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A 100-day incubation experiment was conducted to (i) trace the fate of rice residue-derived 13C in the amino sugar (AS) pool in 0-1-cm (oxic) and 1-5-cm (anoxic) layers of paddy soil and (ii) evaluate the effects of inorganic N ((NH4)2SO4) fertilization on the formation of AS at early and late incubation times (5 and 100 days, respectively). The accumulation of rice residue-derived AS occurred at 5 and 100 days in both soil layers as a result of AS stabilization. Inorganic N addition increased the contents of rice residue-derived muramic acid, glucosamine, and galactosamine in the 0-1-cm soil layer for both incubation times by average on 14.7-20.8%, 23.7-31.8%, and 11.6-23.3%, respectively. In contrast, no effects of N fertilization on AS content in the 1-5-cm soil layer were found. The amount of rice residue-derived AS was higher in the 1-5-cm than in the 0-1-cm soil layer at early incubation time, probably due to the higher contents of ammonium here compared to the upmost oxic layer where nitrate was the dominated N form. Thus, the preferential uptake of ammonium but not nitrate by microorganisms led to the higher formation of rice residue-derived AS in the anoxic soil layer. The ratio of fungal to bacterial residues (fungal glucosamine/muramic acid) ranged between 1.0 and 1.7 for rice residue-derived AS and was 12.5-14.6 for total AS, indicating that fungi and bacteria have similar contributions to the decomposition of fresh rice residue whereas native soil organic matter (SOM) is a fungi-predominated process. This study emphasized that coupling of C and N cycles in paddy soils is different in oxic and anoxic layers, resulting in variation of plant residue decomposition and formation of SOM. [ABSTRACT FROM AUTHOR]

Microbial processes involving in nitrogen (N) assimilation mediate the continuous supply of mineral N for crop growth and the reactive N loss in intensively managed agriculture soils. This study aimed to reveal the microbial assimilation role in N accumulation in paddy soils under partial substitution of mineral fertilizer with organic materials (straw or manure), by quantifying the microbial N derived from living biomass and necromass. Rice rhizosphere and bulk soils were collected at a 31-year field trial including five fertilization treatments: no fertilizer (Control), mineral fertilizer alone (NPK), 20% mineral fertilizer substituted by rice straw (NPKS), and 30% and 60% mineral fertilizer substituted by manure (NPKLM and NPKHM, respectively). Incorporation of organic materials increased the total soil N by 22–76% compared to NPK, with a greater increase under manure- than rice straw-treated soils. Both organic inputs enhanced the amounts of living biomass and necromass N by 1.1–1.5 times via stimulating the activity of N cycling genes (chiA , AOA, narG , nirK , nirS , nosZ , nifH) in bulk soils. Although the pools of living biomass and necromass N in soils with manure amendments were 12–34% and 13–22% higher, than those with rice straw incorporation, their corresponding contributions to total N were 7.2–7.4% and 6.2–15% lower, respectively, suggesting a weaker relative microbial contribution in soil N accrual with manure amendments. Rice rhizosphere processes strengthened the role of microbial participation in soil N accumulation, particularly with organic inputs. The relative contribution of fungal necromass N to total N as well as the ratios of fungi to bacteria in both living biomass and necromass were lower under manure amendments than rice straw incorporation. As a whole, as compared with the incorporation of rice straw, the greater N accumulation in paddy soil with manure amendments is associated with the relatively weaker accumulation of fungal necromass and stronger retention of organic debris without microbial processing. • Organic inputs increase microbial derived-N relative to mineral fertilization. • Manure induces lower relative contribution of microbial necromass to soil N than rice straw. • Rhizosphere process strengthens the microbial participation in soil N accumulation. [ABSTRACT FROM AUTHOR]

This study aimed to trace the incorporation of 13C-labeled hydrophilic and hydrophobic compounds extracted from maize straw into bacterial and fungal phospholipid fatty acids (PLFAs) in upland and paddy soils. In both soils, the amounts of bacterial 13C-PLFAs recovered from hydrophilic C were 1.2–2.8 times higher than those from hydrophobic C. By contrast, the amounts of fungal 13C-PLFAs recovered from hydrophobic C were 3.8–12.8 times greater than those from hydrophilic C. These results indicate that bacteria have a preference for the uptake of hydrophilic compounds, whereas fungi prefer hydrophobic compounds in both soils. The fungal preference for hydrophobic compounds is stronger than bacterial preference for hydrophilic compounds. This is partly owing to the efficient incorporation of hydrophobic compounds into actinomycete PLFA of 10Me18:0 in upland soil and bacterial PLFA of 18:1ω7c in paddy soil. Consequently, the specific uptake of hydrophilic and hydrophobic compounds from plant residues by bacteria and fungi will lead to divergent C pathways in soil. • Bacteria prefer to take up hydrophilic C. • Fungi prefer to take up hydrophobic C. • Fungal preference for hydrophobic C is stronger than bacterial preference for hydrophilic C. [ABSTRACT FROM AUTHOR]

• We describe microbial biomass role in C sequestration in fertilized paddy soil. • Organic inputs increase microbial growth and necromass accumulation. • Organic inputs do not alter microbial CUE in bulk soil, but reduce CUE in rhizosphere soil. • Microbial CUE is lower in rhizosphere soil than in bulk soil. Microbial anabolism relative to catabolism, reflected by the C use efficiency (CUE), determines the fate of C transformation in soil. Understanding how the microbial CUE and microbial necromass respond to fertilization is crucial for the evaluation of the C sequestration potential in intensively managed paddy soils. We examined the microbial CUE, microbial biomass turnover, and necromass accumulation in rice rhizosphere and bulk soils subjected to long-term (31 years) fertilizations: no fertilizers (control), mineral fertilizers alone (NPK), mineral fertilizers plus rice straw incorporation (NPK-Straw), and mineral fertilizers combined with a low or a high amount of organic manure (NPK-lowM or NPK-highM). The microbial CUE was determined by 18O incorporation into DNA. Microbial necromass accumulation was quantified by the biomarker analysis of amino sugars. Rice straw and manure incorporation reduced the microbial CUE in the rhizosphere soil, whereas the CUE remained constant in the bulk soil. CUE was lower in the rhizosphere soil than in the bulk soil due to nutrients uptake and root exudate release by rice plants, leading to a higher C/nutrient ratio in the rhizosphere. Organic inputs strengthened these rhizosphere processes and could thus weaken the relative potential of C sequestration. The microbial CUE decreased with the increase of the available C/N ratio in the rhizosphere but not in the bulk soil. The microbial CUE mainly depended on the respiration in the bulk soil and on the microbial growth in the rhizosphere soil, indicating the divergent microbial utilization of organic substrates between rhizosphere and bulk soils. In both rhizosphere and bulk soils, organic inputs promoted the microbial biomass growth rate and further increased the amount of microbial necromass by 27–52 % compared with NPK alone, which was highly correlated with the soil organic C pools. Despite enhancing rhizosphere respiration, our findings highlight that rice straw and manure applications increase C sequestration in paddy soils by enhancing the net flux of microbial biomass formation, and consequently promoting necromass accumulation. [ABSTRACT FROM AUTHOR]

Labile organic carbon (C) substrates could accelerate microbial transformation of soil N pool by stimulating the decomposition of large molecule organic N. However, it remains unclear how gross N transformation processes (protein depolymerization, amino acid uptake, microbial N mineralization and NH 4 +-N uptake rates) in response to individual C substrates. Typical paddy soil was incubated with the supplement of oxalic acid or glucose under simulated field water conditions for 16 days to assess the gross N transformation rates by 15N pool dilution assays. A mixture of 15N labeled amino acid was applied to gross protein depolymerization and amino acid uptake rates measurement, and 15N-(NH 4) 2 SO 4 was used to gross microbial N mineralization and NH 4 +-N uptake rates analyses. Oxalic acid supplement promoted the gross protein depolymerization, gross microbial uptake of amino acid, and gross N mineralization rates at the early stage. It was attributed that oxalic acid supplement urged microbes to decompose large molecular organic N to acquire amino acid derived C and excluded the superfluous N via mineralization as evidenced by the increase of NH 4 +-N. By contrast, glucose supplement diminished the gross N transformation processes, since microbes prefer to utilize the native NH 4 +-N to meet their N demand supported by the decreasing NH 4 +-N concentration in soil, and consequently inhibited the decomposition for the large molecule organic N. With the increase of microbial growth, especially for bacteria, glucose amendment stimulated the large molecular organic N depolymerization to acquire amino acid to maintain the microbial C/N stoichiometric balance. Compared to glucose treatment, oxalic acid supplement stimulated more N allocation into microbial growth but not for mineralization, and thus led to higher microbial N use efficiency, which was adverse for available inorganic N supply for rice growth in paddy ecosystem. Overall, this study emphasizes that low molecular organic C substrates of organic acid and glucose exerted contrasting influences on gross N transformation, and help to improve our understanding of the mechanism of the coupling biotransformation of C and N in paddy soil. [Display omitted] • Oxalic acid supplement enhanced amino acid uptake to acquire C in early stage. • Glucose supplement urged amino acid uptake to maintain C/N balance in middle stage. • Oxalic acid supplement induced greater N use efficiency via increasing microbial N uptake. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) mineralization is affected by various abiotic and biotic factors, as well as the input of exogenous organic substances. Our previous studies have shown that SOM mineralization in flooded rice paddies is lower than that in adjacent upland soils in subtropical agro-ecosystems. However, the main factors contributing to the differences in SOM mineralization remain unclear. To compare the effects of biotic and abiotic factors on SOM mineralization between upland and paddy soils, we incubated upland and paddy (flooded) soils with three low molecular weight organic substances (LMWOS, i.e., glucose, acetic acid, or oxalic acid) for 30 days under field conditions. Generally, the average CO 2 efflux from upland soil was higher than that in paddy soil with the same LMWOS addition. The total content of phospholipid fatty acids (PLFAs) in paddy soil was 2–5 times higher than that of upland soil, irrespective of the LMWOS added. Redundancy analyses indicated that microbial community composition was influenced mainly by the low redox potential (Eh) and dissolved organic carbon in paddy soil. Structural equation modeling revealed that, among abiotic factors, temperature exerted indirect effects on SOM mineralization by influencing biotic factors in both soils; Eh has a positive and direct effect on SOM mineralization in paddy soil. In terms of biotic factors, SOM mineralization in upland soil was mainly regulated by the quantity of bacteria. In paddy soil, SOM mineralization was largely influenced by the ratio of fungal to bacterial PLFAs and peroxidase activity. [ABSTRACT FROM AUTHOR]

Although soil organic matter (SOM) and microbial biomass pools in flooded paddy soils are generally larger than they are in upland soils, the processes (i.e., slower mineralisation, other types of C stabilization, and a negative priming effect) underlying higher SOM stocks in paddy soil are unclear. To elucidate these processes, three 13 C labelled low molecular weight organic substances ( 13 C-LMWOS) (i.e., glucose, acetic acid, and oxalic acid) were incubated in upland and paddy soils under simulated field conditions. Within 30 days of incubation, acetic acid exhibited the highest mineralisation in both soils. The amount of mineralisation of glucose in upland soil was higher than that of oxalic acid ( p < 0.05), whereas the opposite was observed for paddy soil. Mineralisation of all three LMWOS was lower in paddy soil than that in upland soil ( p < 0.05), illustrating that the molecular structure of the LMWOS as well as soil management determined the mineralisation rate. The priming effect evoked by oxalic acid and glucose was lower in paddy than in upland soil ( p < 0.05). Therefore, the generally weaker mineralisation and priming effect of LMWOS observed in paddy soil contributed to higher carbon accumulation than they did in upland soil. Priming effect was positively correlated with fungal abundance, which was lower in paddy soil than in upland soil. Thus, slow organic C turnover in paddy soil is partly attributed to the suppression of fungal activity by flooding. [ABSTRACT FROM AUTHOR]

Iron-bound organic carbon (Fe-OC) complexes are important for stabilizing soil organic carbon (SOC) against biodegradation. However, it is unclear how the stabilization of OC and its release from Fe minerals subsequently affect the priming effect on SOC mineralization. To address the knowledge gap, we incubated typical paddy soil for 60 days by adding 2-line ferrihydrite (2LFh) or 6-line Fh (6LFh)-bound glucose, each with both high and low amounts of glucose, under anaerobic conditions. Approximately 21% more CO 2 was derived from 2LFh-bound glucose than from 6LFh-bound glucose. Glucose addition alone stimulated SOC mineralization and caused a positive priming effect (0.27% of SOC). In contrast, 2LFh- and 6LFh-bound glucose inhibited SOC mineralization to both CO 2 and CH 4 and subsequently induced a negative priming effect, ranging from −0.33% to −0.55% SOC. Compared to 2LFh-bound glucose, 6LFh-bound glucose induced a lower priming effect on CO 2 emissions (2-fold lower), which was attributed to the lower Fe-reduction rate of 6LFh and OC released. In addition, the available nutrients adsorbed by 6LFh were more difficult to release than those by 2LFh, which aggravated microbial nutrient limitation, and further decreased microbial activity. The priming effect for CH 4 emissions was directly proportional to the glucose level loaded. The Fe reduction rates were higher in Fh-bound high amount of glucose than that in the Fh-bound low amount of glucose, which subsequently provided more available C sources for methanogens. Thus, Fe minerals have a high capacity for SOC accumulation, as they prevent bound OC from mineralization and decrease native SOC priming. Moreover, the protection of SOC by Fe minerals depended on its crystalline structure and the amount of OC loading. Our results show that promoting the transformation from weakly crystalline Fe oxides to more crystalline forms would increase SOC accumulation and stability over the complete rice-growing period. [Display omitted] • Ferrihydrite (Fh) addition reduced paddy soil organic carbon (SOC) mineralization. • 6-line Fh (6LFh) restricted OC release because it is difficult to reduce 6LFh. • OC bound by 6LFh had a lower mineralization rate than that by 2LFh. • Only glucose addition caused positive SOC priming. • 6LFh-bound glucose caused stronger negative SOC priming than 2LFh-bound glucose. [ABSTRACT FROM AUTHOR]

Denitrification is important for nitrogen balance in agricultural ecosystems. It is well established that the abundance and structure of denitrifier are strongly influenced by environmental factors. However, knowledge of how nirS - and nirK -harboring microbial communities vary in paddy soils of different climatic regions is lacking, along with how these microbes are associated with denitrification potential. In this study, forty paddy soils from tropical, sub-tropical, warm-temperate, and mid-temperate climate zones were collected. The results showed that soils from the sub-tropical zone had the highest denitrification rates. The abundances of nirS and nirK had a strong negative association with C/N ratio and clearly varied among the four climate zones. Variation in nirS- and nirK- containing communities existed across sampling sites from each climate zone in terms of α- and β-diversity. Soil pH and climate factors significantly affected community diversity. Network analysis revealed that different climate regions had similar keystone taxa like Azospira , and Achromobacter , which were significantly related to denitrification rates. Structural equation modeling indicated that the differences in denitrifying enzyme activity among the four climate zones were mostly explained by climate factors, soil pH, and nirS biodiversity. Specifically, the biodiversity of nirS was more important than that of nirK in regulating potential denitrification activity in paddy soil, suggesting that nirS- type denitrifiers may have high activity under anaerobic conditions. Our results allow a deeper insight into the relative contribution of nirS- and nirK- containing communities to the soil denitrification activity in paddy soils across climate zones. This study highlighted the need of manipulation experiment to explain how denitrifier biodiversity affect potential denitrification activity in the future. • Soils from the sub-tropical zone have the highest denitrification rates. • nirS and nirK gene abundances show clear variations among the four climate zones. • nirS and nirK gene abundances have strong negative associations with C/N ratio. • The pH, C/N and climate factor significantly affected the community structure. • nirS gene biodiversity is important to regulate denitrification rates in paddy soil. [ABSTRACT FROM AUTHOR]