Combined Effects of Straw Return with Nitrogen Fertilizer on Ion Balance, Photosynthetic Characteristics, Leaf Water Status and Rice Yield in Saline-sodic Paddy Fields

doi:10.21203/rs.3.rs-2545065/v1

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Research Article

Combined Effects of Straw Return with Nitrogen Fertilizer on Ion Balance, Photosynthetic Characteristics, Leaf Water Status and Rice Yield in Saline-sodic Paddy Fields

https://doi.org/10.21203/rs.3.rs-2545065/v1

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published 28 Aug, 2023

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Aims salinisation severely limits crop growth and yield. In recent years, the effect of nitrogen fertilisation and different management practices on the mitigation of saline-sodic stress in crops has been less studied. Therefore, we conducted a three-year field experiment in Jilin Province, China, to investigate the effects of combined straw and nitrogen fertilizer application on the physiological and photosynthetic characteristics of rice in saline-sodic paddy fields.

Methods The experiment was designed as a split-zone trial, with straw (S) as the main zone and nitrogen fertilizer (N) as the secondary zone. The amount of straw returned was 0 t ha^-1 (B) and 7 t ha^-1 (T). Nitrogen treatments of 0, 150, 250 and 350 kg ha^-1 were recorded as N0, N1, N2 and N3, and each treatment was repeated three times.

Results Straw combined with nitrogen fertilizer can effectively reduce the Na⁺/K⁺ value, malondialdehyde content and relative electric leakage of rice leaves in salt-alkali soil, and ensure the integrity of cell membrane. At the same time, the leaf water potential, relative water content and chlorophyll content were increased, which promoted rice photosynthesis and improved rice yield. In addition, it was found that straw combined with nitrogen fertilizer had the greatest positive effect on rice yield at 250kg ha^-1.

Conclusions Straw combined with nitrogen fertilizer can improve the physiological and photosynthetic characteristics of rice in saline-alkali paddy field and provide a theoretical basis for rice yield increase in this area

rice

saline-sodic stress

straw

nitrogen fertilization

yield

Soil salinization is a common environmental problem in the world. Approximately 831 million hectares of land are affected by saline-alkali worldwide (Mostofa et al. 2015). The western Songnen Plain of China is one of the world's three major saline-sodic concentration areas, with more than 3.0 × 10⁶ ha of saline-sodic land (Yang et al. 2016). NaHCO₃ and Na₂CO₃ are the main soil salts in saline-sodic land, and the soil pH value is between 8.5–10.5 (Liu et al. 2011). Songnen Plain is one of the major grain producing areas in China, where saline-sodic soil severely limits crop yields (Wang et al. 2009). Saline-sodic soil affects plant growth and development mainly through osmotic imbalance, ion toxicity and high pH stress (Chi et al. 2012). On the one hand, the large amount of salt ions in saline-sodic soil causes the soil osmotic potential to decrease, while the plant cells have a relatively high osmotic potential, which causes osmotic stress on the plant membrane system and limits the water absorption of the plant (Roy et al. 2014). On the other hand, saline-sodic soil containing a large amount of Na⁺ can cause the imbalance of ion flux in plant tissue cells (Ruiz et al. 2015, Liu et al. 2022), which promotes the accumulation of MDA. Resulting in plasma membrane peroxidation and destruction of membrane structural and functional integrity (Jini and Joseph 2017, Willekens 1997). At the same time, the high PH stress of saline-sodic soil inhibits the absorption of essential ions Ca²⁺, Mg⁺ and NO₃⁻ in crop roots, affects the synthesis of chlorophyll, and also inhibits photosynthesis, which affects the formation of crop yield (Feng et al. 2021). With the increase of soil salinization, crop production is seriously threatened. How to improve saline-sodic soil and reduce the harm of saline-sodic stress on crops is the key to develop and utilize saline-sodic soil rationally.

Nitrogen fertilizer is the most widely produced and used fertilizer in the world, and can harmonize the nutritional and reproductive growth of crops and increase the yield of crops (Wang et al. 2019). At the same time, a large number of studies have shown that nitrogen accumulation is an important guarantee to improve crop stress resistance: i) Effective use of nitrogen can improve the antioxidant metabolism of plants by increasing the activities of key antioxidant enzymes and the contents of non-enzyme components. (Ahanger et al. 2019). ⅱ) Applying the right amount of nitrogen can increase the content of soluble sugars and proline in leaves and raise the osmotic pressure in the crop, which enables the crop to reduce the inhibitory effect of saline-sodic stress on root water uptake through osmoregulation (Iqbal et al. 2015). iii) Nitrogen can affect the activity of a series of related enzymes during photosynthetic reactions and is involved in the synthesis of chlorophyll, promoting photosynthesis of the crops in saline-sodic land and improving crop yield (Cong et al. 2020). In addition, it has been shown that appropriate nitrogen application to the crops such as maize (Akram et al. 2011), rice (Dionisio-Sese and Tobita 2000) and wheat in saline-sodic land (Elgharably et al. 2009), and it can affect crop nitrogen uptake and metabolism (Parida and Das 2005) to improve crop saline-sodic tolerance (Chen et al. 2009) and reduce the growth inhibition of crops in saline-sodic land. Although nitrogen application is an important measure for high yield in saline-sodic soil crops in which nitrogen is extremely deficient, it is particularly difficult to implement proper nitrogen application in crops due to improper nitrogen application amounts and methods (Ahmed et al. 2017). The study showed that too low nitrogen fertilizer would lead to low rice yield, which could not play the yield advantage of rice varieties, but also lead to excessive consumption of soil nitrogen and the decrease of soil fertility level. (Sanchez 2002). The application of large amounts of nitrogen fertilizer alone will not only increase the number of ineffective tillers in rice and reduce the tiller tassel rate, but also disrupt the hormone balance in the plant, which will inhibit plant growth and affect the formation of rice yield (Xiao et al. 2016). Long-term unreasonable application of nitrogen fertilizer can cause environmental pollution and ecological damage, and increase the degree of salinization and soil compaction in saline-sodic fields (Ju et al. 2009). Therefore, how to apply nitrogen rationally is an urgent problem to be solved in saline-sodic land.

Straw return has gained importance as a green straw treatment method in China (Bhattacharyya et al. 2003). Straw return can not only reduce soil bulk density, but also improve soil aeration and soil aggregate quantity (Xiao et al. 2016). Straw return increased soil microbial quantity and activity by effectively regulating surface soil temperature, thereby increasing biodiversity(Ju et al. 2009). At the same time, straw has a significant improvement effect on saline-sodic land. Straw return can not only improve the stability of soil structure, but also inhibit the accumulation of salt to the soil surface and reduce the rhizosphere soil salt content (Li et al. 2016). Studies have shown that straw return improves soil porosity and aeration in saline-sodic soil, which is conducive to nutrient and water absorption by crops (Sui et al. 2015, Wang et al. 2018). Zhao showed that the placement of straw layers in 20 or 30 cm soil layers could cut off soil capillaries, reduce Na⁺ accumulation in surface soil, and optimize the growing environment of crops (Zhao et al. 2016). In addition, straw contains a lot of nutrients such as carbon, nitrogen, phosphorus and potassium, which can improve soil nutrient supply and soil fertility in saline-sodic soil and promote crop growth after return to the field (Saroa and Lal 2003, Zhao et al. 2020). However, other studies have shown that straw return has a adverse effect (Brennan et al. 2014) on crop nitrogen uptake and yield formation. Studies have shown that the large amount of carbon in straw can greatly stimulate the growth of soil microorganisms, which causes intense nitrogen competition between crops and soil microorganisms, and nitrogen adsorption is formed through soil microbial immobilization, resulting in insufficient supply of crop nitrogen fertilizer and affecting the formation of crop yield (Li et al. 2021, Yadvinder -Singh et al. 2004). The slow decomposition of straw in the early stage aggravated the soil cavity, affected the seed germination of crops and the root growth of seedlings, and reduced the formation of crop yield. Therefore, it is necessary to reconsider the way of straw application in saline-sodic land, reduce the negative effects of straw return and achieve the sustainability of straw return.

In the study of corn-winter wheat rotation model, it is shown that straw combined with nitrogen fertilizer is the best measure to maintain soil nitrogen accumulation and balance crop yield (Manevski et al. 2016). The simultaneous addition of straw and nitrogen fertilizer can improve the water status and chlorophyll content of the crop, which ultimately affects the yield of the crop (Bahrani et al. 2007). It shows that straw return combined with appropriate nitrogen fertilizer is beneficial to crop growth and yield formation. However, the effects of straw combined with nitrogen fertilizer on the photosynthetic and physiological characteristics of rice in saline-sodic paddy fields are still unclear. Therefore, the purpose of this 3-year study was to study the effects of straw combined with nitrogen fertilizer on the improvement of photosynthetic characteristics, ion balance, leaf water status and yield of rice in saline-sodic paddy fields, and to explore the mechanism of action. We assume the following: i) The combination of straw and nitrogen fertilizer maintained the balance of Na⁺ and K⁺ in rice leaves in saline-sodic paddy fields and ensured the integrity of membrane structure and function. ⅱ) The combined application of straw and nitrogen could improve the water condition and photosynthetic performance of rice leaves in saline-sodic paddy fields. ⅲ) The combination of straw and nitrogen fertilizer increased the rice yield and reduced the nitrogen application rate on saline-sodic paddy fields.

Experimental site

Field trials were conducted in Da'an City, Jilin Province, China (N 45° 35′58″-N 45°36′28″, E 123° 50′27″-123°51′31″), from April 2017 to October 2019. The region has a typical mid-temperate monsoon climate and a transitional zone from a semi-humid to semi-arid climate. Spring is dry and windy, summer is hot and rainy, autumn is cool, and shortwinter is dry and cold, with little snow. The average annual rainfall in this area is 399.9 mm, and most of the rainfall is concentrated in June to August, with an average annual evaporation of 1840 mm, which is 4.5 times higher than the annual rainfall. The annual average sunshine hours are 2915 h, the annual average temperature is 5.2℃, the annual average accumulated temperature is 2996.2℃, and the frost-free period is 144 d. The average precipitation and temperature during the test year are shown in Fig. 1. The basic physical and chemical properties of the soil (0–20 cm) prior to the test are listed in Table 1. According to the classification standards of the United States Department of Agriculture (1954), the soil type of this test site was high-pH saline-sodic soil. Therefore, this test site is representative of regional hydrogeological and climatic conditions.

Table 1

Basic physical and chemical properties of the tested soil
Parameter	Unit	Mean	Parameter	Unit	Mean
Bulk density	g/cm³	1.61	Total N	g/ kg	0.27
pH	-	10.10	Total K	g/ kg	22.36
Salinity	g/kg	4.78	Organic matter	g/ kg	7.31
ENa⁺	cmol_c /kg	3.21	Alkalihydrolysis N	mg/kg	16.3
CEC	cmol_c /kg	12.97	Available P	mg/kg	16.90
ESP	%	24.74	Available K	mg/kg	107.25

Experimental Design

The experiment was designed as a split-plot trial, with straw (S) as the main zone and nitrogen fertilizer (N) as the secondary zone. The amount of straw returned was 0 t ha^− 1 (B) and 7 t ha^− 1 (T) (the total incorporation is calculated based on the local rice grain-straw ratio, which was 7 t ha^− 1). Nitrogen treatments of 0, 150, 250 and 350 kg ha^− 1 were recorded as N0, N1, N2 and N3, and each treatment was repeated three times. In this experiment, a total of 24 plots were established, each with an area of 36 m² (8 m × 4.5 m), and the plots were separated by ridges, which were 50 cm wide and 30 cm high. Simultaneously, each plot had an independent single row and a single irrigation system to minimize the impact of fertilizer between treatments. The nitrogen fertilizer used was urea, and the N content was 46%. Nitrogen fertilizer was applied at a ratio of base fertilizer : mid-tillering fertilizer : panicle fertilizer at 6 : 3 : 1. The application amount of superphosphate phosphate fertilizer was 50 kg ha ^− 1, which was applied as a base fertilizer at one time; and the P₂O₅ content was 16%. The application rate of potassium fertilizer was 75 kg ha ^− 1, which was applied according to the ratio of base fertilizer : panicle fertilizer of 6 : 4. and the K₂O content was 60%.

In this study, straw returning test was carried out in the same plot for three consecutive years. The straw used in the previous season was collected manually from a nearby farmland. After the straw was air-dried under natural conditions, it was cut into pieces 5–7 cm in length with a straw chopper. With beating in spring, the field water layer was controlled at to 1–2 cm, and the straw was evenly spread on the field surface. A small beating machine was used to beat the straw to mix with 0–20 cm soil, and to ensure that the effect of straw return to the field was even and consistent. Base fertilizer was applied along with the return of the straw to the field, mid-tillering fertilizer was applied in the middle and late June (around June 20), and panicle fertilizer was applied in the middle of July (around July 15).

The rice varieties were selected from the classic control variety Changbai 9 on saline-sodic land. Transplanting was carried out around May 20, with a density of 20 hills and a hill spacing of 16.5 × 30.0 cm with five seedlings per hill. Harvest took place around October 1st over three years. The paddy fields were flooded after transplanting, and a floodwater depth of 3–5 cm was maintained until two weeks prior to harvest in order to prevent the loss of biomass and yield. Insects, diseases, and weeds were strictly controlled throughout the rice growth period.

Sampling Methods and Measurements

Determination of Na⁺ and K⁺ content in rice leaves

At the heading stage of rice, three rice leaves were selected from each plot, and three rice leaves with the same growth trend were selected from each plot. Rice leaves were oven-dried at 105℃ for 0.5 h and then oven-dried at 80℃ for 48 h. Subsequently, rice leaves were ground into fine powder and screened. After heating and digesting 0.5 g of accurately weighed samples with H₂SO₄-H₂O₂, Na⁺ and K⁺ contents in leaves were determined by flame photometry (Chen et al. 2021).

Determination of malondialdehyde (MDA) content and relative electrolyte leakage (REL), Superoxide (SOD) and peroxidase (POD) activities in rice leaves

At the heading stage of rice, three spots were selected from each plot, and three rice plants with the same growth trend were selected from each spot, and the contents of MDA, REL, SOD and POD were determined to use the topmost whole explants leaves. The content of MDA was determined by thiobarbituric acid staining (Assaha et al. 2017). The REL of rice leaves was determined according to DionisioSese and Tobita 1998 (Dionisio-Sese and Tobita 1998). The determination of superoxide dismutase (SOD) and peroxidase (POD) was determined according to the method of Jini and Joseph (2017) (Jini and Joseph 2017).

Determination of leaf relative water content (RWC) and water potential (Ψ_w)

At the rice heading stage, three points were selected from each plot, and three rice plants with the same growth trend were selected from each point to determine the RWC and Ψ_w of leaves. Ψ_w of the first unfurling leaf of rice (1 leaf per plant, 3 leaves per treatment) was measured between 4 h and 6 h using an HR-33T dew point microvoltmeter (Wescor Inc., UT,Logan, UT,UT,UT,UT) (Ran et al. 2019). The relative water content was determined by Machado and Paulsed methods (Machado and Paulsen 2001). First, the fresh weight (WF) of rice leaves was determined. Then, the leaves were soaked in a plastic bag with clean water and sealed for 24 h. The surface water was blotted dry with absorber paper, and the saturated fresh weight (WS) of the leaves was weighed. Dry weight (WD) was measured after drying in oven at 80℃ for 48 h. Formula for calculating leaf relative water content:

Determination of photosynthetic parameters and SPAD value of rice leaves

At the heading stage of rice, Three spots were selected from each plot, and three rice plants with the same growth trend were selected from each spot to determine SPAD values and photosynthetic parameters of rice flag leaves. the net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of flag leaves were measured by Li-6400 photosynthetic apparatus from 9:00 am to 11:00 am on a clear day without wind. The chlorophyll content of flag leaves (leaves used to determine photosynthetic parameters) was measured with a SPAD-502 portable chlorophyll meter (Minolta, Japan).

Determination of rice yield

At the rice maturity stage, three sites were randomly selected from each treated plot and 3 m² were harvested at each site to determine the yield per unit area (t ha^− 1). The rice yield at harvest time has been adjusted to 14% grain water content.

Statistical analysis

SPSS 22.0 (IBM Inc., USA) was used for ANOVA. The mean test showed the smallest difference at the P < 0.05 level (LSD0.05). The mean and standard deviation of measurement parameters were tested by descriptive statistics. Simulated linear regression analysis was carried out using Origin 2021 (Origin Lab Corporation, Northampton, Ma, USA).

Analysis of variance

Table 2

Analysis of variance (ANOVA) showing the significance probability of each effects for grain yield, rice physiological parameters.
Source of variation	Yield	Na⁺	K⁺	Na⁺/K⁺	MDA	REL	SPAD	Ψ_w	RWC
Nitrogen(N)	**	**	**	**	**	**	**	*	**
Straw (S)	**	**	**	**	**	**	**	**	**
N×S	**	*	**	*	*	ns	ns	ns	ns

ns —non significant, * — significant at P < 0.05, ** — significant at P < 0.01. MDA: Malondialdehyde; SOD: Superoxide; POD: Peroxidase; REL: Relative electrolyte leakage; Ψ_w: Water potential; RWC: Relative water content.

The ANOVA results of rice yield, Na⁺, K⁺, Na⁺/K⁺, MDA, REL, SPAD, Ψ_w and RWC are shown in Table 2. Nitrogen application rate (N) and straw return rate (S) had significant effects on all parameters (P < 0.05). There were significant N × S interactions for all parameters except REL, SPAD, Ψ_w, and RWC.

Na⁺ and K⁺ contents and the Na⁺/K⁺ ratio of rice leaves

As shown in Fig. 2 (a, b, c), Under the same nitrogen fertilizer level, straw return treatment reduced Na⁺ concentration and Na⁺/K⁺ ratio. Under the condition of straw return to the field, the concentration of Na⁺ and the ratio of Na⁺/K⁺ increased first and then decreased with the increase of nitrogen application rate, and reached the lowest level under N2 treatment in 2019. The concentration of Na⁺ in N2 treatment was significantly lower than that in N0 and N1 treatment by 20.0% and 12.4%. And the Na⁺/K⁺ ratio was significantly lower than that of N0 and N1 treatments by 34.1% and 20.3%.

Under the same nitrogen fertilizer level, with the increase of nitrogen application rate, K⁺ concentration was N0 < N1 < N3 < N2 (Fig. 2d, e, f). In 2019, the K⁺ concentration in N2 treatment was 21.4% and 9.8% higher than that in N0 and N1 treatments. In these three years, there were no significant differences in Na⁺ concentration, K⁺ concentration and Na⁺/K⁺ ratio between N2 and N3 treatments.

Under the same nitrogen fertilizer level, soil Na⁺ concentration and Na⁺/K⁺ ratio decreased and K⁺ concentration increased with the increase of straw return years (Fig. 2g, h, i).

Malondialdehyde (MDA) and Relative Electricity Leakage (REL) of rice leaves

Under the same nitrogen fertilizer level, straw return reduced the MDA content and REL of rice (Fig. 3). The MDA content of rice under straw return treatment and no straw return treatment was N0 > N1 > N3 > N2, and in 2019, N2 treatment was significantly lower than N0 and N1 treatment by 6.7% and 2.4%. There was no significant difference between N2 and N3 treatments. The REL of straw return treatment decreased with the increase of nitrogen application. In 2019, the REL of N0 treatment was significantly higher than that of N1 and N2 treatments by 6.1% and 6.2%, but there was no significant difference between N2 and N3 treatments. With the increase of straw return years, the MDA content and REL of rice leaves decreased under the same nitrogen fertilizer level.

Superoxide dismutase (SOD) and peroxidase (POD) activities of rice leaves

The SOD and POD activities of rice with straw return were significantly reduced under the same nitrogen fertilizer level (Fig. 4). SOD and POD activities decreased with increasing nitrogen application under the straw return treatment. The POD activity of rice in 2019 was the lowest under N2 treatment. Under the condition of straw return, SOD activity of rice treated with N2 was significantly decreased by 17.7% and 12.2%, POD activity was significantly decreased by 15.2% and 9.3% compared with those treated with N0 and N1. However, there was no significant difference in SOD and POD activities between N2 and N3 treatments. Under the same nitrogen fertilizer level, the activities of SOD and POD decreased with the increase of straw return years.

Relative water content (RWC) and leaf water potential (Ψ_w) of rice leaves

The results showed that under the straw return treatment in 2019, the RWC and Ψ_w of N2 treatment were the highest, which were 60.83% and 1.21 MPa, respectively (Fig. 5). Under the same nitrogen fertilizer level, the RWC and Ψ_w of straw return treatment were higher than those of no straw return treatment. The RWC and Ψ_w of both straw return and no straw return treatments showed a trend of first increasing and then decreasing with the increase of nitrogen application rate. The RWC and Ψ_w were the largest in the N2 treatment, but there was no significant difference between the N2 and N3 treatments. At the same nitrogen fertilizer level, RWC and Ψ_w showed an increasing trend with the increase of straw return years (Fig. 5).

SPAD value of rice leaves

As shown in Fig. 6 (a, b, c), Under the same nitrogen fertilizer level, the SPAD value of straw return treatment was 6.0%-28.9% higher than that of no straw return treatment. SPAD values increased with increasing N application for both straw return and no straw return treatments. In 2019, SPAD values were significantly higher in the N2 treatment than in the N0 and N1 treatments by 34.4% and 9.7%, while there were no significant differences in SPAD values between the N2 and N3 treatments. The SPAD value increased with the increase of the year of straw return under the same nitrogen fertilizer level.

Net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of rice

Under the same nitrogen fertilizer level, the Pn, Gs and Tr of rice leaves under straw return treatment were higher than those under no straw return treatment (Fig. 7). The Pn, Gs and Tr of rice leaves increased first and then decreased with the increase of nitrogen application under the condition of straw return and no straw return. Under the condition of straw return, the Pn, Gs and Tr of rice leaves under N2 treatment in 2019 were 21.8%, 16.7% and 13.9% higher than those under N0 treatment, but there was no significant difference between N3 and N2 treatments. Under the same nitrogen fertilizer level, the Pn, Gs and Tr of rice leaves increased with the increase of straw return years.

Grain yield of rice

As shown in Fig. 6 (d, e, f), in 2019, the rice yield of N2 treatment was the highest, reaching 9.64 t ha^− 1 (Fig. 6). Under the same nitrogen fertilizer level, rice yield in the straw return treatment was 9.8%-27% higher than that in the no straw return treatment, except for N0. Under the N0 treatment, the yield of straw return treatment was 26.0% and 8.6% lower than that of no straw return treatment in 2017 and 2018. In 2019, the yield of straw return treatment was 20.4% higher than that of no straw return treatment. With the increase of nitrogen application rate, rice yield increased first and then decreased in the treatment of straw return and no straw return. The rice yield of N2 treatment was significantly higher than that of N0 and N1 treatment, but there was no significant difference between N3 and N2 treatment. Under the same nitrogen fertilizer level, the rice yield increased with the increase of straw return years.

The relationship between rice yield and N fertilizer application before and after straw addition was explored by correlation analysis (Fig. 8). A highly significant quadratic correlation (P < 0.01) was found between rice yield and nitrogen application from 2017–2019. The correlation coefficients under no straw return conditions were 0.985, 0.997 and 0.999, respectively. The correlation coefficients were 0.992, 0.965 and 0.999 under the straw-returned condition, respectively. Nitrogen reduction refers to the difference between the amount of nitrogen fertilizer used when straw is added to achieve the maximum rice yield without straw and the amount of nitrogen fertilizer used when the maximum rice yield without straw is added. According to the regression analysis, the reduction in N fertiliser use from 2017 to 2019 was 49.3%, 50.4% and 60.8% respectively. The regression analysis also showed that under straw returning conditions, the maximum theoretical yields (peak of the curve) from 2017 to 2019 were 6.53, 7.54 and 9.56 t ha^− 1 respectively, corresponding to 283.73, 284.69 and 307.45 t ha^− 1 of nitrogen fertiliser application, respectively. This indicates that the optimum N fertilizer application rate varied with increasing years of straw return.

Combined application of straw and nitrogen fertilizer maintained the ion balance of rice tissue in saline-sodic land

Saline-sodic affects plant growth and development mainly through osmotic imbalance, ion toxicity and high pH stress (Chi et al. 2012). In saline-sodic land, the high concentration of Na⁺ in soil will lead to excessive absorption of Na⁺ by plants, inhibit the absorption of K⁺, NO₃⁻ and other nutrient ions by plants, affect the growth and development of plants, and even lead to their death. At the same time, high concentration of Na⁺ can displace Ca²⁺ in the cell intima system, damage the integrity and function of the membrane structure, and lead to the exosmosis of intracellular K⁺ and other nutrient ions and organic solets, breaking the ion stable state of plant cells (Demidchik et al. 2002). In this study, it was found that straw return reduced the Na⁺ concentration of rice (Fig. 2a, b, c), which may be due to: (ⅰ) Long-term application of straw increased the content of cations such as Ca²⁺, Mg²⁺ and NH₄⁺ in the soil of saline-sodic land, improved the cation exchange capacity of rice root cell membranes, and reduced the uptake of Na⁺ by rice roots (Thomas et al. 2013). (ⅱ) The decomposition of straw produces colloids that are adsorbent to Na⁺, which prevents Na⁺ uptake by the rice root system (Nunes et al. 2017). (ⅲ) Long-term straw return changed the structure of soil aggregates, reduced the diffusion of Na⁺ from deep soil to cultivated layer, optimized the rhizosphere soil of rice, and reduced the concentration of Na⁺ in rice tissues (Khamzina et al. 2008). In addition, the combined application of straw and nitrogen fertilizer is better under the condition of returning to the field. The reason may be that nitrogen can stimulate the vitality of soil microorganisms and drive microbial decomposition of straw, thus promoting soil carbon cycle, improving cation exchange capacity and soil structure, and reducing the uptake of Na⁺ by rice roots in soil (Li et al. 2016). At the same time, nitrogen effectively increased transporter protein expression and prevented Na uptake by the root system of rice (Ahanger et al. 2019). Na⁺ enters plant root cells mainly through two pathways: non-selective cation channels and high-affinity K⁺ channels (Munns 2002). Under saline-sodic stress, due to the similar hydration radius of Na⁺ and K⁺, a large amount of Na⁺ compete for K⁺ channels, which inhibits K⁺ absorption (Demidchik et al. 2002, Blumwald 2000). However, the straw return combined with the application of nitrogen fertilizer limited the absorption of Na⁺, reduced the concentration of Na⁺ in rice tissues (Fig. 2a, b, c), and increased the absorption of K⁺ (Fig. 2d, e, f). In addition, Potassium is an important plant nutrient, and increased K⁺ concentration in soil is considered a key mechanism to combat saline-sodic stress and accelerate crop aging (Ahanger and Agarwal 2017). the decomposition of straw increased the content of K⁺ in the soil and promoted the uptake of K⁺ by rice roots. Therefore, maintaining a low Na⁺/K⁺ ratio in plant tissues is the key to plant saline-sodic resistance (Guo et al. 2009). The Na⁺/K⁺ of rice tissue was decreased by straw return combined with nitrogen fertilizer, and the saline-sodic resistance of rice was increased (Fig. 2g, h, i).

Combined application of straw and nitrogen fertilizer improved the oxidative damage and water status of rice leaves in saline-sodic lan

The malondialdehyde (MDA) content can reflect the damage degree of plant cell membrane (Shao et al. 2013). Under saline-sodic stress, the excessive amount of Na⁺ in plant tissues destroys the integrity of membrane structure and its function, leads to the imbalance of reactive oxygen species (ROS), and increases the content of MDA, a product of membrane lipid peroxidation (Mittler 2002). In this study, the combined application of straw and nitrogen fertilizer reduced the MDA content of rice leaves in the saline-sodic paddy fields (Fig. 3a, b, c). This may be related to the fact that straw return and nitrogen fertilizer application reduced the concentration of Na⁺ in rice leaves, as shown in Fig. 9A, which showed a significant positive correlation with MDA content.

The relative electricity leakage (REL) of tissue is an important index to reflect the integrity of cell membrane. The higher the REL of plant tissue, the greater the damage of cell membrane integrity. The study showed that the higher the MDA content of plant tissues, the greater the REL of the tissues (Farhangi-Abriz and Torabian 2017), which was consistent with the results of this study. As shown in Fig. 9B, the MDA content of rice was significantly positively correlated with its REL. With the combined application of straw and nitrogen fertilizer, the saline-sodic resistance of rice was increased, the production of MDA was inhibited, and the REL of rice was reduced, so as to ensure the integrity of cell membrane.

Under saline-sodic stress, ROS also acts as a stress signal to activate the activities of antioxidant enzymes such as superoxide dismutase (SOD) and peroxidase (POD) to maintain the dynamic balance of ROS in cells under stress conditions (Gross et al. 2013). Studies have shown that SOD and POD are the most important enzymes in the antioxidant enzyme system. When plants are subjected to saline-sodic stress, the activity of SOD is firstly enhanced to eliminate O₂⁻, followed by POD (Jini and Joseph 2017, Li et al. 2017). In this study, saline-sodic stress increased the activities of SOD and POD enzymes (Fig. 4). With the combined application of straw and nitrogen fertilizer, the saline-sodic resistance of rice was increased and the dynamic balance of ROS was maintained, thus reducing the activities of SOD and POD enzymes.

Under saline-sodic stress, the accumulation of harmful ions such as Na⁺ in plant leaves significantly increases the content of ions in plant cells and the permeability of cell membrane, which makes it difficult for cells to absorb water and leads to the decrease of leaf water potential (Ψ_w) (Katerji et al. 2004). In this study, the combined application of straw and nitrogen fertilizer reduced the concentration of Na⁺ and the content of MDA in the rice tissues in saline-sodic rice area, maintained the integrity and permeability of the cells, improved the water absorption capacity, and thus increased Ψ_w and RWC of the leaves (Fig. 5). It is also possible that the application of nitrogen fertilizer enhances the accumulation of amino acids in plant tissues. Amino acids act as osmotic protectant, which can offset the increase in osmotic potential of saline-sodic soil and increase the RWC and the Ψ_w of leaves (Vyrides and Stuckey 2017).

Combined application of straw and nitrogen fertilizer increased the SPAD value and photosynthetic capacity of rice leaves in saline-sodic land

Under saline-sodic stress, the excess of Na⁺ in plant leaves destroys the integrity of membrane structure and its function, leading to an imbalance of reactive oxygen species (ROS), increasing the products of membrane lipid peroxidation, damaging the chloroplast membrane, and eventually leading to chlorophyll degradation (Khavari-Nejad and Mostofi 1997). The SPAD value indicates that the chlorophyll content. In this study, the combined application of straw and nitrogen fertilizer significantly increased SPAD values of rice in saline-sodic paddy fields (Fig. 6a, b, c). This may be because the combined application of straw and nitrogen fertilizer reduced the Na⁺ content of plant leaves, maintained the integrity of the membrane, prevented the degradation of chlorophyll, and provided a good environment for chlorophyll synthesis to ensure the smooth progress of chlorophyll synthesis (Pardo 2010). In addition, The decrease in chlorophyll content under saline-sodic stress may also be caused by a decrease in nitrogen uptake (van Hoorn et al. 2001). the combined application of straw and N fertilizer promoted rice root vigour and increased N content in the leaves, which increased leaf chlorophyll content (Nieva et al. 1999, Wang et al. 2020).

Photosynthesis is the basis of plant growth and development. Through photosynthesis, plants can convert light energy into stable chemical energy, and convert inorganic matter absorbed by the outside world into organic matter, providing material and energy for plant physiological metabolism and growth and development (van de Weg et al. 2013). saline-sodic stress can inhibit the synthesis of photosynthetic pigments, leading to the reduction of photosynthetic characteristics of leaves (Feng et al. 2021). saline-sodic stress can also inhibit the absorption of nitrogen by plants, which is closely related to the synthesis of photosynthetic enzymes in plants, so it can inhibit the synthesis of photosynthase and affect the activity of photosynthase (Gaude et al. 2007, Iqbal and Ashraf 2005). In this study, the combined application of straw and nitrogen increased the net photosynthetic rate, stomatal conductance and transpiration rate of rice in saline-sodic paddy fields (Fig. 7). Under stress, plants need to maintain a high K⁺ concentration in epidermal cells and guard cells to regulate stomatal opening and closing. The combination of straw and nitrogen fertilizer increased the K⁺ concentration in rice tissue and alleviated stomatal closure caused by saline-sodic stress(Fig. 2d, e, f). It promoted the improvement of net photosynthetic rate and transpiration rate of plants (Yang et al. 2011). It is also possible that the combined application of straw and nitrogen fertilizer reduces the content of Na⁺ in plant tissues and maintains the integrity of the membrane (Fig. 2a, b, c), which not only ensures the smooth progress of chlorophyll synthesis, but also provides a favorable place for the smooth progress of photosynthesis. In addition, the increase of photosynthetic pigment content by nitrogen addition strengthens the absorption of light energy, electron transfer and utilization efficiency of pigments, which can improve the photosynthetic capacity of plant leaves (Maslenkova et al. 1993).

Combined application of straw and nitrogen fertilizer improved the rice yield in saline-sodic land

saline-sodic soil not only limits photosynthesis and water use efficiency of plants, but also causes physiological drought and ion toxicity of plants, thus reducing agricultural productivity and yield (Liu et al. 2022). In this study, the combined application of straw and nitrogen fertilizer increased rice yield in saline-sodic soil (Fig. 6d, e, f), as shown in Table 2, there was a highly significant interaction between N×S and yield. The mechanism of combining straw and nitrogen fertilizer to improve rice yield in saline-sodic paddy fields is shown in Fig. 10: (1) The combined application of straw and nitrogen fertilizer effectively reduced the Na⁺/K⁺ value(Fig. 2), maintained the ion balance of rice tissues, reduced MDA content and REL (Fig. 3), and ensured the integrity of cell membrane. (2) In the present study, there was a significant positive correlation between rice yield and RWC (Fig. 9D) as the combination of straw and nitrogen fertilizers improved the water status of the rice, reduced osmotic stress and increased the rice yield. (3) The combined application of straw and nitrogen fertilizer increased SPAD value(Fig. 6a, b, c), promoted photosynthesis of rice, and improved rice yield. As indicated in Fig. 9C, there was a significant positive correlation between rice yield and SPAD value. In addition, straw decay interprets the release of important macro and micro nutrients, such as Ca, K, N, P and Zn, which promote rice growth and increase rice yield (Guan et al. 2020). This study also found that rice yield increased with the number of years of straw return (Fig. 8), indicating that straw return has a cumulative effect. However, it is important to note that straw decomposition requires a nutrient supply and in the absence of N fertiliser application can compete with the crop for N, resulting in an inadequate N supply to the crop (Li et al. 2021), which is alleviated with the combined application of N fertiliser (Fig. 7d, e, f). In addition, the yield of N3 (350 kg ha^− 1 N) treatment was lower than that of N2 (250 kg ha^− 1 N), and the yield of rice increased and then decreased as the amount of nitrogen fertilizer applied increased. This may be due to the fact that excessive N application will lead to the occurrence of greying and late maturity of rice, extensive collapse and induced pests and diseases, which in turn will reduce rice yield by (Sanchez 2002).

The correlation analysis between rice yield and nitrogen application before and after straw addition showed (Fig. 8) that straw addition could reduce nitrogen application during 2017–2019 in saline-sodic paddy fields, possibly because straw returning could maintain soil nitrogen storage in saline-sodic paddy fields and provide a good soil chemical, physical and biological environment (Wang et al. 2021). Meanwhile, straw returning to field can improve nitrogen use efficiency and reduce the dependence on chemical fertilizers (Chen et al. 2014). Under straw returning condition, the optimal nitrogen fertilizer application rate in saline-sodic paddy fields changed with the increase of returning years in 2017–2019. Therefore, it is necessary to conduct a long-term study on straw and nitrogen application in saline-sodic soil to further explore the optimal amount of nitrogen fertilizer under straw returning condition, and the influence on nitrogen utilization rate and environmental in saline-sodic paddy fields.

The results of the 3-year study showed that the combined application of straw and nitrogen fertilizer effectively reduced the Na⁺/K⁺ value of rice in saline-sodic paddy fields, maintained the ion balance of rice tissue, reduced the content of malondialdehyde and the relative electricity leakage, and ensured the integrity of the cell membrane, thus improving the water potential and relative water content of leaves, and alleviating the stress damage of rice in saline-sodic paddy fields. At the same time, the combined application of straw and nitrogen fertilizer increased the chlorophyll content, promoted the photosynthesis of rice, and increased the yield of rice. With the increase of straw returning years, the optimal nitrogen application rate of rice would also change. In addition, under the condition of straw returning, the amount of nitrogen fertilizer should not be too large, which not only ensures the stability of rice yield in saline-sodic paddy fields, but also has a positive effect on the application of chemical fertilizer and the deterioration of soil environment in saline-sodic.

Author statement

Kun Dang: Conceptualization, Methodology, Software, Data curation, Writing-original draft preparation. Cheng Ran, Dapeng Gao, Hao Tian, Jinmeng Mou, Zhenyu Zhang: Conceptualization, Methodology, Visualization, Investigation. Yanqiu Geng, Liying Guo: Reviewing and Editing, Supervision. Xiwen Shao: Reviewing and Editing.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

We gratefully acknowledge funding support by National key research and development program (2022YFD1500501), Open Project of the Key Laboratory of Germplasm Innovation and Physiological Ecology of Coldland Grain Crops, Ministry of Education (CXSTOP202202).

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Combined Effects of Straw Return with Nitrogen Fertilizer on Ion Balance, Photosynthetic Characteristics, Leaf Water Status and Rice Yield in Saline-sodic Paddy Fields

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Materials And Methods

Experimental site

Experimental Design

Sampling Methods and Measurements

Determination of Na⁺ and K⁺ content in rice leaves

Determination of leaf relative water content (RWC) and water potential (Ψ_w)

Determination of photosynthetic parameters and SPAD value of rice leaves

Determination of rice yield

Statistical analysis

Results

Analysis of variance

Na⁺ and K⁺ contents and the Na⁺/K⁺ ratio of rice leaves

Malondialdehyde (MDA) and Relative Electricity Leakage (REL) of rice leaves

Superoxide dismutase (SOD) and peroxidase (POD) activities of rice leaves

Relative water content (RWC) and leaf water potential (Ψ_w) of rice leaves

SPAD value of rice leaves

Net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of rice

Grain yield of rice

Discussion

Combined application of straw and nitrogen fertilizer maintained the ion balance of rice tissue in saline-sodic land

Combined application of straw and nitrogen fertilizer improved the oxidative damage and water status of rice leaves in saline-sodic lan

Combined application of straw and nitrogen fertilizer increased the SPAD value and photosynthetic capacity of rice leaves in saline-sodic land

Combined application of straw and nitrogen fertilizer improved the rice yield in saline-sodic land

Conclusion

Declarations

References

Status:

Journal Publication

Version 1

Combined Effects of Straw Return with Nitrogen Fertilizer on Ion Balance, Photosynthetic Characteristics, Leaf Water Status and Rice Yield in Saline-sodic Paddy Fields

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Materials And Methods

Experimental site

Experimental Design

Sampling Methods and Measurements

Determination of Na+ and K+ content in rice leaves

Determination of leaf relative water content (RWC) and water potential (Ψw)

Determination of photosynthetic parameters and SPAD value of rice leaves

Determination of rice yield

Statistical analysis

Results

Analysis of variance

Na+ and K+ contents and the Na+/K+ ratio of rice leaves

Malondialdehyde (MDA) and Relative Electricity Leakage (REL) of rice leaves

Superoxide dismutase (SOD) and peroxidase (POD) activities of rice leaves

Relative water content (RWC) and leaf water potential (Ψw) of rice leaves

SPAD value of rice leaves

Net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of rice

Grain yield of rice

Discussion

Combined application of straw and nitrogen fertilizer maintained the ion balance of rice tissue in saline-sodic land

Combined application of straw and nitrogen fertilizer improved the oxidative damage and water status of rice leaves in saline-sodic lan

Combined application of straw and nitrogen fertilizer increased the SPAD value and photosynthetic capacity of rice leaves in saline-sodic land

Combined application of straw and nitrogen fertilizer improved the rice yield in saline-sodic land

Conclusion

Declarations

References

Status:

Journal Publication

Version 1

Determination of Na⁺ and K⁺ content in rice leaves

Determination of leaf relative water content (RWC) and water potential (Ψ_w)

Na⁺ and K⁺ contents and the Na⁺/K⁺ ratio of rice leaves

Relative water content (RWC) and leaf water potential (Ψ_w) of rice leaves