NPKS Treatment Research Articles

Long-term different fertilization practices restructured the functional carbon pools in a paddy soil through distinct mechanisms

Fertilization is a common management practice for improving soil organic matter (SOM), which is a crucial indicator of soil fertility. Nevertheless, how long-term fertilization affects functional SOM fractions and its mechanisms remain unclear. In this study, a 12-year field experiment, initiated in 2011, was conducted in Xinzhu village, Zhejiang province, China. Four different fertilizer treatments (without any fertilizers application, Control; sole application of inorganic fertilizers, NPK; combined application of NPK with cattle manure, NPKM; and combination of NPK fertilizers and return of rice straw, NPKS) were selected in this field trial. Functional carbon (C) pools, microbial community structure and SOM composition in both topsoil (0−15 cm) and subsoil (15−30 cm) were explored using the physical and chemical fractionation, phospholipid fatty acids (PLFA) analysis and solid-state 13C-Nuclear Magnetic Resonance Spectrometry (13C NMR) assays. Compared to the control, the NPK combined with organic amendments had significantly higher organic carbon contents of unprotected pools (cPOM and fPOM) in both topsoil (39−277 %) and subsoil (43.1−91.3 %), respectively. Organic carbon accumulated the most within iPOM (1.99 g kg−1 soil) and H-d(Silt+Clay) fractions (1.41 g kg−1 soil) under NPKM in the topsoil, suggesting that physical and chemical protection played the pivotal role after manure addition. Furthermore, NPKM increased the abundance of bacterial and fungal PLFAs in both topsoil (31.4 % and 24.5 %) and subsoil (173 % and 175 %), while NPKS elevated the alkyl/O-alkyl C ratio but reduced the aromaticity in topsoil, compared to inorganic fertilization alone. Partial least squares path modelling showed that fertilization had a direct impact on silt+clay associated C (0.6) in topsoil, POM-C (0.76) in subsoil, and microbial community structure (0.88 and 0.87) in both soil layers. Redundancy analysis revealed that in the NPKM treatment, the key factors affecting functional carbon pools were the total PLFA and the ratio of gram-positive to gram-negative bacterial PLFA, while for the NPKS treatment, it was the alkyl C/O-alkyl C ratio. In conclusion, long-term fertilization had a strong effect on functional carbon fractions through multiple mechanisms in different soil layers, involving changing microbial community and SOM composition.

Straw Addition Enhances Crop Yield, Soil Aggregation, and Soil Microorganisms in a 14-Year Wheat-Rice Rotation System in Central China.

Straw return utilizes waste resources to reduce the use of chemical fertilizers worldwide. However, information is still lacking on the relative impact of straw return on soil fertility, the nutrient composition of different soil aggregates, and soil microbial communities. Therefore, this study aimed to understand the effects of different management practices on the crop yield, soil fertility, and soil community composition in a 14-year wheat-rice rotation system. The treatments included a control (without fertilizer and straw addition), chemical fertilization (NPK), straw return without fertilizer (S), and straw addition with chemical fertilizer (NPKS). The results showed that NPKS improved the wheat and rice yield by 185.12% and 88.02%, respectively, compared to the CK treatment. Additionally, compared to the CK treatment, the N, P, and K contents of the wheat stem were increased by 39.02%, 125%, and 20.23% under the NPKS treatment. Compared to the CK treatment, SOM, TN, TP, AN, AP, AK, CEC, AFe, AMn, ACu, and AZn were increased by 49.12%, 32.62%, 35.06%, 22.89%, 129.36%, 48.34%, 13.40%, 133.95%, 58.98%, 18.26% and 33.33% under the NPKS treatment, respectively. Moreover, straw addition promoted the creation and stabilization of macro-aggregates in crop soils. The relative abundance of macro-aggregates (0.25-2 mm) increased from 37.49% to 52.97%. Straw addition was associated with a higher proportion of aromatic and carbonyl carbon groups in the soil, which, in turn, promoted the formation of macro-aggregates. Redundancy analysis showed that straw return significantly increased the microbial community diversity. These findings demonstrate that straw addition together with chemical fertilizer could increase the crop yield by improving soil fertility, soil aggregate stability, and the diversity of fungi.

Microbial Properties Depending on Fertilization Regime in Agricultural Soils with Different Texture and Climate Conditions: A Meta-Analysis

Over-fertilization has a significant impact on soil microbial properties and its ecological environment. However, the effects of long-term fertilization on microbial properties on a large scale are still vague. This meta-analysis collected 6211 data points from 109 long-term experimental sites in China to evaluate the effects of fertilizer type and fertilization duration, as well as soil and climate conditions, on the effect sizes on various microbial properties and indices. The organic fertilizers combined with straw (NPKS) and manure (NPKM) had the highest effect sizes, while the chemical fertilizers N (sole N fertilizer) and NPK (NPK fertilizer) had the lowest. When compared with the control, NPKM treatment had the highest effect size, while N treatment had the lowest effect size on MBN (111% vs. 19%), PLFA (110% vs. −7%), fungi (88% vs. 43%), Actinomycetes (97% vs. 44%), urease (77% vs. 25%), catalase (15% vs. −11%), and phosphatase (58% vs. 4%). NPKM treatment had the highest while NPK treatment had the lowest effect size on bacteria (123% vs. 33%). NPKS treatment had the highest while N treatment had the lowest effect sizes on MBC (77% vs. 8%) and invertase (59% vs. 0.2%). NPKS treatment had the highest while NPK treatment had the lowest effect size on the Shannon index (5% vs. 1%). The effect sizes of NPKM treatment were the highest predominantly in arid regions because of the naturally low organic carbon in soils of these regions. The effect sizes on various microbial properties were also highly dependent on soil texture. In coarse-textured soils the effect sizes on MBC and MBN peaked sooner compared with those of clayey or silty soils, although various enzymes were most active in silty soils during the first 10 years of fertilization. Effect sizes on microbial properties were generally higher under NPKM and NPKS treatments than under NPK or N treatments, with considerable effects due to climate conditions. The optimal field fertilizer regime could be determined based on the effects of fertilizer type on soil microorganisms under various climate conditions and soil textures. This will contribute to the microbial biodiversity and soil health of agricultural land. Such controls should be used for adaptation of fertilization strategies to global changes.

Straw is more effective than biochar in mobilizing soil organic phosphorus mineralization in saline-alkali paddy soil

Fertilizer management can alter the availability and biological cycle of soil phosphorus (P). The objective of this study was to explore the relationship among soil P availability, organic phosphorus (Po) mineralization and soil bacteria characteristics under organic and inorganic fertilization in saline-alkali paddy soil. Four treatments were designed, namely no fertilizer (Control), mineral fertilizer (NPK), mineral fertilizer with rice straw (NPKS) or rice straw-derived biochar (NPKB). Compared with NPK treatment, straw and biochar addition greatly increased P uptake by rice while decreased soil organic P/total P ratio. The contents of soil available P (AP) and alkaline phosphatase (ALP) in NPKS treatment were higher than those in NPKB treatment. This was most likely due to the higher abundance of Streptomyces in NPKS treatment than that in NPKB treatment, which was known as a potential Po mineralizer. NPKS and NPKB treatments significantly increased diversity and abundance of phoD-harboring bacteria but have no significant effect on the total bacteria. Based on SEM analysis, the composition of phoD-harboring bacteria community was more important in ALP activity than the increased phoD-harboring bacterial abundance and diversity. Soil dissolved organic carbon (DOC) content and DOC: AP ratio were the key factors affecting the formation of total bacterial and phoD-harboring bacterial community, which were significantly increased in NPKS treatment. Compared with biochar application, straw was more efficient in increasing the abundance of some phoD-harboring bacteria, which preferred carbohydrate-rich environments, thereby improving the availability of P and its potential biological transformation in saline-alkali paddy soil.

Effect of soil supplementation with mineral-organic mixtures on the amount of maize biomass and the mobility of trace elements in soil

The application of calcium fertilisers and organic fertilisers or materials is an alternative allowing to reduce soil acidification and increase the organic matter content. Under Polish conditions, where soils belonging to the agronomic category of light and acidic soils predominate, such an approach can bring measurable benefits in improving physical, chemical, and biological properties of soils. However, it should be noted that until now these treatments have usually been applied separately; therefore, the aim of this study was to determine the effect of the combined application of calcium carbonate fertiliser with leonardite (CaLO) and with poultry litter (CaPL) on the amount of maize biomass, the content of trace elements in the biomass, and selected soil properties, including the mobility of trace elements in soil. The study was conducted as a pot experiment. The experimental design included 4 treatments: soil with no fertilisers (Control), soil with mineral fertilisers (NPKS), and soil with mineral fertilisers and the addition of a mixture of CaCO3 and leonardite (NPKS+CaLO) as well as CaCO3 and poultry litter (NPKS+CaPL). The study showed a substantial increase in the amount of mainly aboveground parts of maize biomass after applying mixtures of CaCO3 and organic materials. Despite the lack of significant differences, the relative increase in the chlorophyll a content in NPKS+CaLO and NPKS+CaPL treatments was over 7 % compared to NPKS treatment. The obtained results are inconclusive as to the effect of the applied mixtures on the trace element content in maize biomass. Soil supplementation with CaLO mixture significantly reduced cadmium, copper, and lead contents in maize aboveground parts, while the zinc content was higher compared to the NPKS-fertilised treatment. Soil supplementation with CaPL mixture decreased the zinc content in maize aboveground parts, while the cadmium, copper, and lead contents were higher compared to the NPKS-fertilised treatment. Soil pH significantly improved after applying CaLO and CaPL mixtures. Soil supplementation with CaLO and CaPL was vital for reducing the content of water-soluble forms of Cd and Zn, while for Pb and Cu, we observed an increase in these forms of trace elements in the soil. The treatments with CaLO and CaPL had lower content of NH4EDTA-extracted elements compared to the control.

Effects of Long-term Straw Returning on Fungal Community, Enzyme Activity and Wheat Yield in Fluvo-aquic Soil

To illustrate the effects of long-term straw returning on the fungal community, soil enzyme activity, and crop yield in a fluvo-aquic soil area typical of the Huang-Huai-Hai Plain, a 10-year field experiment (established in 2010) located in Dezhou City, Shandong province, was performed, including three fertilization regimes (NF, no fertilization control; NPK, fertilization with chemical N, P, and K fertilizers; NPKS, straw returning combined with chemical N, P, and K fertilizers). This study aimed to explore the regulation mechanisms of fungal communities on soil fertility, enzyme activities, and crop yield by employing co-occurrence network and structural equation model analyses. Our results showed that long-term straw returning significantly improved soil nutrients, enzyme activity, and wheat yield. Compared with the NPK and NF treatments, soil organic matter (SOM) increased by 9.20% and 34.75%, alkali-hydrolyzed nitrogen (AN) increased by 12.03% and 39.17%, dehydrogenase (DHA) increased by 37.21% and 50.91%, β-glucosidase (β-GC) increased by 17.29% and 73.48%, and wheat production increased by 16.22% and 125.53%, respectively. Different long-term fertilization regimes did not significantly change soil fungal α-diversity but resulted in significant differences in β-diversity. Available phosphorus (AP), SOM, and AN were the main driving factors of fungal community differentiation based on redundancy analysis and hierarchical partitioning analysis. Different abundance analyses revealed significantly different fungal community compositions among fertilization regimes. The long-term NF treatment resulted in a significant enrichment of phosphate/potassium-solubilizing species (i.e., Mortierella, Aspergillus, Ceriporia, and Acremonium) and symbiotic species (i.e., Leohumicola and Hyalodendriella). The relative abundance of pathogenic fungi, namely Sarocladium, Fusarium, and Fusicolla, increased significantly in the NPK treatment. Long-term straw returning in the NPKS treatment significantly stimulated the growth of plant growth-promoting species (i.e., Pseudogymnoascus and Schizothecium) and straw-degrading species (i.e., Trichocladium and Lobulomyces). Co-occurrence network analysis showed that the fungal network was composed of four main modules; the cumulative relative abundance of module 2 was significantly increased under the NPKS treatment and showed a positive linear correlation with DHA and β-GC. The structural equation model further indicated that the wheat yield was mainly regulated by SOM, whereas species of module 2 could indirectly affect SOM and wheat yield by positively regulating DHA and β-GC. Taken together, long-term straw returning to the fluvo-aquic soil area of the Huang-Huai-Hai Plain could regulate fungal interspecific interactions, stimulate the growth of specific species groups, inhibit the activity of pathogens, increase the activity of soil enzymes, promote the accumulation of SOM, and achieve high crop yield.

Crop Yield, Nitrogen Recovery, and Soil Mineral Nitrogen Accumulation in Extremely Arid Oasis Cropland under Long-Term Fertilization Management

Crop yield stability and soil mineral nitrogen (Nmin) have rarely been evaluated from a long-term perspective in the extremely arid cropland regions of China. Therefore, a nationwide experiment aimed to optimize fertilizer application and increase productivity and nitrogen use efficiency in gray desert soils was initiated in 1990. Eight combinations of chemical fertilizers (CK, N, NK, NP, and NPK), straw return (NPKS), and manure amendments (NPKM and NPKM+) were tested for 24 years on spring wheat, winter wheat, and maize. The results displayed that the yield of three crops from balanced fertilizer treatments (NPK, NPKS, NPKM, and NPKM+) did not differ significantly after 24 years; however, reliable yield stability due to lower coefficient of variation (CV) and higher nitrogen harvest index (NHI) were recorded for manure amendment treatments. Compared to NPKM, NHI was lower for the NPKM+ treatment, but crop yield and stability did not improve, suggesting that the appropriate choice for manure amendment is important for guaranteeing food security in extremely arid regions. Balanced fertilizer treatments resulted in lower Nmin residual in the 300 cm soil profile, compared to unbalanced fertilizer treatments. The NPKS treatment gave the lowest value. In the 0–100 cm soil profile, Nmin was higher in NPKM than in the NPK treatment, suggesting that straw or manure amendment can effectively maintain Nmin in the topsoil undercurrent cropland management in arid areas. The NPKM treatment had the highest crop nitrogen recovery rate and the lowest nitrogen losses, further illustrating that manure amendment has higher N retention potential. Overall, although Nmin residues are relatively high in these regions, balanced fertilizer treatments, especially NPKM and NPKS, are the optimum strategies in extremely arid regions.

Response of wheat (Triticum aestivum L) to sulphur under balanced and imbalanced fertilizer application

A field experiment was conducted during rabi season of 2015-16 and 2016-17 at Panwari village of Agra district to study the response of wheat (Triticum aestivum L.) to sulphur under balanced and imbalanced fertilizer application. Treatments consisted of application of balanced (NPK) and imbalanced (NP and NK) fertilizer application along with sulphur. The experiment was laid out in a randomized block design with six treatments and three replications. Results revealed that balanced fertilizers (NPK) application proved superior in respect of growth yield and utilization of nutrients over imbalanced fertilizer application. The application of NPKS (150 kg N + 60 kg P2O5 + 40 kg K2O + 30 kg S ha-1) significantly increased the plant height (95 cm) length of ear (103.cm grain/ear (51.8), ear/m2 (50.8) and test weight (40.4g) compared with other treatments. Significantly higher grain (56.39 q ha-1) and straw (71.15 q ha-1) yield were recorded with NPKS treatment over other treatments. Application of NPKS significantly increased the uptake of nitrogen by wheat crop over other treatments Protein content in grain (13%) and protein yield (733.0 kg ha-1) were also higher with NPKS treatment. The lowest values of all these parameters were recorded under NK treatment. However, NK and NP treatments statistically at par in respect of growth and yield of wheat crop.

Long-term straw return influenced ammonium ion retention at the soil aggregate scale in an Anthrosol with rice-wheat rotations in China

Soil aggregates are an important controlling factor for the physico-chemical and biological processes such as ammonium (NH4+) retention. Straw return to the field is increasingly recommended to promote soil carbon (C) sequestration and improve crop yields. However, the effects of straw return on NH4+ retention at soil aggregate level in agricultural soils have seldom been investigated. This study aimed to evaluate the influences of long-term straw return on NH4+ adsorption and fixation in microaggregates (<0.25 mm) with or without soil organic carbon (SOC) oxidization. Soil samples were collected from plots of three treatments, i.e., no fertilizer (CK), inorganic NPK fertilizers (NPK), and inorganic NPK fertilizers with rice straw return (NPKS), from a 20-year-old field trial with rice-wheat rotations in Taihu Lake Region, China. Soil aggregates were separated using wet-sieving method. The SOC of microaggregates was oxidized by H2O2. The results showed that long-term straw return significantly increased SOC and NH4+ adsorption, but inhibited NH4+ fixation in microaggregates. NH4+ adsorption potential and strength - obtained from adsorption isotherms - increased, but NH4+ fixation decreased along with increasing SOC in microaggregates, indicating the important role of SOC in NH4+ adsorption and fixation. This was verified by the SOC oxidization test that showed a relative decrease in NH4+ adsorption potential for the NPKS treatment and an increase in NH4+ fixation in all three treatments. Therefore, long-term straw return influences NH4+ adsorption and fixation by enhancing SOC content and could improve N availability for crop uptake and minimize applied N fertilizer losses in rice-wheat cropping systems.

Effect of Soil-Applied L-tryptophan on the Amount of Biomass and Nitrogen and Sulfur Utilization by Maize

As natural plant growth stimulators, amino acids are widely used to improve crop yield and quality. There are numerous studies documenting the influence of amino acids on plants, which is not always positive. This study was conducted to determine the effect of soil-applied L-tryptophan (L-TRP) on the accumulation and utilization of nitrogen and sulfur by maize. The study was carried out under the conditions of a pot experiment. The experimental design included three treatments: soil without fertilization (control), soil with mineral fertilization (NPKS), and soil with mineral fertilization and L-tryptophan addition (NPKS + L-TRP). The application of tryptophan to the soil, supported by mineral fertilization, caused a significant increase in maize biomass. Although no significant differences in nitrogen and sulfur contents in maize biomass were found between treatments without and with the addition of L-tryptophan, significantly higher intakes of both elements were observed in the NPKS + L-TRP treatment. The application of L-tryptophan increased the biosynthesis of Chlorophyll a. Utilization of nitrogen and sulfur by maize in the NPKS + L-TRP treatment was more than 27% and 17% higher, respectively, compared to the NPKS treatment. Maintaining the recommended contents of individual nutrients in the rhizosphere is not a guarantee of optimal quantitative and qualitative intake of nutrients. Problems with maintaining optimal relationships between individual nutrients may be compounded by soil properties.

Effects of Long-Term Straw Management and Potassium Fertilization on Crop Yield, Soil Properties, and Microbial Community in a Rice–Oilseed Rape Rotation

The present study aims to assess the influences of long-term crop straw returning and recommended potassium fertilization on the dynamic change in rice and oilseed rape yield, soil properties, bacterial and fungal alpha diversity, and community composition in a rice–oilseed rape system. A long-term (2011–2020) field experiment was carried out in a selected paddy soil farmland in Jianghan Plain, central China. There were four treatments with three replications: NP, NPK, NPS, and NPKS, where nitrogen (N), phosphate (P), potassium (K), and (S) denote N fertilizer, P fertilizer, K fertilizer, and crop straw, respectively. Results showed that long-term K fertilization and crop straw returning could increase the crop yield at varying degrees for ten years. Compared with the NP treatment, the long-term crop straw incorporation with K fertilizer (NPKS treatment) was found to have the best effect, and the yield rates increased by 23.0% and 20.5% for rice and oilseed rape, respectively. The application of NPK fertilizer for ten years decreased the bacterial and fungal alpha diversity and the relative abundance of dominant bacterial and fungal taxa, whereas continuous straw incorporation had a contradictory effect. NPKS treatment significantly increased the relative abundance of some copiotrophic bacteria (Firmicutes, Gemmatimonadetes, and Proteobacteria) and fungi (Ascomycota). Available K, soil organic matter, dissolved organic carbon, and easily oxidized organic carbon were closely related to alterations in the composition of the dominant bacterial community; easily oxidized organic carbon, dissolved organic carbon, and slowly available K were significantly correlated with the fungal community. We conclude that long-term crop straw returning to the field accompanied with K fertilizer should be employed in rice-growing regions to achieve not only higher crop yield but also the increase in soil active organic carbon and available K content and the improvement of the biological quality of farmland.

Dynamics and composition of soil organic carbon in response to 15 years of straw return in a Mollisol

Crop straw return is being widely applied as a sustainable soil management practice to improve soil organic carbon (SOC) storage in intensive agricultural ecosystems. However, the dynamics and chemical composition of SOC under long-term straw return are not fully understood. Based on a 15-year field experiment with soybean (Glycine max (L.) Merrill.)–maize (Zea mays L.) cropping system, we studied SOC temporal dynamics under no fertilization (NF), mineral fertilizers (NPK) and mineral fertilizers with straw return (NPKS). Meanwhile, we determined five labile carbon (C) pools, i.e., microbial biomass C (MBC), water-soluble organic C (WSOC), light fraction C (LFC), readily oxidizable organic C (ROC), and particulate organic C (POC), as well as δ13C values and organic functional groups of soil organic matter (SOM). After 15-year application of continuous straw return with mineral fertilizer application, SOC content increased by 14.2%. The δ13C values of SOC increased with time under all treatments, and its positive relationship with cumulative maize C input indicated a larger C contribution to SOC from maize than soybean residues. The NPKS treatment significantly increased the contents of MBC, WSOC, LFC, ROC, and POC and their proportion in SOC, compared with the NF treatment. The aliphatic and aromatic relative peak areas were significantly affected by straw return, with an increasing relative peak area at 2930 cm−1 and decreasing at 1620 cm−1, respectively. Our results demonstrate that straw return could continuously increase SOC in an inter-annual rotation of soybean and maize cropping system in Mollisol. Meanwhile, long-term fertilization can alter SOM chemical composition, with lower humification degree under the combined application of straw and mineral NPK fertilizers.

Effects of long-term straw returning on distribution of aggregates and nitrogen, phosphorus, and potassium in paddy

In order to understand the composition and stability of soil aggregate in paddy filed, as well as the changes of soil aggregate-associated nitrogen (N), phosphorus (P) and potassium (K) after straw addition combined with chemical fertilization, soil samples were collected from a 34-year positioning experiment with three treatments, including no chemical fertilizer (CK), chemical fertilizer only (NPK), and straw addition plus chemical fertilizer (NPKS). The composition of water-stable aggregates at the soil layers of 0-20 cm and 20-40 cm were analyzed with the wet sieving method, as well as the distribution characteristics, contribution rate and activation rate of soil aggregate-associated N, P, and K. Results showed that the fractions of >2 mm and 0.25-1 mm aggregates dominated the soil water-stable aggregates in paddy field, while the contribution of <0.053 mm aggregates was lowest. Compared with CK, NPKS treatment increased the contents of >2 mm and 1-2 mm aggregates at the layers of 0-20 and 20-40 cm, and reduced the contents of 0.053-0.25 mm and <0.053 mm. Similar result in NPK treatment was observed at the layer of 0-20 cm. Compared with tat under the NPK treatment, mean weight diameter (MWD) and geometric mean diameter (GMD) increased by 3.9%-15.5% and 6.3%-41.7% in NPKS treatment, respectively. However, the unstable aggregate index (ELT) reduced by 5.7%-28.7% in the NPKS treatment. NPKS significantly increased the contents of total N (TN), available P (AP), and available K (AK) in soil aggregates, especially in the >0.25 mm aggregates. There were no significant diffe-rences about alkali-hydrolysable N (AN) and total K (TK) between NPK and NPKS treatments. The nutrient contribution of soil aggregates in paddy field was affected by aggregate composition. NPKS significantly increased the contribution of AN, AP, and AK within >1 mm aggregates. In all, straw addition combined with chemical fertilizer could increase the stability of soil aggregates at the layers of 0-20 cm and 20-40 cm, and increase the contents of soil aggregate-associated N, P and K, especially for the >1 mm aggregates. Our results provided insights into ensuring soil quality and sustainable development of resources in paddy field by adjusting the ratio of soil C to N.

Understanding the effects of long-term different fertilizer applications on methylmercury accumulation in rice (Oryza sativa L.) plants

The effects of long-term applications of fertilizer form and rate on methylmercury concentration in rice grain have been previously reported. However, there remains a large gap in understanding fertilizer treatments affect the transformation of mercury (Hg) in paddy soils and the translocation of Methylmercury (MeHg) in rice plants. Thus, we investigated the effects of four fertilizer treatments (NK, nitrogen and potassium; NPK, nitrogen, phosphorus and potassium; NPKM, nitrogen, phosphorus, potassium and swine manure; and NPKS, nitrogen, phosphorus, potassium and rice straw) on mercury (Hg) methylation in paddy soils and on MeHg translocation in rice plants after a 24-year period; specifically, we investigated differences in MeHg production in paddy soils and MeHg mass flow in rice plants. The results showed that Geobacteracea and Methanosarcinacea were the major Hg-methylating microorganisms and their relative abundances were significantly promoted by rice straw return, which led to the significantly higher MeHg concentration both in paddy soils and in the rice grain of NPKS treatment compared with control. In addition, the phosphate and organic fertilizers increased MeHg translocation from the roots to the leaves and grain more than the nitrogen and potassium fertilizers did. In detail, compared with the control, NK increased MeHg concentrations in the rice grain by approximately 1-fold; NPK and NPKM, by approximately 3-fold; and NPKS, by approximately 8-fold. Last, we suggest that policy-makers worldwide should comprehensively assess the high potential risk of MeHg in rice grain in large production areas where rice straw is returned.

Higher soil organic carbon accumulation in the subsoil layer by 37 years combined application of inorganic fertilizers with manure than with rice straw in a double-rice paddy soil

ABSTRACT Organic incorporation is widely recommended to increase Soil Organic Carbon (SOC) content, but little was done to compare the effects of using manure and straw in different soil layers. The study was set up to assess the changes of SOC and oxidizable organic carbon fractions in three soil depths (0–15, 15–30 and 30–45 cm) in a double-rice paddy field subjected to 37 years fertilization schemes [control (CK), combined inorganic fertilization (NPK), inorganic fertilizer NK with pig manure (NKM), and inorganic fertilizer NP or NPK incorporated with rice straw (NPS and NPKS)]. The 37 years’ application of NPK, NKM, NPS and NPKS increased SOC content up to 16.5%, 38.7%, 24.4% and 24.2% across the three soil depths, respectively, compared with CK. Both NPS and NPKS treatment had 77% higher SOC accumulation by sequestering more labile SOC fraction (CHL) in topsoil but not the subsoil layers compared with NPK. Long-term NKM application performed best in SOC accumulation (threefold higher compared with NPK) in all studied soil depths, and in the content of CHL in topsoil. Overall, the integration of inorganic fertilizer with organic manure is more effective than straw incorporation on improving SOC accumulation and stability in not only the topsoil but also the subsoil layers.

Characteristics of amino sugar accumulation in rhizosphere and non rhizosphere soil of rice under different fertilization treatments

Soil samples were collected from paddy ecosystem under five long-term fertilization treatments, including control without fertilizer (CK), chemical fertilization alone (NPK), rice residue combined with NPK (NPKS), 30% manure plus 70% chemical fertilizer (LOM), and 60% manure plus 40% chemical fertilizer (HOM) in Ningxiang City, Hunan Province. The cha-racteristics of amino sugars accumulation in the rhizosphere and non-rhizosphere soils at rice tillering stage were analyzed. Results showed that the contents of soil organic carbon, total amino sugars and three amino monosaccharides (muramic acid, glucosamine and galactosamine) with long-term application of organic materials (rice residue or manure) were significantly higher compared with CK and NPK. The inconsistent accumulation trends of the three amino monosaccharides under different fertilization treatments indicated that different responses of microbial groups to various fertilization treatments. The content of total amino sugars was not significantly different between the rhizosphere soil and the non-rhizosphere soil, probably because the agricultural operations such as plowing could homogenize paddy soils. The contribution of amino sugar derived carbon to soil organic carbon ranged from 24.0 to 28.3 mg·g-1, which was highest in NPKS, and lowest in HOM and CK. The ratio of fungal to bacterial residues (fungal glucosamine/muramic acid) ranged from 24.4 to 36.6, indicating that fungi dominated the degradation and transformation of organic matter in all the soils. Compared with that under NPK and CK, the participation of organic matter transformation from fungi under NPKS treatment was increased, whereas the bacteria involved in organic matter transformation under HOM treatment was enhanced.

Response of activity, abundance, and composition of anammox bacterial community to different fertilization in a paddy soil

The anaerobic oxidation of ammonium (anammox) process plays a critical role in the loss of nitrogen (N) in paddy soils, yet the response of anammox to different fertilization is not well documented. In this study, three different fertilized (control, CK; soil treated with inorganic fertilizers, NPK; soil treated with inorganic fertilizer and involving the incorporation of straw, NPKS) paddy soils were selected to investigate the activity, functional gene abundance, diversity, and composition of anammox bacterial community using isotope-tracing technique, quantitative PCR assays, and Illumina sequencing. The anammox rate in the NPKS treatment was 2.4 nmol N g−1 soil h−1, significantly higher than that in CK and NPK treatments (1.7 and 1.8 nmol N g−1 soil h−1, respectively). Potential anammox contributed 6.2–7.8% to total N loss with the remainder being due to denitrification. Significant differences in the number of hydrazine synthase β-subunit (hzsB) gene were observed in three treatments with the highest value being observed in the NPK treatment. The anammox rate of per functional gene in the NPKS treatment (11.4 fmol day−1) was higher than that in CK and NPK treatments (8.3 and 7.0 fmol day−1, respectively). Three genera of anammox bacteria were identified: Candidatus Brocadia, Candidatus Anammoxoglobus, and Candidatus Scalindua, with Candidatus Brocadia being the dominant. Anammox bacteria diversity was significantly lower in the NPK than in CK and NPKS treatments as shown by Shannon, Simpson, Chao 1, and ACE indices (p < 0.05). The results showed that activity, abundance, and composition of anammox bacterial community depended on the type of fertilization.

Contrasting impacts of long-term application of manure and crop straw on residual nitrate-N along the soil profile in the North China Plain

The effects of long-term animal manure application and crop straw incorporation on the migration of carbon (C) and nitrogen (N) deep into the soil profile and the associated N leaching risk in particular have not been thoroughly elucidated to date. Soil profile samples were collected from depths of up to 200 cm from the following four treatments in a 27-year field experiment on the North China Plain: N + phosphorus (P) + potassium (K) fertilizers (NPK), NPK + 22.5 t ha−1 swine manure (NPKM), NPK + 33.7 t ha−1 swine manure (NPKM+) and NPK + straw incorporation (NPKS). The results revealed that long-term manure application and straw incorporation significantly enhanced the soil organic C (SOC) and total N (TN) contents in the upper 20 cm and that this effect was weaker in the deeper soil layers (P < 0.05). Residual nitrate-N (NO3−-N) contents at 0 to 40 cm and 120 to 200 cm in the NPKM and NPKM+ were 4–16 and 2–9 times higher than those in the NPK and NPKS, respectively. These results indicated a greater potential for N leaching from manure addition and a higher propensity for NO3−-N leaching out of the 40–100 cm soil layer. Pearson relationship analysis demonstrated that NO3−-N content was clearly affected by SOC and dissolved organic N (DON) contents along the soil profile (20–200 cm), implying that the higher residual NO3−-N contents in the deeper soil from manure addition were partially attributable to the mineralization and nitrification of the downward SOC and DON. Interestingly, a low level of residual NO3−-N combined with negative mineralization in the 120–200 cm soil layers of the NPKS treatment was observed, suggesting that straw incorporation promotes soil NO3−-N retention. Thus, we concluded that long-term manure application is beneficial for soil NO3−-N content retention, whereas long-term straw incorporation benefits NO3−-N retention.

Fungal communities and functions response to long-term fertilization in paddy soils

Soil fungi play an important role in agricultural ecosystems, yet the understanding of the responses of fungi community and function to different fertilization is limited. Here we report the responses of fungal communities and functions to 34 years of application of inorganic fertilizer (NPK), rice straw combined with inorganic fertilizer (NPKS), and no fertilizer (CK). The fungal communities were identified by Illumina high-throughput sequencing and the functional groups were inferred with FUNGuild. Three predominant Phyla of Ascomycota, Basidiomycota, Zygomycota were identified in three treatments. A significantly lower proportion of Ascomycota (47%) but higher relative abundance of Basidiomycota (28%) were observed in the NPKS treatment compared with the CK (71% and 14%, respectively). A low proportion of Basidiomycota (10%), and no significant changes in Ascomycota (74%) were found in the NPK treatment in comparison to the CK (14% and 71%, respectively). The relative abundance of Zygomycota in the NPKS (8%) and NPK (9%) treatments were significantly higher than in the CK (5%). Diversity analysis showed that indexes including Shannon, Simpson, Chao 1 and ACE all significantly declined in NPK but increased in NPKS compared with CK. FUNGuild revealed that symbiotrophic fungi in the NPKS treatment (8%) were significantly higher than in the NPK (3%) and CK (3%) treatments, and dominated by the genera Redeckera and Tricholoma, the species Trichoderma atroviride and the order Scleroderma. However, a higher proportion of pathotrophic fungi in the NPK treatment (19%) were detected than in the NPKS (11%) and CK (10%) treatments, and dominated by the species Schizangiella serpentis. Redundancy analysis (RDA) showed that fungal community structure and function were more strongly related to moisture, EC and porosity in the soil. We suggest that the sole application of inorganic fertilizer results in great changes in fungal community composition and the hazard of excess growth of pathogenic fungi, whereas combined organic fertilizer and rice straw is beneficial to maintain a healthy ecological environment and the diversity of fungal communities in paddy soil.

Soil N transformation mechanisms can effectively conserve N in soil under saturated conditions compared to unsaturated conditions in subtropical China

The connection between moisture and nitrogen (N) transformation in soils is key to understanding N losses, particularly nitrate (NO3−) losses, and also provides a theoretical framework for appropriate water management in agricultural systems. Thus, we designed this study to provide a process-based background for management decision. We collected soil samples from the long-term field experiment in subtropical China, which was designed to examine tobacco and rice rotations under a subtropical monsoon climate. The field experiment was established in 2008 with four treatments: (1) no fertilization as control; (2) N, phosphorus (P), and potassium (K) fertilizers applied at recommended rates; (3) N fertilizers applied at rates 50% higher than the recommended amounts and P and K fertilizers applied at recommended rates; and (4) N, P, and K fertilizers applied at recommended rates with straw incorporated (NPKS). Soil samples were collected during the unsaturated tobacco-cropping season and saturated rice-cropping season and were incubated at 60% water holding capacity and under saturated conditions, respectively. Two 15N tracing treatments (15NH4NO3 and NH415NO3) and a numerical modeling method were used to quantify N transformations and gross N dynamics. Autotrophic nitrification was stimulated by N fertilizer both under unsaturated and saturated conditions. The rate of NO3− consumption (via immobilization and denitrification) increased under the NPKS treatment under saturated conditions. Secondly, the rates of processes associated with ammonium (NH4+) cycling, including mineralization of organic N, NH4+ immobilization, and dissimilatory NO3− reduction to NH4+, were all increased under saturated conditions relative to unsaturated conditions, except for autotrophic nitrification. Consequently, NO3−-N and NH4+-N concentrations were significantly lower under saturated conditions relative to unsaturated conditions, which resulted in reduced risks of N losses via runoff or leaching. Our results suggest that under saturated conditions, there is a soil N conservation mechanism which alleviates the potential risk of N losses by runoff or leaching.