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Coastal saline soil aggregate formation and salt distribution are affected by straw and nitrogen application: A 4-year field study
Soil & Tillage Research 198 (2020) 104535
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Coastal saline soil aggregate formation and salt distribution are affected by straw and nitrogen application: A 4-year field study
T
Wenjun Xiea,*, Qingfeng Chenb, Lanfang Wuc, Hongjun Yanga, Jikun Xua, Yanpeng Zhanga a
School of Biological & Environmental Engineering, Key Laboratory of Instrumental Analysis of Binzhou City, Binzhou University, Binzhou, Shandong, 256603, China College of Geography and Environment, Shandong Normal University, Jinan, Shandong, 250358, China c Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Aggregate formation and stability Straw and N application Saline soil
Little is known about the effects of straw incorporation on saline soil aggregate formation or salt distribution in coastal zones. In this study, a 4-year coastal wheat/maize rotation field experiment was employed. In each growing season, maize/wheat straw was applied at the rates of 5.0 × 103 kg ha−1 (S) and 1.0 × 104 kg ha−1 (2S), and inorganic N was applied at the rates of 75 kg ha−1 (N1/2), 150 kg ha−1 (N), and 300 kg ha−1 (N2). Treatment without straw addition and applied with 150 kg ha−1 inorganic N was used as the control (CK). Dryand wet-sieving techniques were used to fractionate the soils into large macroaggregates (> 2 mm, LM), small macroaggregates (0.25–2.0 mm, SM), microaggregates (0.053–0.25 mm, MI), and silt-plus-clay particles (< 0.053 mm, CS). Results revealed that the proportion of dry and water-stable macroaggregate fractions (> 0.25 mm, LA + SA) ranged from 74.2 %–88.3 % and 28.4 %–37.6 % in straw applied treatments, which significantly increased by 14.7 %–19.0 % and 21.1 %–32.4 % compared to CK, respectively (p < 0.05). The mean weight diameter (MWD) and aggregate stability rate (AR) enhanced as straw and N application rate increased. After straw application, soil organic carbon (SOC) preferentially accumulated in MI fractions, which was significantly higher than other aggregate fractions, except SN1/2 and CK (p < 0.05). MWD and AR were significantly negatively correlated with soil salinity (p < 0.05), and soil salinity decreased by 20.5 %–26.9 % in straw treatments compared to the initial soil salinity. In the CK and SN1/2 treatments, soil salinity of LA + SA fractions was significantly higher than < 0.25 mm fractions (MI + CS) (p < 0.05). The same difference in soluble Ca2+ was also observed in CK, SN1/2, or SN. In conclusion, saline soil aggregate formation and stability improved after straw and N application, which caused the salinity to decrease and affected salt distribution in aggregates. The findings of this study suggest that adequate N should be applied in order to maximize saline soil reclamation efficiency with straw.
1. Introduction Globally, soil degradation by salinization has become a serious threat to agricultural production, the provisioning of ecosystem services, and economic welfare (Amundson et al., 2015; Keesstra, 2016). Due to the shallow underground water table and high salinity, salt accumulation in topsoil occurs repeatedly in coastal zones. During salinization, Ca2+ and Mg2+ are replaced by Na+ at cation exchange sites, which cause the dispersion of soil particles and a reduction in microbial activities and plant growth (Lozupone and Knight, 2007; Dendooven et al., 2010; Rath and Rousk, 2015; Singh, 2016). Limited plant production in these regions has led to soil with a lower organic
matter content and poor structure. Seawater intrusion and upward saline groundwater movement are the dominant sources of salt in coastal areas (Rengasamy, 2006), with NaCl accounting for ∼80 % of total soluble salts (Weng and Gong, 2006). In previous studies, many endeavors were devoted to saline soil remediation. For instance, irrigation with freshwater was used to enhance soil salt leaching (Zhao et al., 2019), and chemical amendments were employed to replace exchangeable Na+ with Ca2+ or other divalent cations (El Hasini et al., 2019). Driven by capillary force, topsoil salt accumulation has easily recurred without soil structure improvement in coastal areas (He et al., 2014). Recently, various organic amendments, including crop straw, manure, and compost, were
Abbreviations: LM, large macroaggregate; SM, small macroaggregate; MI, microaggregate; CS, silt-plus-clay particles; MWD, mean weight diameter; AR, aggregate stability rate; DOC, dissolved organic carbon; SOC, soil organic carbon ⁎ Corresponding author. E-mail address: [email protected] (W. Xie). https://doi.org/10.1016/j.still.2019.104535 Received 13 October 2019; Received in revised form 2 December 2019; Accepted 6 December 2019 0167-1987/ © 2019 Elsevier B.V. All rights reserved.
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investigated for their effectiveness in saline soil improvement (Xie et al., 2017a; Wu et al., 2018). It has been demonstrated that straw application can enhance soil salt leaching (Kim et al., 2017), increase soil microbial biomass and activities, and decrease soil salinity (Zhao et al., 2016; Xie et al., 2017b). In particular, straw amendments decrease soil bulk density and increase soil pore space (Kim et al., 2017; Getahun et al., 2018). While straw decomposes, soil organic matter content increases, which boosts soil aggregate formation and stability (Bandyopadhyay et al., 2010; Mizuta et al., 2015). Zhang et al. (2014) reported that straw incorporation at rate of 9 × 103 kg ha−1 significantly increased topsoil water-stable macroaggregate content and the total porosity by 63.39 %–64.51 % and 2.03 %, and decreased topsoil bulk density by 2.32 %. As soil aggregate structure improves and porosity increases substantially, soil’s ability to inhibit salt migrating from deep soil solum or underground water is enhanced, which is a complicated and inevitable problem during coastal saline soil reclamation (Kim et al., 2017; Xie et al., 2017a; Getahun et al., 2018). Straw is an abundant and renewable source in China; however, many of these resources have been burned, which has resulted in ecological environmental problems (Li et al., 2017). Thus, it appears that saline soil remediation with straw was an available and effective strategy in coastal zones. Straw transformation in soil can be influenced by N application. The previous studies had proved that straw decomposition rate was significantly enhanced with N addition, and N application could effectively reduce mineralization of soil organic matter derived from straw and enrich proteinaceous compounds of soil organic matter (Gillespie et al., 2014; Qiu et al., 2016; Xie et al., 2017b). The change of soil organic matter composition and status would further influence aggregate formation and stability. Saline soil remediation with straw has been reported in previous studies. Most of these studies focused on decreasing salinity and increasing soil productivity (Zhao et al., 2016; Kim et al., 2017). Currently, little is known about the effects of straw application on saline soil aggregate formation or salt distribution in each size fraction. To address this issue, a 4-year coastal field experiment was employed to elucidate the effects of straw incorporation on 1) saline soil aggregate formation, and 2) C, N, and salt distribution in each aggregate size fraction. The goal of this study was to explore the basic soil processes during saline soil reclamation and improve the reclamation efficiency of straw incorporation.
Table 1 Different straw and nitrogen application rates of the six treatments from one growing season (kg ha−1). Treatments
SN1/2
SN
SN2
2SN
2SN2
CK
Wheat/maize straw Inorganic nitrogen
5 × 103 75
5 × 103 150
5 × 103 300
1 × 104 150
1 × 104 300
0 150
ha−1 in each treatment. N fertilizer was applied as urea; 50 % of the applied N was used as a basal fertilizer, and the other half as a supplemental fertilizer. The straw of maize/wheat was broken into 5–15 cm fragments with a combine harvester, then incorporated into the soil with a rotary tiller. For each treatment, three replicate 6 × 8 m2 plots were placed randomly. 2.3. Soil sampling and analysis Soil samples of each treatment were collected in June 2018. At each sampling event, five soil blocks (6 cm × 4 cm × 20 cm, length × width × depth) were collected in each plot and mixed to produce a composite sample, then air dried for soil aggregate analysis. To avoid losses of dissolved organic carbon (DOC) and soluble salts, dry soil aggregates were determined by the dry-sieving method (Savinov, 1936). Briefly, 100 g dried soil was sieved through a sequence of sieves (2.0, 0.25, and 0.053 mm). After sieving, 4 aggregate fractions were acquired: large macroaggregate (> 2 mm, LM), small macroaggregate (0.25–2.0 mm, SM), microaggregate (0.053–0.25 mm, MI), and siltplus-clay particles (< 0.053 mm, CS). Each soil aggregate fraction was weighed separately to calculate the dry soil aggregate contents. Similar to the dry-sieving processing, wet-sieving was conducted using 50 g soil from each treatment. Soil samples were immersed in distilled water and passed through a sequence of sieves (2.0, 0.25, and 0.053 mm) after mechanical shaking at 30 cycles min−1 for 5 min. The water-stable fraction of CS was calculated by taking the difference between the initial soil weight and the sum of the weights of the remaining fractions. The mean weight diameter (MWD) was calculated as follows: MWD = ∑ Xi × Wi,
(1)
2. Materials and methods
where Xi indicates the mean diameter of each size fraction, and Wi indicates the proportion of soil aggregate weights in the corresponding size. The soil aggregate stability rate (AR) was calculated as follows:
2.1. Site description
AR (%) =
From October 2014 to June 2018, this study was conducted in a coastal field located in Wudi County (26°45ʹN, 111°52ʹE), Shandong Province, China, along the south coast of Bohai Bay. The mean annual temperature is 12.5 °C, the mean annual precipitation is 600 mm, and the mean annual evaporation is ∼1800 mm. The ground water level ranges from 0.9 to 1.1 m. The soil has a sandy loam texture comprising 26.3 % sand, 65.1 % silt, and 8.6 % clay, and is classified as Aquic Inceptisol. The initial soil salinity is 2.77 g kg−1 with a pH of 8.52. The initial soil available N, P, and K contents are 23.8, 20.4, and 537 mg kg−1, respectively. The organic matter content is 13.5 g kg−1.
where WR0.25 indicates the proportion of soil water-stable aggregates > 0.25 mm, and DR0.25 indicates the dry aggregates > 0.25 mm. Salinity of bulk soils and their dry aggregate fractions were determined by the gravimetric method (Lu, 2000). The soluble cation concentrations (i.e., K+, Na+, Ca2+, and Mg2+) were measured by inductively coupled plasma optical emission spectrophotometry (ICPOES) using an Optima 8000 spectrometer (PerkinElmer, MA, USA). Soil pH was measured in a 1:2.5 soil: water suspension. Available N content was extracted with 2 mol L−1 KCl and analyzed with an AA3 segmented flow analyzer (Seal, Norderstedt, Germany). DOC was extracted with ultrapure water and determined using a liquiTOC II analyzer (Elementar, Huanau, Germany) after filtration through a 0.45 μm Millipore filter. After inorganic carbon being removed by 1 N HCl, soil organic carbon (SOC) and total N contents were measured using a vario EL III analyzer (Elementar, Hanau, Germany).
2.2. Experimental design In October 2014, six treatments were arranged in a coastal winter wheat and summer maize rotation field (Table 1). In each growing season, maize/wheat straw was applied at the rates of 5.0 × 103 kg ha−1 (S) and 1.0 × 104 kg ha−1 (2S), and inorganic N was applied at the rates of 75 kg ha−1 (N1/2), 150 kg ha−1 (N), and 300 kg ha−1 (N2). Treatment without straw addition was applied with 150 kg ha−1 inorganic N and used as the control (CK). In the wheat/maize growing season, inorganic P was applied as superphosphate at the rate of 35 kg P
WR0.25 × 100, DR0.25
(2)
2.4. Statistical analyses All data were presented as mean values from triplicate experiments. An analysis of variance (ANOVA) and correlation analysis were performed using SPSS v12.0 software. Significant differences between 2
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Table 2 Soil aggregate size distribution and stability in different treatments. Treatments
LA (%) Dry
SN SN1/2 SN2 2SN 2SN2 CK
44.5 43.8 46.8 49.0 38.9 34.7
ab ab a a bc c
SA (%) Wet
Dry
8.6 bc 7.4 c 9.0 bc 11.0 ab 11.9 a 6.7 c
43.8 41.3 39.7 38.5 46.7 39.5
LA + SA (%) Wet
ab ab b b a b
25.8 27.0 28.6 24.8 23.5 21.7
Dry ab a a ab ab b
88.3 85.1 86.5 87.5 85.6 74.2
MI (%) Wet
a b ab a b c
34.4 34.4 37.6 35.8 35.4 28.4
b b a ab ab c
CS (%)
Dry
Wet
7.2 b 8.8 b 8.5 b 7.8 b 8.3 b 19.6 a
19.7 22.1 20.3 19.9 20.9 18.3
Dry ab a ab ab a b
4.5 6.4 4.9 4.7 6.1 6.1
MWD Wet
a a a a a a
45.9 43.5 42.2 44.4 43.8 53.3
Dry b b b b b a
2.06 2.01 2.10 2.16 1.90 1.68
AR (%) Wet
ab ab a a b c
0.66 0.63 0.70 0.73 0.75 0.54
bc c ab a a d
39.3 39.3 42.9 40.8 40.4 32.5
b b a ab ab c
LA, > 2.0 mm; SA, 0.25–2.0 mm; LA + SA, > 0.25 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm. Dry, dry aggregate; Wet, water-stable aggregate; MWD, mean weight diameter; AR, aggregate stability rate. Different letters within columns indicate significant differences (p < 0.05).
observed in MI fractions and the least in CS fractions, with the exception of the SN and SN2 treatments. Soil available and total N contents increased with N fertilizer application (Fig. 2). The highest available N contents of the bulk and aggregate fraction soils were observed in the SN2 and 2SN2 treatments, and the least in SN1/2. The total N contents of the bulk soil in the SN2 and 2SN2 treatments were significantly higher than that in SN1/2 and CK (p < 0.05). The highest total N contents in LA, SA, MI, and CS fractions were observed in the SN, SN2, SN, and SN2 treatments, respectively; the corresponding lowest contents were observed in CK, SN, SN1/2, and SN1/2. Similar to SOC, the highest total N content was in MI fractions and the least in CS fractions in each treatment.
treatments were determined by the least significant difference (LSD) method at a significance level of p < 0.05. 3. Results 3.1. Soil aggregate size distribution and stability Dry aggregate fractions of maroaggregate (> 0.25 mm, LA + SA) were the predominant fractions in all treatments. The proportions in straw treatments were > 85 %, which significantly increased by 14.7 %–19.0 % when compared to the CK treatment (p < 0.05) (Table 2). For water-stable aggregates, the proportions of LA + SA fractions decreased greatly and ranged from 28.4%–37.6%. Treatments were ranked in following the order: SN2 > 2SN > 2SN2 > SN > SN1/ 2 > CK. The fractions significantly increased by 21.1 %–32.4 % in straw treatments when compared to CK (p < 0.05) (Table 2). The proportions of LA fractions in 2SN2 were significantly higher than other treatments (p < 0.05), except 2SN. MI and CS distributions differed among the straw treatments, but the difference was insignificant. The lowest proportion of MI and largest proportion of CS were observed in the CK treatment. MWD and AR are often used to evaluate soil aggregate formation and stability. Straw application significantly increased MWD and AR (p < 0.05) (Table 2). The greatest MWD for dry aggregates was observed in the 2SN treatment, which was significantly higher than 2SN2 and CK (p < 0.05). For the wet-sieving aggregates, the greatest MWD was observed in the 2SN2 treatment, showing a significant increase when compared to the treatments that received low straw and N application (i.e., SN, SN1/2, and CK) (p < 0.05). The greatest AR was observed in the SN2 treatment, which significantly increased by 9.2 %, 9.2 %, and 32.0 % when compared to the SN, SN1/2, and CK treatments, respectively (p < 0.05).
3.3. Soil salinity and distribution of soluble cations After 4 years, compared to the initial values, bulk soil salinity of the SN, SN1/2, SN2, 2SN, 2SN2, and CK treatments decreased by 23.6 %, 20.5 %, 25.0 %, 26.6 %, 26.9 %, and 18.1 %, respectively. Soil salinity of the SN1/2 and CK treatments was significantly higher than the other treatments (p < 0.05) (Fig. 3). Therefore, straw application effectively reduced topsoil salinity. In aggregate fractions, soil salinity varied insignificantly in the straw treatments. The salinity of LA + SA fractions was higher than < 0.25 mm fractions (MI + CS) in the SN, SN1/2, and CK treatments, which reached a significant level in CK and SN1/2 (p < 0.05). In the SN2, 2SN, and 2SN2 treatments, soil salinity between the two fraction groups varied slightly. The soluble K+, Na+, Ca2+, and Mg2+ concentrations in bulk and aggregate fraction soils are presented in Fig. 4. Bulk soil soluble K+ and Mg2+ concentrations in the 2SN and 2SN2 treatments were significantly higher than the other treatments (p < 0.05), except for Mg2+ in SN2. Bulk soil soluble Na+ and Ca2+ concentrations in the CK treatment were significantly higher than the straw treatments (p < 0.05), except for Ca2+ in 2SN and 2SN2. In soil aggregate fractions, soluble K+ and Mg2+ concentrations increased as fraction size decreased. Soluble Na+ concentrations decreased as fraction size decreased. Soluble Ca2+ concentration in LA fractions was significantly higher than the other fractions in the SN, SN1/2, and 2SN treatments (p < 0.05). In the CK treatment, the highest soluble Ca2+ concentration was observed in SA fractions. Additionally, the soluble Ca2+ concentrations in LA + SA fractions were significantly higher than in MI + CS fractions of the lower straw and N treatments, i.e., SN, SN1/2, and CK (p < 0.05).
3.2. Soil C and N content Straw application significantly increased the DOC contents in bulk soils (p < 0.05) (Fig. 1A), with the greatest increase in the SN2 treatment, followed by 2SN2, and the least in SN1/2. Similar change trends of DOC content were also observed in aggregate fractions of LA and SA. For MI and CS fractions, the difference in DOC contents among the 6 treatments decreased. Meanwhile, DOC contents increased as aggregate sizes decreased in the SN, SN1/2, and CK treatments, while the opposite trend was observed in SN2. The highest bulk SOC contents were observed in the SN2 and 2SN2 treatments, which were significantly higher than that in SN, SN1/2, and CK (Fig. 1B). The distribution of SOC within aggregate fractions differed significantly among the 6 treatments (p < 0.05). In LA fractions, the highest SOC content was observed in the 2SN2 treatment, which was significantly higher than that in SN1/2, 2SN, and CK (p < 0.05). In SA, MI, and CS fractions, the highest SOC content was observed in the SN2 treatment. In each treatment, the highest SOC contents were
4. Discussion 4.1. Straw and N application influence saline soil aggregate formation and stability Soil aggregation is the main indicator of soil with good structure. In coastal areas, Ca2+ is gradually replaced by Na+ at soil exchangeable sites during soil salinization, which worsen soil structure (Wong et al., 3
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Fig. 1. Soil dissolved organic carbon (A) and organic carbon (B) contents in aggregate fractions of different treatments. Columns represent the mean (n = 3) and bars represent the standard deviation (SD); different letters within groups are significantly different (p < 0.05). LA, > 2.0 mm; SA, 0.25–2.0 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm.
incorporation of straw into saline soil, N was the most important nutrient for straw transformation. The combination of straw and N application boosts saline soil microbial growth and development (Xie et al., 2017b), and soil microbial remains and products are vital sources of SOC (Cotrufo et al., 2015). Additionally, N application efficiently mitigates the “prime effect” and decreases straw-derived organic carbon mineralization, thereby improving carbon sequestration (Kirkby et al., 2014; Qiu et al., 2016). N addition alters the characteristics and status of SOC and increases the quantity of polar groups containing N atoms in SOC (Grandy and Neff, 2008; Gillespie et al., 2014). The increase in polar N groups improves the formation of organo-mineral complexes through adsorption between the soil mineral matrix and organic molecules. Collectively, these factors contribute to soil aggregate formation and stability under saline conditions.
2010). In this study, straw application significantly increased saline soil aggregation formation and stability (Table 2), which resulted from the increase in soil organic matter that can bind mineral particles into aggregates (Fig. 1) (Gentile et al., 2011; Karami et al., 2012; Blankinship et al., 2016). This can be supported by the significant positive correlation between SOC/DOC and MWD (p < 0.05) (Table 3). However, the increase in magnitude is dependent on straw quality, quantity, and soil properties (Chivenge et al., 2011; Gentile et al., 2011). In this study, greater MWD and AR were in the treatments with higher straw rate (Table 2), suggesting that saline soil aggregation was enhanced as straw incorporation rates increased. According to the differences between the SN1/2, SN, and SN2 treatments, increased N application rates boosted saline soil aggregation formation and stability. Compared to the CK treatment, the proportion of water-stable macroaggregates, MWD, and AR in SN1/2, SN, and SN2 increased by 21.1 %–32.4 %, 16.7 %–29.6 %, and 20.9 %–32.0 %, respectively. The correlation analysis revealed that SOC was significantly positively correlated with the total N contents in bulk, SA, MI, and CS fraction soils (p < 0.01) (Table 4). Thus, adequate N application effectively promoted carbon sequestration in bulk and different saline aggregate fraction soils (Qiu et al., 2016; Liu et al., 2019). With the
4.2. C and N distribution within saline soil aggregates DOC mainly includes carbohydrates, proteins, and microbial products, which act as aggregate-stabilizing agents that ultimately improve soil aggregate formation (Degens, 1997). Due to the wet-sieving analysis often used in previous studies, little is known about the DOC 4
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Fig. 2. Soil available N (A) and total N (B) contents in aggregate fractions of different treatments. Columns represent the mean (n = 3) and bars represent the SD; different letters within groups are significantly different (p < 0.05). LA, > 2.0 mm; SA, 0.25–2.0 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm.
Following N addition, the available and total N contents in bulk soil and aggregate fractions increased (Fig. 2). Due to the high pH values in the study area, nitrate N is the main component of available N, and is weakly adsorbed onto soil particles. Thus, the lowest available N content observed was in CS fractions. Over 90 % of N in soils is in organic form (Kelley and Stevenson, 1995), causing the total N to possess similar distributions as SOC in soil aggregate fractions.
distribution within soil aggregate fractions. This study found that the DOC content increased as aggregate size decreased in lower content treatments (i.e., SN, SN1/2, and CK) but decreased in a higher content treatment (i.e., SN2) (Fig.1 A). These findings suggest that DOC preferentially bound to soil clay and silt particles, then induced microaggregate and macroaggregate formations as its quantity increased. SOC increased after straw application was unevenly distributed among the different aggregate size fractions. The highest SOC content and increase were observed in MI fractions in straw treatments (Fig. 1B). Because microaggregates have longer turnover times and higher stability than macroaggregates (Denef et al., 2007; Huang et al., 2010), higher SOC contents in microaggregates improve physical-chemical protection (Zhao et al., 2012). This is beneficial for carbon sequestration in saline soils. The formation of soil macroaggregates was due to the combination of microaggregates and particles. Thus, higher SOC contents in microaggregates increased the accumulation of SOC in macroaggregates. In this study, the contribution of SOC in dry macroaggregates to total SOC accumulation was 84.4 %–86.5 % in straw treatments. Thus, it appears that SOC preferentially accumulated in microaggregates after straw incorporation and mainly sequestrated in macroaggregates of saline soil.
4.3. Soluble salt distribution within saline soil aggregates Due to the shallow groundwater table, decreasing upward salt movement and accumulation in the topsoil was a crucial step for coastal saline soil improvement. Straw application efficiently enhances salt leaching and inhibits upward salt movement from underground water or deep soils by increasing soil porosity and aggregate formation (Mulumba and Lal, 2008; Kim et al., 2017). The correlation analysis indicated that soil salinity was significantly negatively correlated with MWD, SOC, DOC, and available and total N (Tables 3 and 4) (p < 0.05).). Thus, improving soil organic matter and aggregate formation with straw was an important mechanism for saline soil reclamation. Saline soil aggregate formation affected soluble salt distribution 5
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Fig. 3. Soil salinity of aggregate fractions in different treatments. Columns represent the mean (n = 3) and bars represent the SD; different letters within groups are significantly different (p < 0.05). LA, > 2.0 mm; SA, 0.25–2.0 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm.
Fig. 4. Soil soluble K+ (A), Na+ (B), Ca2+ (C), and Mg2+ (D) concentrations in aggregate fractions of different treatments. Columns represent the mean (n = 3) and bars represent the SD; different letters within groups are significantly different (p < 0.05). LA, > 2.0 mm; SA, 0.25–2.0 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm.
within aggregates. In the study area, NaCl is the main constituent of soluble salts, comprising ∼80 % of the total (Weng and Gong, 2006). Thus, the high soluble Na+ concentrations in bulk and aggregate fraction soils of the CK treatment were due to its high salinity. Na+ is a highly dispersive agent that directly or indirectly results in the breakup of soil aggregates (Wong et al., 2010; Huang et al., 2016), leading to worse aggregate formation and stability. The concentration of Na+
increased as aggregate size increased, which may be because the soil colloid possessed more anionic groups as aggregate size increased and weakly adsorbed Na+. They are easily separated from anionic groups when extracted using water (Wang et al., 2017). The high Ca2+ concentrations in bulk and aggregate soils of the CK treatment were an unexpected and interesting result. Ca2+ can improve soil structure through cationic bridging with clay particles and soil organic matter 6
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Table 3 Correlation analysis between saline soil properties.
Salinity SOC DOC AN TN MWD AR K+ Na+ Ca2+ Mg2+
pH
Salinity
SOC
DOC
AN
TN
MWD
AR
K+
Na+
Ca2+
0.65** −0.60** −0.22 −0.61** −0.52* −0.55* −0.26 −0.76** 0.72** −0.24 −0.79**
−0.56* −0.56* −0.67** −0.66** −0.56* −0.43 −0.83** 0.87** 0.38 −0.39
0.73** 0.67** 0.77** 0.52* 0.44 0.68** −0.71** −0.12 0.53*
0.65** 0.76** 0.51* 0.61** 0.40 −0.67** −0.11 0.21
0.63** 0.58** 0.49* 0.60** −0.79** −0.56* 0.52*
0.56* 0.41 0.60** −0.68** −0.26 0.27
0.82** 0.60** −0.73** −0.20 0.56*
0.36 −0.55* −0.48* 0.31
−0.81** −0.12 0.63**
0.21 −0.56*
0.40
SOC, soil organic carbon; DOC, dissolved organic carbon; AN, available N; TN, total N; MWD, mean weight diameter; AR, aggregate stability rate. *, significance level at p < 0.05; **, significance level at p < 0.01. Table 4 Significance levels of correlation analysis between properties of different saline soil fractions. Soils
DOC AN TN Salinity
Bulk soil
LA
SOC
DOC
+** +** +** –*
+** +** –*
AN
+** –**
SA
TN
SOC
DOC
–**
+ +** + –*
+** + –**
AN
+ –**
MI
TN
SOC
DOC
–
+ + +** –
+* + –
AN
+* –
CS
TN
SOC
DOC
–*
+ +* +** +
– – +
AN
+ –
TN
SOC
DOC
AN
TN
–
+ + +** +
+ + +*
+ –
+
LA, > 2.0 mm; SA, 0.25–2.0 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm; SOC, soil organic carbon; DOC, dissolved organic carbon; AN, available N; TN, total N. +**, positive correlation at significance level of p < 0.01; +*, positive correlation at significance level of p < 0.05; +, insignificant positive correlation; –**, negative correlation at significance level of p < 0.01; –*, negative correlation at significance level of p < 0.05; –, insignificant negative correlation.
Declaration of Competing Interest
(Huang et al., 2016; Kim et al., 2017). The low DOC and SOC contents in the CK treatment prevented Ca2+ from combining with organic matter and remained free in soil solutions. The correlation analysis revealed that the soluble Ca2+ concentration was negatively correlated with SOC, DOC, and MWD, and significantly negatively correlated with AR (p < 0.05) (Table 3). Soluble Ca2+ concentrations in LA + SA fractions were significantly higher than in MI + CS fractions in the SN, SN1/2, and CK treatments. This difference was obviously reduced in the SN2, 2SN, and 2SN2 treatments, suggesting that sufficient straw or N application could improve the binding effects between Ca2+ and soil constituents, possibly by influencing the quantity and chemical properties of soil organic matter. Soluble K+ and Mg2+ concentrations were higher in the 2SN and 2SN2 treatments, which was likely caused by the high straw application rates, as K and Mg nutrients are released into the soil during straw decomposition (Zhu et al., 2010; Xie et al., 2017a). The increase in soluble K+ and Mg2+ concentrations as aggregate fraction size decreased indicated that more K+ and Mg2+ were involved in the formation of aggregates, reducing their availability in soil solutions. The distribution of soluble ions in each aggregate fraction ultimately influenced the salinity of different soil aggregates, which in turn regulated soil aggregate formation and stability.
None. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (41877101; 31901456), the Key Research and Development Program of Shandong Province (2019GNC106003), and the Taishan Scholars Program of Shandong, China (tsqn201812086). References Amundson, R., Berhe, A.A., Hopmans, J.W., Olson, C., Sztein, A.E., Sparks, D.L., 2015. Soil and human security in the 21st century. Science 348 (6235), 1261071. Bandyopadhyay, P.K., Saha, S., Mani, P.K., Mandal, B., 2010. Effects of organic inputs on aggregate associated organic carbon concentration under long-term rice-wheat cropping system. Geoderma 154, 379–386. Blankinship, J.C., Fonte, S.J., Six, J., Joshua, P.S., 2016. Plant versus microbial controls on soil aggregate stability in a seasonally dry ecosystem. Geoderma 272, 39–50. Cotrufo, M.F., Soong, J.L., Horton, A.J., Campbell, E.E., Haddix, M.L., Wall, D.H., Parton, W.J., 2015. Formation of soil organic matter via biochemical and physical pathways of litter mass loss. Nat. Geosci. 8, 776–779. Degens, B.P., 1997. Macro-aggregation of soils by biological bonding and binding mechanisms and the factors affecting these: a review. Aust. J. Soil Res. 35, 431–459. Dendooven, L., Alcántara-Hernández, R.J., Valenzuela-Encinas, C., Luna-Guido, M., Perez-Guevara, F., Marsch, R., 2010. Dynamics of carbon and nitrogen in an extreme alkaline saline soil: a review. Soil Biol. Biochem. 42, 865–877. Denef, K., Zotarelli, L., Boddey, R.M., Six, J., 2007. Microaggregate-associated carbon as a diagnostic fraction for management-induced changes in soil organic carbon in two Oxisols. Soil Biol. Biochem. 39, 1165–1172. El Hasini, S., Halima, O.I., Azzouzi, M., El Douaik, A., Azim, K., Zouahri, A., 2019. Organic and inorganic remediation of soils affected by salinity in the Sebkha of Sed El Mesjoune-Marrakech (Morocco). Soil Till. Res. 193, 153–160. Gentile, R., Vanlauwe, B., Six, J., 2011. Litter quality impacts short- but not long-term soil carbon dynamics in soil aggregate fractions. Ecol. Appl. 21 (3), 695–703. Getahun, G.T., Kätterer, T., Munkholm, L.J., Parvage, M.M., Keller, T., Rychel, K., Kirchmann, H., 2018. Short-term effects of loosening and incorporation of straw slurry into the upper subsoil on soil physical properties and crop yield. Soil Till. Res. 184, 62–67. Gillespie, A.W., Diochon, A., Ma, B.L., Morrison, M.J., Kellman, L., Walley, F.L., Regier, T.Z., Chevrier, D., Dynes, J.J., Gregorich, E.G., 2014. Nitrogen input quality changes
5. Conclusions The present study demonstrated that the combination of straw and inorganic N application was an effective strategy for improving coastal saline soil. Straw decomposition enhanced saline soil aggregate formation and stability by increasing soil DOC and SOC contents, which were manifested by improved proportions of macroaggregates, MWD, and AR. Straw and inorganic N application efficiently decreased soil salinity. These processes were promoted as straw and N application increased. SOC preferentially accumulated in microaggregates, which was conducive to carbon sequestration in saline soil. Saline soil aggregate formation influenced the distribution of soluble salts and soil salinity in coastal zones. 7
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W. Xie, et al. the biochemical composition of soil organic matter stabilized in the fine fraction: a long-term study. Biogeochemistry 117, 337–350. Grandy, A.S., Neff, J.C., 2008. Molecular C dynamics downstream: the biochemical decomposition sequence and its impact on soil organic matter structure and function. Sci. Total Environ. 404, 297–307. He, B., Cai, Y.L., Ran, W.R., Jiang, H., 2014. Spatial and seasonal variations of soil salinity following vegetation restoration in coastal saline land in eastern China. Catena 118, 147–153. Huang, S., Peng, X., Huang, Q., Zhang, W., 2010. Soil aggregation and organic carbon fractions affected by long-term fertilization in a red soil of subtropical China. Geoderma 154, 364–369. Huang, X.R., Li, H., Li, Song, Xiong, H.L., Jiang, X.J., 2016. Role of cationic polarization in humus-increased soil aggregate stability. Eur. J. Soil Sci. 67, 341–350. Karami, A., Homaee, M., Afzalinia, S., Ruhipour, H., Basirat, S., 2012. Organic resource management: impacts on soil aggregate stability and other soil physico-chem-ical properties. Agric. Ecosyst. Environ. 148, 22–28. Keesstra, S.D., 2016. The significance of soils and soil science towards realization of the United Nations Sustainable Development Goals. Soil 2, 111–128. Kelley, K.R., Stevenson, F.J., 1995. Forms and nature of organic N in soil. Fertil. Res. 42, 1–11. Kim, Y.J., Choo, B.K., Cho, J.Y., 2017. Effect of gypsum and rice straw compost application on improvements of soil quality during desalination of reclaimed coastal tideland soils: ten years of long-term experiments. Catena 156, 131–138. Kirkby, C.A., Richardson, A.E., Wade, L.J., Passioura, J.B., Batten, G.D., Blanchard, C., Kirkegaard, J.A., 2014. Nutrient availability limits carbon sequestration in arable soils. Soil Biol. Biochem. 68, 402–409. Li, L., Zhao, Q.Y., Zhang, J., Li, H.P., Liu, Q., Li, C.Y., Chen, F., Qiao, Y.Z., Han, J.Z., 2017. Air pollutant emissions from straw open burning: a case study in Tianjin. Atmos. Environ. 171, 155–164. Liu, K.L., Huang, J., Li, D.M., Yu, X.C., Ye, H.C., Hu, H.W., Hu, Z.H., Huang, Q.H., Zhang, H.M., 2019. Comparison of carbon sequestration efficiency in soil aggregates between upland and paddy soils in a red soil region of China. J. Integr. Agric. 18 (6), 1348–1359. Lozupone, C.A., Knight, R., 2007. Global patterns in bacterial diversity. Proc. Natl. Acad. Sci. U. S. A. 104 (27), 11436–11440. Lu, R.K., 2000. Soil Agro-Chemical Analysis. Agricultural Scientech Press, Beijing. Mizuta, K., Taguchi, S., Sato, S., 2015. Soil aggregate formation and stability induced by starch and cellulose. Soil Biol. Biochem. 87, 90–96. Mulumba, L.N., Lal, R., 2008. Mulching effects on selected soil physical properties. Soil Till. Res. 98 (1), 106–111.
Qiu, Q.Y., Wu, L.F., Ouyang, Z., Li, B.B., Xu, Y.Y., Wu, S.S., Gregorich, E.G., 2016. Priming effect of maize residue and urea N on soil organic matter changes with time. Appl. Soil Ecol. 100, 65–74. Rath, K.M., Rousk, J., 2015. Salt effects on the soil microbial decomposer community and their role in organic carbon cycling: a review. Soil Biol. Biochem. 81, 108–123. Rengasamy, P., 2006. World salinization with emphasis on Australia. J. Exp. Bot. 57 (5), 1017–1023. Savinov, N.O., 1936. Soil physics. Sielchozgiz Press, Moscow. Singh, K., 2016. Microbial and enzyme activities of saline and sodic soils. Land Degrad. Dev. 27, 706–718. Wang, R.Z., Dungait, J.A.J., Buss, H.L., Yang, S., Zhang, Y.G., Xu, Z.W., Jiang, Y., 2017. Base cations and micronutrients in soil aggregates as affected by enhanced nitrogen and water inputs in a semi-arid steppe grassland. Sci. Total Environ. 575 (1), 564–572. Weng, Y.L., Gong, P., 2006. Soil salinity measurements on the Yellow River Delta (in chinese). J. Nanjing Univ. (Natural Sciences) 42 (6), 602–610. Wong, V.N.L., Greene, R.S.B., Dalal, R.C., Murphy, B.W., 2010. Soil carbon dynamics in saline and sodic soils: a review. Soil Use Manage. 26, 2–11. Wu, Y.P., Li, Y.F., Zhang, Y., Bi, Y.M., Sun, Z.J., 2018. Responses of saline soil properties and cotton growth to different organic amendments. Pedosphere 28 (3), 521–529. Xie, W.J., Wu, L.F., Zhang, Y.P., Wu, T., Li, X.P., Ouyang, Z., 2017a. Effects of straw application on coastal saline topsoil salinity and wheat yield trend. Soil Till. Res. 169, 1–6. Xie, W.J., Wu, L.F., Wang, J.S., Zhang, Y.P., Ouyang, Z., 2017b. Effect of salinity on the transformation of wheat straw and microbial communities in a saline soil. Commun. Soil Sci. Plant Anal. 48, 1455–1461. Zhang, P., Wei, T., Jia, Z.K., Han, Q.F., Ren, X.L., 2014. Soil aggregate and crop yield changes with different rates of straw incorporation in semiarid areas of northwest China. Geoderma 230-231, 41–49. Zhao, H., Lv, Y., Wang, X., Zhang, H., Yang, X., 2012. Tillage impacts on the fractions and compositions of soil organic carbon. Geoderma 189-190, 397–403. Zhao, Q.Q., Bai, J.H., Gao, Y.C., Zhao, H.X., Huang, Y.J., Zhang, W., Wang, J.N., Chen, G.H., 2019. Effects of freshwater inputs on soil quality in the Yellow River Delta. China. Ecol. Indic. 98, 619–626. Zhao, Y.G., Li, Y.Y., Wang, J., Pang, H.C., Li, Y., 2016. Buried straw layer plus plastic mulching reduces soil salinity and increases sunflower yield in saline soils. Soil Till. Res. 155, 363–370. Zhu, H.H., Wu, J.S., Huang, D.Y., Zhu, Q.H., Liu, S.L., Su, Y.R., Wei, W.X., Syers, J.K., Li, Y., 2010. Improving fertility and productivity of a highly-weathered upland soil in subtropical China by incorporating rice straw. Plant Soil 331, 427–437.
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Coastal saline soil aggregate formation and salt distribution are affected by straw and nitrogen application: A 4-year field study
T
Wenjun Xiea,*, Qingfeng Chenb, Lanfang Wuc, Hongjun Yanga, Jikun Xua, Yanpeng Zhanga a
School of Biological & Environmental Engineering, Key Laboratory of Instrumental Analysis of Binzhou City, Binzhou University, Binzhou, Shandong, 256603, China College of Geography and Environment, Shandong Normal University, Jinan, Shandong, 250358, China c Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Aggregate formation and stability Straw and N application Saline soil
Little is known about the effects of straw incorporation on saline soil aggregate formation or salt distribution in coastal zones. In this study, a 4-year coastal wheat/maize rotation field experiment was employed. In each growing season, maize/wheat straw was applied at the rates of 5.0 × 103 kg ha−1 (S) and 1.0 × 104 kg ha−1 (2S), and inorganic N was applied at the rates of 75 kg ha−1 (N1/2), 150 kg ha−1 (N), and 300 kg ha−1 (N2). Treatment without straw addition and applied with 150 kg ha−1 inorganic N was used as the control (CK). Dryand wet-sieving techniques were used to fractionate the soils into large macroaggregates (> 2 mm, LM), small macroaggregates (0.25–2.0 mm, SM), microaggregates (0.053–0.25 mm, MI), and silt-plus-clay particles (< 0.053 mm, CS). Results revealed that the proportion of dry and water-stable macroaggregate fractions (> 0.25 mm, LA + SA) ranged from 74.2 %–88.3 % and 28.4 %–37.6 % in straw applied treatments, which significantly increased by 14.7 %–19.0 % and 21.1 %–32.4 % compared to CK, respectively (p < 0.05). The mean weight diameter (MWD) and aggregate stability rate (AR) enhanced as straw and N application rate increased. After straw application, soil organic carbon (SOC) preferentially accumulated in MI fractions, which was significantly higher than other aggregate fractions, except SN1/2 and CK (p < 0.05). MWD and AR were significantly negatively correlated with soil salinity (p < 0.05), and soil salinity decreased by 20.5 %–26.9 % in straw treatments compared to the initial soil salinity. In the CK and SN1/2 treatments, soil salinity of LA + SA fractions was significantly higher than < 0.25 mm fractions (MI + CS) (p < 0.05). The same difference in soluble Ca2+ was also observed in CK, SN1/2, or SN. In conclusion, saline soil aggregate formation and stability improved after straw and N application, which caused the salinity to decrease and affected salt distribution in aggregates. The findings of this study suggest that adequate N should be applied in order to maximize saline soil reclamation efficiency with straw.
1. Introduction Globally, soil degradation by salinization has become a serious threat to agricultural production, the provisioning of ecosystem services, and economic welfare (Amundson et al., 2015; Keesstra, 2016). Due to the shallow underground water table and high salinity, salt accumulation in topsoil occurs repeatedly in coastal zones. During salinization, Ca2+ and Mg2+ are replaced by Na+ at cation exchange sites, which cause the dispersion of soil particles and a reduction in microbial activities and plant growth (Lozupone and Knight, 2007; Dendooven et al., 2010; Rath and Rousk, 2015; Singh, 2016). Limited plant production in these regions has led to soil with a lower organic
matter content and poor structure. Seawater intrusion and upward saline groundwater movement are the dominant sources of salt in coastal areas (Rengasamy, 2006), with NaCl accounting for ∼80 % of total soluble salts (Weng and Gong, 2006). In previous studies, many endeavors were devoted to saline soil remediation. For instance, irrigation with freshwater was used to enhance soil salt leaching (Zhao et al., 2019), and chemical amendments were employed to replace exchangeable Na+ with Ca2+ or other divalent cations (El Hasini et al., 2019). Driven by capillary force, topsoil salt accumulation has easily recurred without soil structure improvement in coastal areas (He et al., 2014). Recently, various organic amendments, including crop straw, manure, and compost, were
Abbreviations: LM, large macroaggregate; SM, small macroaggregate; MI, microaggregate; CS, silt-plus-clay particles; MWD, mean weight diameter; AR, aggregate stability rate; DOC, dissolved organic carbon; SOC, soil organic carbon ⁎ Corresponding author. E-mail address: [email protected] (W. Xie). https://doi.org/10.1016/j.still.2019.104535 Received 13 October 2019; Received in revised form 2 December 2019; Accepted 6 December 2019 0167-1987/ © 2019 Elsevier B.V. All rights reserved.
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investigated for their effectiveness in saline soil improvement (Xie et al., 2017a; Wu et al., 2018). It has been demonstrated that straw application can enhance soil salt leaching (Kim et al., 2017), increase soil microbial biomass and activities, and decrease soil salinity (Zhao et al., 2016; Xie et al., 2017b). In particular, straw amendments decrease soil bulk density and increase soil pore space (Kim et al., 2017; Getahun et al., 2018). While straw decomposes, soil organic matter content increases, which boosts soil aggregate formation and stability (Bandyopadhyay et al., 2010; Mizuta et al., 2015). Zhang et al. (2014) reported that straw incorporation at rate of 9 × 103 kg ha−1 significantly increased topsoil water-stable macroaggregate content and the total porosity by 63.39 %–64.51 % and 2.03 %, and decreased topsoil bulk density by 2.32 %. As soil aggregate structure improves and porosity increases substantially, soil’s ability to inhibit salt migrating from deep soil solum or underground water is enhanced, which is a complicated and inevitable problem during coastal saline soil reclamation (Kim et al., 2017; Xie et al., 2017a; Getahun et al., 2018). Straw is an abundant and renewable source in China; however, many of these resources have been burned, which has resulted in ecological environmental problems (Li et al., 2017). Thus, it appears that saline soil remediation with straw was an available and effective strategy in coastal zones. Straw transformation in soil can be influenced by N application. The previous studies had proved that straw decomposition rate was significantly enhanced with N addition, and N application could effectively reduce mineralization of soil organic matter derived from straw and enrich proteinaceous compounds of soil organic matter (Gillespie et al., 2014; Qiu et al., 2016; Xie et al., 2017b). The change of soil organic matter composition and status would further influence aggregate formation and stability. Saline soil remediation with straw has been reported in previous studies. Most of these studies focused on decreasing salinity and increasing soil productivity (Zhao et al., 2016; Kim et al., 2017). Currently, little is known about the effects of straw application on saline soil aggregate formation or salt distribution in each size fraction. To address this issue, a 4-year coastal field experiment was employed to elucidate the effects of straw incorporation on 1) saline soil aggregate formation, and 2) C, N, and salt distribution in each aggregate size fraction. The goal of this study was to explore the basic soil processes during saline soil reclamation and improve the reclamation efficiency of straw incorporation.
Table 1 Different straw and nitrogen application rates of the six treatments from one growing season (kg ha−1). Treatments
SN1/2
SN
SN2
2SN
2SN2
CK
Wheat/maize straw Inorganic nitrogen
5 × 103 75
5 × 103 150
5 × 103 300
1 × 104 150
1 × 104 300
0 150
ha−1 in each treatment. N fertilizer was applied as urea; 50 % of the applied N was used as a basal fertilizer, and the other half as a supplemental fertilizer. The straw of maize/wheat was broken into 5–15 cm fragments with a combine harvester, then incorporated into the soil with a rotary tiller. For each treatment, three replicate 6 × 8 m2 plots were placed randomly. 2.3. Soil sampling and analysis Soil samples of each treatment were collected in June 2018. At each sampling event, five soil blocks (6 cm × 4 cm × 20 cm, length × width × depth) were collected in each plot and mixed to produce a composite sample, then air dried for soil aggregate analysis. To avoid losses of dissolved organic carbon (DOC) and soluble salts, dry soil aggregates were determined by the dry-sieving method (Savinov, 1936). Briefly, 100 g dried soil was sieved through a sequence of sieves (2.0, 0.25, and 0.053 mm). After sieving, 4 aggregate fractions were acquired: large macroaggregate (> 2 mm, LM), small macroaggregate (0.25–2.0 mm, SM), microaggregate (0.053–0.25 mm, MI), and siltplus-clay particles (< 0.053 mm, CS). Each soil aggregate fraction was weighed separately to calculate the dry soil aggregate contents. Similar to the dry-sieving processing, wet-sieving was conducted using 50 g soil from each treatment. Soil samples were immersed in distilled water and passed through a sequence of sieves (2.0, 0.25, and 0.053 mm) after mechanical shaking at 30 cycles min−1 for 5 min. The water-stable fraction of CS was calculated by taking the difference between the initial soil weight and the sum of the weights of the remaining fractions. The mean weight diameter (MWD) was calculated as follows: MWD = ∑ Xi × Wi,
(1)
2. Materials and methods
where Xi indicates the mean diameter of each size fraction, and Wi indicates the proportion of soil aggregate weights in the corresponding size. The soil aggregate stability rate (AR) was calculated as follows:
2.1. Site description
AR (%) =
From October 2014 to June 2018, this study was conducted in a coastal field located in Wudi County (26°45ʹN, 111°52ʹE), Shandong Province, China, along the south coast of Bohai Bay. The mean annual temperature is 12.5 °C, the mean annual precipitation is 600 mm, and the mean annual evaporation is ∼1800 mm. The ground water level ranges from 0.9 to 1.1 m. The soil has a sandy loam texture comprising 26.3 % sand, 65.1 % silt, and 8.6 % clay, and is classified as Aquic Inceptisol. The initial soil salinity is 2.77 g kg−1 with a pH of 8.52. The initial soil available N, P, and K contents are 23.8, 20.4, and 537 mg kg−1, respectively. The organic matter content is 13.5 g kg−1.
where WR0.25 indicates the proportion of soil water-stable aggregates > 0.25 mm, and DR0.25 indicates the dry aggregates > 0.25 mm. Salinity of bulk soils and their dry aggregate fractions were determined by the gravimetric method (Lu, 2000). The soluble cation concentrations (i.e., K+, Na+, Ca2+, and Mg2+) were measured by inductively coupled plasma optical emission spectrophotometry (ICPOES) using an Optima 8000 spectrometer (PerkinElmer, MA, USA). Soil pH was measured in a 1:2.5 soil: water suspension. Available N content was extracted with 2 mol L−1 KCl and analyzed with an AA3 segmented flow analyzer (Seal, Norderstedt, Germany). DOC was extracted with ultrapure water and determined using a liquiTOC II analyzer (Elementar, Huanau, Germany) after filtration through a 0.45 μm Millipore filter. After inorganic carbon being removed by 1 N HCl, soil organic carbon (SOC) and total N contents were measured using a vario EL III analyzer (Elementar, Hanau, Germany).
2.2. Experimental design In October 2014, six treatments were arranged in a coastal winter wheat and summer maize rotation field (Table 1). In each growing season, maize/wheat straw was applied at the rates of 5.0 × 103 kg ha−1 (S) and 1.0 × 104 kg ha−1 (2S), and inorganic N was applied at the rates of 75 kg ha−1 (N1/2), 150 kg ha−1 (N), and 300 kg ha−1 (N2). Treatment without straw addition was applied with 150 kg ha−1 inorganic N and used as the control (CK). In the wheat/maize growing season, inorganic P was applied as superphosphate at the rate of 35 kg P
WR0.25 × 100, DR0.25
(2)
2.4. Statistical analyses All data were presented as mean values from triplicate experiments. An analysis of variance (ANOVA) and correlation analysis were performed using SPSS v12.0 software. Significant differences between 2
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Table 2 Soil aggregate size distribution and stability in different treatments. Treatments
LA (%) Dry
SN SN1/2 SN2 2SN 2SN2 CK
44.5 43.8 46.8 49.0 38.9 34.7
ab ab a a bc c
SA (%) Wet
Dry
8.6 bc 7.4 c 9.0 bc 11.0 ab 11.9 a 6.7 c
43.8 41.3 39.7 38.5 46.7 39.5
LA + SA (%) Wet
ab ab b b a b
25.8 27.0 28.6 24.8 23.5 21.7
Dry ab a a ab ab b
88.3 85.1 86.5 87.5 85.6 74.2
MI (%) Wet
a b ab a b c
34.4 34.4 37.6 35.8 35.4 28.4
b b a ab ab c
CS (%)
Dry
Wet
7.2 b 8.8 b 8.5 b 7.8 b 8.3 b 19.6 a
19.7 22.1 20.3 19.9 20.9 18.3
Dry ab a ab ab a b
4.5 6.4 4.9 4.7 6.1 6.1
MWD Wet
a a a a a a
45.9 43.5 42.2 44.4 43.8 53.3
Dry b b b b b a
2.06 2.01 2.10 2.16 1.90 1.68
AR (%) Wet
ab ab a a b c
0.66 0.63 0.70 0.73 0.75 0.54
bc c ab a a d
39.3 39.3 42.9 40.8 40.4 32.5
b b a ab ab c
LA, > 2.0 mm; SA, 0.25–2.0 mm; LA + SA, > 0.25 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm. Dry, dry aggregate; Wet, water-stable aggregate; MWD, mean weight diameter; AR, aggregate stability rate. Different letters within columns indicate significant differences (p < 0.05).
observed in MI fractions and the least in CS fractions, with the exception of the SN and SN2 treatments. Soil available and total N contents increased with N fertilizer application (Fig. 2). The highest available N contents of the bulk and aggregate fraction soils were observed in the SN2 and 2SN2 treatments, and the least in SN1/2. The total N contents of the bulk soil in the SN2 and 2SN2 treatments were significantly higher than that in SN1/2 and CK (p < 0.05). The highest total N contents in LA, SA, MI, and CS fractions were observed in the SN, SN2, SN, and SN2 treatments, respectively; the corresponding lowest contents were observed in CK, SN, SN1/2, and SN1/2. Similar to SOC, the highest total N content was in MI fractions and the least in CS fractions in each treatment.
treatments were determined by the least significant difference (LSD) method at a significance level of p < 0.05. 3. Results 3.1. Soil aggregate size distribution and stability Dry aggregate fractions of maroaggregate (> 0.25 mm, LA + SA) were the predominant fractions in all treatments. The proportions in straw treatments were > 85 %, which significantly increased by 14.7 %–19.0 % when compared to the CK treatment (p < 0.05) (Table 2). For water-stable aggregates, the proportions of LA + SA fractions decreased greatly and ranged from 28.4%–37.6%. Treatments were ranked in following the order: SN2 > 2SN > 2SN2 > SN > SN1/ 2 > CK. The fractions significantly increased by 21.1 %–32.4 % in straw treatments when compared to CK (p < 0.05) (Table 2). The proportions of LA fractions in 2SN2 were significantly higher than other treatments (p < 0.05), except 2SN. MI and CS distributions differed among the straw treatments, but the difference was insignificant. The lowest proportion of MI and largest proportion of CS were observed in the CK treatment. MWD and AR are often used to evaluate soil aggregate formation and stability. Straw application significantly increased MWD and AR (p < 0.05) (Table 2). The greatest MWD for dry aggregates was observed in the 2SN treatment, which was significantly higher than 2SN2 and CK (p < 0.05). For the wet-sieving aggregates, the greatest MWD was observed in the 2SN2 treatment, showing a significant increase when compared to the treatments that received low straw and N application (i.e., SN, SN1/2, and CK) (p < 0.05). The greatest AR was observed in the SN2 treatment, which significantly increased by 9.2 %, 9.2 %, and 32.0 % when compared to the SN, SN1/2, and CK treatments, respectively (p < 0.05).
3.3. Soil salinity and distribution of soluble cations After 4 years, compared to the initial values, bulk soil salinity of the SN, SN1/2, SN2, 2SN, 2SN2, and CK treatments decreased by 23.6 %, 20.5 %, 25.0 %, 26.6 %, 26.9 %, and 18.1 %, respectively. Soil salinity of the SN1/2 and CK treatments was significantly higher than the other treatments (p < 0.05) (Fig. 3). Therefore, straw application effectively reduced topsoil salinity. In aggregate fractions, soil salinity varied insignificantly in the straw treatments. The salinity of LA + SA fractions was higher than < 0.25 mm fractions (MI + CS) in the SN, SN1/2, and CK treatments, which reached a significant level in CK and SN1/2 (p < 0.05). In the SN2, 2SN, and 2SN2 treatments, soil salinity between the two fraction groups varied slightly. The soluble K+, Na+, Ca2+, and Mg2+ concentrations in bulk and aggregate fraction soils are presented in Fig. 4. Bulk soil soluble K+ and Mg2+ concentrations in the 2SN and 2SN2 treatments were significantly higher than the other treatments (p < 0.05), except for Mg2+ in SN2. Bulk soil soluble Na+ and Ca2+ concentrations in the CK treatment were significantly higher than the straw treatments (p < 0.05), except for Ca2+ in 2SN and 2SN2. In soil aggregate fractions, soluble K+ and Mg2+ concentrations increased as fraction size decreased. Soluble Na+ concentrations decreased as fraction size decreased. Soluble Ca2+ concentration in LA fractions was significantly higher than the other fractions in the SN, SN1/2, and 2SN treatments (p < 0.05). In the CK treatment, the highest soluble Ca2+ concentration was observed in SA fractions. Additionally, the soluble Ca2+ concentrations in LA + SA fractions were significantly higher than in MI + CS fractions of the lower straw and N treatments, i.e., SN, SN1/2, and CK (p < 0.05).
3.2. Soil C and N content Straw application significantly increased the DOC contents in bulk soils (p < 0.05) (Fig. 1A), with the greatest increase in the SN2 treatment, followed by 2SN2, and the least in SN1/2. Similar change trends of DOC content were also observed in aggregate fractions of LA and SA. For MI and CS fractions, the difference in DOC contents among the 6 treatments decreased. Meanwhile, DOC contents increased as aggregate sizes decreased in the SN, SN1/2, and CK treatments, while the opposite trend was observed in SN2. The highest bulk SOC contents were observed in the SN2 and 2SN2 treatments, which were significantly higher than that in SN, SN1/2, and CK (Fig. 1B). The distribution of SOC within aggregate fractions differed significantly among the 6 treatments (p < 0.05). In LA fractions, the highest SOC content was observed in the 2SN2 treatment, which was significantly higher than that in SN1/2, 2SN, and CK (p < 0.05). In SA, MI, and CS fractions, the highest SOC content was observed in the SN2 treatment. In each treatment, the highest SOC contents were
4. Discussion 4.1. Straw and N application influence saline soil aggregate formation and stability Soil aggregation is the main indicator of soil with good structure. In coastal areas, Ca2+ is gradually replaced by Na+ at soil exchangeable sites during soil salinization, which worsen soil structure (Wong et al., 3
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Fig. 1. Soil dissolved organic carbon (A) and organic carbon (B) contents in aggregate fractions of different treatments. Columns represent the mean (n = 3) and bars represent the standard deviation (SD); different letters within groups are significantly different (p < 0.05). LA, > 2.0 mm; SA, 0.25–2.0 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm.
incorporation of straw into saline soil, N was the most important nutrient for straw transformation. The combination of straw and N application boosts saline soil microbial growth and development (Xie et al., 2017b), and soil microbial remains and products are vital sources of SOC (Cotrufo et al., 2015). Additionally, N application efficiently mitigates the “prime effect” and decreases straw-derived organic carbon mineralization, thereby improving carbon sequestration (Kirkby et al., 2014; Qiu et al., 2016). N addition alters the characteristics and status of SOC and increases the quantity of polar groups containing N atoms in SOC (Grandy and Neff, 2008; Gillespie et al., 2014). The increase in polar N groups improves the formation of organo-mineral complexes through adsorption between the soil mineral matrix and organic molecules. Collectively, these factors contribute to soil aggregate formation and stability under saline conditions.
2010). In this study, straw application significantly increased saline soil aggregation formation and stability (Table 2), which resulted from the increase in soil organic matter that can bind mineral particles into aggregates (Fig. 1) (Gentile et al., 2011; Karami et al., 2012; Blankinship et al., 2016). This can be supported by the significant positive correlation between SOC/DOC and MWD (p < 0.05) (Table 3). However, the increase in magnitude is dependent on straw quality, quantity, and soil properties (Chivenge et al., 2011; Gentile et al., 2011). In this study, greater MWD and AR were in the treatments with higher straw rate (Table 2), suggesting that saline soil aggregation was enhanced as straw incorporation rates increased. According to the differences between the SN1/2, SN, and SN2 treatments, increased N application rates boosted saline soil aggregation formation and stability. Compared to the CK treatment, the proportion of water-stable macroaggregates, MWD, and AR in SN1/2, SN, and SN2 increased by 21.1 %–32.4 %, 16.7 %–29.6 %, and 20.9 %–32.0 %, respectively. The correlation analysis revealed that SOC was significantly positively correlated with the total N contents in bulk, SA, MI, and CS fraction soils (p < 0.01) (Table 4). Thus, adequate N application effectively promoted carbon sequestration in bulk and different saline aggregate fraction soils (Qiu et al., 2016; Liu et al., 2019). With the
4.2. C and N distribution within saline soil aggregates DOC mainly includes carbohydrates, proteins, and microbial products, which act as aggregate-stabilizing agents that ultimately improve soil aggregate formation (Degens, 1997). Due to the wet-sieving analysis often used in previous studies, little is known about the DOC 4
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Fig. 2. Soil available N (A) and total N (B) contents in aggregate fractions of different treatments. Columns represent the mean (n = 3) and bars represent the SD; different letters within groups are significantly different (p < 0.05). LA, > 2.0 mm; SA, 0.25–2.0 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm.
Following N addition, the available and total N contents in bulk soil and aggregate fractions increased (Fig. 2). Due to the high pH values in the study area, nitrate N is the main component of available N, and is weakly adsorbed onto soil particles. Thus, the lowest available N content observed was in CS fractions. Over 90 % of N in soils is in organic form (Kelley and Stevenson, 1995), causing the total N to possess similar distributions as SOC in soil aggregate fractions.
distribution within soil aggregate fractions. This study found that the DOC content increased as aggregate size decreased in lower content treatments (i.e., SN, SN1/2, and CK) but decreased in a higher content treatment (i.e., SN2) (Fig.1 A). These findings suggest that DOC preferentially bound to soil clay and silt particles, then induced microaggregate and macroaggregate formations as its quantity increased. SOC increased after straw application was unevenly distributed among the different aggregate size fractions. The highest SOC content and increase were observed in MI fractions in straw treatments (Fig. 1B). Because microaggregates have longer turnover times and higher stability than macroaggregates (Denef et al., 2007; Huang et al., 2010), higher SOC contents in microaggregates improve physical-chemical protection (Zhao et al., 2012). This is beneficial for carbon sequestration in saline soils. The formation of soil macroaggregates was due to the combination of microaggregates and particles. Thus, higher SOC contents in microaggregates increased the accumulation of SOC in macroaggregates. In this study, the contribution of SOC in dry macroaggregates to total SOC accumulation was 84.4 %–86.5 % in straw treatments. Thus, it appears that SOC preferentially accumulated in microaggregates after straw incorporation and mainly sequestrated in macroaggregates of saline soil.
4.3. Soluble salt distribution within saline soil aggregates Due to the shallow groundwater table, decreasing upward salt movement and accumulation in the topsoil was a crucial step for coastal saline soil improvement. Straw application efficiently enhances salt leaching and inhibits upward salt movement from underground water or deep soils by increasing soil porosity and aggregate formation (Mulumba and Lal, 2008; Kim et al., 2017). The correlation analysis indicated that soil salinity was significantly negatively correlated with MWD, SOC, DOC, and available and total N (Tables 3 and 4) (p < 0.05).). Thus, improving soil organic matter and aggregate formation with straw was an important mechanism for saline soil reclamation. Saline soil aggregate formation affected soluble salt distribution 5
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Fig. 3. Soil salinity of aggregate fractions in different treatments. Columns represent the mean (n = 3) and bars represent the SD; different letters within groups are significantly different (p < 0.05). LA, > 2.0 mm; SA, 0.25–2.0 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm.
Fig. 4. Soil soluble K+ (A), Na+ (B), Ca2+ (C), and Mg2+ (D) concentrations in aggregate fractions of different treatments. Columns represent the mean (n = 3) and bars represent the SD; different letters within groups are significantly different (p < 0.05). LA, > 2.0 mm; SA, 0.25–2.0 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm.
within aggregates. In the study area, NaCl is the main constituent of soluble salts, comprising ∼80 % of the total (Weng and Gong, 2006). Thus, the high soluble Na+ concentrations in bulk and aggregate fraction soils of the CK treatment were due to its high salinity. Na+ is a highly dispersive agent that directly or indirectly results in the breakup of soil aggregates (Wong et al., 2010; Huang et al., 2016), leading to worse aggregate formation and stability. The concentration of Na+
increased as aggregate size increased, which may be because the soil colloid possessed more anionic groups as aggregate size increased and weakly adsorbed Na+. They are easily separated from anionic groups when extracted using water (Wang et al., 2017). The high Ca2+ concentrations in bulk and aggregate soils of the CK treatment were an unexpected and interesting result. Ca2+ can improve soil structure through cationic bridging with clay particles and soil organic matter 6
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Table 3 Correlation analysis between saline soil properties.
Salinity SOC DOC AN TN MWD AR K+ Na+ Ca2+ Mg2+
pH
Salinity
SOC
DOC
AN
TN
MWD
AR
K+
Na+
Ca2+
0.65** −0.60** −0.22 −0.61** −0.52* −0.55* −0.26 −0.76** 0.72** −0.24 −0.79**
−0.56* −0.56* −0.67** −0.66** −0.56* −0.43 −0.83** 0.87** 0.38 −0.39
0.73** 0.67** 0.77** 0.52* 0.44 0.68** −0.71** −0.12 0.53*
0.65** 0.76** 0.51* 0.61** 0.40 −0.67** −0.11 0.21
0.63** 0.58** 0.49* 0.60** −0.79** −0.56* 0.52*
0.56* 0.41 0.60** −0.68** −0.26 0.27
0.82** 0.60** −0.73** −0.20 0.56*
0.36 −0.55* −0.48* 0.31
−0.81** −0.12 0.63**
0.21 −0.56*
0.40
SOC, soil organic carbon; DOC, dissolved organic carbon; AN, available N; TN, total N; MWD, mean weight diameter; AR, aggregate stability rate. *, significance level at p < 0.05; **, significance level at p < 0.01. Table 4 Significance levels of correlation analysis between properties of different saline soil fractions. Soils
DOC AN TN Salinity
Bulk soil
LA
SOC
DOC
+** +** +** –*
+** +** –*
AN
+** –**
SA
TN
SOC
DOC
–**
+ +** + –*
+** + –**
AN
+ –**
MI
TN
SOC
DOC
–
+ + +** –
+* + –
AN
+* –
CS
TN
SOC
DOC
–*
+ +* +** +
– – +
AN
+ –
TN
SOC
DOC
AN
TN
–
+ + +** +
+ + +*
+ –
+
LA, > 2.0 mm; SA, 0.25–2.0 mm; MI, 0.053–0.25 mm; CS, < 0.053 mm; SOC, soil organic carbon; DOC, dissolved organic carbon; AN, available N; TN, total N. +**, positive correlation at significance level of p < 0.01; +*, positive correlation at significance level of p < 0.05; +, insignificant positive correlation; –**, negative correlation at significance level of p < 0.01; –*, negative correlation at significance level of p < 0.05; –, insignificant negative correlation.
Declaration of Competing Interest
(Huang et al., 2016; Kim et al., 2017). The low DOC and SOC contents in the CK treatment prevented Ca2+ from combining with organic matter and remained free in soil solutions. The correlation analysis revealed that the soluble Ca2+ concentration was negatively correlated with SOC, DOC, and MWD, and significantly negatively correlated with AR (p < 0.05) (Table 3). Soluble Ca2+ concentrations in LA + SA fractions were significantly higher than in MI + CS fractions in the SN, SN1/2, and CK treatments. This difference was obviously reduced in the SN2, 2SN, and 2SN2 treatments, suggesting that sufficient straw or N application could improve the binding effects between Ca2+ and soil constituents, possibly by influencing the quantity and chemical properties of soil organic matter. Soluble K+ and Mg2+ concentrations were higher in the 2SN and 2SN2 treatments, which was likely caused by the high straw application rates, as K and Mg nutrients are released into the soil during straw decomposition (Zhu et al., 2010; Xie et al., 2017a). The increase in soluble K+ and Mg2+ concentrations as aggregate fraction size decreased indicated that more K+ and Mg2+ were involved in the formation of aggregates, reducing their availability in soil solutions. The distribution of soluble ions in each aggregate fraction ultimately influenced the salinity of different soil aggregates, which in turn regulated soil aggregate formation and stability.
None. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (41877101; 31901456), the Key Research and Development Program of Shandong Province (2019GNC106003), and the Taishan Scholars Program of Shandong, China (tsqn201812086). References Amundson, R., Berhe, A.A., Hopmans, J.W., Olson, C., Sztein, A.E., Sparks, D.L., 2015. Soil and human security in the 21st century. Science 348 (6235), 1261071. Bandyopadhyay, P.K., Saha, S., Mani, P.K., Mandal, B., 2010. Effects of organic inputs on aggregate associated organic carbon concentration under long-term rice-wheat cropping system. Geoderma 154, 379–386. Blankinship, J.C., Fonte, S.J., Six, J., Joshua, P.S., 2016. Plant versus microbial controls on soil aggregate stability in a seasonally dry ecosystem. Geoderma 272, 39–50. Cotrufo, M.F., Soong, J.L., Horton, A.J., Campbell, E.E., Haddix, M.L., Wall, D.H., Parton, W.J., 2015. Formation of soil organic matter via biochemical and physical pathways of litter mass loss. Nat. Geosci. 8, 776–779. Degens, B.P., 1997. Macro-aggregation of soils by biological bonding and binding mechanisms and the factors affecting these: a review. Aust. J. Soil Res. 35, 431–459. Dendooven, L., Alcántara-Hernández, R.J., Valenzuela-Encinas, C., Luna-Guido, M., Perez-Guevara, F., Marsch, R., 2010. Dynamics of carbon and nitrogen in an extreme alkaline saline soil: a review. Soil Biol. Biochem. 42, 865–877. Denef, K., Zotarelli, L., Boddey, R.M., Six, J., 2007. Microaggregate-associated carbon as a diagnostic fraction for management-induced changes in soil organic carbon in two Oxisols. Soil Biol. Biochem. 39, 1165–1172. El Hasini, S., Halima, O.I., Azzouzi, M., El Douaik, A., Azim, K., Zouahri, A., 2019. Organic and inorganic remediation of soils affected by salinity in the Sebkha of Sed El Mesjoune-Marrakech (Morocco). Soil Till. Res. 193, 153–160. Gentile, R., Vanlauwe, B., Six, J., 2011. Litter quality impacts short- but not long-term soil carbon dynamics in soil aggregate fractions. Ecol. Appl. 21 (3), 695–703. Getahun, G.T., Kätterer, T., Munkholm, L.J., Parvage, M.M., Keller, T., Rychel, K., Kirchmann, H., 2018. Short-term effects of loosening and incorporation of straw slurry into the upper subsoil on soil physical properties and crop yield. Soil Till. Res. 184, 62–67. Gillespie, A.W., Diochon, A., Ma, B.L., Morrison, M.J., Kellman, L., Walley, F.L., Regier, T.Z., Chevrier, D., Dynes, J.J., Gregorich, E.G., 2014. Nitrogen input quality changes
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