Long-term Field Fertilization Research Articles

Long-term fertilization promotes soil organic nitrogen accumulation by increasing the abundance of keystone microbial cluster across aggregates

Nitrogen (N) in soil mainly occurs in organic forms, with soil organic nitrogen (SON) playing an essential role in supplying N to crops. However, it is unclear how the content and distribution of SON vary among different aggregate sizes, and how SON content is related to soil microorganisms. In this study, we investigated the associations between SON content and various microbial community characteristics across different aggregate sizes using data from a long-term field fertilization experiment in paddy soils. We also performed functional prediction to explore potential microbial mechanisms underlying SON accumulation. The results showed that SON content decreased as aggregate size decreased, with macroaggregates having the highest SON content and the silt-clay fraction having the lowest. Furthermore, our linear regression and structural equation modeling analysis revealed that keystone microbial clusters (highly interconnected taxa within Firmicutes, Proteobacteria, Chloroflexi, Actinobacteriota, and Ascomycota) were significantly correlated with SON content. Finally, functional prediction analysis suggested that this relationship may be due, at least in part, to the large number of metabolic pathways involved in biosynthesis of N-containing compounds present in keystone microbial clusters. Overall, our study highlights the importance of keystone microbial clusters in influencing the accumulation of SON and contributes to our understanding of soil N biogeochemical cycling.

Phosphorus Fertilization Boosts Mineral-Associated Soil Organic Carbon Formation Associated with Phagotrophic Protists.

Long-term fertilization affects soil organic C accumulation. A growing body of research has revealed critical roles of bacteria in soil organic C accumulation, particularly through mineral-associated organic C (MAOC) formation. Protists are essential components of soil microbiome, but the relationships between MAOC formation and protists under long-term fertilization remain unclear. Here, we used cropland soil from a long-term fertilization field trial and conducted two microcosm experiments with 13C-glucose addition to investigate the effects of N and P fertilizations on MAOC formation and the relationships with protists. The results showed that long-term fertilization (especially P fertilization) significantly (P < 0.05) increased 13C-MAOC content. Compared with P-deficient treatment, P replenishment enriched the number of protists (mainly Amoebozoa and Cercozoa) and bacteria (mainly Acidobacteriota, Bacteroidota, and Gammaproteobacteria), and significantly (P < 0.001) promoted the abundances of bacterial functional genes controlling C, N, P, and S metabolisms. The community composition of phagotrophic protists prominently (P < 0.001) correlated with the bacterial community composition, bacterial functional gene abundance, and 13C-MAOC content. Co-occurrence networks of phagotrophic protists and bacteria were more connected in soil with the N inoculum added than in soil with the NP inoculum added. P replenishment strengthened bacterial 13C assimilation (i.e., 13C-phospholipid fatty acid content), which negatively (P < 0.05) correlated with the number and relative abundance of phagotrophic Cercozoa. Together, these results suggested that P fertilization boosts MAOC formation associated with phagotrophic protists. Our study paves the way for future research to harness the potential of protists to promote belowground C accrual in agroecosystems.

Organic-inorganic fertilization promotes paddy soil macroaggregate organic carbon accumulation associated with key bacterial populations in the subtropical area of China

Macroaggregate organic carbon (Macro-OC) accumulation in paddy soil is of great significance in promoting multiple agroecosystem services. However, the effects of different fertilization practices on Macro-OC accumulation in paddy soil at the regional scale have not been comprehensively investigated. Here, we selected four long-term fertilization field experiments at Taoyuan, Wangcheng, Jinxian, and Suzhou in the subtropical area of China to reveal the effects of inorganic fertilization and organic-inorganic fertilization on Macro-OC accumulation and the relationships with important microbial traits in paddy soil. The results showed that long-term fertilization (particularly organic-inorganic fertilization) significantly increased the content of Macro-OC, which mainly consisted of particulate organic carbon (POC). Organic-inorganic fertilization decreased the percentage of O-alkyl C but increased the percentages of alkyl C, aromatic C, and phenolic C. Organic-inorganic fertilization promoted the abundance of bacterial cellulose-degrading gene retrieved from macroaggregate. At the order level, Anaerolineales, Bacillales, and Clostridiales were identified as keystone bacterial taxa in macroaggregate, and significantly correlated with the physical fraction and chemical structure of Macro-OC. Structural equation modeling revealed that fertilization-induced changes in soil pH and C:N ratio affected the richness of Anaerolineales, Bacillales, and Clostridiales, which was strongly associated with the increase of percentages of aromatic C and phenolic C and further facilitated Macro-OC accumulation. Together, these results suggested that organic-inorganic fertilization promotes Macro-OC accumulation associated with key bacterial populations in paddy soil. The results provide an important basis for boosting soil carbon accrual in the subtropical rice-growing areas.

Wheat yield and nitrogen use efficiency enhancement through poly(aspartic acid)-coated urea in clay loam soil based on a 5-year field trial.

The innovation of N fertilizer and N management practices is essential to maximize crop yield with fewer N inputs. A long-term field fertilization experiment was established in 2015 on the North China Plain (NCP) to determine the effects of a control treatment (CN) and the eco-friendly material poly(aspartic acid)-coated urea (PN), applied as a one-time basal application method, on winter wheat yield and N use efficiency at four N application rates: 0 (N0), 63 (N63), 125 (N125), and 188 (N188) kg N ha–1. The results indicated that compared to CN, PN resulted in a significant increase in wheat yield by 9.6% and 9.2% at N63 and N125, respectively, across the three experimental years, whereas no significant (p < 0.05) difference was detected at N188. Leaf area duration (LAD), crop growth rate (CGR), and dry matter accumulation (DMA) increased with increasing N rates, while PN significantly increased LAD and CGR by 5.1%–16.4% and 5.4%–64.3%, respectively, during the anthesis-ripening growth stage and DMA by 13.7% and 10.1% at N63 and N125, respectively, after the anthesis stage compared to CN. During the grain-filling stage, PN significantly increased the kernel maximum grain-filling rate (Gmax) by 21.7% and the kernel weight at the maximum grain-filling rate (Wmax) by 6.7% at N125 compared to CN. Additionally, compared to CN, PN significantly improved the stover and grain N content at harvest and increased NUT, NPFP, and NAE by 5.7%–40.1%, 2.5%–23.3%, and 3.9%–42.8%, respectively, at N63–N125. Therefore, PN applied using a single basal nitrogen fertilizer application method showed promising potential in maintaining a stable wheat yield and increasing N use efficiency with a 33% urea cut (approximately 63 kg N ha–1) compared to CN at the current wheat yield level on the NCP.

Effect of Long-term Straw Returning on the Mineralization and Priming Effect of Rice Root-carbon

Long-term straw returning to the field changes the environmental conditions of rice paddy soil, which affects the mineralization and priming effect of residual rice roots in the soil, but the direction and intensity of its influence is not clear. Therefore, based on a long-term fertilization field experiment, 13C-CO2 isotopic labeling technology and laboratorial incubation were used to analyze the characteristics of mineralization of rice roots and native soil organic carbon, the intensity and direction of the priming effect, and the source partitioning of CO2 emissions in three treatments, consisting of no fertilization (CK), chemical fertilizer (CF), and straw returning with chemical fertilizer (CFS). The results showed that after 120 days of flooding incubation, the root residue (R) increased the cumulative CO2 emissions by 617.41-726.27 mg·kg-1. The cumulative CO2 emissions from roots and root mineralized proportions in the CFS+R and CF+R treatments were 470.82 and 444.04 mg·kg-1, respectively, and 18.8% and 17.8%, respectively. These were significantly higher than those in the CK+R treatment (384.19 mg·kg-1, 15.4%). There was no significant difference in the cumulative CO2 emissions from native soil organic carbon among the three treatments. However, the mineralized proportion of native soil organic carbon in the CFS+R treatment (4.2%) was significantly lower than that in the CF+R and CK+R treatments (5.4% and 5.8%). The priming effect in the CFS+R treatment was 29.6%, which was significantly lower than that in the CK+R treatment (42.5%) and higher than that in the CF+R treatment (14.4%). A total of 23.47% to 27.59% of the cumulative CO2 emission of the flooded paddy soil was from the roots, and the remainder was from the soil. In addition, the proportion of CO2 emission caused by the priming effect was smaller in the CFS+R treatment than that in the CK+R treatment and larger than that in the CF+R treatment. In summary, the long-term straw returning in the flooded paddy soil will increase the mineralization potential of rice roots, but it is more conducive to the stability of the native soil organic carbon.

Responses of Labile Organic Carbon and Extractable Cadmium Fractions in an Agricultural Soil Following Long-Term Repeated Application of Pig Manure and Effective Microbes.

Long-term pig manure addition has been widely applied in red soil to improve soil fertility. However, the influence of combined utilization of pig manure and effective microbes (EM) on soil organic carbon (SOC) and Cd are not well understood. This study conducted a 23-year (1996-2019) long-term fertilization field trial to investigate the changes of different fractions of SOC and Cd under chemical fertilization (CF), pig manure (PM), and pig manure with effective microbes (PM + EM) treatments in an agricultural soil of Jiangxi Province, South China. The results showed that the pig manure addition significantly enhanced the contents of SOC and Cd in the soils compared with the CF treatments. Furthermore, with the increment of SOC, the PM + EM treatment significantly increased the contents of soil microbial biomass carbon, dissolved organic carbon and easily oxidizable carbon compared with the pig manure application alone. Meanwhile, compared with the CF treatments, the EM addition significantly enhanced the exchangeable and oxidizable fractions of Cd, thus the potential Cd environment risk due to pig manure application should be carefully assessed.

Optimized fertilizer recommendation method for nitrate residue control in a wheat–maize double cropping system in dryland farming

The recommendations for soil fertilization based on soil testing are time-critical and labor-intensive for farmers in the winter wheat–summer maize double cropping system. Understanding the relationships between the crop yield, nitrogen requirement, and nitrate residue level under the combined N and P fertilizer application is necessary to optimize the fertilizer recommendation method in order to reduce the nitrate residue levels. In 2009, we established a long-term field fertilization experiment with five N application rates and four P application rates in the winter wheat-summer maize double cropping system. The grain yield, N requirement, and soil nitrate residue were determined during 2016–2019 to optimize the N inputs for nitrate residue control. The crop yield was increased by N application and it increased further when combined with P fertilizer. The N requirement of crops increased by N application but decreased when combined with P. The calculated maximum winter wheat yield was 7235 kg ha−1 and the theoretical N requirement for 1000 kg grain formation (NR) was 28.68 kg Mg−1. The maximum summer maize yield was 7866 kg ha−1, and the theoretical NR was determined as 23.43 kg Mg−1. The residual nitrate-N was increased by N application but decreased by combined P fertilizer. The correlation between the nitrate-N content in 0–20 cm top soil (SNC) and nitrate-N residue in 0–100 cm soil (SNR) was linear at the winter wheat harvest but exponential at the summer maize harvest. Thus, the SNR can be predicted by the SNC according to their correlation at the harvest. The predicted SNR at the harvest of the previous crop, the target yield and NR for the subsequent crop obtained from long-term in-situ observations can be used to guide the fertilizer amounts applied to the following crop. Therefore, we developed a convenient method to optimize the fertilizer recommendation method for the winter wheat–summer maize cropping system.

Long-term manure application to improve soil macroaggregates and plant-available nitrogen in a Mollisol

Long-term incorporation of manure has shown many benefits to improve soil productivity, but its effects on nitrogen (N) partitioning in soil aggregates and availability to plants is not well understood. The aim of this study was to examine the influence of different long-term field fertilization regimes on N partitioning and availability in soil aggregates. Soils from a field after 25-yr treatments of no fertilization (CK), mineral fertilizer (NPK), NPK with stover return (NPKS), and NPK with manure application (NPKM) were examined for N partitioning after adding 15N-labelled urea and incubating for 42 days. Using a wet sieving procedure, soil was separated into macroaggregates (> 250 μm, coarse particulate organic matter or cPOM), microaggregates (53–250 μm), and silt + clay (< 53 μm, inter mineral-associated OM or inter-MOM). The OM in microaggregates was further separated into fine free POM (ffPOM), physically protected intra-aggregate POM (iPOM), and intra mineral-associated OM (intra-MOM). Total N and 15N concentrations were determined. All fractions except ffPOM (limited mass) were used to cultivate ryegrass (Lolium perenne L.) and 15N uptake was determined. Mass of macroaggregate was 10 % higher from NPKM and NPKS with significantly higher use efficiency than that from NPK while the opposite was true for intra-MOM fraction with no differences in other fractions. The 15N retained in cPOM, intra-MOM, and inter-MOM ranged from 24 to 34%, 28–37 %, and 33–38 %, respectively, and that from ffPOM and iPOM was 1–4% of all retained in aggregates. Ryegrass N uptake was in the range of 14–19 %, 14–24 %, and 3–5% for the cPOM, inter-MOM, and the intra-MOM, respectively. However, the total amount of 15N fertilizer retained in aggregates from the NPKM treatment (14 %) was the lowest due to its higher amount (86 % of 15N) as in soluble phase compared to other treatments (78–83 %). This research concludes that long-term manure application resulted in increased formation of macroaggregates, higher plant-available N in the aggregates, and larger amount of soluble N that contributed to the high yield in the field illustrating the role of manure to improve soil productivity and sustain crop production.

Effects of Long-Term Application of Chemical Fertilizers and Organic Fertilizers on Heavy Metals and Their Availability in Reddish Paddy Soil

To investigate the effects of long-term fertilization on the accumulation and availability of heavy metals in reddish paddy soil and to analyze the major influencing factors, soil samples were collected after the later rice was harvested in 2018 from a long-term fertilization field experiment that began in 1984. Six treatments were selected, namely CK (control without fertilization), PK (P and K fertilizer), and NPK (N, P, and K fertilizer), and different proportions of organic fertilizer plus chemical fertilizer (M1NPK: 30%M+70%NPK; M2NPK: 50%M+50%NPK; and M3NPK: 70%M+30%NPK), soil chemical properties, total and available heavy metal contents, and the relationships between the available forms of heavy metals and soil chemical parameters and total heavy metals (THM) were analyzed. The results showed that ① long-term fertilization changed the soil chemical properties; compared with those of CK, PK significantly increased the contents of soil available phosphorus (AP) and available potassium (AK), NPK significantly increased the soil organic matter (SOM), cation exchange capacity (CEC), AP, and AK, and the organic fertilizer treatments significantly increased the contents of SOM, CEC, AP, AK, and nitrate (NO3--N). ② There were slight variations in the THM contents under the chemical fertilizer treatments (PK and NPK), whereas the organic fertilizer treatments significantly increased the total contents of Cu, Zn, and Cd. ③ The chemical fertilizer treatments significantly increased the available Cr and As, whereas the organic fertilizer treatments significantly increased the available Cu, Zn, Cr, Cd, As, and Fe. ④ There were significant positive correlations between the available Cu, Zn, Cr, Cd, As, and Fe and the SOM, CEC, AP, and NO3--N. In addition, the available Zn and Cd were significantly positively correlated with the soil pH, whereas the available Pb was significantly negatively correlated with soil pH, SOM, CEC, and NO3--N. ⑤ There were significant positive correlations between the available and total contents of Cu, Zn, and Cd, whereas there were significant negative correlations between the available and total contents of Cr and Fe. ⑥ Redundancy analysis showed that SOM and pH accounted for 80.7% and 5.5% of the variation in THM, whereas the soil CEC, AP, and pH accounted for 81.1%, 4.9%, and 3.3% of the variation in the available heavy metals, respectively. ⑦ The partial least squares path model analysis showed that the path coefficients of the THM, CEC, and AP on the available state of heavy metals were 0.459, 0.417, and 0.293, respectively. Long-term application of organic manure, such as pig manure, significantly improved the soil chemical properties and affected the availability of heavy metals, and soil CEC and AP may play key roles in regulation.

Saprotrophic fungal communities in arable soils are strongly associated with soil fertility and stoichiometry

The importance of soil saprotrophic fungi in agroecosystems is receiving increasing attention. However, the linkages between saprotrophic fungal communities and soil fertility and stoichiometry in agroecosystems are poorly understood. Here, four long-term field fertilization experiments with three treatments (i.e., no fertilizer (control), chemical nitrogen (N), phosphorus (P), and potassium (K) fertilizer (NPK), chemical combined with organic fertilizer (NPKM)) were performed to evaluate the associations of saprotrophic fungal communities with the soil fertility index (SFI) and stoichiometric carbon (C):N:P ratios. Long-term NPKM application increased the SFI by 57.66%–129.50% at the four sites, and the SFI was strongly correlated with the saprotrophic fungal community composition, which indicated that exogenous organic matter improved soil fertility to a large extent and stimulated the growth of saprotrophic fungi in arable soils. The soil C:P and N:P ratios were notably decreased in the fertilization treatment, indicated that P was sufficient. Further, the composition of saprotrophic fungi was significantly related to the C:N and N:P ratios, suggested that N status is an important factor for soil saprotrophic fungal communities. Our study provides empirical evidence that the saprotrophic fungal communities in arable soils are strongly associated with soil fertility and stoichiometry and offers guidance for improving soil fertility by regulating the saprophytic fungal community.

The driving factors of nematode gut microbiota under long-term fertilization.

Animal bodies are colonized by many microorganisms which can provide indispensable services to their hosts. Although nematode gut microbiota has been extensively studied in recent years, the driving factors of gut microbiome of soil nematodes from a long-term fertilization field are unclear. Here, using 16S rRNA gene amplicon sequencing, we explored the nematode gut microbiota under different fertilization patterns (control, inorganic fertilizers and mixed fertilizers) and fertilization durations (5 y, 8 y and 10 y). Our results revealed that nematode gut microbiota was dominated by core bacterial taxa AF502208 (anaerobic bacteria), Enterobacter (plant litter decomposition) and Ancylobacter (organic matter decomposition and nitrogen cycling), significantly distinct from soil microbiome, and the assembly of that was a non-random process, which suggested host conditions contributed to maintaining the gut microbiota. Moreover, fertilization pattern had a greater influence on nematode gut microbiome than fertilization duration. Inorganic fertilization (5.19) significantly reduced the diversity of the nematode gut microbiota (6.68) shown by Shannon index (P <0.05). Canonical correspondence analysis demonstrates that soil properties such as pH, organic matter, total phosphorus, available phosphorus, ammonium nitrogen, moisture content, nitrate nitrogen and total nitrogen have significant effects on the nematode microbiome. Structured equation models further revealed that fertilization could obviously affect the nematode gut microbiota, and the effects were maintained even when accounting simultaneously for the drivers of soil bacteria and soil properties. This study provides a solid evidence that the shifting of nematode gut microbiota under long-term fertilization was resulted from environmental factors and host conditions, and advance the insights into host-microbiome in the agricultural ecosystems.

Multiple long-term observations reveal a strategy for soil pH-dependent fertilization and fungal communities in support of agricultural production

Agricultural fertilization plays a crucial role in crop production, and the fungal communities catalyze transformation of soil nutrients in support of crop production. However, it remains controversial about the optimal strategy for fertilizer inputs and the adaptive mechanisms of fungal communities across China. By using seven long-term field fertilization experiments in China, we analyzed crop yields, soil properties and fungal communities in soils that were treated for > 25 years with no fertilizer (control), inorganic fertilizers (NPK) and organic-inorganic fertilizers (NPKM). Long-term NPK resulted in significant acidification up to a decline by 1.20 pH units, while NPKM prevented acidification and increased pH up to 6.39 in three acidic soils with pH < 5.70. NPKM increased crop yields by 1.19–8.72 folds in acidic soils, being significantly higher than NPK. Specific saprotroph Mortierella and Pseudaleuria in acidic soils were exclusively enriched by NPKM. Soil pH was directly related to the abundance of Mortierella, and the enrichment of Mortierella species further caused a positive direct effect on crop yield. In four alkaline soils with pH > 8.11, both NPK and NPKM led to only marginal decline of soil pH, and NPK and NPKM showed comparable crop yields. Some members of Ascomycota in alkaline soils were both enriched by NPKM and NPK. Soil available P and C:N ratio, rather than pH, directly or indirectly affect crop yield in alkaline soils. High crop yield can be achieved by the sole use of inorganic fertilizers in alkaline soils, but acidic soil productivity should be maintained by organic amendment to counteract acidification by inorganic fertilization. Our study advances a mechanistic understanding for optimizing fertilization strategies towards sustainable agriculture under increasingly intensified fertilizer inputs.

Change of soil productivity in three different soils after long-term field fertilization treatments

Soil productivity (SP) without external fertilization influence is an important indicator for the capacity of a soil to support crop yield. However, there have been difficulties in estimating values of SPs for soils after various long-term field treatments because the treatment without external fertilization is used but is depleted in soil nutrients, leading to erroneous estimation. The objectives of this study were to estimate the change of SP across different cropping seasons using pot experiments, and to evaluate the steady SP value (which is defined by the basal contribution of soil itself to crop yield) after various long-term fertilization treatments in soils at different geographical locations. The pot experiments were conducted in Jinxian of Jiangxi Province with paddy soil, Zhengzhou of Henan Province with fluvo-aquic soil, and Gongzhuling of Jilin Province with black soils, China. Soils were collected after long-term field fertilization treatments of no fertilizer (control; CK-F), chemical fertilizer (NPK-F), and combined chemical fertilizer with manure (NPKM-F). The soils received either no fertilizer (F0) or chemical fertilizer (F1) for 3–6 cropping seasons in pots, which include CK-P (control; no fertilizer from long-term field experiments for pot experiments), NPK-P (chemical fertilizer from long-term field experiments for pot experiments), and NPKM-P (combined chemical and organic fertilizers from long-term field experiments for pot experiments). The yield data were used to calculate SP values. The initial SP values were high, but decreased rapidly until a relatively steady SP was achieved at or after about three cropping seasons for paddy and fluvo-aquic soils. The steady SP values in the third cropping season from CK-P, NPK-P, and NPKM-P treatments were 37.7, 44.1, and 50.0% in the paddy soil, 34.2, 38.1, and 50.0% in the fluvo-aquic soil, with the highest value observed in the NPKM-P treatment for all soils. However, further research is required in the black soils to incorporate more than three cropping seasons. The partial least squares path mode (PLS-PM) showed that total N (nitrogen) and C/N ratio (the ratio of soil organic carbon and total N) had positive effects on the steady SP for all three soils. These findings confirm the significance of the incorporation of manure for attaining high soil productivity. Regulation of the soil C/N ratio was the other main factor for steady SP through fertilization management.

Effects of Rhizosphere and Long-Term Fertilization Practices on the Activity and Community Structure of Denitrifiers Under Double-Cropping Rice Field

ABSTRACTThe soil physicochemical properties, soil denitrification rates (PDR), denitrifiers via nitrite reductases (nirK and nirS) and nitrous oxide reductase (nosZ), abundance and community composition of denitrifiers in both the rhizosphere and bulk soil from a long-term (32 year) fertilizer field experiment conducted during late rice season were investigated by using the MiSeq sequencing, quantitative PCR, terminal restriction fragment polymorphism (T-RFLP). The experiment including four treatments: without fertilizer input (CK), chemical fertilizer alone (MF), rice straw residue and chemical fertilizer (RF), and organic manure and chemical fertilizer (OM). The results showed that the application of rice straw residue and organic manure increased soil organic carbon (C), total nitrogen (N), and NH4+-N contents. The nirK, nirS, and nosZ copy numbers with OM and RF treatments were significant higher than that of the MF and CK treatments in the rhizosphere and bulk soil (p < 0.05). The principal coordinate analysis (PCoA) analysis showed that the different parts of root zone are the most important factors for the variation of denitrifying bacteria community, and the different fertilization treatments is the second important factors for the variation of denitrifying bacteria community. The MiSeq sequencing result showed that nirK, nirS and nosZ-type denitrifiers communities within bulk soil had lower species diversity compared with rhizosphere soil, and were dominated by Rhizobiales, Rhodobacterales, Burkholderiales, and Pseudomonadales. As a result, the application of fertilization practices had significant effects on soil N and PDR levels, and affected the abundance and community composition of N-functional microbes.

Prokaryotic Community Structure of Long-Term Fertilization Field Andisols in Central Japan

Long-term fertilization experiments are a useful way to elucidate the impacts of fertilization on soil ecosystems. Here, we report the prokaryotic community structure in experimental field soil after 80 years of successive fertilization. Our 16S rRNA gene sequencing detected 20,996 amplicon sequence variants, including major phyla such as Proteobacteria, Acidobacteria, and Actinobacteria.

Impact of 36 years of nitrogen fertilization on microbial community composition and soil carbon cycling-related enzyme activities in rhizospheres and bulk soils in northeast China

Nitrogen (N) deposition can change ecosystem functions but little is known of long-term N-deposition and rhizosphere effects on the microbial community composition and enzymes activities related to the carbon (C) cycle in the black soil common to northeastern China. Here, we studied two enzyme activities involved in C cycles and microbial community composition in both the rhizosphere and bulk soil from a long-term (36-year) fertilization field experiment. N-addition significantly decreased bacterial abundance and phenol oxidase activity, but enhanced fungal abundance and peroxidase activity, in both the rhizosphere and bulk soil. The fungal diversity exhibited more obvious shifts than the bacterial diversity after long-term N-addition, resulting in significantly decreased bacterial and fungal diversity levels, except for bacterial diversity in the rhizosphere, which was not significantly changed. Moreover, the enzyme activities and the bacterial and fungal abundance levels were higher in the rhizosphere than in the bulk soil, suggesting a rhizosphere effect on microbial activities involved in the C cycle. There was a significant difference in the microbial community compositions among different N-addition levels. A lesser β-diversity response to N-addition was observed in the rhizosphere than in the bulk soil, and the responses of fungal communities were greater than those of bacterial communities. Our findings suggested that rhizosphere effects and fertilization regimes both have significant influences on microbial communities and soil enzyme activities, and that fungi were more sensitive than bacteria in responding to N-deposition.

Soil aggregate size and long-term fertilization effects on the function and community of ammonia oxidizers

Long-term field fertilization trials have suggested that reduced chemical nitrogen (N) plus organic fertilization can effectively reduce N loss without sacrificing crop yield, while the knowledge of how organic fertilizers regulate soil microorganisms and their function in N transformation are limited. In this study, the response of net nitrification rate and the ammonia-oxidizer community within soil aggregates to long-term combined organic N and reduced chemical N fertilization was evaluated to understand the underlying mechanism of the practice in mitigating soil N loss. The fertilization experiment included an unfertilized control; chemical N fertilizer (N), superphosphate (P) and potassium sulfate (K) fertilizer (NPK); NPK plus straw (NPKS); and NPK plus manure (NPKM). The results showed that the large macro-aggregates mass (>2 mm) in soil significantly increased from 34.1% in NPK to 47.2% in NPKS (P < 0.05). NPKS and NPKM both had positive effects on soil moisture retention, total N (TN), soil organic carbon (SOC), ammonium (NH4+-N) and nitrate (NO3−-N) accumulation, particularly within micro- (<0.25 mm) and small macro-aggregates. Compared with the NPK treatment, soil net nitrification rate (NNR) and ammonia-oxidizing bacterial (AOB) abundance decreased by 67.1% and 40.7% respectively under NPKS (P < 0.05), and the decrease mainly appeared in large macro-aggregate and micro-aggregates. The net nitrification rate was significantly correlated with ammonia-oxidizing archaea (AOA) abundance only in small macro-aggregates (r = 0.642, n = 12, P < 0.05). In contrast, NRR was positively correlated with AOB abundance within small macro- and micro-aggregate size classes (r values ranged from 0.654 to 0.813, P < 0.05). The community structure of AOB varied among different fertilization treatments, while AOA community differentiation was mainly dependent on aggregate size. The shift of the AOA and AOB communities corresponded with high moisture and lower ammonia content in the large macro-aggregates and were potentially responsible for the suppressed nitrification activity in the NPKS treatment. These results indicated that reduced chemical N plus organic fertilization is beneficial for increasing N concentration in large macro-aggregates.

Long-term field fertilization experiment with energy willow (Salix sp.) − Elemental composition and chlorophyll fluorescence in the leaves

A small-plot long-term field fertilization experiment was set up in 2011 with willow (Salix triandra x Salix viminalis ’Inger’) grown as an energy crop in Nyíregyháza, Hungary. The brown forest soil was treated three times (in June 2011, May 2013, May 2016) with municipal biocompost (MBC), municipal sewage sludge compost (MSSC) or willow ash (WA), and twice (June 2011, May 2013) with rhyolite tuff (RT). In late May – early June 2016 urea (U) and sulphuric urea (SU) fertilizers were also applied to the soil as top-dressing (TD). These fertilizers and amendments were also applied to the soil in 2016 in the combinations; MBC+SU, RT+SU, WA+SU and MSSC+WA. All the treatments were repeated four times. In July 2016 the highest nitrogen concentrations in willow leaves were measured in the U (3.47 m/m%) and SU (3.01 m/m%) treatments, and these values were significantly higher than the control (2.46 m/m%). An excess of nitrogen considerably reduced the Zn uptake of the leaves, with values of 39.5 μg g-1 in the U treatment, 53.4 μg g-1 in the SU treatment, and 63.5 μg g-1 in the control. All other amendments or TDs, except for WA, enhanced the specific potassium concentrations in willow leaves compared to the control. No significant quantities of toxic elements (As, Ba, Cd, Pb) were transported from soil amendments or TDs to the willow leaves. In July 2016 the most intensive leaf chlorophyll fluorescence was observed in the MSSC and MSSC+WA treatments.

Effect of long-term application of NPK fertilizer on maize yield and yellow soil nutrients sustainability in Guizhou, China

A long-term fertilization field experiment was conducted to investigate the effect of nitrogen (N), phosphorus (P), and potassium (K) fertilizer on maize relative yield, yield-increasing effect and the changes of nutrients in yellow soil in Guizhou Province. Five fertilizer combinations were evaluated, including balanced fertilization (NPK) and nutrient deficiency treatments (N, NK, NP, and PK). The maize relative yield, contribution efficiency of N, P, K fertilizer application, sustainability index of soil N, P, K nutrients, and other indicators were measured. The results revealed that the balanced fertilization (NPK) significantly increased maize yield, and the average yield under each treatment ranked as: NPK>NP>NK>PK>CK. The contribution efficiency and agronomic efficiency of N, P, K fertilizer application was N>P>K. The fertilization dependence was ranked as: combined application of N, P and K>N>P>K. But in the lack of P treatment (NK), the maize relative yield significantly decreased at a speed of 1.4% per year, with the contribution efficiency and fertilization dependence of applied P significantly increasing at a speed of 2.3% per year and 1.4% per year, respectively. Over time, the effect of P fertilizer on maize yield gradually became equal to that of N fertilizer. The pH and soil organic matter content were the lowest in the P-lack treatment (NK), while they were higher in the N-lack treatment (PK). The application of chemical P significantly improved the sustainability index of soil P, but the application of chemical N and K did not significantly change the sustainability index of soil N and K nutrients compared to the N- and K-lack treatments, respectively. In summary, the use of balanced fertilizer application is critical for achieving high maize yield in typical yellow soil regions in Guizhou Province. P and N fertilizers are equally important for improving maize yield, and long-term application of unbalanced chemical fertilizer, especially the lack of P, would not benefit the sustainable use of nutrients in yellow soil.

Does long-term application of fertilizers enhance the micronutrient density in soil and crop?—Evidence from a field trial conducted on a 47-year-old rice paddy

To date, how optimized, long-term fertilizer application affects the concentration of micronutrients in the milling fractions of rice grains (bran or edible grain part) with details of their dietary exposure upon consumption has not been much explored. In this study, we report results from a 47-year-old field experiment conducted in rice paddy. Soil and rice grain samples were collected from a 47-year-old long-term fertilization field trial (treatments included NPK, compost, NPK + compost, and no fertilizer/control) conducted at Miryang, South Korea. Density of total micronutrients namely As, B, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Se, V, and Zn in soil and rice milling fractions were quantified using ICP-MS and inductively coupled plasma optical emission spectrometry (ICPOES). Concentrations of the exchangeable and water-soluble micronutrients in the soil were also determined by ICP-MS and ICPOES. Potentially bioavailable soil micronutrient levels, micronutrient transfer from soil to rice grain, and dietary intake level of micronutrients by rice consumption were calculated. The correlations between soil properties and crop micronutrient availability were analyzed. In general, long-term fertilization (NPK + compost > NPK > compost) yielded a significant effect on grain As, B, Co, Cr, Cu, Fe, Mn, Mo, Ni, Se, V, and Zn fortification and promoted bioavailable soil micronutrient density. The highest contributors to micronutrient intake could be rice grains from long-term NPK + compost fertilized plots. However, estimated levels were still lower than the recommended dietary intake levels for micronutrients established by FAO/WHO. From our study, it is important to note that 80% of total micronutrient content (Fe, Mn, Zn, Co, Ni, Se, and V) were concentrated in rice bran. This has important implications for human nutrition and health. Furthermore, correlation analysis indicates that available soil P could be considered a predictor of crop micronutrient intake. Optimized long-term application of compost with NPK fertilizers to soils is a potential grain fortification strategy. Long-term fertilization will not reasonably benefit individuals who consume only white rice, because bran with enriched micronutrients will be removed by milling. We suggest replacing the consumption of white rice by brown rice so that individuals have a better micronutrient intake.