Effects of straw returning with potassium fertilizer on the stem lodging resistance, grain quality and yield of spring maize (Zea mays L.)

The effects of straw returning with potassium fertilizer on the stem lodging resistance, grain quality and yield of spring maize were investigated to provide a scientific basis for the rational utilization of Inner Mongolia spring maize straw and potassium fertilizer resources. This study utilized Xianyu 335 as the test material, and a split plot design was carried out in three ecological regions from eastern to western Inner Mongolia (Tumochuan Plain irrigation area, Hetao Plain irrigation area and Lingnan warm dry zone), with the straw returning method as the main plot and potassium fertilizer dosage as the subplot. The stem resistance index, grain quality and yield were systematically identified. Both application of potassium fertilizer and straw returning improved the resistance and yield indicators of spring maize. Straw returning increased the effectiveness of potassium fertilizer application on spring maize plant height, ear height, fresh weight of stems, brix of stems and stem puncture strength by 2.82%-5.22%, 3.11%-5.90%, 15.96%-19.78%, 4.35%-4.50% and 8.89%-14.82%, respectively. Straw returning increased the effectiveness of potassium fertilizer application on the spring maize grain protein content, spring maize grain crude fat content, maize yield and yield variation coefficient by 3.49%-6.50%, 2.09%-4.43%, 4.87%-12.50% and 5.07%-7.55%, respectively. Straw returning can be combined with reasonable application of potassium fertilizer to increase the effectiveness of potassium fertilizer and enhance lodging resistance. Along with increased maize yield, straw returning also improves grain quality and enhances yield stability, providing a theoretical basis for high-yield and stress-resistant cultivation of Inner Mongolia spring maize, which can be popularized and applied in the spring maize planting areas of Inner Mongolia.

latitude, sunshine hours from April to October, average temperature, and rainfall at each test site are listed in Table 1. Table 2 shows the soil type and basic soil fertility of each test site.  The field study was carried out on the official land which belonged to the key laboratory of crop cultivation and genetic improvement of Inner Mongolia Autonomous Region, permission was given after research application passing verification. During the field study none of endangered or protected species were involved. No specific permissions were required for conducting the field study because it was not carried out in protected area.

Experimental design
This study utilized Xianyu 335 as the test material in a split plot design, with the straw returning method as the main plot and potassium fertilizer dosage as the subplot. The 4 treatments used in the study were straw returning + potassium fertilizer (ST+6K), straw returning + no potassium fertilizer (ST+0K), no straw returning + potassium fertilizer (NST+6K), and no straw returning + no potassium fertilizer (NST+0K). The row length was 30 m, the row width was 5 m, and the row spacing was 0.60 m.
Five replicates were used in this experiment, and the planting density was 82500 plants/hm 2 . The straw returning treatments utilized pulverized straw that was returned to the field in the autumn of the previous year. The non-returning treatments were all household shallow rotation modes. Potassium was applied as 90 kg/hm 2 potassium sulfate (K 2 O 50%) and 228 kg/hm 2 diammonium phosphate (P 2 O 5 46%) once as the base fertilizer before sowing. For the treatments without potassium, only 228 kg/hm 2 diammonium phosphate (P 2 O 5 46%) was applied once as the base fertilizer before sowing. The top dressing of each treatment was 652 kg/hm 2 (N46%), which was applied in the jointing stage and the bell stage at a ratio of 3:7. Other management procedures followed typical field production practices.

Measurement items and methods
Before sowing, 0-20 cm soil samples were taken for each treatment, ventilated and dried in a cool place, after which they were ground to pass through a 0.15-0.25 mm soil sieve. According to the measurement requirements, soil samples with different particle sizes were used to determine soil essential nutrients [22].
(1) Soil organic matter determination was performed using the potassium dichromate titration method.
(2) Soil total nitrogen determination was performed using a Kjeldahl nitrogen analyzer (K-9840, Jinan) and the semi-micro Kjeldahl method.
(3) Soil available phosphorus determination was performed using the NaH 2 CO 3 (0.5 mol/L) Mo-Sb colorimetric method. (4) Soil available potassium determination was performed using the NH 4 Ac (1 mol/L) extraction 30-min flame photometric method. (5) Soil alkaline hydrolysis determination was performed using the alkaline hydrolysis diffusion-absorption method.
The following stem indicators were measured during the silking period. (1) Plant height was measured by using a steel ruler to measure the distance from the top of the tassel to the ridge side.
(2) Ear height was measured by using a steel ruler to measure the distance between the first ear internode and the ridge side.
(3) Stem diameter was measured by using a Vernier caliper to measure the third stem node at the stem base part. (4) Ear stem length was determined by measuring the maize ear node length. (5) Stem fresh weight was determined by measuring the maize stem fresh weight. (6) Stem dry weight was determined by measuring the weight of dried maize stems. (7) The water content of the stems was calculated as the ratio of (stem fresh weight-stem dry weight) and stem fresh weight. (8) To measure the brix of the stems, the maize stems were extracted and mixed, and 1-2 mL of the mixture was measured with a handheld digital sugar meter (PAL-1, Japan ATAGO, accuracy = ±0.2%). (9) To assess stem   lodging resistance mechanical indicators, the stem puncture strength, compressive strength and bending   strength of the third stem node at the maize stem base were measured with a plant stem strength   instrument (YYD-1 Straw-return treatments, potassium fertilizer treatments, and ecological regions were the independent variables, and the stem lodging resistance, grain quality and yield of spring maize were dependent variables in this test. In order to determine the impact of independent variables on dependent variables, statistically significant variance was tested using three-way analysis of variance, and multiple comparisons were made using the least significant difference (LSD) test with α = 0.05 [24]. Histograms were conducted by using Sigma Plot 12.5. And different letters on histograms indicated that means statistically different at P<0.05 level.

Effects of straw returning combined with potassium fertilizer on the morphological indexes of spring maize stems
As shown in Table S1  diameter and ear stem length by 3.54%, 3.88%, 0.31%, 6.07% and 3.96%, respectively.

Effects of straw returning with potassium fertilizer on the phenotypic traits of spring maize stems
As shown in Table S2   Effects of the straw returning method and potassium fertilizer dosage on the lodging resistance mechanical properties of spring maize stems As shown in Table S3, the effects of the single factor, two-factor interactions and three-factor interactions of the straw returning method, potassium fertilizer dosage, and ecological region on the maize stem puncture strength, compressive strength and bending strength were extremely significant. As shown in Figure 1 and

Effects of the straw returning method and potassium fertilizer dosage on maize grain quality
As shown in Table S5 Figure 2 and Table S6, the protein content, starch content, crude fat content and water content of maize grains differed significantly among the treatments. In the Tumochuan Plain irrigation area, Hetao Plain irrigation area, and Lingnan warm dry zone, straw returning treatment increased the protein content of grains by 11.78%, 13.68%, and 13.53%, respectively, with potassium application in comparison with no potassium application, whereas the starch content of grains increased by 1.34%, 1.68%, and 1.31%, while the crude fat content of grains increased by 12.71%, 14.72%, and 14.73%, and the water content of grains decreased by 7.30%, 7.22%, and 7.66%. With no straw returning treatment, the protein content of grains in the Tumochuan Plain irrigation area, Hetao Plain irrigation area, and Lingnan warm dry zone increased by 7.52%, 10.19%, and 7.03%, respectively, with potassium application in comparison with no potassium application, whereas the starch content of grains increased by 1.01%, 1.25%, and 0.99%, while the crude fat content of grains increased by 10.21%, 12.62%, and 10.30%, and the water content of grains decreased by 2.63%, 2.67%, and 4.91%. In the Tumochuan Plain irrigation area, Hetao Plain irrigation area, and Lingnan warm dry zone, straw returning increased the effectiveness of potassium application on the protein content of grains by 4.26%, 3.49%, and 6.50%, respectively, whereas its effectiveness on the starch content of grains increased by 0.33%, 0.43%, and 0.32%, while its effectiveness on the crude fat content of grains increased by 2.51%, 2.09%, and 4.43%, and its effectiveness on the water content of grains increased by 4.67%, 4.55%, and 2.75%.  As shown in Table S7, the effects of the single factor, two-factor interaction and three-factor interactions 3 of the straw returning method, potassium fertilizer dosage, and ecological region on the maize grain number 4 per ear, 1000-grain weight, water content and yield all reached a significant or extremely significant level, 5 whereas their effects on ear number did not reach a significant level. 6