Response of height, dry matter accumulation and partitioning of oat (Avena sativa L.) to planting density and nitrogen in Horqin Sandy Land

Nitrogen fertilizer application and planting densities were key limiting factor for the physiological metabolism and growth of Avena sativa L. In the paper, it was set five nitrogen fertilizer application (including: N1:225 kg/hm2, N2:300 kg/hm2, N3:375 kg/hm2, N4:450 kg/hm2, N5:525 kg/hm2) and four planting densities (including M1:3 million plants/hm2, M2:3.5 million plants/hm2, M3:4 million plants/hm2, M4:4.5 million plants/hm2), and measure plant height, aboveground biomass, underground biomass, total biomass, stem biomass, leaf biomass, root biomass and ear biomass, and calculate stem/leaf ratio, root/crown ratio, stem contribution rate, leaf contribution rate, root contribution rate and spike contribution rate, to study the effect of density and nitrogen fertilizer on height, biomass and material distribution of Avena sativa L to provide scientific basis for water and fertilizer management of Avena sativa L in Horqin Sandy Land (Science and Technology Park of Inner Mongolia university for nationalities). The results showed, The main purpose was to harvest aboveground biomass such as oat stem and leaf, the optimal density and nitrogen fertilizer combination was M4 and N3, and aboveground biomass was 4254.96 g/m2 (2017) and 4226.21 g/m2 (2018). Under the same planting density, the height, aboveground biomass and underground biomass of Avena sativa L all showed first increased and then decreased with the rising of fertilizer application and the biggest height value appeared at N3. Under the same nitrogen fertilizer application, the height indicated increased gradually with the rising of density, and higher biomass was obtained under lower nitrogen fertilizer application and higher planting density conditions (such as:N1 conditions, the biggest value of underground biomass and total biomass was obtained under M1 condition), and under higher nitrogen fertilizer application conditions the higher value of biomass was obtained under lower planting density condition (M1 (2017) and M2 (2018) under the condition of N5 treatment, the aboveground biomass and total biomass were the highest, while M4 (2017) and M3 (2018) underground biomass was the highest.). Under same nitrogen fertilizer application conditions, the root/shoot ratio and stem/leaf ratio all showed increased first and then decreased with the rising of density. Under same density conditions, the root/shoot ratio and stem/leaf ratio indicated different variation laws with different nitrogen fertilizer application amount. The changes laws of root contribution rate, stem contribution rate, leaf contribution rate and ear contribution rate were different due to density and nitrogen fertilizer application.

make hay. It was an important forage fodder for winter and spring supplementation, which plays a very important role in stabilizing livestock production 1-3 . Nitrogen was a key limiting factor for the physiological metabolism and growth of crops 4 . Nitrogen fertilizer plays an extremely important role in improving the yield per unit area of food crops. And nitrogen fertilizer application amount accounts for about 2/3 of total fertilizer application amount, but utilization rate is less than 40%. Excessive application of nitrogen fertilizer not only aggravates greenhouse gas emissions from ammonia volatilization and denitrification, but also aggravates eutrophication due to nitrogen leaching [5][6][7] . For panting density was an important factor for crop yield, and it was one of important way to improve crop yield by appropriately increasing planting density 8 . However, the competition of crops on water, light, fertilizer and other resources become more intense with the increasing of planting density and resulting in the plant stem becoming thin and the root growth being restrained, rising intraspecific competition and the risk of lodging 9 . It was important influence factors on oat production for nitrogen fertilizer and planting densities, and some studies have showed that there was significantly influence of nitrogen fertilizer and planting densities on oat hay yield, such as reproductive allocation, effective tiller number and spikelet number etc 10,11 . Therefore, studies on nitrogen fertilizer and density of oat have important significance for improving oat yield and quality.
There were a lot of research on Avena sativa L. on domestic and foreign scholars, such as how to improve the yield and quality seeds, silage and hay oat vegetation modulation, variety breeding, high-yield cultivation techniques etc. [10][11][12][13][14] . And the density and nitrogen fertilizer are the main factors to limit the yield of Avena sativa L., which was hot question too. For nitrogen fertilizer, which was an important factor limiting yield and quality of Avena sativa L. But large amount of nitrogen fertilizer, which will lead to grow rapidly in stem, and diameter of stem thinner and easy to lodging, and thus affect the harvest Avena sativa L and hay quality; Insufficient application of nitrogen fertilizer will slow the growth of Avena sativa L, decrease in leaf area index, decrease in light and efficiency, and decrease in yield. For density, it mainly Avena sativa L yield by the root nutrition competition and leaf density light energy use efficiency, unreasonable planting density can cause light energy using efficiency drop, the low level of production. High density, inhibition of Avena sativa L tillering, thereby reducing stem diameter and production.
In China, Avena sativa L. was mainly planted in alpine pastoral areas, such as Tibet, Qinghai, Gansu, Ningxia etc, and which was as one of the important measures to solve the problem of high efficiency production and sustainable development of animal husbandry in the region 15 . The study area were mainly concentrated in the region, and there was relatively few about related research of Avena sativa L in Horqin sandy Land, and the main research content concentrated variety selection, nitrogen, spacing and suitable seeding rate ect. And there was few study on the response of dry matter accumulation and distribution of Avena sativa L. on planting density and nitrogen addition. So the article discuss the effect of planting density and nitrogen addition on biomass, material distribution, stem/leaf ration and root/shoot and so on in Horqin sandy land to provide theoretical basis and technical support.

Materials and Methods
the natural conditions of the experiment. The experiment was conducted at the Inner Mongolia University for the Nationalities experimental field station in Tongliao (Inner Mongolia, China). It is a semi-arid region with a temperate monsoon climate. The annual precipitation is approximately 399 mm, with 50-60% of total precipitation falling in August and September. The mean annual temperature is 6.4 °C. The accumulated temperature (>10 °C) is 3184 °C. The frost-free period is approximately 150 d, from May to September. The soil is gray meadow, with a pH 8.2 (pH water), organic matter content is 18 g/kg, and available nitrogen, phosphorus, and potassium is 62, 39 and 185 mg/kg, respectively. Rainfall and temperatures during the experimental period. experiment design. Avena sativa L. was planted on April 20, 2017 and April 25, 2018, and depth 3 cm.
Root, stem, leaf and spike biomass measurement: 10 strains were randomly selected in each district, the whole plant (including root, stem, leaf and spike) excavated in organ separation, drying, the determination of each organ biomass, and used to calculate the related indicators.
Data analysis. Relevant calculation. Total biomass = terrestrial biomass + root biomass.
Root ratio = root biomass/total biomass. Stem/leaf ratio = stem/leaf weight. Single plant total weight = root weight + stem weight + leaf weight + ear weight. The contribution rate of stem is equal to the weight of the stem and the total weight of the single plant. Blade contribution = blade weight/single plant weight. The root contribution rate = root weight/single plant weight. Ear contribution rate = ear weight/single plant total weight. www.nature.com/scientificreports www.nature.com/scientificreports/ Data analysis. DPS14.0 was used for single factor analysis and correlation analysis.

Results
The effect of planting densities and nitrogen fertilizer on height of Avena sativa L. The height of oat showed first increased and then decreased with rising of nitrogen fertilizer under the same densities and different nitrogen fertilizer condition in 2017, and the biggest value was appeared under N3 conditions. In 2018, the change law of height was similar with 2017 except for the M4 planting density, and the height of N1 and N5 was significantly higher than N2, N3 and N4, N2 and N3 remarkable higher than N3. The height of oat indicated a trend of increasing with the rising of densities under the same nitrogen fertilizer and different densities conditions (Fig. 1).

The effect of planting densities and nitrogen fertilizer on biomass of Avena sativa L. Under
the same densities and different nitrogen fertilizer, the biomass (including aboveground biomass, underground biomass and total biomass) appeared different changing trend. Such as aboveground biomass and total biomass was the highest value under M1 condition, and underground biomass of M3 (2017) and M4 (2018) under N1 conditions. And under N2 condition, aboveground biomass and total biomass of M4 were the highest, and underground biomass of M3 was the highest. And under N3 condition, the highest value of underground biomass, aboveground biomass and total biomass was M3 in 2017 and 2018. And under N4 condition, the highest value of aboveground biomass and total biomass appeared in M1 (2017) and M3 (2018), underground biomass of M2 (2014) and M3 (2018) was the biggest. And under N5 condition, aboveground biomass and total biomass appeared in M (2017) and M2 (2018), underground biomass of M4 (2017) and M (2018) was the highest. Aboveground biomass and underground biomass all showed first increasing and then decreasing with rising of nitrogen fertilizer under the same densities, and the highest value appeared in N2, N3 and N4 due to different densities ( Table 1).

The effect of planting densities and nitrogen fertilizer on contribution rate of Avena sativa L.
Root contribution rate. Under N1 condition, the contribution rate of M3 was the biggest value in 2017 and 2018, and there was no different among densities treatments in 2017. Under N2 condition, the contribution rate of M4 was the biggest, and the value of M4 was significantly higher than M1 and M3 in 2017, and there was no different among densities treatment in 2018. Under N3 condition, there was no different among densities treatments in 2018 and M1 value was the biggest which remarkable higher than M3 in 2017. Under N4 condition, the root contribution rate of M2 and M4 was significantly higher than M1 and M3 in 2017, there was no different among densities treatments in 2018. Under N5 condition, there was no different among densities treatment in 2018, and M1 value was remarkable lower than other treatments in 2017 (Table 2).
Under M1 condition, the root contribution of N1 and N3 was remarkable higher than N2 and N5 in 2017, and no different among nitrogen fertilizer application in 2018. Under M2 condition, the value of N1 was significantly higher than N3 in 2017, and N1 was remarkable lower than other treatments in 2018. Under M3 condition, the value of N1 was significantly higher than N2, N3, N4 and N5, N5 remarkable higher than N2, N3 and N4 in 2017, and there was no different among treatments in 2018. Under M4 condition, N4 was significantly higher than N3 in 2017, and no different among treatments in 2018 (Table 2).
Stem contribution rate. Under N1 and N3 condition, there was no different among different densities treatments for stem contribution rate in 2017 and 2018. Under N2 condition, M3 was significantly higher than M2 and M4 in 2017, and no different among treatments in 2018. Under N4 condition, M1 and M3 were significantly higher than M4 in 2017, and M2 and M4 were remarkable higher than M1 and M3 in 2018. Under N5 condition, M1 and M3 were remarkable higher than M2 in 2017, and no different among treatment in 2018 (Table 2).
Under M1 condition, there were no significantly difference among treatments in 2017 and 2018. Under M2 condition, N3 was significantly higher than N5, with no significant difference among other treatments in 2017, www.nature.com/scientificreports www.nature.com/scientificreports/ and N4 was significantly higher than other nitrogen treatments in 2018. Under M3 condition, N1 was significantly lower than N2 and N3 in 2017, and there was no significant difference in nitrogen fertilizer treatments in 2018. Under M4 conditions, N3 was significantly lower than N1, N2, N4 and N5 in 2017, and N4 was significantly higher than other treatments in 2018 (Table 2).  www.nature.com/scientificreports www.nature.com/scientificreports/ leaf contribution rate. Under N1 condition, M1 was significantly higher than M3 in 2017, and M3 was significantly lower than other density treatments in 2018. Under the condition of N2, M4 in 2017 was significantly lower than N1, N2, N3 and N5, and M3 in 2018 was significantly lower than other density treatments. In 2017, there was no significant difference between M1, M2, M3 and M4 under N3 and N5 conditions. In 2018, M1 was significantly higher than other treatments under N3 treatment, and M2 was significantly higher than other treatments under N5 treatment. Under N4 conditions, M2 in 2017 was significantly lower than M1, M3 and M4, and M1 in 2018 was significantly higher than other treatments ( Table 2).
Under M1 condition, there was no significantly difference among nitrogen fertilizer application treatments in 2017, and the leaf contribution rate of M4 was remarkable higher than other nitrogen fertilizer application treatments in 2018. Under M2 condition, the value of N1 was significantly higher than others in 2017, and N4 and N5 were remarkable higher than others in 2018. Under M3 condition, N2 was significantly higher than N1, N3 and N5, N1 was remarkable lower than N2 and N4 in 2017, and N4 and N5 were remarkable higher than others in 2018. Under M4 condition, there was no difference among nitrogen fertilizer application in 2017 and 2018 (Table 2).
Panicle contribution rate. In 2017, there were no significant differences among M1, M2, M3 and M4 under N1 and N2 conditions. The ear contribution rate of M4 was significantly lower than other treatments under N3 condition, and under N4 condition M4 was remarkable lower than M3, and M4 significantly lower than M2 under N5 condition. In 2018 there was no significantly different among treatments under N4 and N5 condition, M1 and M3 were significantly higher than M2 and M4 under N1 condition, and M3 was remarkable lower than others under N2 condition, and M4 was significantly higher than M2 and M3 under N3 condition (Table 2).
Under the condition of M1, N2, N4 and N5 in 2017 were significantly higher than that of N1 and N3, and in 2018, N1 was significantly higher than that of N2, N3 and N4. Under the M2 condition, N3, N4 and N5 were significantly higher than N1 in 2017, and N5 was significantly higher than N1, N3 and N4 in 2018. Under the condition of M3, N4 was significantly higher than N1, N2 and N5 in 2017, N1 was significantly lower than N2, N3, N4 and N5, and in 2018, N1 was significantly higher than N2, N3 and N4. Under M4 conditions, there was no significant difference between N1, N2, N3, N4 and N5 in 2017, and N3 was significantly higher than other treatments in 2018 ( Table 2).
The effect of planting densities and nitrogen fertilizer on root/shoot ratio and stem/leaf ratio of Avena sativa L. Root/shoot ratio. Under same nitrogen fertilizer and different densities conditions, the root/shoot ratio indicated first increased and then decreased with rising densities under N1 and N2 conditions in 2017, and the biggest value appeared in M3, and the value of M3 and M4 was remarkable higher than M1 and M2 in N1 conditions, and there was no different among M1, M2, M3 and M3 in N2 conditions. The trend of root/ shoot ratio showed first decreased and then increased under N3 conditions in 2017, and the lowest value appeared in M3, and which significantly lower than M1, M2 and M4, and the value of M1 was remarkable higher than M2, M3 and M4, and there was showed first increased and then decreased in 2018, and the value of M3 was the biggest. The root/shoot ratio of M2 was significantly higher than M1, M3 and M4 under N4 conditions in 2017. And the change trends of root/shoot ration showed gradually increased with rising densities under N5 conditions in 2017, and there was no different among M1, M2, M3 and M4 (Figs 2, 3).
Under M1 condition, the root/shoot ration value of N2 and N3 was significantly higher than N5 in 2017 and there was no different among different nitrogen fertilizers treatments in 2018. Under M2 condition, the value of N4 was remarkable higher than N1, N2, N3 and N5, and N2 higher than N1, N3 and N5 in 2017, and there was no different among nitrogen fertilizer treatments in 2018. Under M3 condition, there was significantly different among different nitrogen fertilizer treatments expect between N4 and N5 in 2017, and the change trend of root/ shoot ration indicated first increased and then decreased with rising of nitrogen fertilizer treatment, and N3 was the biggest higher which remarkable higher than others in 2018. Under M4 condition, the value of N1 and N4 was www.nature.com/scientificreports www.nature.com/scientificreports/ significantly higher than N2, N3 and N5 in 2017, and there was no different among nitrogen fertilizer treatments in 2018 (Figs 2, 3).
Stem/leaf ratio. Under N1 condition, the change trend of stem/leaf ration showed first increased and then decreased with the rising of densities treatments, and the biggest value appeared M3 (2017) and M2 (2018). Under N2 condition, the change trend of stem/leaf ration indicated gradually increased with rising of densities treatment, and the value of M4 was significantly higher than M1, M2 and M3 in 2017, and first increased and then decreased with rising of densities treatment, and M3 was the biggest value in 2018. Under N3 condition, there was no different among M1, M2, M3 and M4 among different densities treatments in 2017 and first increased and then decreased with rising of densities treatments in 2018. Under N4 condition, it showed first increased and then decreased with rising of densities treatment in 2017 and gradually increased with rising of densities treatment in 2018. Under N5 condition, M2 was the biggest value, but there was no different among treatments in 2017, and first decreased and then increased with rising of densities in 2018 (Figs 2, 3).
Under M1 condition, the stem/leaf ratio value of N3 was significantly higher than N1, N2 and N5, and N2 was the lowest value which remarkable lower than N3 and N4 in 2017, and the change trend indicated first increased and then decreased with rising of nitrogen fertilizer application in 2018. Under M2 condition, N4 was the biggest value and which remarkable higher than others and N2 was the lowest which significantly lower than others in 2017, and in 2018 N5 was the lowest and remarkable lower than others. Under M3, N1 was the biggest value which was remarkable higher than N2, N4 and N5, and N2 was the lowest and remarkable lower than N1 and N3 in 2017, in 2018, the change trend indicated first increased and then decreased with rising of nitrogen fertilizers application in 2018. Under M4 condition, there was no different among treatments in 2017 and first increased and then decreased in 2018 (Figs 2, 3).

Discussion
Nitrogen fertilizer is an important factor and which can affect the growth and development of Avena sativa L. The lach of nitrogen fertilizer application can cause leaves to lost chlorophyll, affecting photosynthesis and growth was restrained. Excessive nitrogen application will lead the stalk spindling and fall easily 15 . And studies have shown that the height of Avena sativa L. showed first increased and then decreased with the increase of the amount of nitrogen fertilizer application. It is shown that the application of appropriate nitrogen fertilizer can effectively improve the Avena sativa L. plant height, while the overdose has certain inhibitory effect. And the change rule is further verified in this experiment. And the biggest value of height was obtained under N3 nitrogen fertilizer application under same planting density. Planting density was the important factors on influencing the Avena sativa L. height, in the article, it showed that height of Avena sativa L. indicated increasing trend with the increase of planting density unding same nitrogen fertilizer application. And the result was similar with previous studies 16 . The main reason was that the coverage and density of Avena sativa L plant were improved and intensification of light competition by high planting density, and lead to the increase of the individual plants for obtaining more light. But there was no same change trend for biomass and height with increase of planting density, and higher biomass was obtained under lower planting density, such as M1 and M2. The reasons was that the yield of Avena sativa L was closely related to the number of tiller in addition to the high impact. And low density promoted the tillering of Avena sativa L, and high density restricted 17 .
Density and nitrogen fertilizer are important factors of yield for crops, and there are complementary between density and nitrogen fertilizer [17][18][19] . High density planting can reduce the effect of low nitrogen on oat yield, High nitrogen can increase the yield of Avena sativa L artificial grassland with low density. In this paper, the results also showed that there were no significant differences between different density Avena sativa L aboveground biomass under the condition of low nitrogen (N1 and N2), and high nitrogen (N3 and N4 interchange and N5) under the condition of low density (M1) planting Avena sativa L ground biomass is significantly higher than the density of planting (M4), and further suggests that under the condition of low density planting high nitrogen Avena sativa L biological effect higher than that of high density to increase production.
Under same planting density, aboveground biomass showed first increased and then decreased with nitrogen fertilizer application. The amount of nitrogen fertilizer applied to the highest aboveground biomass was slightly www.nature.com/scientificreports www.nature.com/scientificreports/ different in different densities, indicating that there was a certain exchange between density and nitrogen fertilizer. Under the condition of low nitrogen (N1 and N2), root-shoot ratio appears to increase with increasing planting density decreases after the change of the trend, perhaps the main cause is the low level of nitrogen in the soil nutrient content has become an important condition limit plant growth, plants depend on the increased amount of root system to get more chance to absorb nutrients. Therefore, the root crown ratio of m1-m3 was gradually increasing in low nitrogen condition. Under the condition of different density and nitrogen fertilizer into the quantity, in the process of data analysis and found no obvious change rule and trend, the reason may be due to certain interactions between nitrogen fertilizer and density effect.