WANG Yong，MENG Qingfeng

(College of Resources and Environment，Northeast Agricultural University，Harbin 150030，China)

Abstract：In order to explore the effects of cattle manure on soil salinity and sodicity on the sodic soil in long-term experiments，the experiments were performed in a randomized complete block design with four treatments；soils that received manure applications for 8，12，18 years were used as the experimental treatments，and soil that did not receive cattle manure application was used as the control treatment(CK).The results showed that the application of cattle manure to saline-sodic soil resulted in a reduction in the bicarbonate ion(HCO3-)contents，the elimination of carbonate ions(CO32-)，the decrease in soil bulk density(ρb)，the increases in soil porosity(ft)and soil organic matter(SOM)，the decreases in the exchangeable and soluble sodium ion (Na+)contents associated with increases in the exchangeable calcium ion(Ca2+)，soluble potassium ion(K+)，and magnesium ion(Mg2+)contents compared to those in untreated soil.The soil exchangeable sodium percentage(ESP)and pH were both significantly and positively correlated with the exchangeable Na+ and HCO3-，and CO32- contents，and soil pH was significantly and negatively correlated with SOM.Regression analysis showed that the dominant factors affecting the sodium absorption ratio(SAR)were the soluble Mg2+ and Na+ contents in the soil.Pearson correlation analysis showed that there were significantly negative correlation between the accumulated amount of cattle manure among the indicators of soil salinization degree，such as pH，EC，ESP and SAR.It was concluded that long-term manure application significantly decreased the soil pH，ESP，electrical conductivity(EC)and SAR due to the replacement of soil colloidal Na+ with Ca2+，the leaching of soil soluble salts from the topsoil and changes in the soil soluble salt ion composition.These outcomes were likely due to the decrease of ρb associated with increase of ft and Ca2+ and Mg2+ contents caused by annual manure application.

Key words：Cattle manure；Long-term experimentation；Solonetz；Exchangeable cations；Soluble salt；Soil sodicity

Soil salinization refers to the increase in the soluble salt concentration in water and soils under natural conditions or due to human activities.The parent materials，topographical features，climatic conditions and anthropogenic factors contribute to the formation and evolution of the salinization of the soil[1-2].Salt-affected soils with high salinity and alkalinity are not suitable for plant growth and have substantially adversely affected agricultural production and the environment[3].As a result，soil salinization has become a common environmental problem worldwide[4].The Songnen Plain in Northeast China is one of the three largest distribution regions of saline-sodic soil in the world；saline-sodic cultivated land accounts for 6.8% of the whole cultivated land area and covers 1.27×106ha of the plain[5].

To solve the problems of salinity and sodicity affecting crop production，many techniques have been applied to salt-affected soils，including tillage practices，diversion irrigation，the application of flue gas desulfurization gypsum or organic manure，etc.Organic manure application is a traditional method for the improvement of salt-affected soils in China.Salt-affected soils have been improved and made usable through the application of organic manure to soils.Compared with those in saline-sodic soil without organic amendments，the EC，exchangeable sodium percentage(ESP)and SAR with organic amendments are clearly lower[10].In addition，Trivedi et al[11]reported that the application of organic manure helped reduce sodicity factors，such as pH，EC and exchangeable Na+，and increased soil fertility.In conclusion，manure application helped improve the physicochemical properties of saline-sodic soils and increased crop yields.However，the results of many of these studies are based on short-term or small-scale experiments，and there are few conclusions from long-term experiments with large-scale organic manure application to soil.Applying organic manure to saline-sodic soil is beneficial for agricultural ecosystems and has the potential to improve the saline-sodic soil.This study aimed to evaluate the continuous effects of long-term cattle manure application on soil salinity and sodicity.Moreover，we also determined how long-term manure application affects the distribution of ions in various soil layers due to the key properties of soils in the study area.

1 Materials and Methods

1.1 Study site

The study was performed on the long-term saline-sodic soil study station located in the Yongle Village of Zhaozhou County，Heilongjiang Province，China(125.06°E，45.04°N，and altitude 149 m)since 1995.According to the FAO World Soil Resources Reference Base，the soils in this area are classified as solonetz soils，and the soil texture is clay(26.2% sand，21.5% silt，52.3% clay)based on the International Society of Soil Science Classification[12].The climate in this area is a continental monsoon climate in the middle temperate zone，and the area is part of the first accumulated temperature zone.The annual active accumulated temperature above 10 ℃ is approximately 2 800 ℃，the frost-free period is 143 d，the annual average precipitation is 436 mm，the annual average evaporation is 1 800 mm，and the annual average temperature is 3.7 ℃.The soil physicochemical properties prior to experimentation at 0—20 cm soil depth were illustrated as follows：the soil pH is 9.56；the EC is 6.23 dS/m；the soil bulk density ρbis 1.35 g/cm3；and the total porosity(ft)is 49.06%.

1.2 Experimental design

In late April of each year，cattle manure was applied to soil at a rate of 10 000 kg/ha on an oven-dry weight basis.The cattle manure was equably scattered spread onto the surface of the soil in the experimental area by hand and was immediately incorporated into the surface of the soil at 0—20 cm by ploughing and ridging before seeding the filed with corn(ZeamaysL.).The basic properties of the manure on an oven-dry weight basis were as follows：the pH was 8.42；the SOM was 602.94 g/kg；the total sodium，potassium，calcium and magnesium contents were 0.41，15.35，9.77，4.58 g/kg，respectively；and the bulk density of the manure was 0.29 g/cm3.Based on their history of cattle manure application，five treatments were established in a randomized complete block design with three replicates in late April 2018.The treatments were：a control treatment of corn grown without manure application(CK)and corn grown with manure application for 8(T8)，12(T12)，18 years(T18).Urea(N=46%)was applied as a topdressing at a rate of 400 kg/ha at the jointing stage of corn.

1.3 Soil sampling

To determine the changes in soils as affected by the manure application，soils were sampled from each plot after harvest of the corn in October 2018，three sampling points were randomly selected in each plot，and soil samples from the 0—20 cm，20—40 cm and 40—60 cm soil layers were used for the determination of the soil salt contents，pH，EC，SOM and exchangeable cation contents，such as exchangeable Na+，K+，Ca2+and Mg2+.Three replicates of soil cores were collected using a cutting ring with a volume of 100 cm3for the determination of ρband calculation of ft.

1.4 Laboratory methods

After soil sample collection，the soil samples were treated with the Quartering method to remove any stones and residual crop roots.Each quartered sample weighed approximately 2 kg.After air-drying in a cool and ventilated location，the air-dried soil samples were ground and sieved through a 1.00，0.25 mm sieve，respectively.

Soil pH was measured at a 2.5∶1.0 water-to-soil radio using a pH-meter electrode and EC was measured at a 5∶1 water-to-soil radio using a conductivity meter，respectively.The soluble and exchangeable Ca2+and Mg2+concentrations were measured by atomic spectrophotometry after extraction with distilled water(5∶1 water to soil ratio)and 1 mol/L NH4OAc，respectively.The soluble and exchangeable K+and Na+concentrations were determined with flame photometry.Soluble HCO3-and CO32-were titrated with double indicator neutralization titration，and silver nitrate was used for the measurements.Soluble chloride(Cl-)and sulfate(SO42-)were titrated with EDTA indirect complexometric titration.

The soil cation exchange capacity(CEC)was determined as described by Bao[13]；after 1 mol/L NaOAc(pH 8.2)saturation of the soil and followed by C2H5OH rinsing，Na+was exchanged with 1 mol/L NH4OAc(pH 7.0)and then determined by flame photometry.

ρbwas measured using core method by undisturbed soil cores with volume of 100 cm3and dried for 48 h at 105 ℃[14]，ftwas calculated usingρband particle density(ρd)according to the equation ①：

①

Whereρdis 2.65 g/cm3[15]；ftis soil total porosity(%)；ρbis bulk density(g/cm3)；ρdis particle density(2.65 g/cm3)；

Soil organic matter(SOM)was determined by dichromate oxidation with 230 ℃ heating(K2Cr2O7-H2SO4)and back titration with 0.2 mol/L FeSO4.

ESP and SAR were calculated using equations ② and ③，respectively.

②

③

1.5 Statistical analysis

All statistical analyses were performed using SPSS 17.0，significant differences among all the measured data were determined with one-way analysis of variance(ANOVA)followed by Duncan test atP<0.05，all figures were drawn using Origin 2019 and tables were listed using Microsoft Word 2016.

2 Results and Analysis

2.1 Soil pH and EC

The soil pH at 0—20 cm，20—40 cm，40—60 cm was obviously affected by manure application and soil pH decreased from 9.35 to 7.92 in topsoil(Fig.1).At 0—20 cm，the soil pH was significantly(P<0.05)higher in the CK treatment than in the treatments with manure application，whereas there was no significant difference among the T8，T12 and T18 treatments.Similarly，the highest value of soil pH was observed in the CK treatment compared with other treatments at 20—40 cm，and the soil pH at 20—40 cm was significantly higher in the CK treatment than in the other treatments.Although the soil pH changed with the number of years of manure application，no significant difference in soil pH was observed among the T8，T12，and T18 treatments.A similar result was observed at 40—60 cm，where the highest value of soil pH was also observed in the CK treatment，followed by the T8 and T12 treatments，and the lowest value was observed in the T18 treatment.The soil pH was significantly higher in the CK treatment than in the T8，T12 and T18 treatments.

Within a soil depth，different letters indicate significant differences using Duncan test at P<0.05.The same as Fig.2—5.

At 0—20 cm，EC decreased by 0.06—0.39 dS/m and EC was significantly(P<0.05) lower in the T18 treatment than in the CK treatment.Although the application of manure to the saline-sodic soil resulted in a decrease in EC，there was no significant difference between the CK，T8 and T12 treatments.At 20—40 cm，the highest value of EC was also observed in the CK treatment compared to other treatments，followed by CK is the T8 and T12 treatments，and the lowest EC value was observed in the T18 treatment.EC was significantly higher in the CK treatment than in the T12 and T18 treatments.EC at 40-60 cm was significantly lower in the manure application treatments than in the CK treatment(Fig.2).

Fig.2 Effects of manure application on electrical conductivity(EC) across all treatments

2.2 Soil exchangeable cations and ESP

The soil ESP of the treatments at 0—20 cm，20—40 cm，40—60 cm was shown in Fig.3.At 0—20 cm，soil ESP decreased from 50.01% to 1.54%, and the soil ESP in the T8，T12 and T18 treatment was 43.33，48.47，47.79 percentage points lower than that in the CK treatment，respectively.The differences between the manure application treatments and the CK treatment at 0—20 cm were statistically significant(P<0.05).Similarly，significant differences were observed at 20—40 cm between the CK treatment and the treatments with manure application.The manure application treatments showed significantly(P<0.05) lower ESP than the CK treatment，and there was no significant difference in ESP among the manure application treatments.At 40—60 cm，the ESP was also significantly higher in the CK treatment than in the T8，T12 and T18 treatments，whereas the differences in ESP were not significant between the T12 and T18 treatments.

Fig.3 Effects of manure application on exchangeable sodium percentage(ESP) across all treatments

Tab.1 showed the soil exchangeable cations at different sampling depths，which were influenced by different treatments.The manure application treatments showed significantly lower exchangeable Na+than the CK treatment，and significantly higher exchangeable Ca2+and exchangeable Mg2+were observed in the treatments with manure application than CK，except for the exchangeable Mg2+was not significant in the T18 treatment at 0—20 cm.Exchangeable K+was significantly lower in the CK treatment than in the T8 treatment，whereas there was no significant difference in exchangeable K+among the CK，T12 and T18 treatments at 0—20 cm.As the exchangeable Ca2+at 20—40 cm increased，the manure application treatments showed significantly decreased exchangeable Na+，by 10.48，12.19，12.28 cmol/kg in the T8，T12 and T18 treatments，respectively，relative to the CK treatment.Exchangeable Mg2+and K+were significantly higher in the T8 treatment than in the CK treatment at 20—40 cm.At 40—60 cm，the exchangeable Na+was significantly higher in the CK treatment than in the manure application treatments(P<0.05).A significant difference in exchangeable Ca2+was observed between the CK and treatments with manure application except for the T18 treatment(P<0.05).In the treatments with manure application，except for the T18 treatment，exchangeable Mg2+showed significantly higher than the CK treatment，whereas no significant difference in exchangeable K+was observed between the CK，T8 and T12 treatments.

Tab.1 Effects of manure application on exchangeable cations across all treatments cmol/kg

2.3 Soil soluble cations

The distribution of soluble cations in the different soil layers after manure application was shown in Tab.2.At 0—20 cm，Na+and K+in the CK treatment were significantly higher than those in the treatments with manure application，except for K+in the T8 treatment.The Mg2+in the CK treatment was significantly lower than that in the treatments with manure application，except for the T18 treatment，and there was no significant difference in Ca2+between CK and the treatments with manure application，except for the T12 treatment.Similarly，the Na+and K+levels decreased significantly with manure application in the 20—40 cm layer compared with those in the CK treatment.A significant increase in Ca2+in the 20—40 cm layer was observed in the T12 treatment compared with the CK treatment，whereas there was no significant difference between the CK treatment and the T8 and T18 treatments.There was a significant difference in Mg2+content between the CK，T8 and T12 treatments.Similar results were observed at 40—60 cm，and Na+and K+in the manure application treatments were significantly different from those in the CK treatment.Although manure application resulted in changes in Ca2+and Mg2+levels in the soil，there was no significant difference in Ca2+and Mg2+levels across all treatments.

Tab.2 Effects of manure application on soluble cations across all treatments mg/kg

2.4 Soil soluble anions

The effects of manure application for different time periods on soil soluble anions were shown in Tab.3.The value of CO32-was high in the CK treatment，whereas it decreased considerably in the manure application treatments such that CO32-was not measured in the three soil layers.At 0—20 cm，HCO3-and Cl-were significant higher in the CK treatment than in the treatments with manure application，and there was no significant change in SO42-between the CK，T8 and T18 treatments.Similarly，HCO3-was significantly lower in the manure application treatments than in CK.A change in Cl-was observed after manure application，but there was no significant difference between treatments with manure application.At 20—40 cm，the CK treatment had significantly lower SO42-content than the treatments with manure application，and the SO42-content gradually decreased with the number of years of manure application.Similar results were observed at 40—60 cm：CO32-was found in the CK treatment；HCO3-and SO42-contents were significantly higher in the CK treatment than in the manure application treatments，except SO42-in the T8 treatment.Although there was a decrease in Cl-in the manure application treatments，significant differences in Cl-compared with the CK treatment were observed only in the T12 and T18 treatments.

Tab.3 Effects of manure application on soluble anions across all treatments mg/kg

2.5 Soil SAR

The soil SAR was remarkably high in the CK treatment in the three soil layers(Fig.4).At 0—20 cm，the SAR decreased by 8.05—9.89 mmol/L and SAR was significantly higher in the CK treatment than in the treatments with manure application，none of the SAR contents in the manure application treatments were higher than 2.33 mmol/L at 0—20 cm.Although the SAR in the T12 and T18 treatments was lower than that in the T8 treatment，there was no significant difference among those at 0—20 cm.As at 0—20 cm，the SAR in the CK treatment in the other soil layers were significantly different from those in the treatments with manure application.Although the application of manure to saline-sodic soil resulted in changes in the SAR，there was no significant difference among the manure application treatments at 20—40 cm，40—60 cm.

Fig.4 Effects of manure application sodium adsorption ratio(SAR)across all treatments

2.6 Soil organic matter

The application of manure to saline sodic soil could increase 31.78%—97.83% SOM content.A similar result was observed in the three soil layers(Fig.5).At 0—20 cm，the SOM content was significantly lower in the CK treatment than in the T8 and T12 treatments，although the application of manure to the saline sodic soil resulted in an increase in SOM，there was no significant difference between the CK and T18 treatments.Similar result was observed at 20—40 cm，the SOM content was significantly(P<0.05)lower in the CK treatment than in the T8 treatment，whereas there was no significant difference among the CK，T12 and T18 treatments.Although the application of manure to saline-sodic soil resulted in changes in the SOM，there was no significant difference among all the application treatments at 40—60 cm.

Fig.5 Effects of manure application soil organic matter(SOM)across all treatments

2.7 Soil bulk density and total porosity

Tab.4 showed the changes of ρband ftcontents at the three layers，which were influenced by cattle manure application.The manure application treatments showed lower ρband higher ftthan the CK treatment and there was a significant increase in ρband decrease in ftin the T18 treatment compared with the CK treatment at 0—20 cm.The ρbwas significantly lower and ftwas significantly higher than those in the CK treatment at 20—40 cm.There was no significant difference in ρband ftvalues across all treatments at 40—60 cm.

Tab.4 Effects of manure application on soil bulk density and total porosity across all treatments

2.8 Relationships between soil indexes

Relationships between soil indexes were shown in Tab.5.A significantly negative correlation was observed between cumulative amount of manure (CM) and EC by correlation analysis with a correlation coefficient of -0.60(P<0.01). SAR was significantly and negatively correlated with CM and positively correlated with EC by correlation analysis with correlation coefficients of -0.75 and 0.85, respectively(P<0.01). HCO3-+CO32-was significantly and negatively correlated with CM and positively correlated with pH by correlation analysis with correlation coefficients of -0.67 and 0.78, respectively(P<0.01). A significantly negative correlation was observed between SOM and pH by correlation analysis with a correlation coefficient of -0.64(P<0.01). Exchangeable Na+was significantly and negatively correlated with CM and positively with pH by correlation analysis with correlation coefficients of -0.87 and 0.91, respectively(P<0.01). A significantly negative correlation was observed between CM and ESP by correlation analysis with a correlation coefficient of -0.84(P<0.01).

Tab.5 Relationships between soil indexes

3 Discussion and Conclusions

The application of manure to saline-sodic soil significantly increased the SOM compared to untreated soil，which is a direct result of the manure，this is consistent with the results of Zhang et al[16]，who reported that soils treated with manure application significantly increased SOM compared to untreated soil；as well as decreased the ρb，particularly at 0—20 cm depth，decreased ρbis associated with increased soil porosity，and the change occurred as a direct result of the addition of the manure，which had a lower bulk density(0.33 g/cm3)than the soil，this result is consistent with Rasool et al[17]，who reported manure application had a positive effect on decrease of ρbassociated with increase of ft.In addition，the manure had higher Ca2+and Mg2+contents than the soil，which was beneficial for soil management and reduction of soil salinization[18].

In our study，treatments in saline-sodic soil with manure application significantly reduced EC content compared to that in the untreated soil.This result was due to annual manure application decreased ρb，which is associated with increased ftthat promote the leaching of soil soluble salts from the topsoil by rain[19-20].Furthermore，this led to changes in the soil soluble salt ion composition due to differences in migration ability among the soil ions.This result is further confirmed by the soluble Na+in different layers in Tab.2.

The SAR is commonly used as a sensitive indicator of soil alkalization and is usually used to assess the potential for Na+and soil alkalization to affect soil structure[21-22].A previous study reported that the application of manure to saline-sodic soil improved the soil structure and that soil soluble salts could be transmitted from the surface soil to the subsoil by soil water；this resulted in a lower soluble Na+content，which led to a reduction in the SAR in soil treated with manure.This is consistent with our results，in our study，the cumulative amount of manure was significantly and negatively related to SAR.In our study，a reduction in the SAR was caused by a decrease in the soil soluble salt content，and the SAR was significantly and positively related to EC.Additionally，the relationships of soluble Na+，Ca2+and Mg2+with SAR were determined by linear regression models；the results showed that SAR was significantly and negatively correlated with soluble Mg2+and Ca2+and had a significant correlation with soluble Na+in equation④.This suggested that after manure application to saline-sodic soil，soluble Na+and especially soluble Mg2+were the dominant factors influencing SAR，based on their discriminant coefficients.This probably occurred due to the input of Mg2+from the manure application and the leaching of soluble Na+from the topsoil.This result verifies the findings observed by Yu et al[23]，who discovered that Na+was the predominant soluble cation and that the content of soluble anions was very low at the 0—50 cm depth on the Songnen Plain before planting.This finding is further confirmed by the contents of soluble cations in Tab.2.

SAR=1.424+9.012Na+-20.303Mg2+-1.887Ca2+

④

whereSARis the sodium adsorption ratio(mmol/L)and the solubleNa+，Ca2+andMg2+contents in soil are provided in mmol/L.

Soil pH is mainly influenced by the free NaHCO3and Na2CO3and exchangeable Na+contents of saline-sodic soil[24]and their hydrolysis.In our study，the soluble HCO3-and CO32-contents were significantly lower in soil treated with manure than in untreated soil and decreased with the number of years of manure application，soluble CO32-was eliminated in the soils treated with manure.In our study，the contents of HCO3-and CO32-were significantly and positively related to soil pH.The soil pH was significantly and negatively related to SOM.The decrease in soil pH with manure application is likely due to the organic acid produced during the decomposition of the SOM transformed CO32-into HCO3-[23]，in addition，the input of Ca2+replaced Na+at soil colloid changed into highly soluble salts that restrained the hydrolysis of alkaline Na+.Furthermore，the reduction in soil pH is also probably due to the manure application resulted in the decrease of ρb，which is associated with increase of ftthat promoted the leaching of soluble Na+from the solution.This result is further confirmed by the correlation among pH，ρband cumulative amount of manure.

The soil exchangeable Na+content decreased with the number of years of manure application.In contrast，the exchangeable Ca2+content increased significantly with the decrease in exchangeable Na+content in soils treated with manure except for the T18 treatment at 40—60 cm.This suggested that the exchangeable Ca2+replaced the exchangeable Na+in the soil colloids due to the input of Ca2+from manure[25].In addition，the increase of cumulative amount of manure led to the decrease of ρbassociated with the increase of ft，respectively promoted the leaching of soluble Na+from the solution and then reducing the hydrolysis of alkaline Na+as a result of decrease in soil pH.Thus，the decrease in soil pH was caused by a reduction in the exchangeable Na+content；soil pH was significantly and positively correlated with exchangeable Na+.A similar result was observed by Meng et al[10]in a laboratory soil column leaching experiment；they reported that amended soils had significantly higher soil exchangeable Ca2+levels than control soils and that soil exchangeable Na+concentrations were significantly lower in organic amendment-treated soils than in control soils after leaching.

Soil ESP is expressed as the saturation of exchangeable Na+adsorbed on soil colloids.In general，the higher the ESP is，the higher the degree of soil alkalization and the poorer the soil properties.A reduction in soil ESP reflects a low level of exchangeable Na+relative to the levels of other exchangeable cations[18].In our study，soil ESP was significantly lower in the treated soils than in the untreated soil.This was likely due to a decrease in the exchangeable Na+content and the leaching of soluble ions from the solution in topsoil as a result of increase of ft，which affected by the manure application between cumulative amount of manure and ESP.In addition,soil ESP decreased significantly with decreasing exchangeable Na+content.These findings are also consistent with the results presented by Mao et al[26]，who reported that an increase in soluble Ca2+in the soil resulted in a reduction in the amount of exchangeable Na+present in soil colloids.

The experiments were carried out by applying cattle manure application on sodic soil on the Songnen Plain，Northeast China，we studied the desalinization effects of long-term cattle manure application in saline-sodic soils and the results showed that the application of cattle manure to saline-sodic soil resulted in an increase in SOM，a reduction in pH，EC，ESP and SAR，compared with CK treatment at 0—20 cm，SOM increased by 31.78%—97.83%；pH and ESP decreased from 9.35 to 7.92 and from 50.01% to 1.54%，EC and SAR decreased by 0.06—0.39 dS/m and 8.05—9.89 mmol/L，respectively.The application of cattle manure to saline-sodic soil resulted in a change the ion composition.The application of cattle manure to saline-sodic soil resulted in an elimination of CO32-，decreases in exchangeable and soluble Na+contents associated with increases in exchangeable Ca2+，a decrease in soluble K+and an increase in soluble Mg2+contents，compared to those in untreated soil.The higher the accumulated amount of cattle manure，the more obvious the desalination effect in the treatments.There was an extremely significant negative correlation between the accumulated amount among the indicators of soil salinization degree，such as pH，EC，ESP and SAR.We concluded that the observed decreases in soil pH and ESP were caused by the replacement of soil colloidal Na+with Ca2+.Moreover，the reductions in soil salinity and sodicity(soil pH，EC and SAR)resulted from the leaching of soil soluble salts from the topsoil and changes in the soil soluble salt ion composition.This result was likely due to the decrease in ρbassociated with increase in ftand the increased Ca2+and Mg2+contents caused by annual manure application.