NPKM Treatment Research Articles

Dynamics and influencing factors of soluble organic nitrogen in paddy soil under different long-term fertilization treatments

Soil soluble organic nitrogen (SON) plays an important role in nitrogen cycling. However, the dynamic and influencing factors of SON in paddy soil are largely unknown. In this study, 36-year fertilization experiments with different fertilized (control (CK), chemical fertilizer (NPK), chemical fertilizer combined with cow manure (NPKM) and chemical fertilizer combined with straw (NPKS)) paddy soils were selected to investigate the dynamic changes and influencing factors of the SON content during the whole rice growth stage. We found that NPK, NPKM and NPKS increased the soil SON content by 20.61 %, 51.33 % and 52.50 %, respectively, compared with CK. Compared with the NPK treatment, the average SON contents in the NPKM and NPKS treatments were significantly increased by 25.46 % and 26.44 %, respectively, but there was no significant difference between the NPKM and NPKS treatments. Soil SON contents in different growth stages under the same fertilization treatments showed maturity stage > tillering stage > flowering stage > jointing stage > seedling stage. Structural equation model (SEM) analysis showed that bacterial biomass was an important factor affecting soil SON content, with a total path coefficient of 0.92, including direct path coefficients of 0.24 for soil SON and indirect path coefficients of 0.68 for soluble organic carbon, microbial biomass nitrogen and protease activity. Redundancy sequencing analysis showed that SON contents during different rice growth stages were significantly affected by the bacterial community, i.e., Chloroflexi in the early growth stage and Proteobacteria and Bacteroidetes in the later stage contributed 5.33 %, 37.40 %, and 2.17 % to the SON content variation, respectively. Conclusively, the dynamics of SON in paddy soil under long-term fertilization treatments are closely related to both fertilization treatments and the rice growth stage and are mainly affected by the bacterial community.

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.

Low N2O emissions from wheat in a wheat-rice double cropping system due to manure substitution are associated with changes in the abundance of functional microbes

Fertilization has been shown to affect nitrogen (N) cycling and its related functional microbes in agricultural soils. However, the linkage between soil N2O emissions and N-related functional genes under different fertilization strategies from wheat in a wheat-rice double cropping system is rarely examined. Here, we carried out a two-year field study to examine the response of soil N2O emissions driven by N cycling functional genes [archaeal and bacterial amoA (AOA + AOB), nirS, nirK and nosZ] to different fertilization strategies in a wheat cropland of subtropical China. Three fertilizer treatments were established consisting of chemical phosphorus (P) and potassium (K) fertilizer application (PK), chemical N (urea) and PK fertilizer application (NPK), and chemical NPK fertilizer application with chemical N partially replaced with manure (composted pig manure) (NPKM). Over the two wheat seasons, seasonal total N2O emissions averaged 0.66, 3.60 and 3.11 kg N ha−1 for PK, NPK and NPKM plots, respectively. Relative to the NPK treatment, the NPKM treatment significantly decreased N2O emissions by 14 % without compromising grain yield, with a lowered combined fertilizer-induced emission factor (EF) of 1.02 %. Compared with the PK treatment, N fertilization consistently and significantly increased the abundance of ammonium-oxidation bacteria (AOB), nirS, nirK and nosZ genes. The nosZ gene, which drives N2O reduction during denitrification, showed a greater extent under NPKM with manure N combination. The AOB had a more sensitive response than ammonium-oxidation archaea (AOA) to chemical N fertilization. Seasonal N2O emissions showed significant positive correlations with AOB gene abundance and the ratio of (nirK+nirS)/nosZ, while had a negative correlation with nosZ gene abundance across N fertilized treatments. The N2O-related microbial composition of functional genes was significantly changed by N fertilizer application and also showed contrasting patterns between treatments of chemical N fertilizer with and without manure N substitution. Together, partially replacing chemical N fertilizer with manure reduced N2O emissions from wheat in a wheat-rice double cropping system, mainly through decreasing AOB associated-nitrifying potential, and particularly stimulating N2O reduction.

Linkages between ecoenzymatic stoichiometry and microbial community structure under long-term fertilization in paddy soil: A case study in China

This study investigated relationships between ecoenzymatic stoichiometry and microbial community structure under long-term (38-years) combined organic and inorganic fertilization. Experiment was initiated in 1982 and seven fertilization treatments were studied that included: CK (no fertilizer), NPK (inorganic N, P and potassium fertilizers), NPM (inorganic N, P fertilizers and manure), NKM (inorganic N, K fertilizers and manure), PKM (inorganic P, K fertilizers and manure), NPKM (inorganic N, P, K fertilizers and manure) and M (manure) treatments. We analyzed soil and microbial biomass of C, N and P, enzyme activities related to C, N and P, and their effects on the microbial community structure in the top layer (0–20 cm) of the paddy soil. The results showed that combined application of manure and inorganic fertilizers significantly increased soil C, N, P contents, extracellular enzyme activities and total phospholipid fatty acid (PLFA) concentration, compared to the NPK and CK treatments. Fertilization treatments significantly changed microbial biomass stoichiometry. Among all fertilization treatments, highest microbial biomass C:N, C:P ratio and N:P ratios were 10.2, 36.0 and 3.59, respectively, under the PKM treatment. Soil C, N and P-cycling enzyme activities were highest under the NPKM treatments. Soil C:N showed positive and soil C:P ratio showed negative relationships with microbial biomass C:N and C:P ratios, respectively. Soil total PLFA concentration was highest under NPKM treatment. The order of individual PLFAs concentrations in all fertilization treatments was: Gram negative bacteria > Gram positive bacteria > Actinomycetes > AM fungi > Fungi> Eukaryotes > anaerobes. Soil organic C, total N, soil N:P and microbial N:P ratios were positively correlated with total PLFA concentration. While, soil C:N and microbial biomass C:N ratios were negatively correlation with total PLFA and individual microbial community structure. Our results suggested that ecoenzymatic stoichiometry and microbial community structure are very sensitive to combined long-term organic and inorganic fertilization that can affect the nutrient acquisition and cycling in agroecosystem.

Dynamics of ammonia oxidizers in response to different fertilization inputs in intensively managed agricultural soils

Ammonia oxidation, the rate-limiting step of autotrophic nitrification, is carried out by ammonia-oxidizing archaea (AOA) and bacteria (AOB) that differ in their physiology and metabolic pathways. However, there has been little field investigations on the dynamic responses of ammonia oxidizers to different fertilizer inputs in intensive crop rotations. Here, we examine the soil potential nitrification rates (PNR), abundance and communities of AOB and AOA across different growth stages of wheat and maize at two soil depths (0–20 and 20–40 cm) under three fertilization regimes, namely zero fertilization (CK), chemical fertilizer (NPK) and partial replacement of the chemical fertilizer nutrients with organic manure (NPKM). Soil PNR was higher in the surface soil than in the subsoil in the wheat season, but no significant difference was observed in the maize season. Fertilization especially NPK treatment increased soil PNR, and the effect was more pronounced in the surface soil in the wheat season. The influence of soil depth on the abundance and communities of AOA and AOB was greater than that of fertilizer regime or plant growth stage across wheat and maize growth stages. The differences of ammonia oxidizers communities were mainly attributable to soil total nitrogen, organic carbon and soil moisture contents in the two depths. The abundance of AOB decreased more sharply than that of AOA with increasing soil depth. In general, application of NPK only increased AOB abundance in both wheat and maize seasons, whereas NPKM treatment increased AOB abundance only in the maize season (except in the subsoil at harvest). Generally, soil PNR was correlated with AOB and AOA abundances except for AOB in the subsoil in the wheat season and AOA in the surface soil during maize growth. Fertilization regime did not increase AOA abundance or alter AOA communities across all soil depths and plant growth stages. Our results indicate that the ammonia oxidizers differ greatly in responses to fertilization regimes and the changes are associated with soil depth and crop growth stage. The dynamics of soil microorganisms should be integrated into fertilizer and crop management to achieve sustainable agricultural systems.

Composition, predicted functions, and co-occurrence networks of fungal and bacterial communities_ Links to soil organic carbon under long-term fertilization in a rice-wheat cropping system

Soil microbial community compositions and soil labile organic carbon (LOC) fractions change rapidly in response to fertilizers and crop rotation, but our understanding of the associations between specific microbial taxa and C fractions is still limited. The objective of this study was to compare the composition and function of soil bacterial and fungal communities in response to fertilization and crop rotation and their correlations with LOC fractions. Communities were analyzed by PCR-amplification of bacterial 16S rRNA gene and fungal ITS sequences from soil DNA. Functions were predicted by in-silico tools. A long-term experimental rice-wheat cropping system was studied, and the soil collected from plots treated with different fertilizers, including a control without fertilizer (CK), inorganic nitrogen, phosphorous, and potassium (NPK), manure (M), and NPKM, was analyzed. Fertilizations, non-crop types, significantly modified the soil bacterial community composition and suspected functions, while both fertilization and crop types significantly affected the fungal community composition. Compared with CK, NPKM treatment significantly increased the relative bacterial abundances of Candidatus Solibacter, Nitrosporia, Geobacter and Bryobacter, but decreased the relative fungal abundances of Paraphaeosphaeria sporulosa, Emericellopsis, Cladosporium tennuissimum, Curvularia lunata, Stachybotrys chlorolonate and Moleospora. All responding OTUs correlated significantly with soil LOC fractions, especially particulate organic C. The suspected functions suggest that organic amendment provided larger quantities of organic C, then promoting the growth of microorganism involved in C degradation, which in turn contributing to C sequestration. Our results indicate that fertilizers and crop rotation alter the diversity and function of soil bacterial and fungal communities, thereby causing an impact on agroecosystems.

Nitrous oxide emissions in response to straw incorporation is regulated by historical fertilization

The incorporation of crop straw with fertilization is beneficial for soil carbon sequestration and cropland fertility improvement. Yet, relatively little is known about how fertilization regulates the emissions of the greenhouse gas nitrous oxide (N2O) in response to straw incorporation, particularly in soils subjected to long-term fertilization regimes. Herein, the arable soil subjected to a 31-year history of five inorganic or organic fertilizer regimes (unfertilized; chemical fertilizer application, NPK; 200% NPK application, 2 × NPK; manure application, M; NPK plus manure application, NPKM) was incubated with and without rice straw to evaluate how historical fertilization influences the impact of straw addition on N2O emissions. The results showed that compared to the unfertilized treatment, historical fertilization strongly increased N2O emissions by 0.48- to 34-fold, resulting from increased contents of hot water-extracted organic carbon (HWEOC), NO3−, and available phosphorus (Olsen-P). Straw addition had little impact on N2O emission from the unfertilized and NPK treatments, primarily due to Olsen-P limitation. In contrast, straw addition increased N2O emissions by 102–316% from the 2 × NPK, M, and NPKM treatments as compared to the corresponding straw-unamended treatments. These results indicated that N2O emissions in response to straw addition were largely regulated by historical fertilization. The N2O emissions were closely associated with the depletion of NO3− and decoupled from change in NH4+ content, suggesting that NO3− was the main substrate for N2O production upon straw addition. The stoichiometric ratios of HWEOC to mineral N and mineral N to Olsen-P were key factors affecting N2O emissions, underscoring the importance of resource stoichiometry in regulating N2O emissions. In conclusion, historical fertilization largely regulated the impacts of crop straw incorporation on N2O emissions via shifts in NO3− depletion and the stoichiometry of HWEOC, mineral N, and Olsen-P.

The chemical nature of soil phosphorus in response to long-term fertilization practices: Implications for sustainable phosphorus management

Developing sustainable phosphorus (P) management strategies is a worldwide challenge in the face of growing food demand, dwindling rock phosphate resources, and deteriorating environmental quality. This requires improved knowledge on the effect of current fertilization practices on long-term P accumulation, bioavailability and mobility, which are functions of the chemical nature of soil P. Using state-of-the-art techniques such as solution 31P-NMR and P K-edge XANES spectroscopy, combined with sequential fractionation, this study investigated the chemical speciation and transformation of P in soils under three fertilization treatments (no fertilization, CK; chemical nitrogen (N), P and potassium (K) fertilization, NPK; and partial substitution of chemical fertilizer with sheep manure, NPKM) from a long-term experiment in China initiated in 1990. Compared to the NPK soil, total P and organic P (Po) concentrations were lower in the NPKM soil in the sampling years of 2000 and 2015, suggesting reduced P accumulation under the NPKM treatment. The NaHCO3-extracted P, considered the most readily plant-available P fraction, significantly increased in the NPKM soil relative to the NPK soil in 2000. The P-XANES results revealed the formation of the magnesium (Mg)–P compound newberyite, which probably contributed to soil P availability with NPKM application. Moreover, P-NMR analysis showed higher deoxyribonucleic acid content and ratio of orthophosphate diesters to monoesters in the NPKM soil relative to the NPK soil, which implied the stronger soil Po lability with NPKM fertilization. This study provides new insights on the significant role of NPKM treatment in optimizing soil P fertilization regimes, which benefits sustainable crop production.

The response of soil Olsen-P to the P budgets of three typical cropland soil types under long-term fertilization.

The Olsen phosphorus (Olsen-P) concentration of soil is generally a good indicator for estimating the bioavailability of P and environmental risk in soils. To maintain soil Olsen-P at adequate levels for crop growth and environmental sustainability, the relationship between soil Olsen-P and the P budget (the P input minus the output) as well as the variations of soil Olsen-P and P budget were investigated from three long-term (22 years) experiments in China. Five treatments were selected: (1) unfertilized control (CK); (2) nitrogen and potassium (NK); (3) nitrogen, phosphorous, and potassium (NPK); (4) nitrogen, phosphorous, potassium and straw; (5) nitrogen, phosphorous, potassium and manure. The results showed that without P fertilizers (CK, NK), there was a soil P deficit of 75-640 kg ha-1, and the lowest P deficit (mean of CK and NK) was in Eutric Cambisol. Soil Olsen-P decreased by 0.11-0.39 mg kg-1 year-1 in the order of Luvic Phaeozems > Eutric Cambisol > Calcaric Cambisol. Soil Olsen-P and the P deficit had a significantly (P<0.01) positive linear relationship. For every 100 kg of P ha-1 of deficit, soil Olsen-P decreased by 0.44-9.19 mg kg-1 in the order of Eutric Cambisol > Luvic Phaeozems > Calcaric Cambisol. Under the P fertilizer treatments (NPK, NPKS, and NPKM), soil Olsen-P showed an obvious surplus (except the NPK and NPKS in Luvic Phaeozems) of 122-2190 kg ha-1, and the largest P surplus was found under the NPKM treatment at each site. The relation between soil Olsen-P and the experimental years could be simulated using quadratic equation of one unknown in Calcaric Cambisol for the lower P input after 14 years of fertilization. And soil Olsen-P increased by 1.30-7.69 mg kg-1 year-1 in the order of Luvic Phaeozems > Eutric Cambisol. The relation between soil Olsen-P and the P surplus could be simulated by a simple linear equation except under NPK and NPKS in Luvic Phaeozems. With 100 kg ha-1 P surplus, soil Olsen-P increased by 3.24-7.27 mg kg-1 in the order of Calcaric Cambisol (6.42 mg kg-1) > Eutric Cambisol (3.24 mg kg-1). In addition, the change in soil Olsen-P with a 100 kg P ha-1 surplus (soil Olsen-P efficiency) was affected by the soil organic matter (SOM), pH, and CaCO3 content, etc. In the practice of fertilization, it's not necessary to increase the amount of P fertilizers, farmers should take measure to solve the local problem, for adjust the soil pH of Eutric Cambisol and Calcaric Cambisol, and apply more nitrogen in Luvic Phaeozems. In the area of serious soil P surplus, it is encouraged to stop applying P fertilizers for a few years to take advantage of soil accumulated P and make the high Olsen-P content decrease to a reasonable level.

Effects of Long-Term Fertilization on Different Nitrogen Forms in Paddy along Soil Depth Gradient

The combined application of organic fertilizer and chemical fertilizer is an effective measure to increase nutrient content of soil plough layer, which must have a profound impact on the deep soil nutrients, especially the contents of nitrogen forms. The purpose of this study was to explore the characteristics of soil nitrogen forms in plough layer and along depth gradient in different fertilization treatments, so as to evaluate the soil quality in spatial dimension, further providing a theoretical basis for scientific fertilization and improvement of paddy soil fertility. Here, a 34-year field experiment was conducted with three treatments: without any fertilizer (CK), pure chemical fertilizer (NPK) and chemical fertilizer combined with organic fertilizer (NPKM). We analyzed the content of nitrogen forms in 0 - 100 cm soil depth and their ratios to total nitrogen (TN), and discussed the correlation between nitrogen forms contents and pH, CEC. Results showed that, compared with CK, both NPK and NPKM significantly increased the contents of nitrogen forms in topsoil (soil layer of 0 - 20 cm), especially nitrate nitrogen (NO3--N) content increased by 70% (NPK) and 111% (NPKM), respectively. Although the contents of different nitrogen forms decreased gradually along soil depth gradient, NPKS slowed down the decline rate of TN and alkali-hydrolysable nitrogen (AN) in 0 - 60 cm soil layer, compared to CK. Compared to NPK, NPKM significantly increased the NO3--N/TN ratio in 0 - 20 cm soil layer, but also decreased the content of -N in 20 - 40 cm, which was beneficial to reduce the risk of nitrogen leaching caused by nitrate leaching into deep layer. The increase of soil pH in NPKM treatment obviously alleviated the problem of soil acidification caused by long-term application of chemical fertilizer. Correlation analysis showed that there was a significant positive correlation between soil nitrogen forms and cation exchange capacity (CEC), but no significant correlation with soil pH. In conclusion, NPKM ensured the nutrients of soil plough layer (0 - 20 cm), also reduced the risk of nitrogen infiltration and nitrogen loss, thus ensuring the fertility of soil profile.

Partial substitution of chemical fertilizers with organic amendments increased rice yield by changing phosphorus fractions and improving phosphatase activities in fluvo-aquic soil

The aim of this study was to investigate the effect of combined use of chemical fertilizers with wheat straw and swine manure on rice yield and phosphorus (P) fractions distribution. We selected two field experiments, both located in Hefei, Anhui province of China. We named these experimental sites as HF-1 and HF-2. The experiments HF-1 and HF-2 were initiated on 2013 and 2015 respectively in fluvo-aquic soil. Treatments included CK (no fertilizer), NPK (chemical fertilizers), NPKS (chemical fertilizers plus wheat straw), and NPKM (chemical fertilizers plus swine manure at HF-1, and chemical fertilizers plus fermented wheat straw at HF-2). Results showed that NPKS and NPKM treatments increased grain yield, P use efficiency (PUE), P recovery efficiency, and partial factor productivity, and decreased apparent P balance compared with CK and NPK at both sites. NPKS and NPKM improved pH, organic matter, and nutrient contents in surface soil (0–20 cm) and also improved pH and organic matter content in sub-surface soil (20–40 cm), compared with CK and NPK treatments. Olsen P content in both soil depths was highest under NPKM, but NPKS had lower Olsen P content in sub-surface soil compared with that of NPK. Organic amendments changed P fractions and increased labile P and moderately labile P fractions, and decreased residual P compared with NPK. Organic amendments with chemical fertilizers also improved acid phosphomonoesterase and phosphodiesterase activities compared with chemical fertilization. In addition, path analysis showed relationship between soil properties and P mobility, and also explained soil properties influencing grain yield by changing PUE. Our results suggested that organic amendments improve crop yield through improving PUE by changing soil P fractions and increasing the phosphatase activities.

Influence of biodiversity and iron availability on soil peroxide: Implications for soil carbon stabilization and storage

AbstractIntensive agriculture results in soil degradation, especially with the depletion of soil organic carbon (SOC). Application of organic fertilizers (e.g., animal manures) alone or combined with chemical fertilizers can alleviate soil degradation by increasing soil C while causing variations in soil hydrogen peroxide (H2O2) and iron availability. However, the underlying relationships among soil H2O2 production, iron availability, and soil C storage following organic fertilization remain poorly understood. By combining results from three agroecosystem experiments from temperate northern to subtropical southern China with 25–29 years of different fertilization treatments (no fertilization, Control; inorganic nitrogen, phosphorus and potassium fertilization [NPK]; NPK plus manure [NPKM]), here, we show that NPK treatments increased soil H2O2 but decreased biodiversity (i.e., Shannon index) and SOC compared with NPKM treatments across all of the three experiments. In all of the examined soils, the contents of H2O2 were approximately 0.7– to 7–fold higher under NPK treatment than under Control or NPKM treatments. The contents of H2O2 were significantly affected by the fertilization regimes, experimental places, and their interaction. Compared with Control and NPKM treatments, NPK treatment decreased Shannon index to the greatest extent (~1.5) at the Qiyang experiment in subtropical southern China, followed by Jinxian (~0.6) in subtropical southern China, and Gongzhuling (~0.3) in temperate northern China. There was a significant and negative correlation between soil H2O2 and Shannon index or mobilized iron, indicating that high biodiversity and high mobilized iron were beneficial to the decay of soil H2O2. Results from microcosm experiments support the field observations, implying that the occurrence of microbial‐mediated Fenton‐like reactions or free radical reactions was influenced by organic inputs. Together, these findings suggest that long‐term manure inputs to soil initialize free radical reactions by activating microbial communities and mobilizing iron, providing benefits for soil C stabilization and storage by increasing recalcitrance and SOC interactions.

Soil carbon (C), nitrogen (N) and phosphorus (P) stoichiometry drives phosphorus lability in paddy soil under long-term fertilization: A fractionation and path analysis study.

Soil C:N:P stoichiometry plays a vital role in nutrient cycling in ecosystems, but its importance to P transformation in paddy soil remains unclear. We investigated the effect of soil C:N:P stoichiometry on P mobility and uptake under long-term fertilization. Three treatments, CK (no fertilization), NPK (inorganic nitrogen, phosphorus and potassium fertilization) and NPKM (combined inorganic NPK fertilizer and manure application), were selected from two long-term experiments of paddy soil that were initiated in 1991 and 1982 in Chongqing and Suining, respectively. The results showed that in comparison the control treatment, under long-term fertilization, soil pH decreased. In comparison with the NPK and CK treatments, the NPKM treatment significantly increased soil nutrient contents, P uptake and phosphatase activities. In comparison to the CK treatment, the NPK and NPKM treatments significantly decreased soil C:N, C:P and N:P ratios. In comparison to NPK and CK treatments, the NPKM treatment decreased residual-P at both sites. Compared with CK treatment, the NPKM treatments increased labile-P and moderately labile-P by 987% and 144%, respectively, and NPK treatment increased these factors by 823% and 125%, respectively, at the Chongqing site. At the Suining site, with NPKM treatment, increases in labile-P and moderately labile-P were 706% and 73%, respectively, and with NPK treatment, the increases were 529% and 47%, respectively. In contrast, non-labile-P was significantly decreased with NPKM treatment in comparison to that with NPK and CK treatments. Moreover, increases in soil C:N and C:P ratios decreased the labile-P pools and increased non-labile-P pools. A path analysis indicated that soil C:N:P stoichiometry indirectly controlled P uptake by directly affecting P transformation from non-labile to labile-P pools. Moreover, the non-labile-P in soil with high SOM and P content directly affected P uptake, indicating that soil P transformation is mainly driven by soil C and P in paddy soil. In conclusion, understanding mechanism of P mobility influenced by soil C:N:P stoichiometry could be helpful to manage soil P fertility under long-term fertilization in paddy soils of these regions.

Response of soil aggregate-associated potassium to long-term fertilization in red soil

Potassium (K) deficiency is commonly observed during crops grown in the red soil (which is a typical Plinthosol based on the IUSS guidelines) of China, but few studies have examined soil aggregate-associated K. In a long-term field experiment (initiated in 1986), the following treatments were applied: no fertilizer (CK), nitrogen and phosphorus fertilizers (NP), NP and K fertilizers (NPK) and a combination of manure and NPK (NPKM). After 30 years of fertilization, the nonexchangeable K (Nonex-K), and exchangeable K (Ex-K) contents or stocks in most aggregates varied among different treatments. Compared with NP treatment, the Nonex-K contents in the aggregate fractions of >2-, 1–2-, 0.5–1-, 0.25–0.5- and 0.053–0.25-mm under NPKM treatment were increased by 40.57%, 40.78%, 42.71%, 40.82% and 55.43%, respectively. The Nonex-K contents in 0.5–1-, 0.25–0.5-, 0.053–0.25- and <0.053-mm aggregates of the NPK treatment were 29.17%, 31.63%, 43.48% and 35.42% higher than those of the NP treatment, respectively. The Ex-K contents in all aggregates of the NPKM treatment were significantly (p < 0.05) higher than those of the other treatments (CK, NP and NPK). Compared with the NP treatment, the Ex-K contents in 1–2-, 0.5–1-, 0.25–0.5- and 0.053–0.25-mm aggregates of NPKM treatment were significantly increased by 30.30%, 32.65%, 33.67% and 33.33%, respectively. Moreover, the Ex-K stocks in the >2-, 1–2-, 0.5–1- and 0.053–0.25-mm aggregates of NPKM treatment were significantly increased by 74.36%, 123.63%, 44.88% and 37.47%, respectively, compared with those of the NPK treatment. Furthermore, a random forest model showed that K stocks in the >2-, 1–2- and 0.5–1-mm aggregates were the main factors affecting the uptake of K by maize. The relationships between the uptake of K by maize and K stocks in the >2-, 1–2- and 0.5–1-mm aggregates could be fitted by linear equations. Therefore, the long-term combination of chemical fertilizers with manure improved K contents and stocks for most aggregate sizes in red soil, especially Ex-K. We found that the turnover rate of K in the >2-mm aggregates was faster than in the other aggregates (1–2- and 0.5–1-mm) through slopes of linear regressions, allowing the soil to meet the requirements for crop K uptake.

Changes in phosphorus fractions associated with soil chemical properties under long-term organic and inorganic fertilization in paddy soils of southern China.

The identification of phosphorus (P) fractions is essential for understanding the transformation and availability of P in paddy soils. To investigate the soil P fractions associated with soil properties under long-term fertilization, we selected three fertilization treatments, including no fertilization (CK), chemical fertilizers (NPK) and chemical fertilizers combined with manure (NPKM), from three long-term experiments located in Nanchang (NC), Jinxian (JX) and Ningxiang (NX). The results showed that chemical fertilizers combined with manure (NPKM) significantly (P ≤ 0.05) increased the soil total phosphorus, Olsen P and soil organic matter (SOM) by 2, 3 and 1 times, respectively, compared with the NPK treatment, and by 4, 17 and 2 times, respectively, compared with the CK treatment. NPKM significantly increased the grain yield compared with CK and NPK at all sites. The apparent P balance with NPK was higher in NC and NX but lower in JX compared with NPKM. Hedley fractionation revealed the predominance of most of the organic and inorganic phosphorus (Po and Pi) fractions with long-term fertilization, especially with the NPKM treatment, at all sites. The nonlabile P pool decreased by 14% and 18% whereas the moderately labile P pool proportions increased by 3 and 6 times with the NPK and NPKM treatments, respectively, compared to the CK treatment. The labile P pool showed a significant positive relationship with the SOM, total P and Olsen P contents. The moderately labile P was positively correlated with the total P and Olsen P. A significant positive correlation was observed between soil pH and the nonlabile P pool. Redundancy analysis revealed that the moderately labile P fraction (HCl dil. Pi fraction) was remarkably increased by the NPKM treatment and significantly correlated with the soil pH and total P concentration. The labile P fraction (NaHCO3-Pi) showed a strong relationship with the Olsen P and total P. However, the residual P fraction was negatively correlated with the HCl. dil. Pi fraction. We concluded that NPKM application improved P availability by many folds compared to NPK, which could lead to environmental pollution; therefore, the rate of combined application of manure and chemical fertilizer should be reduced compared to chemical fertilizer inputs to minimize the wastage of resources and environmental P losses.

Soil aggregate size modifies the impacts of fertilization on microbial communities

Soil microorganisms play an essential role in the redistribution of soil aggregate sizes due to the microenvironmental changes, but little is known about the changes in microbial communities (especially fungi) and their activities within different microenvironments due to agronomic managements. Here, we examined the influence of fertilization (inorganic and inorganic plus organic amendments) on soil aggregate distribution and the microbial communities within aggregates at the site of a long-term fertilization experiment. Significant impacts on soil aggregate distribution and distinct changes in fungal and bacterial communities in soil aggregates due to fertilization were observed, with the fungal community exhibiting more marked changes compared with the bacterial community. Soil carbon and nitrogen contents in aggregates were the most closely related to changes in microbial communities associated with soil aggregates. The changes of microbial taxa abundance in aggregates were generally accompanied by the redistributions of these taxa among all size aggregates, showing the preference for specific ecological niches for these taxa under fertilizations. Both applications of nitrogen alone (N) and inorganic plus organic amendment (NPKM) contributed to mega-aggregate formation. N treatment caused the low total fungal counts in mega-aggregates and carbon depletion, whereas it increased the abundance of N2O-producing fungi in both mega- and macro-aggregates, denitrifying bacteria in micro-aggregates and nitrifying bacteria in macro-aggregates. In contrast, NPKM treatment mitigated/prevented the sharp shifts of the abundance of these microbes mentioned above with nitrogen fertilization, and increased the ectomycorrhizal fungi abundance in all soil fractions as well as carbon accumulation in soil. Our results suggest that the responses of soil bacteria and fungi to fertilizations are modified by their ecological niches, which responses are responsible for the variation of ecosystem services.

Response of soil Olsen-P to P budget under different long-term fertilization treatments in a fluvo-aquic soil

The concentration of soil Olsen-P is rapidly increasing in many parts of China, where P budget (P input minus P output) is the main factor influencing soil Olsen-P. Understanding the relationship between soil Olsen-P and P budget is useful in estimating soil Olsen-P content and conducting P management strategies. To address this, a long-term experiment (1991–2011) was performed on a fluvo-aquic soil in Beijing, China, where seven fertilization treatments were used to study the response of soil Olsen-P to P budget. The results showed that the relationship between the decrease in soil Olsen-P and P deficit could be simulated by a simple linear model. In treatments without P fertilization (CK, N, and NK), soil Olsen-P decreased by 2.4, 1.9, and 1.4 mg kg–1 for every 100 kg ha–1 of P deficit, respectively. Under conditions of P addition, the relationship between the increase in soil Olsen-P and P surplus could be divided into two stages. When P surplus was lower than the range of 729–884 kg ha–1, soil Olsen-P fluctuated over the course of the experimental period with chemical fertilizers (NP and NPK), and increased by 5.0 and 2.0 mg kg–1, respectively, when treated with chemical fertilizers combined with manure (NPKM and 1.5NPKM) for every 100 kg ha–1 of P surplus. When P surplus was higher than the range of 729–884 kg ha–1, soil Olsen-P increased by 49.0 and 37.0 mg kg–1 in NPKM and 1.5NPKM treatments, respectively, for every 100 kg ha–1 P surplus. The relationship between the increase in soil Olsen-P and P surplus could be simulated by two-segment linear models. The cumulative P budget at the turning point was defined as the “storage threshold” of a fluvo-aquic soil in Beijing, and the storage thresholds under NPKM and 1.5NPKM were 729 and 884 kg ha–1 P for more adsorption sites. According to the critical soil P values (CPVs) and the relationship between soil Olsen-P and P budget, the quantity of P fertilizers for winter wheat could be increased and that of summer maize could be decreased based on the results of treatments in chemical fertilization. Additionally, when chemical fertilizers are combined with manures (NPKM and 1.5NPKM), it could take approximately 9–11 years for soil Olsen-P to decrease to the critical soil P values of crops grown in the absence of P fertilizer.

Different Response of Grain Yield to Soil Organic Carbon, Nitrogen, and Phosphorus in Red Soil as Based on the Long-Term Fertilization Experiment

Soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) are essential nutrients for plant growth. Thus, soil amendments that increase the amounts of these nutrients can improve crop yield in poor growing soils such as Red soils in subtropical China, which were classified as Plinthosols in the WRB (World Reference Base for Soil Resources). It is unknown whether any of the three nutrients are more influential than the others on crop yield. A 30-year-long field experiment was established in 1986 to investigate the response of grain yield to changes in SOC, TN and TP levels, identified by regression equations, due to various fertilizer treatments applied to corn crops grown in red soil. Corn yields varied significantly under four different fertilizer treatments. For most of the experimental years, the two fertilizer treatments of application of manure combined with inorganic nitrogen, phosphorus, and potassium (NPKM) and a higher rate of NPK (HNPK) fertilizer produced significantly higher yields than the control (no addition of fertilizer, CK) by 441–548 and 380–703%, respectively. Differences between yields increased over time, particularly between NPKM and CK treatments. The long-term application of NPKM increased SOC, TN and TP contents more so than the other treatments and contents were greater than those in the CK by 48.6–57.1, 26.4–37 and 81.6–142%, respectively. There were no significant differences between the treatments with the low and high rates of NPK (LNPK and HNPK). Additional results showed that NPKM raised the C : N ratio and lowered the C : P ratio. The relationships between SOC, TN, TP and grain yield were best represented by the Michaelis–Menten equation rather than the simple linear equation. Moreover, the equations estimated different maximum yields of corn; yields estimated by SOC and TP were higher than the yield estimated by TN. Furthermore, The estimated maximum yield that could be attained is at a C : N ratio of 10.22 in red soil. Therefore, SOC and TP are likely the more important nutrients to consider altering in soil management of red soil.

Growth and Distribution of Maize Roots in Response to Nitrogen Accumulation in Soil Profiles after Long-Term Fertilization Management on a Calcareous Soil

The replacement of inorganic fertilizer nitrogen by manure is highlighted to have great potential to maintain crop yield while delivering multiple functions, including the improvement of soil quality. However, information on the dynamics of root distributions in response to chemical fertilizers and manure along the soil profile is still lacking. The aim of this study was to investigate the temporal-spatial root distributions of summer maize (Zea mays L.) from 2013 to 2015 under four treatments (unfertilized control (CK), inorganic fertilizer (NPK), manure + 70% NPK (NPKM), and NPKM + straw (NPKMS)). Root efficiency for shoot N accumulation was increased by 89% in the NPKM treatment compared with the NPK treatment at V12 (the emergence of the twelfth leaf) of 2014. Root growth at 40–60 cm was consistently stimulated after manure and/or straw additions, especially at V12 and R3 (the milk stage) across three years. Root length density (RLD) in the diameter &lt;0.2 mm at 0–20 cm was significantly positively correlated with soil water content and negatively with soil mineral N contents in 2015. The RLD in the diameter &gt;0.4 mm at 20–60 cm, and RLD &lt;0.2 mm, was positively correlated with shoot N uptake in 2015. The root length density was insensitive in response to fertilization treatments, but the variations in RLD along the soil profile in response to fertilization implies that there is a great potential to manipulate N supply levels and rooting depths to increase nutrient use efficiency. The importance of incorporating a manure application together with straw to increase soil fertility in the North China Plain (NCP) needs further studies.

Total and available metal concentrations in soils from six long-term fertilization sites across China.

Approximately 19% of agricultural soils in China are contaminated by heavy metals. However, the effects of agricultural management practices on soil contamination are not well understood. Taking advantage of six long-term (23-34years) field sites across China, this study examined the effects of different agricultural fertilization treatments, including control (no fertilization), inorganic nitrogen, phosphorus and potassium fertilization (NPK), manure fertilization (M), and NPK plus manure fertilization (NPKM), on the total and available metal concentrations in soils. The results showed that after 23-34years of fertilization, the M and NPKM treatments significantly increased the total concentration of cadmium (Cd), copper (Cu), and zinc (Zn) in soils compared with the concentrations measured for the control and NPK treatments. In contrast, the fertilization treatments had almost no influence on soil lead (Pb) and nickel (Ni) concentrations. The results of analysis via diffusive gradients in thin films demonstrated that long-term sheep or cattle manure fertilization increased the available metals, especially Cd, Cu, and Zn, but long-term swine manure application decreased the available metals, except for Cu and Zn, in soils. Further analysis revealed that the manure source, soil pH level, and biogeochemical properties of metals affected the availability of Cd, Cu, Pb, Zn, and Ni in soils. Collectively, organic fertilizers had the potential to reduce metal uptake by crops, but caution should be taken to reduce metal concentrations in manure.