Functional diversity of soil microbial communities in response to supplementing 50% of the mineral N fertilizer with organic fertilizer in an oat field

ZHANG Mei-jun,JIA Ju-qing,LU Hua,FENG Mei-chen,YANG Wu-de

College of Agriculture,Shanxi Agricultural University,Taigu 030801,P.R.China

Abstract The effects of supplementing 50% of the mineral N fertilizer with organic fertilizer on the metabolism and diversity of soil microbial communities in an oat field were investigated using Biolog-Eco plates. The experiment consisted of five treatments:no fertilizer (CK),mineral N fertilizer applied at 90 and 45 kg ha-1 N in the form of urea (U1 and U2,respectively),and U2 supplemented with organic fertilizer in the form of sheep manure at 90 and 45 kg ha-1 N (U2OM1 and U2OM2,respectively).Each treatment had three replications. The experiment was conducted in 2018 and 2019 in Pinglu District,Shanxi Province,China. The carbon source utilization by soil microbial communities,such as amino acids,amines,carbohydrates,carboxylic acids,and polymers,increased when 50% of the mineral N fertilizer was replaced with organic fertilizer in both years. This result was accompanied by increased richness,dominance,and evenness of the microbial communities. The utilization of amino acid,amine,and carboxylic acid carbon sources and community evenness were further improved when the organic fertilizer amount was doubled in both years. Biplot analysis indicated that amines and amino acids were the most representative of the total carbon source utilization by the soil microbial communities in both years. The highest oat yield was achieved at a total N application rate of 135 kg ha-1 in the treatment involving 45 kg ha-1 N in the form of urea and 90 kg ha-1 N in the form of sheep manure in both years. It was concluded that the application of 50% of the conventional rate of mineral N fertilizer supplemented with an appropriate rate of organic fertilizer enhanced both the functional diversity of soil microbial communities and oat yield. Amine and amino acid carbon sources may be used as a substitute for total carbon sources for assessing total carbon source utilization by soil microbial communities in oat fields in future studies.

Keywords:organic fertilizer,Biolog-Eco,soil microbial community,carbon source utilization,diversity,oats

1.Introduction

China is the largest user of N fertilizer in the world;its mineral N fertilizer consumption typically accounts for more than 30% of total world consumption (Liuet al.2016). Excessive use of mineral N fertilizer such as urea may not only result in the waste of N resources,but also adversely affect the soil ecology,due to soil acidification,compaction,and contamination (Guoet al.2010;Stuartet al.2014). One solution to this problem,especially in China,may be to reduce the use of mineral N fertilizer and replace it with organic N fertilizer (Shiet al.2019).

Soils supplied with organic fertilizer have increased organic carbon content to support the microbial community,relative to those supplied with mineral N fertilizer alone (Sulet al.2013). Soil microbes are an important component of the soil ecosystem,playing an essential role in the maintenance of soil biology,chemistry and structure (Trivediet al.2017;Luanet al.2020). Understanding their response to the replacement of mineral N fertilizer with organic N fertilizer may help to improve soil quality and increase crop yield in the development of fertilization strategies (Fierer 2017;Geisseleret al.2017).

Studies show that soil microbes are affected by mineral N fertilizer and organic fertilizer management programs. Mineral N fertilizer addition to soil from a broad range of ecosystems in North America decreased microbial respiration rates and microbial biomass by 11 and 35%,respectively,and also altered bacterial community composition over a year-long incubation period (Ramirezet al.2012). Long-term repeated mineral N fertilizer applications were also found to alter microbial community composition in fertilization trials in cropping systems (Geisseler and Scow 2014;Geisseleret al.2017). Similarly,Liuet al.(2019) found that the diversity of both soil methanogenic and methanotrophic communities were decreased by mineral N fertilizer input in early rice paddy,while Yevdokimovet al.(2008) found a significant decrease in the specific growth rate of the microbial community in soil treated with high mineral N fertilizer in a laboratory incubation experiment. The appropriate rate of mineral N fertilizer application greatly modulated the transformation of soil microbial biomass throughout the growth period of wheat in Northeast China (Yanget al.2016). In contrast,other studies have shown that including organic fertilizer can enhance soil microbial community characteristics.The functional diversity of soil microorganisms,as shown by differences in the community level physiological profile,was found to be increased as a result of long-term farmyard manure application in a heterogeneous sandy soil (Sradnicket al.2013). Long-term organic fertilization management maintained the bacterial diversity of an extremely lowproductivity arable soil in South China (Xunet al.2016). Organic fertilizer supplementation was found to improve the phospholipid fatty acid (PLFA) of bacteria,actinomycetes,and fungi based on meta-analysis (Loriet al.2017;Xiaoet al.2018). A meta-analysis by Wanget al.(2017) indicated that organic amendments can significantly improve soil microbial biomass,which is helpful for soil biological fertility improvement. Importantly,substituting mineral N fertilizer with organic fertilizer significantly improved crop yield and N use efficiency (Xieet al.2016;Liet al.2018).

Biolog-Eco plates may be used to determine the selective utilization of different carbon sources by soil microbial communities,which reflects the metabolic characteristics and the diversity of the microbes present (Fracet al.2012;Rutgerset al.2016;Feiglet al.2017;Yuanet al.2019). Assessing the metabolic reactions performed by the soil microbial communities is essential for understanding and managing soil ecosystems (Manjunathet al.2018). A change in the diversity of soil microbes reflects changes in soil ecological function (Loreauet al.2001). Furthermore,the diversity of soil microbial communities is commonly used as an indicator for soil quality and fertility (Islamet al.2011).

Oats (AvenasativaL.) is a minor crop.Research on oats has been focused on its resistance to abiotic stress (Sánchez-Martínet al.2015) and functional food aspects (Sikoraet al.2013;Zhanget al.2019). The present research was conducted in oats to explore its response to organic versus mineral N fertilizer application and related changes in soil microbial communities. The main objectives of the work were:(i) to assess the impact of reduced mineral N fertilizer supplemented with organic fertilizer on the utilization of carbon sources by soil microbial communities in oat fields;(ii) to investigate the diversity of soil microbial communities in oat fields in response to reduced mineral N fertilizer supplemented with organic fertilizer;and (iii) to identify the representative carbon source utilization by the soil microbial community.

2.Materials and methods

2.1.Experimental site,crop and fertilizers

The study was conducted in an experimental field of Shanxi Agricultural University,Pinglu District,Shanxi Province,China in 2018 and 2019. The geographical location is about 112°07´-112°38´E,39°41´-40°17´N. The soil was a calcareous cinnamon,and in 2018 it was determined to include 9.15 g kg-1organic matter,1.08 g kg-1total N,43.12 mg kg-1alkaline hydrolyzed N,6.38 mg kg-1available P,164.54 mg kg-1available K,and pH of 7.96. The oat variety used was Bayou 18. The N fertilizer used was urea (containing 46% N). The organic fertilizer used was sheep manure which contained 0.62% N,0.30% P2O5,and 0.20% K2O,and 0.65% N,0.30% P2O5,and 0.18% K2O in 2018 and 2019,respectively.

2.2.Experimental design

Five treatments were compared in a randomized complete block design with three replications. The plot size was 2 m by 5 m (8 rows of 5 m long with 25 cm between rows),with a 100 cm distance between plots. The seeding rate was 400 viable seeds m-2. The sowing dates were 25 and 21 May,and the harvesting dates were 17 and 15 September in 2018 and 2019,respectively.

The five treatments were:no fertilizer (CK),conventional N fertilizer (U1,90 kg ha-1N in the form of urea),50% of conventional N fertilizer (U2,45 kg ha-1N in the form of urea),50% of conventional N fertilizer supplemented with 14 516 and 13 846 kg ha-1sheep manure (U2OM1,with an estimated total N of 135 kg ha-1) in 2018 and 2019,respectively,and 50% of conventional N fertilizer supplemented with 7 258 and 6 923 kg ha-1sheep manure (U2OM2,with an estimated total N of 90 kg ha-1) in 2018 and 2019,respectively. The P and K levels were kept the same for all treatments through supplementation of P with the superphosphate (containing 16% P2O5),and K with potassium chloride (containing 60% K2O). The nutritional profile of each treatment is listed in Table 1. The fertilizers were applied before sowing to 25 cm depth. Local routine management measures were applied after sowing.

Table 1 The fertilizer treatments tested in 2018 and 2019 (kg ha-1)

2.3.Soil sampling

Soil samples were taken from the top 0-20 cm in between the oat rows in each plot at the heading stage (a key growth stage of oats) in 2018 and 2019. Five soil samples were taken and mixed to form a single sample for each plot.After removing rubble and plant roots,the soil samples were passed through a 2-mm sieve and mixed thoroughly for use in a Biolog test.

2.4.Biolog test

The soil samples were analyzed using a Biolog-Eco plate (Manjunathet al.2018) to determine the functional diversity of the soil microbial communities present. Each Biolog-Eco plate consisted of 8 rows (A to H) and 12 columns (1 to 12) of wells,for a total of 96 wells. Each set of four adjacent columns was used as a replicate;thus the 96 wells were divided into three replications. The first well in each replicate was a blank check,and each of the remaining 31 wells was filled with a different organic carbon source and a fixed amount of tetrazolium violet dye.

The carbon sources in each replicate of the Biolog-Eco plate included six types of amino acids (A4,B4,C4,D4,E4 and F4),two types of amines (G4 and H4),12 types of carbohydrates (A2,A3,B2,B3,C2,D2,E2,F2,G1,G2,H1 and H2),five types of carboxylic acids (B1,E3,F3,G3 and H3),four types of polymers (C1,D1,E1 and F1) and two types of phenolic acids (C3 and D3).

A mixture of 10 g of the soil sample and 95 mL of 0.145 mol L-1sterile sodium chloride solution was oscillated on a shaking table for 15 min. The mixture was then diluted to 10-3. A total of 125 μL aliquot of the diluted suspension was injected into each well in the plate. The plate was incubated in a standing-temperature incubator at 25°C for 96 h (until the end of the microbial logarithmic growth stage) (Schutter and Dich 2001;Kumaret al.2017). Optical density of the suspensions was determined using a microplate reader at 590 nm.

2.5.Data analysis

Carbon source utilizationThe utilization of different carbon sources by the soil microbial communities was represented by the average well color development (AWCD) and calculated as follows:AWCD=∑(Ci-R)/n,whereCiis the optical density of theith carbon source well,Ris the optical density of the blank well,andnis the number of carbon sources within each of the six carbon sources.

Calculation of the diversity parameters of the soil microbial communitiesDiversity parameters,including the Shannon-Wiener (H´),Simpson (D),and McIntosh (U) indexes,were calculated.

The Shannon-Wiener index is used to measure the species richness of microbial communities and is calculated asH´=-∑Pi(lnPi),wherePirepresents the ratio of the relative optical density of theith carbon source to the sum of the relative optical densities of all carbon sources;it is determined asPi=(Ci-R)/∑(Ci-R).

The Simpson index is used to measure the species dominance of microbial communities and is calculated asD=1-∑(Pi)2.

The McIntosh index is used to measure the species evenness of microbial communities and is calculated as

2.6.Statistical analysis

The analysis of variance and biplot analysis were conducted using SPSS 16.0 and GGEbiplot 8.1,respectively.

3.Results

3.1.Utilization of the carbon sources by soil microbial communities under different fertilizer treatments

The utilization of all carbon sources except for polymers by the soil microbial communities in the oat field was significantly (P＜0.05) enhanced by fertilization in 2018 and 2019 compared to the CK treatment (Fig.1). The utilization of the six types of carbon sources by the soil microbial communities did not differ significantly (P＜0.05) between the mineral N fertilizer treatments. Similarly,no significant difference was observed in the utilization of phenolic acids by soil microbial communities among the fertilizer treatments in either year. However,compared with mineral N fertilizer treatments,soil microbial communities in the U2OM1 and U2OM2 treatments significantly (P＜0.05) improved the utilization of amino acids,amines,carbohydrates,carboxylic acids,and polymers in both years. The utilization of amino acids,amines,carbohydrates,carboxylic acids,and polymers by the soil microbial communities in U2OM2 treatments were significantly (P＜0.05) improved by 187.24,7.26,109.14,125.36,and 319.25% (2018);and by 52.88,9.86,170.76,142.04,and 208.99% (2019),respectively,in comparison with the U1 treatments. Compared with U2OM2 treatments,soil microbial communities in U2OM1 treatments significantly (P＜0.05) improved the utilization of amino acids,amines,and carboxylic acids in both years. The utilization of these carbon sources were increased by 24.60,13.54,and 14.62%,respectively,in 2018;and by 8.68,11.18,and 17.12%,respectively,in 2019. Moreover,soil microbial communities in U2OM1 treatments also significantly (P＜0.05) improved the utilization of carbohydrate carbon sources by 24.92% in 2019.

Fig.1 Utilization of various carbon sources (represented by average well color development;AWCD) by soil microbial communities under different fertilizer treatments in 2018 and 2019. CK,no fertilizer;U1,90 kg ha-1 N in the form of urea;U2,45 kg ha-1 N in the form of urea;U2OM1,45 kg ha-1 N in the form of urea supplemented with 90 kg ha-1 N in the form of sheep manure;U2OM2,45 kg ha-1 N in the form of urea supplemented with 45 kg ha-1 N in the form of sheep manure. Vertical bars indicate standard deviation of the means (n=3). Bars with different letters within each year are significantly different at P＜0.05.

3.2.Identification of the best fertilizer management in terms of total carbon utilization

Biplot analysis showed that the treatments where 50% of the mineral N fertilizer was supplemented with organic fertilizer were separated from the CK and mineral N fertilizer alone treatments (Fig.2-A and B). The fertilizer treatments were ranked in the order of U2OM1,U2OM2＞U2,U1＞CK,in terms of total carbon utilization,in both 2018 and 2019.

Fig.2 Biplots showing the rank of fertilizer treatments in total carbon utilization by soil microbial communities in 2018 (A) and 2019 (B). CK,no fertilizer;U1,90 kg ha-1 N in the form of urea;U2,45 kg ha-1 N in the form of urea;U2OM1,45 kg ha-1 N in the form of urea supplemented with 90 kg ha-1 N in the form of sheep manure;U2OM2,45 kg ha-1 N in the form of urea supplemented with 45 kg ha-1 N in the form of sheep manure. The circle represents the average utilization of the six carbon sources by the soil microbial communities in oat fields with different fertilizer treatments. The straight line with a single arrow passes through the origin and small circle,and is called the average carbon source vector axis (Yan 2010);the arrow points to higher average carbon utilization by the soil microbial communities in each of the fertilizer treatments.

Statistical analysis indicated that the utilization of total carbon sources by the soil microbial communities in the U2OM2 and U2OM1 treatments were significantly (P＜0.05) greater than those in the U1,U2,and CK treatments.Furthermore,the utilization of total carbon sources increased significantly (P＜0.05) with an increase in the organic fertilizer amount (U2OM1＞U2OM2).

3.3.Identification of the representative carbon sources utillized by the microbial communities

There was a positive correlation among the six types of carbon sources (Fig.3-A and B). The six types of carbon sources and the average carbon source vector axis with the smallest angles included,and were in the order of:amines,amino acids,carboxylic acids,and carbohydrates in both 2018 and 2019. Thus,amines and amino acids were the most representative of the total carbon utilization by the soil microbial communities.

Fig.3 Biplots showing the correlations among the carbon source utilization by soil microbial communities under different fertilizer treatments in 2018 (A) and 2019 (B). CK,no fertilizer;U1,90 kg ha-1 N in the form of urea;U2,45 kg ha-1 N in the form of urea;U2OM1,45 kg ha-1 N in the form of urea supplemented with 90 kg ha-1 N in the form of sheep manure;U2OM2,45 kg ha-1 N in the form of urea supplemented with 45 kg ha-1 N in the form of sheep manure. The line connecting the origin to each carbon source in the biplots is called the“vector”of the carbon source (Yan 2010). The cosine of the angle between two carbon sources approximates the correlation between them across the fertilizer treatments. An angle smaller than 90° indicates a positive correlation. The cosine of the angle between a carbon source vector and the average carbon source vector axis indicates the representativeness of the carbon source for the total carbon utilization. The smaller the angle,the stronger the representativeness.

3.4.Diversity index of soil microbial communities under different fertilizer treatments

In both 2018 and 2019,the Shannon-Wiener,Simpson,and McIntosh indexes of the soil microbial communities in the treatments with any fertilizer were significantly (P＜0.05) greater than those in the CK treatments (Table 2). No significant differences in the three indexes were observed between the treatments with mineral N fertilizer,however,all three indexes in the U2OM1 and U2OM2 treatments were significantly (P＜0.05) greater than those with mineral N fertilizer alone. In 2018,the Shannon-Wiener,Simpson,and McIntosh indexes of soil microbial communities in U2OM2 treatments were significantly (P＜0.05) increased by 2.38,5.68,and 46.51%,respectively,compared with those in U1 treatments. The results showed that when 50% of the mineral N fertilizer supplied as urea was replaced with organic fertilizer,the richness,dominance,and evenness of soil microbial communities were significantly (P＜0.05) improved. Further,the McIntosh index in the U2OM1 treatment was significantly (P＜0.05) greater than that in the U2OM2 treatment in both years,indicating that the evenness of the soil microbial communities increased with an increase in the amount of organic fertilizer applied.

3.5.Oat yield under different fertilizer treatments

In both years,all fertilizer treatments significantly (P＜0.05) increased the oat yield above that of the control (Table 3).Oat yield was significantly (P＜0.05) greater at the higher rate (U1 treatment) than at the lower rate (U2 treatment) of mineral N fertilizer application. The results also showed that when 50% of the mineral N fertilizer was replaced by organic fertilizer (U2OM2 treatment),oat yield was significantly (P＜0.05) lower than when the mineral N fertilizer alone was applied (U1 treatment),with decreases of 9.98 and 7.99% in 2018 and 2019,respectively. When the application of organic fertilizer was doubled (U2OM1 treatment),the oat yield reached the level obtained when the mineral N fertilizer was applied alone (U1 treatment) in 2018;and exceeded that of the U1 treatment in 2019 by 5.12% (P＜0.05).

Table 2 Diversity indexes of soil microbial communities under different fertilizer treatments in 2018 and 2019

Table 3 Oat yield under different fertilizer treatments in 2018 and 2019 (kg ha-1)

4.Discussion

The AWCD value can be used to evaluate the ability of the soil microbial communities to utilize soil carbon sources,and it is an important indicator of the metabolic characteristics of the soil microbial communities. In the present study,it is noteworthy that N fertilizer reduction by 50% (U2 treatment) caused no reduction in the utilization of the six carbon sources by the soil microbial community in the oat field,compared to conventional N application (U1 treatment). Segalet al.(2017) found that total microbial biomass and bacterial biomass,assessed using fatty acid methyl esters,did not differ among N rates (0,40,80,120,and 160 kg ha-1yr-1) at a continuous maize site (26 years). However,reductions in microbial biomass under high N addition levels (32 and 64 g urea N m-2yr-1) were found by Zhanget al.(2008) in a semi-arid temperate steppe in the northern China. In their study,these reductions were partly attributed to a decrease in soil pH. Also canonical correspondence analysis (CCA) showed that pH was the most important environmental factor influencing variations in the utilization of microbial carbon sources at different N levels in subtropical forest soils (Diaoet al.2019). Low pH can suppress microbial activities and the growth of soil microbes in long-term fields in southern England (Kemmittet al.2006).Therefore,the lack of a difference between the two mineral N fertilizer treatments in the present study suggests that a threshold level for mineral N fertilizer may exist,above which the utilization of soil carbon sources by the microbial communities will not be affected.

In the present study,the utilization of amino acids,amines,carbohydrates,carboxylic acids and phenolic acids by the soil microbial communities in the treatments where mineral N fertilizer was applied alone (U1 and U2 treatments) were increased significantly compared with that of the control in both years. One explanation for the positive effects of N is that N is a nutrient required by both soil microbes and the crop,and the crop affects the soil microbial communities through the provision of root residues and root exudates (Lupwayiet al.2004;Denniset al.2010). Moreover,a meta-analysis based on long-term trials from around the world revealed that mineral N fertilizer application led to a 15.10% increase in microbial biomass carbon or a 13.60% increase in microbial biomass above the levels in unfertilized control treatments (Geisseler and Scow 2014;Geisseleret al.2016). Geisseler and Scow (2014) also reported that mineral N fertilizer had a significantly positive effect on microbial biomass carbon when soil pH was above 5. The basic soil pH in the present experiment was 7.96.Therefore,the present results are in agreement with those of the previous studies.

In the present study,the utilization of amino acids,amines,carbohydrates,carboxylic acids,and polymers by the soil microbial communities was significantly improved by organic fertilizer application. In addition,the utilization of amino acids,amines,and carboxylic acids was further improved with an increase in the organic fertilizer amount. Xuet al.(2018) also reported that organic fertilizer which was supplemented to a red soil significantly changed the utilization preference of carbon sources by soil microbial communities compared with the application of chemical fertilizer alone. Biplot analysis in the present study showed that total carbon source utilization by the soil microbial community was increased by the application of organic fertilizer. Similar results were obtained by Heet al.(2018),where reduced N fertilizer experiments showed that the utilization of carbon sources by soil microbial communities was significantly improved with an increase in organic fertilizer input in a winter wheat-summer maize cropping system. Long-term fertilizer experiments have also shown that the total utilization capacity of carbon sources by soil microbial communities when mineral N fertilizer alone was used was decreased compared to when mineral N fertilizer was combined with organic fertilizer in a sub-humid tropical rice-rice cropping system (Kumaret al.2017). These results were consistent with the findings of the present study and show that soil amended with organic fertilizer generally maintains a higher level of soil microbial biomass compared to soils in which mineral fertilizers are used alone (Mandalet al.2007;Heinzeet al.2011).

Biplot analysis is an effective method for visual analysis of carbon source utilization by soil microbial communities under different fertilizer treatments (Manjunathet al.2018). In a study on the effects of different fertilization treatments on the functional diversity of soil microbial communities in a sub-humid tropical rice-rice cropping system,biplot analysis showed that the utilization of amino acid and polymer carbon sources were significantly correlated with the principal components,and it was concluded that amino acid and polymer utilizing microbes were dominant,irrespective of treatment (Kumaret al.2017). In the present study,the results of biplot analysis indicated that amines and amino acids were the main representatives of carbon sources utilization by the microbial communities. This result differed somewhat with the findings of Kumaret al.(2017),possibly because of differences in soil characteristics,experimental fertilizers,and crop types in the different studies. These representative carbon sources can be used as a substitute for the total soil carbon utilization by soil microbial communities in similar studies in the future,which may reduce the cost of research.

The increased or decreased utilization of carbon sources as a result of different experimental treatments can be used to represent an increase or decrease in the population of soil microbial communities (Caoet al.2019). The variation in the population of soil microbial communities can be reflected by the diversity of soil microbial community. In the present work,the richness,dominance,and evenness of soil microbial communities in the oat field were significantly increased by organic fertilizer application,and the evenness was increased with an increased quantity of organic fertilizer applied in both years. Kumaret al.(2017) reported that the use of organic fertilizer improved the richness and evenness of soil microbial communities compared with communities where mineral N fertilizer alone was applied to soil in a long-term study. This effect of organic fertilizer was confirmed in the present study. The poor diversity of microbial communities in soils where non-organic fertilizer were used may be attributed mainly to the decrease in abundance of total soil organic carbon and soil microbial biomass (Moharanaet al.2012;Naheret al.2013). In a study on the effects of increased N input from organic fertilizer on carbon source utilization characteristics by soil microbial communities,the evenness of the soil microbial communities was reduced significantly with increased N replacement by organic fertilizer in winter wheat-summer corn rotation systems (Heet al.2018). This contrasts with the results from the present study where the evenness of soil microbial communities in the oat field increased with an increased amount of organic fertilizer. This point,therefore,needs further clarification.

Soil microbes are involved in nutrient activation,transformation,and transportation,and thereby affect the productivity and nutrient utilization of plants (Vimalet al.2017). Community structures and metabolic functions of the soil microbes and their carbon source utilization mode will inevitably affect the cycling and transformation of various soil nutrients,thus affecting the quantity and availability of soil nutrients and ultimately the crop yield (Luoet al.2018). In the present study,a 50% reduction in mineral N fertilizer compensated with organic fertilizer addition significantly increased the microbial utilization of carbon resources and the richness,dominance,and evenness of the soil microbial communities. In contrast,oat yield was reduced in the treatment with the same total N (U2OM2 treatment) compared with the conventional mineral N fertilizer application (U1 treatment) in both years. However,oat yield either reached or significantly exceeded that of the conventional mineral N fertilizer application (U1 treatment) when the application of organic fertilizer was doubled. Therefore,the application of mineral N fertilizer at 50% of the conventional rate supplemented with the application of organic fertilizer at a rate to supply 90 kg N ha-1(total N=135 kg ha-1) could increase the yield of oats as a result of increasing the functional diversity of soil microbial communities as observed in the present study.

5.Conclusion

The soil microbial communities in an oat field supplied with mineral N fertilizer (urea) at 50% of the conventional rate,and supplemented with organic fertilizer,significantly increased the utilization of some carbon sources in both years. The diversity of soil microbial communities was enhanced when 50% of the mineral N fertilizer was replaced with organic fertilizer in the oat field in both years. Also the oat yield in the field supplied with 50% of the mineral N fertilizer supplemented with an appropriate amount of organic fertilizer (to give a total of 135 kg ha-1N) reached or exceeded that from the conventional mineral N fertilizer application (90 kg ha-1) in both years. Amines and amino acids may be used to represent the total carbon source in evaluating the change of functional diversity of the soil microbial communities in oat fields in future studies.

Acknowledgements

This research was supported by the Key Research and Development Program of Shanxi Province,China (201703D211001-03-01 and 201703D211001-03-03) and the Key Research and Development Program of Shanxi Province,China (201903D221061).

Declaration of competing interest

The authors declare that they have no conflict of interest.