Nutrient Accumulation and Distribution Assessment in Response to Potassium Application under Maize–Soybean Intercropping System

: Intercropping is an intensive agricultural cropping system widely practiced for enhanced yield and nutrient acquisition advantages. A two-year maize–soybean intercropping (MSI) ﬁeld study was performed in 2018 and 2019 to assess the e ﬀ ects of potassium (K) fertilizer application on biomass accumulation and distribution of essential nutrients in the various plant parts (root, green biomass and seed) of maize–soybean intercropping (MSI). Three di ﬀ erent treatments of K fertilizer applications (T 0 : no potassium application; T 1 : maize 40, soybeans 30 and T 2 : maize 80, soybeans 60 kg ha − 1 ) were designed with 2 rows of maize by wide, narrow row planting in row arrangements of 160 cm + 40 cm. Soybeans were grown in 2 wide rows at a width of 40 cm and a row spacing of 60 cm between the rows of maize and soybeans, while the sole maize (SM) and sole soybean (SS) were grown with 70-cm and 50-cm row spacing, respectively. The results of the two-year study conﬁrmed that, as compared to T 0 , T 2 signiﬁcantly increased nitrogen, phosphate and potassium (NPK) accumulation in all maize parts by 27%, 16% and 20% grain, 23%, 22% and 14% green biomass and 30%, 17% and 15% root, respectively. In soybean treatments, T 2 signiﬁcantly increased NPK accumulation by 23%, 22% and 24% grain, 16%, 15% and 12% green biomass and 18%, 19% and 20% root, respectively. The increased accumulation of nutrients under T 2 raised the overall biomass and its distribution to root, green biomass and grain in maize and soybeans by 11% and 18% and 16% and 19%, 20% and 12%, respectively, compared to T 0 . On average, after two years of experiments, the T 2 intercropped maize and the soybeans showed 103% and 64% of the sole yield and attained the maximum LER of 1.66 and 1.68, respectively. Our results reveal that managing optimum K level application (80:60 kg ha − 1 ) can accelerate biomass accumulation and distribution of other essential nutrients in the plant parts of intercropped maize and soybeans. Therefore, it is immensely important to concern potassium application levels in developing a sustainable maize–soybean intercropping systems for achieving higher productivity and land equivalent ratio (LER).


Introduction
With population expansions, intensification of urbanization, climate change, the worldwide food crises have been observed recently in the world [1,2]. Traditional cropping no longer meets the needs Currently, more than 66% of China's agricultural areas were stated to be scarce in K availability, which markedly limits the crops' growth and production [37].
Many researchers have shown that under moisture stress, K improves the tolerance of drought [38], crop production, the partitioning of dry matter [39], and significantly increases yields [39][40][41][42][43]. Various physiological mechanisms, such as stomatal regulation and photosynthesis, depend on K [44]. Lodging can reduce plant growth severely, although researchers found that agronomic management can alleviate lodging incidence, especially by potassium (K) application, which is the primary nutrient considered important for crop growth and yield development [34]. Potassium is resistant to crop lodging, its scarcity is known to increase the incidence of lodging [45,46].
In intercropping systems, previous research in literature has mainly focused on crops' light interception, nutrient allocation and yield. However, no specific research has been carried out to analyze the essential nutrients' N, P and K accumulation and its distribution to various maize and soybean parts in MSI under K applications. Some scientists have reported the accumulation of P and N in maize under different row configuration of wheat and maize intercropping system [47], but up till now, potassium effects on dry matter, nutrients accumulation, and distribution are not stated in maize and soybeans. Consequently, there is a need to examine the nutrients accumulation and distribution trend in soybeans and maize for agricultural sustainability. In this experiment, we have accessed the effects of K nutrient manipulated by different treatment levels of maize and soybeans in MSI. This research-based on hypothesis (a) to explore the effects of K application on nutrients accumulation such as NPK, dry matter production and yield of maize and soybean plants; (b) to investigate the dynamic distribution of biomass and nutrients in maize and soybeans under specific K applications; (c) to propose optimum K application for better use of nutrients NPK and competitiveness and productivity of available resources. This hypothesis was investigated by comparing treated ones (T 2 and T 1 ) with non-treated (T 0 ). Further, we have compared intercropping control with maize and soybean sole cropping control. The research aim was to realize whether nutrient accumulation in intercrops and higher biomass production under MSI can be obtained with optimum K application, to support a theoretical ground for greater intercrop production in the MSI.

Plant Materials and Site Description
Field experiments comprising over two years were conducted during 2018 and 2019 at Sichuan Agricultural University experimental Farm, Sichuan China (31 • 46 N, 119 • 00 E, 535 m elevation). The climate condition was humid and subtropical and had a mean temperature of approximately 19.5 • C, with annual mean rainfall 106.89 mm, yearly sunlight 3.95 h and 310 days of frost-free period. The growing season climate included precipitation and an average temperature of each month, as indicated in Figure 1. The experimental site soil was comprised of clay loam texture 54% sand, 28% silt and 17% clay (hydrometer method), with total nitrogen 0.90 g kg −1 by Kjeldahl acid digestion method [48], total phosphorus 0.62 g kg −1 [49] total potassium 6.28 g kg −1 [50], available nitrogen 63.5 mg kg −1 automatic discontinuous analyzer (Clever chem200, Germany), available phosphorus 40.57 mg kg −1 [51] accessible potassium 96.36 mg kg −1 [52] and organic matter 30.34 g kg −1 in the top 0-25 cm soil layer and pH 6.6 [53].

Experiment Design
Zheng Hong 505 semi compact maize crop and Nandou 25 soybeans were planted in this experiment, in the southwest of China such cultivars of maize and soybeans are commonly cultivated [54]. The field trial was comprised of a randomized complete block design (RCBD) with three potassium treatments (T0: no potassium application; T1: maize 40, soybeans 30 and T2: maize 80, soybeans 60 kg ha −1 ) with three replications, whereas sole maize (SM) and sole soybeans (SS) K applied viz K: 00 kg ha −1 , respectively. The potassium application was applied to maize at the V6 stage, while soybeans were treated during the first fully trifoliate leaf V1 stage. In this experiment, maize-soybean intercropping (MSI) consisted of 2 rows' maize and 2 rows' soybeans (2:2) with the row-to-row distance between maize to maize and soybean to soybean was 40 cm. The plant-plant distance between maize and soybeans was 17 and 10 cm, respectively, while the inter-row spacing distance from maize to soybeans was 60 cm. Simultaneously, sole maize and sole soybeans were also planted at an interval 70 and 50 cm, respectively as shown in Figure 2. Each plot had a length and width of 6 × 6 m. Due to overseeding of both cultivars, the plant population was 60 K and 100 K ha −1 for maize and soybeans.

Experiment Design
Zheng Hong 505 semi compact maize crop and Nandou 25 soybeans were planted in this experiment, in the southwest of China such cultivars of maize and soybeans are commonly cultivated [54]. The field trial was comprised of a randomized complete block design (RCBD) with three potassium treatments (T 0 : no potassium application; T 1 : maize 40, soybeans 30 and T 2 : maize 80, soybeans 60 kg ha −1 ) with three replications, whereas sole maize (SM) and sole soybeans (SS) K applied viz K: 00 kg ha −1 , respectively. The potassium application was applied to maize at the V 6 stage, while soybeans were treated during the first fully trifoliate leaf V 1 stage. In this experiment, maize-soybean intercropping (MSI) consisted of 2 rows' maize and 2 rows' soybeans (2:2) with the row-to-row distance between maize to maize and soybean to soybean was 40 cm. The plant-plant distance between maize and soybeans was 17 and 10 cm, respectively, while the inter-row spacing distance from maize to soybeans was 60 cm. Simultaneously, sole maize and sole soybeans were also planted at an interval 70 and 50 cm, respectively as shown in Figure 2. Each plot had a length and width of 6 × 6 m. Due to overseeding of both cultivars, the plant population was 60 K and 100 K ha −1 for maize and soybeans.

Experiment Design
Zheng Hong 505 semi compact maize crop and Nandou 25 soybeans were planted in this experiment, in the southwest of China such cultivars of maize and soybeans are commonly cultivated [54]. The field trial was comprised of a randomized complete block design (RCBD) with three potassium treatments (T0: no potassium application; T1: maize 40, soybeans 30 and T2: maize 80, soybeans 60 kg ha −1 ) with three replications, whereas sole maize (SM) and sole soybeans (SS) K applied viz K: 00 kg ha −1 , respectively. The potassium application was applied to maize at the V6 stage, while soybeans were treated during the first fully trifoliate leaf V1 stage. In this experiment, maize-soybean intercropping (MSI) consisted of 2 rows' maize and 2 rows' soybeans (2:2) with the row-to-row distance between maize to maize and soybean to soybean was 40 cm. The plant-plant distance between maize and soybeans was 17 and 10 cm, respectively, while the inter-row spacing distance from maize to soybeans was 60 cm. Simultaneously, sole maize and sole soybeans were also planted at an interval 70 and 50 cm, respectively as shown in Figure 2. Each plot had a length and width of 6 × 6 m. Due to overseeding of both cultivars, the plant population was 60 K and 100 K ha −1 for maize and soybeans. Plant populations for sole maize and sole soybeans were similarly, maintained, keeping a distance of 16.7 cm and 20 cm, respectively. Both crops were planted simultaneously on the 10th ± 6 days of June. Maize was harvested in the last week of September, while the soybeans were harvested on the 28th ± 4 day of October in both years. Basal fertilizer application was also applied to maize at 130 and 72 kg ha −1 of nitrogen and phosphorus at planting time in sole and intercrops. The second nitrogen Agronomy 2020, 10, 725 5 of 18 dose was applied at the V 6 stage of maize at 80 kg ha −1 as urea in all plots. For both intercropped and sole soybeans, the dosage nitrogen application was 50 kg ha −1 as urea, P with 60 kg ha −1 as calcium superphosphate, while at the flowering stage of soybeans, a 2nd dose of nitrogen was applied to all plots as urea at 25 kg ha −1 [8]. In addition, agricultural practices in all areas were the same as in crop demand and farming practices in the region [55]. The entire crop relied on rainfall.

Nutrient Accumulation and Distribution
At harvesting stage, 15 maize and 30 soybeans plants were sampled from every replication of nitrogen, phosphorus and potassium analysis. The harvested maize and soybean plants were divided into grain, green straw biomass and root; all plant parts were dried for 1 h at 105 • C for tissue destruction and then dehydrated at 80 • C in an oven for constant weight. Then, each part was crushed by Wiley Mill and passed over Udy Mill (Thomas Scientific, Swedesboro, NJ, USA) Fort Collins, CO fitted with a strainer of (0.5 mm), and each part nitrogen concentration (g plant −1 part) of maize and soybeans was found using the Kjeldahl method [56]. Estimations of each part P concentration (g plant −1 part) were determined by vanadomolybdate procedure [57]. The potassium content in each part (g plant −1 ) was determined by FAAS (flame atomic absorption spectrophotometry, Varian 250 plus). In each part of the maize and soybean plant, NPK concentrations were obtained by the product of total biomass of each plant part with the NPK concentrations of each part and presented as kg ha −1 [58]. The evaluative amounts of NPK in all of the plant organs in each plant (g plant −1 ) were determined total nitrogen accumulation (TNA), total phosphate accumulation (TPA) and total potassium accumulation (TKA) [59].

Statistical Analyses
All data obtained for every parameter were analyzed in Microsoft Excel-2013; and variance tests were accomplished using statistics (version, 8.1. Statistix, USA). Analysis of variance (ANOVA) methodologies and LSD were used for the estimation of treatment results on different parameters; both measures were measured at 0.05 probability level [60]. In comparison, the software Microsoft Excel-2013 used standard error (±SE) for visual visualization of results.

K Effect on Plant Biomass Accumulation
The results of the present study suggests that potassium has a profound effect on the total biomass production of maize and soybeans under the MSI (Table 1). In the T 2 treatment, maize and soybean plants produced higher TBP than other treatments. In both years, the average in sole soybean treatment produced the highest biomass (6.01 t ha −1 ), whereas the T 2 maize plant had the maximum biomass accumulated (15.50 t ha −1 ), followed by T 1 (13.19 t ha −1 ). In contrast, the average biomass produced by sole maize was 15.01 t ha −1 .

K Effect on Plant Biomass Distribution
In addition, potassium treatments altered the biomass pattern distribution across various maize and soybean plant parts. Moreover, K treatments changed the biomass distribution trends in various parts of the maize and soybean plant. In general, the biomass of (root, green biomass and grain) in maize and soybeans decreased in T 1 and T 0 (Figure 3a-f). Seemingly, the highest biomass of maize and soybeans grain were (113.23 g plant −1 ) and (13.83 g plant −1 ) in T 2 and the lowest (86.62 g plant −1 ) and (10.81 g plant −1 ) in T 0 treatment, over the two years. Overall, the highest biomass was achieved in SS root (8.91 g plant −1 ), green biomass (34.49 g plant −1 ) and grain (16.71 g plant −1 ), while in MSI, T 2 treatment enhanced the root (11% and 18%), green biomass 16% and 19%), and the grain (20% and 12%) biomass of an intercropped maize-soybean relative to T 0 , respectively. Agronomy 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/agronomy parts of the maize and soybean plant. In general, the biomass of (root, green biomass and grain) in maize and soybeans decreased in T1 and T0 (Figure 3a-f). Seemingly, the highest biomass of maize and soybeans grain were (113.23 g plant −1 ) and (13.83 g plant −1 ) in T2 and the lowest (86.62 g plant −1 ) and (10.81 g plant −1 ) in T0 treatment, over the two years. Overall, the highest biomass was achieved in SS root (8.91 g plant −1 ), green biomass (34.49 g plant −1 ) and grain (16.71 g plant −1 ), while in MSI, T2 treatment enhanced the root (11% and 18%), green biomass 16% and 19%), and the grain (20% and 12%) biomass of an intercropped maize-soybean relative to T0, respectively.

K Effects on Nitrogen Accumulation
Data recording total nitrogen accumulation (TNA) of maize and soybean plants of MSI in response to different K applications are presented in Table 1. All K applications subsequently improved the TNA in various plant parts during two growing seasons (2018 and 2019). The results show that in T2 the average highest total nitrogen accumulation at maturity was 2.64 g of maize plant −1 and 1.93 g of soybean plant −1 , respectively. Maize TNA showed T2 > SM > T1 > T0 and the soybeans showed a SS > T2 > T1 >T0 pattern, as presented in (Table 1). In comparison with T2 treatment, maize TNA increased by 26% and soybeans by 22%, relative to MSI T0.

K Effects on Nitrogen Distribution
The different nitrogen concentration of maize and soybean plant parts of various treatments is indicated in Figure 4a-f. Overall, the highest nitrogen content accumulation was observed in SS grain (1.23 g plant −1 ), green biomass (0.67) and root (0.06). Under all treatments' of MSI nitrogen contents of maize and soybean grain (1.53 and 1.01 g plant −1 part), green biomass (0.96 and 0.55 g plant −1 part) and root (0.16 and 0.05 g plant −1 part) remained higher in T2, respectively than other respective treatments.

K Effects on Nitrogen Accumulation
Data recording total nitrogen accumulation (TNA) of maize and soybean plants of MSI in response to different K applications are presented in Table 1. All K applications subsequently improved the TNA in various plant parts during two growing seasons (2018 and 2019). The results show that in T 2 the average highest total nitrogen accumulation at maturity was 2.64 g of maize plant −1 and 1.93 g of soybean plant −1 , respectively. Maize TNA showed T 2 > SM > T 1 > T 0 and the soybeans showed a SS > T 2 > T 1 >T 0 pattern, as presented in (Table 1). In comparison with T 2 treatment, maize TNA increased by 26% and soybeans by 22%, relative to MSI T 0 .

K Effects on Nitrogen Distribution
The different nitrogen concentration of maize and soybean plant parts of various treatments is indicated in Figure 4a-f. Overall, the highest nitrogen content accumulation was observed in SS grain (1.23 g plant −1 ), green biomass (0.67) and root (0.06). Under all treatments' of MSI nitrogen contents of maize and soybean grain (1.53 and 1.01 g plant −1 part), green biomass (0.96 and 0.55 g plant −1 part) and root (0.16 and 0.05 g plant −1 part) remained higher in T 2 , respectively than other respective treatments.

K Effects on Phosphorus (P) Accumulation
Results regarding total phosphorus accumulation in response to potassium fertilizer are illustrated in Table 1. Considerably, K application levels significantly improved the TPA of maize and soybeans during the two-year experimental period. The highest average TPA was recorded in sole soybeans 0.315 g plant −1 and sole maize 0.507 g plant −1 . Maximum phosphorus concentrations in MSI were measured in T2 with 0.25 g plant −1 and 0.54 g plant −1 and minimum TPA in T0 treatment ( Table 2) with lowest mean 0.18 g plant −1 and 0.38 g plant −1 in soybean and maize plants, respectively.

K effects on Phosphorus (P) Distribution
Figure 5a-f shows the distribution of phosphorus contents in maize and soybean plant parts in response to different K treatment over the period 2018 and 2019. Under T2 treatment, maize achieved maximum average phosphate in grain 0.34 g plant −1 part, green biomass 0.18 g plant −1 part and root 0.02 g plant −1 part, while soybean grain 0.15 g plant −1 parts had the highest average P-value, green biomass 0.13 g of organ plant −1 and root 0.02 g of organ plant −1 in SS. Compared to T0, the T2 treatment enhanced phosphorus content of the grain, green biomass and root by 22%, 15% and 19% in soybeans and 16%, 22% and 17% in maize, respectively, in MSI.

K Effects on Phosphorus (P) Accumulation
Results regarding total phosphorus accumulation in response to potassium fertilizer are illustrated in Table 1. Considerably, K application levels significantly improved the TPA of maize and soybeans during the two-year experimental period. The highest average TPA was recorded in sole soybeans 0.315 g plant −1 and sole maize 0.507 g plant −1 . Maximum phosphorus concentrations in MSI were measured in T 2 with 0.25 g plant −1 and 0.54 g plant −1 and minimum TPA in T 0 treatment ( Table 2) with lowest mean 0.18 g plant −1 and 0.38 g plant −1 in soybean and maize plants, respectively.

K Effects on Phosphorus (P) Distribution
Figure 5a-f shows the distribution of phosphorus contents in maize and soybean plant parts in response to different K treatment over the period 2018 and 2019. Under T 2 treatment, maize achieved maximum average phosphate in grain 0.34 g plant −1 part, green biomass 0.18 g plant −1 part and root 0.02 g plant −1 part, while soybean grain 0.15 g plant −1 parts had the highest average P-value, green biomass 0.13 g of organ plant −1 and root 0.02 g of organ plant −1 in SS. Compared to T 0 , the T 2 treatment enhanced phosphorus content of the grain, green biomass and root by 22%, 15% and 19% in soybeans and 16%, 22% and 17% in maize, respectively, in MSI.

K Effects on Potassium (K) Accumulation
Total potassium content in maize and soybean plants are presented in Table 1. During twoyears, sole soybeans exhibited higher potassium content of 1.08 g plant −1 , while in T2 4.05 g plant −1 for maize. The average lowest maize TKA was 3.19 g plant −1 in T1 and 2.95 g plant −1 in T0. Together, soybeans and maize TKU increased by 16% and 19% in MSI, T2, compared to T0, 4.8.

K Effects on Potassium (K) Distribution
The potassium content of different plant parts of maize and soybeans is provided in Figure 6a-f. The average K concentration of maize grain 0.59 g plant −1 part, green biomass 3.14 g plant −1 part and root 0.33 g plant −1 part was significantly (p < 0.05) higher in T2 than in SM, T1 and T0. Moreover, the average SS potassium concentration measured 0.32 g plant −1 part in grain, 0.74 g plant −1 part in green biomass and 0.03 g plant −1 part in the root of soybeans plant were significantly higher than in T2 with root 0.03 g plant −1 , green biomass 0.19 g plant −1 and seed 1.08 g plant −1 .

K Effects on Potassium (K) Accumulation
Total potassium content in maize and soybean plants are presented in Table 1. During two-years, sole soybeans exhibited higher potassium content of 1.08 g plant −1 , while in T2 4.05 g plant −1 for maize. The average lowest maize TKA was 3.19 g plant −1 in T 1 and 2.95 g plant −1 in T 0 . Together, soybeans and maize TKU increased by 16% and 19% in MSI, T 2 , compared to T 0 , 4.8.

K Effects on Potassium (K) Distribution
The potassium content of different plant parts of maize and soybeans is provided in Figure 6a-f. The average K concentration of maize grain 0.59 g plant −1 part, green biomass 3.14 g plant −1 part and root 0.33 g plant −1 part was significantly (p < 0.05) higher in T 2 than in SM, T 1 and T 0 . Moreover, the average SS potassium concentration measured 0.32 g plant −1 part in grain, 0.74 g plant −1 part in green biomass and 0.03 g plant −1 part in the root of soybeans plant were significantly higher than in T 2 with root 0.03 g plant −1 , green biomass 0.19 g plant −1 and seed 1.08 g plant −1 .

K Effect on Yield and Land Equivalent Ratio
Grain yield of maize in T2 (6.29 t ha −1 and 6.44 t ha −1 ) was significantly higher than in T0 (4.97 t ha −1 and 5.38 t ha −1 ) and T1 (5.15 t ha −1 and 5.50 t ha −1 ), in 2018 and 2019, respectively and significantly greater than sole maize (6.10 t ha −1 and 6.20 t ha −1 ). The maximum mean of soybean grain yield (1.99 t ha −1 ) was found in SS and the minimum mean yield of (0.97 t ha −1 ) was noted in T0 (p < 0.05; Table 2). In particular, the only treatment T2 that increased the yield of maize by (12% in 2018 and 9% in 2019) and soybeans (by 15% in 2018 and 11% in 2019) above normal MSI (T0). Maize grain yield exhibited a trend of T2> SM > T1 > T0 (Table 2) and soybean yield showed a trend of SS > T2 > T1 > T0 ( Table 2). The LER values of maize were higher than the corresponding soybean values for all K treatments. Moreover, in T2 treatment, soybean LER was increased significantly

K Effect on Yield and Land Equivalent Ratio
Grain yield of maize in T 2 (6.29 t ha −1 and 6.44 t ha −1 ) was significantly higher than in T 0 (4.97 t ha −1 and 5.38 t ha −1 ) and T 1 (5.15 t ha −1 and 5.50 t ha −1 ), in 2018 and 2019, respectively and significantly greater than sole maize (6.10 t ha −1 and 6.20 t ha −1 ). The maximum mean of soybean grain yield (1.99 t ha −1 ) was found in SS and the minimum mean yield of (0.97 t ha −1 ) was noted in T 0 (p < 0.05; Table 2). In particular, the only treatment T 2 that increased the yield of maize by (12% in 2018 and 9% in 2019) and soybeans (by 15% in 2018 and 11% in 2019) above normal MSI (T 0 ). Maize grain yield exhibited a trend of T 2 > SM > T 1 > T 0 ( Table 2) and soybean yield showed a trend of SS > T 2 > T 1 > T 0 ( Table 2). The LER values of maize were higher than the corresponding soybean values for all K treatments. Moreover, in T 2 treatment, soybean LER was increased significantly. The T 0 , T 1 and T 2 represent the no potassium treatment, 40:30 kg ha −1 and 80:60 kg ha −1 , respectively, in MSI. The SS and SM refer to sole cropping system of soybeans and maize, respectively. Means are averaged over three replicates ± standard error. Means do not share the same letters in the columns differ significantly at ≤0.05.

Correlation Analysis
In MSI regression, the interdependence relationship of the maize and soybean yield and NPK concentration in response to different K levels were investigated. NPK concentrations were significantly correlated with a yield during the two-year field experiment (Figure 7). We concluded that maize seed yield R 2 = 0.86, R 2 = 0.93 R 2 = 0.91, were strongly and positively (p < 0.05) related to the nitrogen, phosphorus and potassium concentration of grain, respectively. Similarly, in the case of soybean seed yield R 2 = 0.91, R 2 = 0.90 and R 2 = 0.85 also had a strong and positive relationship with NPK concentration in response to different K levels. The mean datasets of 2018 and 2019 correlation coefficient between all parameters were found higher than 0.84 (p < 0.05).

Correlation Analysis
In MSI regression, the interdependence relationship of the maize and soybean yield and NPK concentration in response to different K levels were investigated. NPK concentrations were significantly correlated with a yield during the two-year field experiment (Figure 7). We concluded that maize seed yield R 2 = 0.86, R 2 = 0.93 R 2 = 0.91, were strongly and positively (p < 0.05) related to the nitrogen, phosphorus and potassium concentration of grain, respectively. Similarly, in the case of soybean seed yield R 2 = 0.91, R 2 = 0.90 and R 2 = 0.85 also had a strong and positive relationship with NPK concentration in response to different K levels. The mean datasets of 2018 and 2019 correlation coefficient between all parameters were found higher than 0.84 (p < 0.05).

Discussion
This study was carried out to investigate whether a change in potassium application would affect nutrients accumulation which results in increased biomass accumulation and yield under MSI. In various plant parts, the increased dry weight accumulation and biomass partitioning resulted due to the optimum supply of K to the crop [61,62]. Furthermore, in both years than T0, the total biomass of maize and soybeans under T2 treatment significantly increased by 14% and 19%), respectively. The decline in total biomass under T0 corresponds to previous literature that different crops along with maize and soybeans are more prone to potassium deficiency, with a strong effect on biomass accumulation, quality and yield [19,63,64]. Some studies have explained that K has a key role in stimulating the photosynthesis process for crop growth and biomass production thus confirming more final yield, which is clear in our data [65]. Therefore, variation of K treatments in MSI attributed to the increased biomass accumulation under T2 followed by T1 and T0.
This experiment illustrates that T2 considerably increased the TNA of both crop species than T1 and T0 under MSI, resulting in higher yield. In comparison with T2 treatment, maize TNA increased by 26% and soybeans by 23% relative to MSI T0. In all treatment's total nitrogen accumulation and distribution to maize and soybeans plant parts such as (grain, green biomass and root) varied significantly, that may be correlated to high and low nitrogen requirements for maize and soybean plants, correspondingly. Previous studies have shown that N distribution is independent of the source size of maize [66]. The study, therefore, suggested that the distribution of N in different maize organs, especially grains, is responsive to source size. Among all treatments, the highest concentration of TNA in maize and soybean grain (1.53 and 1.01 g plant −1 part), green biomass (0.96 and 0.55 g plant −1 part) and root (0.14 and 0.05 g plant −1 part) were measured in T2 treatment. This

Discussion
This study was carried out to investigate whether a change in potassium application would affect nutrients accumulation which results in increased biomass accumulation and yield under MSI. In various plant parts, the increased dry weight accumulation and biomass partitioning resulted due to the optimum supply of K to the crop [61,62]. Furthermore, in both years than T 0 , the total biomass of maize and soybeans under T 2 treatment significantly increased by 14% and 19%), respectively. The decline in total biomass under T 0 corresponds to previous literature that different crops along with maize and soybeans are more prone to potassium deficiency, with a strong effect on biomass accumulation, quality and yield [19,63,64]. Some studies have explained that K has a key role in stimulating the photosynthesis process for crop growth and biomass production thus confirming more final yield, which is clear in our data [65]. Therefore, variation of K treatments in MSI attributed to the increased biomass accumulation under T 2 followed by T 1 and T 0 .
This experiment illustrates that T 2 considerably increased the TNA of both crop species than T 1 and T 0 under MSI, resulting in higher yield. In comparison with T 2 treatment, maize TNA increased by 26% and soybeans by 23% relative to MSI T 0. In all treatment's total nitrogen accumulation and distribution to maize and soybeans plant parts such as (grain, green biomass and root) varied significantly, that may be correlated to high and low nitrogen requirements for maize and soybean plants, correspondingly. Previous studies have shown that N distribution is independent of the source size of maize [66]. The study, therefore, suggested that the distribution of N in different maize organs, especially grains, is responsive to source size. Among all treatments, the highest concentration of TNA in maize and soybean grain (1.53 and 1.01 g plant −1 part), green biomass (0.96 and 0.55 g plant −1 part) and root (0.14 and 0.05 g plant −1 part) were measured in T 2 treatment. This consequence may be linked to higher biomass, which increased soil nitrogen absorption and transported higher nitrogen amounts to maize grains [67], making grain a major part of nitrogen storage and accumulation in the plant [66].
In the legume crops, nodulation is a common phenomenon and strongly connected to plant growth itself. Hence, there is a possibility that the extra nitrogen released by soybean nodules accumulated in nearby deep root maize in MSI [68].
Further research has shown that K application led to the release of fixed ammonium ion from the soil, which made the plant more capable of nitrogen accumulation [69]. However, there is a noticeable improvement in TNA and distribution of soybeans in various plant parts, primarily due to increased K applications in T 1 and T 2 . Under T2, the maximum TNA in soybeans and maize can be attributed to the influential diffuse contact mechanism, which transfers CO 2 , water and heat through plants [70], which could contribute to the optimum absorption of plant nutrients like N, as previously reported [71].
Sole soybeans exhibit superiority over intercropping due to higher yield attributes such as a greater number of pods, seed index, number of seeds per plant, however in MSI due to high resource competition, shading effect of maize on soybeans ultimately reduces the yield attributes of intercropped soybeans. In SS P contents in various soybeans, parts are increased than MSI. Notably, in this experiment, found reduced differences in TPA and distribution in different parts of soybeans under T 2 T 1 and T 0 treatments. Increased P accumulation in maize and soybean parts under T 2 , may be due to the reason (i) appropriate K application improve nutrients uptake efficiency by extending root surface area [72] increasing the P uptake available in the soil (ii) improving translocation of P and the accumulation of photosynthates [73]. Therefore, resulting in enhanced TPA in maize and soybean seed by 16% and 6% in T 2 , 4% and 6% in T 1 , respectively as a comparison to T 0 . Interestingly, our experiment revealed that adequate K application (T2) substantially increased TPA and distribution in maize and soybeans than T 0 , while the TPA reduced in T 1 maize and soybeans under MSI. Meanwhile, in MSI the less accumulation of P was observed in root under T 1 than T 2 , the results are consistent with [74]. Thus, K increases phosphorus use efficiency that was documented to have a key role in improving seed yield [57].
Very few studies stated the K accumulation and distribution in various organs of the maize and soybeans into different plant organs in MSI. Commonly, TKA in maize plants accumulated higher than soybean plants, whereas T 2 increased maize TKA by 16% in 2018 and 19% in 2019 compared to T 0 in all treatments. In comparison, T 1 and T 0 resulted in lower TKA, which may be partially correlated with the root receiving lower carbohydrates (lower plant biomass), further decrease in maize grain K accumulation. Our results showed a higher K accumulation in intercropped maize and similar results were recorded in intercropping systems of wheat and wheat-maize, maize-soybean, respectively [56].
Interestingly, all K treatments significantly increased the TKA of soybeans, indicating reduced K competition between maize and soybean plants under T 2 and the T 1 of MSI. An intense reduction of the leaf surface and sunlight interception was observed when the K application was below the plant's required level [75]. Hence, in soybeans, the K application increases the availability and distribution of nutrients under the shading condition and controls photosynthesis through sunlight interception. Our results show that the accumulation ability of nutrients was significantly affected at different K applications in MSI and that the acquisition of NPK in soybean plants was higher than in maize plants. Overall, an increased level of nitrogen and phosphorus was observed in grain compared to green biomass and root, while green biomass accumulates more K than grain. Nutrient accumulation in the root of maize and soybeans could be explained because of nutrients competition to support optimum growth and biomass production and between plant parts near to nutrient sources. Interspecific interactions and the light conditions typically played an essential role in the improvement of resource efficiency by intercropping species, e.g., by improving nutrient recovery in comparison with the monocropping system by fixing nitrogen in the maize-soybean intercropping system [57].
In MSI, intensified competition and shade density by maize plants resulted in reduced soybean nutrient accumulation [56] and subsequently improved nitrogen fixation [76]. Various effects of potassium treatment on grain yields of two intercrop species in MSI were considered and the results of our studies have shown that the yield of the grain of both intercrop species has increased with the optimum use of potassium (T 2 ) in both years [57,68]. Furthermore, we observed that the treatment T 2 substantially increased plant −1 kernels (by 10% from 400 kernel numbers plant −1 in T 0 to 439 kernel numbers Plant −1 in T 2 ) and plant −1 kernel numbers (from 86.31 g in T 0 to 106.14 g in T 2 ) of maize in MSI. Moreover, lack of K lowers the grain filling, possibly due to the role of potassium in translocating sugar to the growing part of the plant resulting in yield decline [28]. Thousand grain's weight was increased by 8% and 4% in maize and soybeans, respectively at the highest rate of K (80 and 60 Kg ha −1 ). The finding is consistent with [77] that increased 1000 grain weight with K application may be due to increased plants' photosynthetic activity that finally moved toward the sink, [78] found increased grain number and grain weight. Potassium helps to enhance the efficient use of carbohydrates and improves the leaf index, which enhances dry matter accumulation, eventually increases the grain yield of rapeseed [79,80]. Therefore, T 2 is the optimum potassium level, which increased the yields of maize and soybeans significantly by 10% and 13%, respectively, compared to T 0 . Altogether, NPK concentration substantially improved in various plant parts with higher K applications. A strong correlation between grain yield and NPK concentration at different K levels indicate that K can be used to predicate maize-soybean intercropped yield.
Notably, the land equivalent ratio (LER) of the two-year experiment in MSI treatments has always been higher than 1 and indicates that MSI grain, yields are advantageous than sole maize and sole soybean crops due to improved water, soil, light and growth of nutrients [81]. The average LER values in T 0 , T 1 and T 2 were 1.35, 1.42 and 1.67, respectively. These findings indicate that (51%-73%) extra agricultural land is needed for a sole maize and soybean crop equal to MSI grain yield, showing better land-use efficiency than sole cropping [82]. The entire intercrop species growth period of this experiment was 141 days, which for maize or soybeans is longer than 113 days. In one growing season, the two crop species may be cultivated in production regions, when the growing season is too short of doubling crop [12], such as Sichuan, Gansu, Chongqing in China. Moreover, our experimental study showed that K is not only responsible for higher production, but also for better harvest efficiency for treatment T 2 and T 0 under MSI. Thus, in MSI, K ensures improved biomass production and use of main nutrients for soybeans and maize crop and ultimately provides higher value crops and benefits for farmers' profits.

Conclusions
Based on two years experiment of MSI, it was concluded that various potassium applications have significant effects on growth, nutrients accumulation, yield and yield-related parameters of intercropped maize and soybeans. T 2 was found more advantageous for improved growth and increased accumulation of nutrients which has raised the overall biomass and its distribution to root, green biomass and grain in maize and soybeans by 11% and 18% and 16% and 19%, 20% and 12%, respectively, compared to T 0 . In this study, our field study investigated three different potassium treatments effects on maize and soybean. Adequate application of K gives rise to nutrients translocation towards grains of maize and soybeans and maximizes the grain yield of maize by improving soybean yield. Overall, T 2 treatment considerably increased the LER by (1.67) of the maize-soybean intercropping system. The study suggests that in maize-soybean intercropping system, optimum K application of (80-60 kg ha −1 ) is a viable nutrient management option. Thus, considering potassium's major role in yield' development, maize-soybean yield can be grown in the intercropping system. Quality research advances should focus on expanding its importance in intercrop species; hence, ensuring optimal utilization of K fertilization enabling growers to achieve provided targets for future yield improvement efforts.