha− 1 (NPKC1 and NPKC2). The aggregate size, labile carbon fractions and root growth were investigated in combination with microbial community structure by high-throughput sequencing. Results: NPKC1 and NPKC2 fertilization decreased Na+ content, but increased available nitrogen, phosphorus and Ca2+ contents. Macroaggregate formation was increased under NPKC1 (by 28 %) and NPKC2 (by 62 %) fertilization through increase of labile carbon fractions (particularly microbial biomass carbon), as well as better root growth. NPKC1 and NPKC2 fertilization increased microbial diversity and enriched the microbial community towards taxa responsible for aggregate stability (e.g., Actinobacteria and Ascomycota). RDA revealed that dissolved organic carbon content was the main factor affecting microbial community composition. Conclusions: Combined mineral and organic fertilizers decreased Na+ content, increased macroaggregates due to broader microbial diversity, increased contents of labile organic carbon and Ca2+. Consequently, combined organic and mineral fertilization were the best practice to increase microbial biomass and labile carbon fractions that strongly contribute to aggregate formation in saline-alkaline soil. Aims: Soil structure is a key factor in sustainable agriculture, especially for saline-alkaline soils, because Na+ destroys clay minearls and colloids, and thus, prevents the aggregate formation. We assessed the effects of organic amendments in combination with mineral fertilizers on chemical properties, microbial community composition and aggregate formation in a typical saline-alkaline soil. [ABSTRACT FROM AUTHOR]

Organic fertilizer can improve phosphorus (P) bioavailability, while the mechanism of microbial-driven clay mineral transformation on P retention and utilization with organic fertilization is still unclear. An 8-year organic fertilization experiment was carried out in saline alkaline paddy soil with five treatments: (1) CK, no fertilization; (2) NK, mineral N and K fertilization; (3) NPK, NK with mineral P fertilization; (4) NPKC1, NPK with 450 kg C ha−1 year−1; (5) NPKC2, NPK with 900 kg C ha−1 year−1. The results showed that, compared with NPK treatment, the percentage of illite with more P adsorption sites was increased by 26.8% and 20.7% in NPKC1 and NPKC2 treatments, respectively. Meanwhile, transmission electron microscopy-energy dispersive X-ray spectroscopy analysis further confirmed that there was more P adsorption on silicate clay minerals. Compared with NK treatment, the content of total P and available P was significantly increased with organic fertilization. Besides, the diversity of bacterial community was also increased by organic fertilizer addition, redundancy analysis showed that soil organic carbon was the main factor in the diversity of bacterial community. The relative abundance of Firmicutes and Gemmatimonadetes was promoted, which drove the transformation of clay minerals and improved P availability, respectively. Compared with NPK treatment, P use efficiency in NPKC1 and NPKC2 treatments was increased by 47.2% and 58.2%, respectively, while no significant difference was found between the two treatments. Therefore, appropriate organic fertilization was a better fertilization practice to improve P retention and utilization through bacteria-driven clay mineral transformation in saline-alkaline paddy soil. [Display omitted] • Organic fertilizer addition promotes clay mineral transformation from kaolinite to illite. • P retention was increased by transformation of clay mineral with organic fertilizer addition. • Soil characteristics and microorganism drive transformation of clay mineral. • The rice yield and P utilization efficiency were increased with organic fertilization. [ABSTRACT FROM AUTHOR]

• Biochar significantly increased the sequestration of SOC in saline soil. • Macroaggregate in organic fertilizer and straw were higher than that in biochar. • CMI values was greater in straw compared with organic fertilizer and biochar. • Organic amendments affected exchangeable cation distribution in aggregates. Organic amendments are a recyclable resource and can improve soil quality and carbon (C) sequestration, however, the difference in their effects on soil aggregate formation and C sequestration in saline-alkaline soils are less studied. We examined the effects of organic amendments to saline rice paddy fields with 4 treatments: (i) basal paddy soil (CK) lacking amendments (ii) organic fertilizer (OF) treatment at 910 kg C ha−1 from soybean litter and beans (iii) rice straw (RS) treatment at 3600 kg C ha−1 and (iv) rice straw-derived biochar (RS-biochar) treatment at 3600 kg C ha−1. All organic amendments significantly increased the soil organic C (SOC) stock in the 0–30 cm soil layer compared to CK. The SOC stock in the biochar treatment increased by 6.2 and 40.4% over the OF and RS amended treatments, respectively. This was most likely due to the greater levels of aromatic C in RS-biochar at 1.22 and 1.24 times greater than for the other organic amendments, respectively. Soil aggregates of 2–8 mm at 0–15 cm in the OF and RS amended soils were 38.3 and 58.2%, respectively, higher than that from the RS-biochar treatment. The mean weight diameter (MWD) in the OF and RS treatments were 35.4% and 45.8% higher than for the RS-biochar, respectively. MWD was significantly negatively correlated with soil exchangeable Na (p < 0.05). This indicated OF and RS were better than RS-biochar in improving soil aggregate stability. In addition, the C management index values for RS were 21.6 and 45.8% greater than for OF and RS-biochar in the 0–15 cm soil layer, respectively. These data indicated that saline soil aggregate formation and C sequestration improved after organic amendments application, which caused the exchangeable Na to decrease and affected its distribution in aggregates, and changed the composition of C functional groups. [ABSTRACT FROM AUTHOR]