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Abstract [Objectives] The aim was to study the effects of humic acid and DMPP on the growth of maize and explore the appropriate added concentration to provide basis for industrial production. [Methods] The pot experiment was used to study the effects of adding different concentrations of humic acid， DMPP and their compound on the growth of maize. [Results] Both humic acid and DMPP could increase plant height， stem diameter， SPAD and biomass of maize. The effect of adding 10 kg of humic acid per t was better than that adding 5 kg， and biomass increased by 5.4% and 3.3% compared with CK. The effect of adding 1 kg of DMPP was better than that adding 0.5 kg， and biomass increased by 5.7% and 2.7% compared with CK. The compound application of the 2 improved the fertilizer efficiency. The best treatment was 5‰ humic acid + 1 ‰ DMPP， which could increase plant height， stem diameter， SPAD， and dry weight by 8.1%， 12.3%， 10.6%， and 10.7%， respectively， and it also had lower economic costs compared with the compounding application of 10‰ humic acid +1‰ DMPP. [Conclusions] The 5‰ humic acid + 1 ‰ DMPP compound fertilizer formula was more suitable for application in maize production.
Key words Humic acid； DMPP； Maize； Growth
The nitrogen in compound fertilizer plays a decisive role in the three major elements of nitrogen， phosphorus and potassium. After nitrogen is applied to the soil， it is lost through gaseous volatilization， leaching and runoff， which not only causes waste of fertilizer and energy， but also pollutes the environment[1-3]. At present， the utilization rate of urea in China is less than 30%， which not only causes a lot of economic losses， but also seriously harms the ecological environment. How to increase the nitrogen utilization rate has become a practical problem in the agricultural field today. As a novel nitrogen fertilizer synergist， nitrification inhibitors can improve nitrogen use efficiency and promote crop growth. Nitrification inhibitors are chemical compounds that slow the nitrification of ammonia， ammonium-containing， or urea-containing fertilizers， which are applied to soil as fertilizers. Then， the nitrogen fertilizers can exist in the form of NH+4 for more time， and thus the accumulation of NO-2 and NO-3 is reduced in the soil， thereby reducing the elution of nitrogen fertilizers in NO-3 form and emissions of N2O. Studies at home and abroad have also shown that nitrification inhibitors can slow the nitrification of nitrogen fertilizers in soil， thereby adjusting the supply form， supply time and supply amount of nitrogen fertilizers. Moreover， nitrification inhibitors are beneficial to crops absorption of nitrogen and trace elements， increase crop yield， improve crop quality， increase nitrogen fertilizer utilization， reduce nitrogen fertilizer application， and simplify fertilizer application[4-6]. 3，4-Dimethylpyrazole Phosphate （DMPP） is a highly effective nitrification inhibitor that is non-toxic and harmless to plants and soils. It can inhibit nitrification by specifically inhibiting the activity of Nitrosomonas which can oxidize ammonia into nitrite in soil[7]. Adding only 1% of the active ingredient in the fertilizer can inhibit nitrification for 4-10 weeks， and it can also be used for solid and liquid fertilizers. Humic acid is mainly the remains of animals and plants. After crop yield， it can increase the utilization rate of fertilizers， enhance the stress resistance of plants， promote the activity of enzymes in plants， and improve soil aggregate structure[8-9]. Humic acid is rich in organic matters and are a new resource for modern fertilizers. More importantly， humic acid also contains many active groups， which have strong ion exchange properties， complexation properties， and biological activity[10]. Humic acid gas many functions and wide adaptability. It can increase fertilizer efficiency when added in fertilizers. In this study， based on the excellent properties of humic acid and DMPP in fertilizers， the 2 were added to the compound fertilizer at rational ratios， becoming single humic acid compound fertilizer， nitrification inhibitor compound fertilizer and humic acid and DMPP compound fertilizer. Maize was used as the test crop to carry out field trials to study the effects of different compound fertilizers on the growth， yield and quality of maize， so as to provide bases for te development of new synergistic fertilizers.
Materials and Methods
Test site overview
The test was set up in the greenhouse of Kingenta Group， Linshu County， Shandong Province. The soil type for pot culture was brown soil， which had the texture of silty loam soil， medium fertility level， pH 6.7， organic matter content of 0.41%， total nitrogen content of 0.94 mg/kg， available phosphorus content of 19.37 mg/kg， and available potassium content of 76.21 mg/kg.
Test fertilizers and crops
The test fertilizer was Jinzhengda 15-15-15 （N-P2O5-K2O） nitrosulfide compound fertilizer， containing 5% of nitrate nitrogen. Humic acid was of liquid state with 45% humic acid； DMPP was in powder form. The above additives were manufactured by Kingenta Group.
The test crop was maize， and the variety was Zhengdan 958， which was produced by Beijing Doneed Seeds.
Test design
The fertilizer used in the CK control was compound fertilizer. Humic acid was added at concentrations of 5， 10 kg/t， and DMPP at 0.5， 1 kg/t. Humic acid and DMPP were compounded in pairs at different concentrations， and sprayed uniformly on the surfaces of fertilizer granules according to different dosages. As shown in Table 1， a total of 9 treatments were set. The test maize was planted on June 15 and harvested on August 20， 2015. Each pot contained 7.5 kg of soil. The amount of fertilizer was N-P2O5-K2O=0.12-0.12-0.12 g/kg， equivalent to 15-15-15 compound fertilizer of 6 g/pot. After thoroughly mixing with the soil， 1 plant was planted in each pot with 6 repetitions for each treatment. Except for experimental factors， the remaining field management and other conditions were the same.
When growing to the jointing stage， plant heights， stem diameters and SPAD values were measured， and when harvested， plant heights， stem diameters and total biomass of dry weights and fresh weights were determined. Roots were measured for root length density， root dry weight， root surface area. After finishing potting， the potted soil was measured for total nitrogen， available nitrogen， available phosphorus， and available potassium. Indicators measurement and data analysis
At the jointing stage， plant heights and stem diameters were measured， and when harvested， plant heights， stem diameters， leaf SPAD values， fresh weights and dry weights of the plants were measured. The statistical data were analyzed using Excel 2010 and SAS 19.0 data processing system.
Results and Analysis
Effect of different treatments on plant height in maize
As shown in Fig. 1， different synergistic treatments had different effects on the plant heights of maize. At the jointing stage of maize， significant differences were observed between treatments. Treatments T1-T4 applied with only humic acid or DMPP had a certain increase in plant heights over the control treatment， in which treatments T1 and T2 with humic acid increased by 3.0% and 4.1%， respectively， compared with the control. The plant height of treatment T3 with DMPP was almost the same with that of the control， while treatment T4 showed an increase of 4.8% from the control. Plant heights increased significantly from the control for treatments T5-T8， which were applied with DMPP and humic acid compound fertilizer. Among them， treatment T8 showed the highest increase of 9.7% from the control. Moreover， at the jointing stage of maize， treatments T7 and T8 showed better growth vigor than T5 and T6.
The increases of plant heights for different treatments at the harvesting time were similar to those at the jointing stage of maize. Treatments T5-T8 applied with humic acid and DMPP compound fertilizer showed an overall better growth， while the growth vigor of treatments with humic acid only and DMPP only was almost the same， all of which were better than that of the control. Different from the growth at the jointing stage， the advantages of treatments T7 and T8 became less and less at the later sate of maize growth. For the compound fertilizers added humic acid only and DMPP only， different adding amounts had different effects on the growth vigor of maize. In the test， higher adding amount was better than lower amount. There were significant differences between the treatments applied with humic acid and DMPP compound fertilizer， and the plants heights from high to low were in the order of T8 （humic acid 10kg/t + DMPP 1 kg/t） > T6 （humic acid 5kg/t + DMPP 1 kg/t） > T7 （humic acid 10kg/t + DMPP 0.5 kg/t）> T5 （humic acid 5kg/ t + DMPP 0.5 kg/t）， with the increase ranging from 5.6% to 9.9%.
Effect of different treatments on stem diameter of maize As shown in Fig. 2， at the jointing stage of maize， compared with CK， each synergistic treatment had a promoting effect on stem diameter of maize， and the overall growth trend was basically the same as the plant height at the jointing stage. Over time， when the maize was harvested， the difference in stem diameter between treatments was gradually reduced. Compared with the treatments added with humic acid only and DMPP only， the stem diameters of treatments T3 and T4 were better than those of treatments T1 and T2， indicating that DMPP was superior to humic acid in promoting maize stem diameter. In addition， the stem diameters of the treatments with humic acid and DMPP compound fertilizer were higher than those of other treatments， indicating that humic acid and DMPP compound played a significant role in promoting stem diameter. The difference between the compounded treatments was small， and low adding amount was more cost-effective. Treatment T8 showed the strongest promoting effect on maize stem diameter （humic acid 10 kg/t + DMPP 1 kg/t） of 19.07mm， which was 13.5% higher than the control， followed by T6， T7， and T5， which were 18.88 mm， 18.87mm and 18.49mm， respectively， increasing by 12.3%， 12.2%， and 10.0% compared with the control. This showed that the combination of humic acid and DMPP enhance the promoting effects on stem diameter， and the promoting effects were enhanced with the increase of the amount of each component.
Effect of different treatments on SPAD value of Maize
As shown in Fig. 3， when harvested， compared with CK， all treatments had certain promoting effects on the SPAD values of maize. The SPAD values of treatments T1 and T2 were similar and small， indicating that humic acid had no significant effect on increasing the chlorophyll content of maize. The SPAD values of T3 and T4 maize were similar but larger than those of T1 and T2， indicating that DMPP had stronger effect on increasing chlorophyll content of maize than humic acid， and the promotion effect was enhanced with increasing dosage. The compound mixture of humic acid and DMPP had better effect on increasing SPAD than their single addition， and the combined application could make the 2 promote each other and increase the SPAD values. The SPAD value of T6 was higher than that of T5， T7， and T8， and was 10.7% higher than that of the control group. Compared with CK， the SPAD values of treatments T5-T8 increased by 6.2%-7.7%. It was obvious that treatment T6 （humic acid 5 kg/t + DMPP 1 kg/t） had the strongest effect on the chlorophyll content of maize. Effect of different treatments on maize biomass
As shown in Fig. 4， when harvested， compared with CK， all treatments showed certain promoting effects on increasing fresh weights and dry weights of the overground parts of maize. Among them， T2>T1>CK， and the dry weights of T2 and T1 had an increase of 5.4% and 3.3% from the CK， respectively， indicating that with the increase of the adding amount of humic acid， the effect became stronger in promoting the increase of fresh weight and dry weight of maize. Similarly， T4> T3> CK， and T4 and T3 showed an increase of 5.7% and 2.7% from the CK， respectively， indicating that with the increase of DMPP amount， the promotion effect on fresh weight and dry weight of maize was enhanced. As for the treatments applied with SMPP and humic acid compound fertilizer， the biomass from high to low was in the order of T6>T8>T7>T5， and the biomass of dry weight increased by 9.7%， 9.4%， 7.9% and 6.1% from the CK， respectively. Moreover， the effect of compound application of humic acid and DMPP was not the best at high concentrations， but was the best with high DMPP amount and low humic acid amount， while the effect was the poorest with low DMPP and low humic acid.
Conclusion and Discussion
In recent years， a new type of nitrification inhibitor， 3，4-dimethylpyrazole phosphate （DMPP）， has gradually attracted the attention of researchers. Nitrification inhibitors are used in agricultural production to keep ammonium nitrogen in the soil for a long time， which reduces the formation of nitrate nitrogen. Crops can absorb and utilize these 2 forms of nitrogen， but different crops have different preferences for ammonium nitrogen and nitrate nitrogen， and nitrification inhibitors have different effects on nitrogen absorption by crops. A large number of field trials have shown that DMPP is a highly effective， safe and non-toxic ideal nitrification inhibitor[11-12]. The application of nitrification inhibitors depends not only on the nature of the inhibitor itself but also on soil types， organic contents， temperature， soil management measures and so on[13]. Nitrification inhibitors have different effects on different types of soil. In paddy soils and fluvo-aquic soils， the application of nitrification inhibitors makes the nitrification process of urea treatment prolong into 28 d from 14 d， and DMPP has significant inhibition of nitrification in alkaline soil[14-15]. The nitrification process in black soil and red soil is relatively slow， and the nitrification process still continues 42 d after fertilization， with the apparent nitrification rate only reacheing about 30%[16]. In this test， DMPP significantly promoted the growth of maize. Overall， the effect of adding 1 kg per ton was better than 0.5 kg. Plant height and stem diameter were significantly increased， and biomass was also significantly increased. The main reason is that DMPP prolongs the conversion of nitrogen in fertilizers and improves the utilization of nitrogen. Weathered coal is rich in humic acid and contains active groups such as carboxyl groups， phenolic hydroxyl groups， and sulfhydryl groups. Therefore， it has adsorption， complexation， and exchange properties， and can improve soil structure. In addition， studies have shown that humic acid has good chemical activity and biological activity[17-18]， and it can promote biomass accumulation and increase crop yield[19]. In this study， the combination of humic acid and inorganic fertilizer makes humic acid be able to increase the fertilizer efficiency through various functions such as hydrogen bonding， complexation and physical adsorption[20-21]， so that it has a certain effect of water and nutrient retention， thereby significantly improving maize biomass.
There have been many studies on the addition of DMPP or humic acid to inorganic compound fertilizers in compound fertilizers， but there have been few reports on the combination of the 2. In this experiment， DMPP and humic acid are combined. The test results show that the combination has significant promoting effects on various biological indicators， all of which show significant increase from the single application. Moreover， the compounding at different concentrations of the 2 has different test results， and the effect is the best in the treatment adding 1 kg of DMPP and 5 kg of humic acid. High adding amount cannot further promote the increase of fertilizer efficiency， but may decrease it. In order to improve the economic benefits of industrial production， low adding amount is more cost-effective. The test results can provide scientific bases for the utilization amount of humic acid and DMPP added to the compound fertilizer， thereby guiding production.
References
[1] ZHU ZL， WEN QX. Soil nitrogen in China[M]. Nanjing： Jiangsu Science and Technology Press， 1992：32-58.
[2] QIU WG， TANG H， WANG C. The characters of N-element leakage and countermeasures in rice fields in Shanghai suburbs[J]. Chinese Environmental Science， 2005，25（5）：558-562.
[3] FENG ML， HU RG， XU KC， et al. Influence of nitrogen application amounts of different scales in small watershed of Three Gorges Reservoir area on water body[J]. China Environmental Science， 2008， 28（2）：168-172.
[4] HUANG YZ， FENG ZW， WANG XK， et al. Research progress of nitrification inhibitors applied in agriculture[J]. Chinese Journal of Soil Science， 2002， 33（4）：310-315.
[5] ZEURLLA W， BARTH T， DRESSEL J， et al. 3，4-dimethylpyrazole phosphate （DMPP）-a new nitrification inhibitor for agricultural and horticulture[J]. Biology and Fertility of Soils， 34：79-84. [6] YU QG， YIN JZ， MA JW， et al. Effects of nitrification inhibitor DMPP application in agricultural ecosystems and their influencing factors： a review[J]. Journal of Agro-Environment Science， 2014， 33（6）： 1057-1066.
[7] WANG YX， LI CX. Green and environmental-friendly humic phosphate fertilizer[M]. Beijing： Chemical Industry Press， 2009.
[8] CHENG L， ZHANG BL， WANG J， et al. Research progress of humic-acid containing fertilizers[J]. Soils and Fertilizers Sciences in China， 2011（5）：1-6.
[9] LIU JP， LIU WY， ZHOU J， et al. Effects of applying humic acids and bio-fertilizer on the qualities and yields of strawberry and soil agrochemical properties[J]. Journal of Agricultural Resources and Environment 2015，32（1）54-59.
[10] CHENG SX. Introduction to humic acids[M]. Beijing： Chemical Industry Press， 2007： 129-180.
[11] PASDA G， HADNDEL R， ZERULLA W. Effect of fertilizers with the new nitrification inhibitor DMPP on yield and quality of agricultural and horticultural crops[J]. Biology and Fertility of Soils. 2001， 34： 85-97.
[12] XU C， WU LH， ZHANG FS. Advances in the study of DMPP in agriculture[J]. Chinese Journal of Soil Sciences， 2003， 34（5）： 478-482.
[13] SUN ZM， WU ZJ， CHEN LJ， et al. Regulation of soil nitrification with nitrification inhibitors and related mechanisms[J]. Chinese Journal of Applied Ecology， 2008a， 19（6）： 1389-1395.
[14] PI HJ， ZENG QR， JIANG CH， et al. Effects of nitrification inhibitors on transformation of urea in different soils[J]. Journal of Soil and Water Conservation， 2009：23（1）：68-72.
[15] SHI M， ZHANG MT， SHEN F， et al. Effects of nitrification inhibitors on nitrification inhibition and nitrite accumulation in calcareous soils[J]. Scientia Agricultura Sinica， 2011， 44（3）： 500-506.
[16] YANG JB. The underlying mechanism of the effect of nitrification inhibitor 3，4-dimethylpyrazole phosphate （DMPP） on nitrogen transformation[D]. Nanjing： Nanjing Agricultural University， 2012.
[17] LI SX， DOU XY. Status and prospects of utilization of weathered coal in China[J]. Coal Chemical Industry， 1996， 74（2）： 28-33.
[18] LIANG ZC， CHENG SX， WU LP， et al. Study on mechanism of the interaction between coal humic acid and urea[J]. Journal of Fuel Chemistry and Technology，1999，27（2）：176-181.
[19] CHEN ZD， HE JM， LI XY， et al. Studies on increasing N utilizing efficiency in maize by applying humic acid[J]. Chinese Journal of Eco-Agriculture， 2007， 15（1）：52-54.
[20] LIU L， WANG P， SUN ZM， et al. Effects of HA compound fertilizer applied in multilayer soil on water content of soil， plant growth and yield of maize[J]. Journal of Agricultural University of Hebei， 2012， 35（1）：7-11.
[21] LIU SY. Effects of humic acid compound fertilizer on rice， wheat and maize[J]. Journal of Northeast Agricultural University， 2005， 36（5）：672-674.
Key words Humic acid； DMPP； Maize； Growth
The nitrogen in compound fertilizer plays a decisive role in the three major elements of nitrogen， phosphorus and potassium. After nitrogen is applied to the soil， it is lost through gaseous volatilization， leaching and runoff， which not only causes waste of fertilizer and energy， but also pollutes the environment[1-3]. At present， the utilization rate of urea in China is less than 30%， which not only causes a lot of economic losses， but also seriously harms the ecological environment. How to increase the nitrogen utilization rate has become a practical problem in the agricultural field today. As a novel nitrogen fertilizer synergist， nitrification inhibitors can improve nitrogen use efficiency and promote crop growth. Nitrification inhibitors are chemical compounds that slow the nitrification of ammonia， ammonium-containing， or urea-containing fertilizers， which are applied to soil as fertilizers. Then， the nitrogen fertilizers can exist in the form of NH+4 for more time， and thus the accumulation of NO-2 and NO-3 is reduced in the soil， thereby reducing the elution of nitrogen fertilizers in NO-3 form and emissions of N2O. Studies at home and abroad have also shown that nitrification inhibitors can slow the nitrification of nitrogen fertilizers in soil， thereby adjusting the supply form， supply time and supply amount of nitrogen fertilizers. Moreover， nitrification inhibitors are beneficial to crops absorption of nitrogen and trace elements， increase crop yield， improve crop quality， increase nitrogen fertilizer utilization， reduce nitrogen fertilizer application， and simplify fertilizer application[4-6]. 3，4-Dimethylpyrazole Phosphate （DMPP） is a highly effective nitrification inhibitor that is non-toxic and harmless to plants and soils. It can inhibit nitrification by specifically inhibiting the activity of Nitrosomonas which can oxidize ammonia into nitrite in soil[7]. Adding only 1% of the active ingredient in the fertilizer can inhibit nitrification for 4-10 weeks， and it can also be used for solid and liquid fertilizers. Humic acid is mainly the remains of animals and plants. After crop yield， it can increase the utilization rate of fertilizers， enhance the stress resistance of plants， promote the activity of enzymes in plants， and improve soil aggregate structure[8-9]. Humic acid is rich in organic matters and are a new resource for modern fertilizers. More importantly， humic acid also contains many active groups， which have strong ion exchange properties， complexation properties， and biological activity[10]. Humic acid gas many functions and wide adaptability. It can increase fertilizer efficiency when added in fertilizers. In this study， based on the excellent properties of humic acid and DMPP in fertilizers， the 2 were added to the compound fertilizer at rational ratios， becoming single humic acid compound fertilizer， nitrification inhibitor compound fertilizer and humic acid and DMPP compound fertilizer. Maize was used as the test crop to carry out field trials to study the effects of different compound fertilizers on the growth， yield and quality of maize， so as to provide bases for te development of new synergistic fertilizers.
Materials and Methods
Test site overview
The test was set up in the greenhouse of Kingenta Group， Linshu County， Shandong Province. The soil type for pot culture was brown soil， which had the texture of silty loam soil， medium fertility level， pH 6.7， organic matter content of 0.41%， total nitrogen content of 0.94 mg/kg， available phosphorus content of 19.37 mg/kg， and available potassium content of 76.21 mg/kg.
Test fertilizers and crops
The test fertilizer was Jinzhengda 15-15-15 （N-P2O5-K2O） nitrosulfide compound fertilizer， containing 5% of nitrate nitrogen. Humic acid was of liquid state with 45% humic acid； DMPP was in powder form. The above additives were manufactured by Kingenta Group.
The test crop was maize， and the variety was Zhengdan 958， which was produced by Beijing Doneed Seeds.
Test design
The fertilizer used in the CK control was compound fertilizer. Humic acid was added at concentrations of 5， 10 kg/t， and DMPP at 0.5， 1 kg/t. Humic acid and DMPP were compounded in pairs at different concentrations， and sprayed uniformly on the surfaces of fertilizer granules according to different dosages. As shown in Table 1， a total of 9 treatments were set. The test maize was planted on June 15 and harvested on August 20， 2015. Each pot contained 7.5 kg of soil. The amount of fertilizer was N-P2O5-K2O=0.12-0.12-0.12 g/kg， equivalent to 15-15-15 compound fertilizer of 6 g/pot. After thoroughly mixing with the soil， 1 plant was planted in each pot with 6 repetitions for each treatment. Except for experimental factors， the remaining field management and other conditions were the same.
When growing to the jointing stage， plant heights， stem diameters and SPAD values were measured， and when harvested， plant heights， stem diameters and total biomass of dry weights and fresh weights were determined. Roots were measured for root length density， root dry weight， root surface area. After finishing potting， the potted soil was measured for total nitrogen， available nitrogen， available phosphorus， and available potassium. Indicators measurement and data analysis
At the jointing stage， plant heights and stem diameters were measured， and when harvested， plant heights， stem diameters， leaf SPAD values， fresh weights and dry weights of the plants were measured. The statistical data were analyzed using Excel 2010 and SAS 19.0 data processing system.
Results and Analysis
Effect of different treatments on plant height in maize
As shown in Fig. 1， different synergistic treatments had different effects on the plant heights of maize. At the jointing stage of maize， significant differences were observed between treatments. Treatments T1-T4 applied with only humic acid or DMPP had a certain increase in plant heights over the control treatment， in which treatments T1 and T2 with humic acid increased by 3.0% and 4.1%， respectively， compared with the control. The plant height of treatment T3 with DMPP was almost the same with that of the control， while treatment T4 showed an increase of 4.8% from the control. Plant heights increased significantly from the control for treatments T5-T8， which were applied with DMPP and humic acid compound fertilizer. Among them， treatment T8 showed the highest increase of 9.7% from the control. Moreover， at the jointing stage of maize， treatments T7 and T8 showed better growth vigor than T5 and T6.
The increases of plant heights for different treatments at the harvesting time were similar to those at the jointing stage of maize. Treatments T5-T8 applied with humic acid and DMPP compound fertilizer showed an overall better growth， while the growth vigor of treatments with humic acid only and DMPP only was almost the same， all of which were better than that of the control. Different from the growth at the jointing stage， the advantages of treatments T7 and T8 became less and less at the later sate of maize growth. For the compound fertilizers added humic acid only and DMPP only， different adding amounts had different effects on the growth vigor of maize. In the test， higher adding amount was better than lower amount. There were significant differences between the treatments applied with humic acid and DMPP compound fertilizer， and the plants heights from high to low were in the order of T8 （humic acid 10kg/t + DMPP 1 kg/t） > T6 （humic acid 5kg/t + DMPP 1 kg/t） > T7 （humic acid 10kg/t + DMPP 0.5 kg/t）> T5 （humic acid 5kg/ t + DMPP 0.5 kg/t）， with the increase ranging from 5.6% to 9.9%.
Effect of different treatments on stem diameter of maize As shown in Fig. 2， at the jointing stage of maize， compared with CK， each synergistic treatment had a promoting effect on stem diameter of maize， and the overall growth trend was basically the same as the plant height at the jointing stage. Over time， when the maize was harvested， the difference in stem diameter between treatments was gradually reduced. Compared with the treatments added with humic acid only and DMPP only， the stem diameters of treatments T3 and T4 were better than those of treatments T1 and T2， indicating that DMPP was superior to humic acid in promoting maize stem diameter. In addition， the stem diameters of the treatments with humic acid and DMPP compound fertilizer were higher than those of other treatments， indicating that humic acid and DMPP compound played a significant role in promoting stem diameter. The difference between the compounded treatments was small， and low adding amount was more cost-effective. Treatment T8 showed the strongest promoting effect on maize stem diameter （humic acid 10 kg/t + DMPP 1 kg/t） of 19.07mm， which was 13.5% higher than the control， followed by T6， T7， and T5， which were 18.88 mm， 18.87mm and 18.49mm， respectively， increasing by 12.3%， 12.2%， and 10.0% compared with the control. This showed that the combination of humic acid and DMPP enhance the promoting effects on stem diameter， and the promoting effects were enhanced with the increase of the amount of each component.
Effect of different treatments on SPAD value of Maize
As shown in Fig. 3， when harvested， compared with CK， all treatments had certain promoting effects on the SPAD values of maize. The SPAD values of treatments T1 and T2 were similar and small， indicating that humic acid had no significant effect on increasing the chlorophyll content of maize. The SPAD values of T3 and T4 maize were similar but larger than those of T1 and T2， indicating that DMPP had stronger effect on increasing chlorophyll content of maize than humic acid， and the promotion effect was enhanced with increasing dosage. The compound mixture of humic acid and DMPP had better effect on increasing SPAD than their single addition， and the combined application could make the 2 promote each other and increase the SPAD values. The SPAD value of T6 was higher than that of T5， T7， and T8， and was 10.7% higher than that of the control group. Compared with CK， the SPAD values of treatments T5-T8 increased by 6.2%-7.7%. It was obvious that treatment T6 （humic acid 5 kg/t + DMPP 1 kg/t） had the strongest effect on the chlorophyll content of maize. Effect of different treatments on maize biomass
As shown in Fig. 4， when harvested， compared with CK， all treatments showed certain promoting effects on increasing fresh weights and dry weights of the overground parts of maize. Among them， T2>T1>CK， and the dry weights of T2 and T1 had an increase of 5.4% and 3.3% from the CK， respectively， indicating that with the increase of the adding amount of humic acid， the effect became stronger in promoting the increase of fresh weight and dry weight of maize. Similarly， T4> T3> CK， and T4 and T3 showed an increase of 5.7% and 2.7% from the CK， respectively， indicating that with the increase of DMPP amount， the promotion effect on fresh weight and dry weight of maize was enhanced. As for the treatments applied with SMPP and humic acid compound fertilizer， the biomass from high to low was in the order of T6>T8>T7>T5， and the biomass of dry weight increased by 9.7%， 9.4%， 7.9% and 6.1% from the CK， respectively. Moreover， the effect of compound application of humic acid and DMPP was not the best at high concentrations， but was the best with high DMPP amount and low humic acid amount， while the effect was the poorest with low DMPP and low humic acid.
Conclusion and Discussion
In recent years， a new type of nitrification inhibitor， 3，4-dimethylpyrazole phosphate （DMPP）， has gradually attracted the attention of researchers. Nitrification inhibitors are used in agricultural production to keep ammonium nitrogen in the soil for a long time， which reduces the formation of nitrate nitrogen. Crops can absorb and utilize these 2 forms of nitrogen， but different crops have different preferences for ammonium nitrogen and nitrate nitrogen， and nitrification inhibitors have different effects on nitrogen absorption by crops. A large number of field trials have shown that DMPP is a highly effective， safe and non-toxic ideal nitrification inhibitor[11-12]. The application of nitrification inhibitors depends not only on the nature of the inhibitor itself but also on soil types， organic contents， temperature， soil management measures and so on[13]. Nitrification inhibitors have different effects on different types of soil. In paddy soils and fluvo-aquic soils， the application of nitrification inhibitors makes the nitrification process of urea treatment prolong into 28 d from 14 d， and DMPP has significant inhibition of nitrification in alkaline soil[14-15]. The nitrification process in black soil and red soil is relatively slow， and the nitrification process still continues 42 d after fertilization， with the apparent nitrification rate only reacheing about 30%[16]. In this test， DMPP significantly promoted the growth of maize. Overall， the effect of adding 1 kg per ton was better than 0.5 kg. Plant height and stem diameter were significantly increased， and biomass was also significantly increased. The main reason is that DMPP prolongs the conversion of nitrogen in fertilizers and improves the utilization of nitrogen. Weathered coal is rich in humic acid and contains active groups such as carboxyl groups， phenolic hydroxyl groups， and sulfhydryl groups. Therefore， it has adsorption， complexation， and exchange properties， and can improve soil structure. In addition， studies have shown that humic acid has good chemical activity and biological activity[17-18]， and it can promote biomass accumulation and increase crop yield[19]. In this study， the combination of humic acid and inorganic fertilizer makes humic acid be able to increase the fertilizer efficiency through various functions such as hydrogen bonding， complexation and physical adsorption[20-21]， so that it has a certain effect of water and nutrient retention， thereby significantly improving maize biomass.
There have been many studies on the addition of DMPP or humic acid to inorganic compound fertilizers in compound fertilizers， but there have been few reports on the combination of the 2. In this experiment， DMPP and humic acid are combined. The test results show that the combination has significant promoting effects on various biological indicators， all of which show significant increase from the single application. Moreover， the compounding at different concentrations of the 2 has different test results， and the effect is the best in the treatment adding 1 kg of DMPP and 5 kg of humic acid. High adding amount cannot further promote the increase of fertilizer efficiency， but may decrease it. In order to improve the economic benefits of industrial production， low adding amount is more cost-effective. The test results can provide scientific bases for the utilization amount of humic acid and DMPP added to the compound fertilizer， thereby guiding production.
References
[1] ZHU ZL， WEN QX. Soil nitrogen in China[M]. Nanjing： Jiangsu Science and Technology Press， 1992：32-58.
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