Furfural and its biochar improve the general properties of a saline soil

Organic materials (e.g., furfural residue) are generally believed to improve the physical and chemical properties of saline soils with low fertility. Recently, biochar has been received more attention as a possible measure to improve the carbon balance and improve soil quality in some degraded soils. However, little is known about their different amelioration of a sandy saline soil. In this study, 56 d incubation experiment was conducted to evaluate the influence of furfural and its biochar on the properties of saline soil. The results showed that both furfural and biochar greatly reduced pH, increased soil organic carbon (SOC) content and cation exchange capacity (CEC), and enhanced the available phosphorus (P) in the soil. Furfural is more efficient than biochar in reducing pH: 5 % furfural lowered the soil pH by 0.5–0.8 (soil pH: 8.3–8.6), while 5 % biochar decreased by 0.25–0.4 due to the loss of acidity in pyrolysis process. With respect to available P, furfural addition at a rate of 5 % increased available P content by 4–6 times in comparison to 2–5 times with biochar application. In reducing soil exchangeable sodium percentage (ESP), biochar is slightly superior to furfural because soil ESP reduced by 51 % and 43 % with 5% furfural and 5 % biochar at the end of incubation. In addition, no significant differences were observed between furfural and biochar about their capacity to retain N, P in leaching solution and to increase CEC in soil. These facts may be caused by the relatively short incubation time. In general, furfural and biochar exhibited a different effect depending on the property: furfural was more effective in decreasing pH and increasing available P, whereas biochar played a more important role in increasing SOC and reducing ESP of saline soil.


Introduction
A large saline soil reserve has been explored in the Yellow River delta.Seasonal accumulation of salt in the surface soil caused by high soil salinity and water shortage restricts the germination of plants, while poor physical and chemical properties of soil are the major obstacles of plant growth.Few categories and small amount of active substances like soil enzyme and microorganisms are important influencing factors of circulation of materials and plants' sustainable utilization of soil resources (Angst and Sohi, 2012).The low productivity of soil and soil environment deterioration in the Yellow River delta further aggravate the soil salinity (Bai et al., 2005;Wang et al., 2010), thus restricting the growth of crops.Amendment with organic materials can improve the soil salinity and increase crop yield significantly (Luo et al., 2008).
As a kind of inexpensive acid organic substance with rich resources, furfural is effective in improving the saline soil.Furfural is the corncob after industrial distillation.It is dark brown and belongs to strong acid organic materials with pH around 2. Furfural contains N(0.5 %-0.6%),P2O5(0.2-0.15 %), K2O(0.15%), humic acid substances(36%), and the content of organic materials is above 98 % (Yang, 2008;Li et al., 2008).Cai et al. (1997) and Li et al. (2008) reported that furfural can lower the soil pH and salinity and increase crop yield.In recent years, incorporation of biochar into low fertility soils has attracted interest because biochar application increases C sequestration while also increasing soil water or nutrient availability, thereby improving plant growth (Lehmann, 2007;Marris, 2006;Renner, 2007;Zhang, 2010).Although there is some research about the influence of Published by Copernicus Publications on behalf of the European Geosciences Union.Y. Wu et al.: Furfural and its biochar improve the general properties furfural on alkaline soils (Li, 2008), no comparative research on the impact of furfural on the physicochemical properties of saline soil before and after its carbonization has been reported yet.Therefore, based on the indoor constant temperature incubation and leaching test, this paper evaluated the effect of furfural in improving saline soil according to the variation trend of acid-base property, water-soluble salt, basic nutrients in soil and cation exchange performance, aiming to provide theoretical basis for the application of furfural and biochar in saline soil improvement.

Materials
In October 2012, the testing soil samples were collected at 0-10 cm depth of saline soil at Yellow River delta (37 • 45 50 N-118 • 59 24 E), which is located in the northeast of Shandong province of China.The sampling site has a warm continental monsoon climate with distinctive seasons and a rainy summer.The soil is typical saline alluvial soil (Fluvisols, FAO) developed on loess material of the Quaternary period, which was carried by water from the Loess Plateau.The collected soil samples were air-dried under room temperature and then sieved in a mesh to 2 mm.The proportion of clay, silt, and sand was 8.4 %, 6.2 %, and 85.4 %, respectively.The physicochemical properties of testing soils are shown in Table 1.The soil showed a pH of 8.3 and an ESP as high as 27 %, known as strong alkaline soil (Lu, 1999).
The involved furfural is the corncob after industrial distillation.It is in dark brown.The furfural biochar (hereinafter referred to as biochar) is made from furfural through 4 h carbonization under 300 • C under the completely or partly anoxic condition.Biochar in association with porous characteristics and high surface area is favorable to accumulating soil moisture, increasing the porosity and reducing bulk density (Wu et al., 2014).The physicochemical properties of furfural and its biochar are listed in Table 1.

Incubation method
A soil incubation test was conducted to investigate the similarities and differences of furfural and biochar in influencing the physicochemical properties of saline soil.The test involved five test treatments: (1) CK, soil without furfural and biochar; (2) T 1 , added with 2.5 % furfural; (3) C 1 , added with 2.5 % biochar; (4) T 2 , added with 5 % furfural; and (5) C 2 , added with 5 % biochar.Each group was repeated four times.In the test, each soil incubation container was filled with 500 g saline soil from the Yellow River delta with a maximum water content of 30 % (evaporated water was replenished every day by weighing method).Soil samples were incubated under constant 25 • C. We took soil samples at 1 d, 3 d, 7 d, 14 d, 21 d, 28 d, 42 d and 56 d of the incubation, respectively, for measuring pH, conductivity, available P, total carbon (TC), NO − 3 -N and NH + 4 -N content.Artificial rainfall was given to the incubated soil at 4 d and 38 d by using distilled water of 200 mL.The leachate was used for measuring the P, NO − 3 -N and NH + 4 -N content.

Data analysis
ESP is a key parameter of saline soil evaluation.ESP=15 is the critical value of soil structural deterioration (So and Aylmore, 1993).
where Na + is the content of exchange sodium (cmol kg −1 ), and CEC is the cation exchange capacity (cmol kg −1 ).Excel 2010 and SPSS 13.0 were used for data statistical analysis.The significant differences among different groups used the one-way ANOVA.The significance level was 0.05.

pH
According to Fig. 1, furfural and biochar added into soil can lower the soil pH, and the more furfural and biochar added into soil, the lower soil pH.This is mainly caused by the far lower pH of furfural and biochar compared with the soil pH.
Compared with same dosage of biochar, furfural can lower the soil pH more significantly, which is mainly caused by its stronger acidity.More evenly, 2.5 % furfural lowered the soil pH more than 5 % biochar.During the incubation period, 5 % furfural lowered the soil pH by 0.5-0.8(soil pH: 8.3-8.6), while 5 % biochar only lowered the soil pH by 0.3-0.T2: soil added with 5 % furfural.The vertical lines are means (n = 3) ± standard error; different letters represent significant differences (Tukey post hoc test, p < 0.05).
of some difficult soluble elements, thus increasing the ionic concentration of soil solution (Yuan and Xu, 2011;Nelson, et al, 2011).

Variation of EC
Soluble salts in the soil are proportional to the electrical conductivity, so the variation of soluble salt can be shown by the               times that of soil.Chan et al. (2011) and Uzoma et al. ( 2012) also reported a significance increase of TOC in soil by adding biochar, which is important for soil quality improvement.

Variation of CEC and ESP
In Fig. 4, biochar (compared with furfural) failed to increase the soil CEC significantly as the incubation time went on, which may be related to the short incubation period (Liang et al., 2006).As the incubation time went on, surface groups of biochar were oxidized, which increased the surface charge density and thereby increased the CEC significantly (Cheng et al., 2008).Both furfural and biochar can increase the soil CEC (P < 0.05) and 5 % biochar increased the soil CEC by 15 %, indicating the involvement of furfural and biochar can increase the buffer performance of soil (Liang et al., 2006).This is because the large specific surface area of organic matter and negatively charged functional groups increased the exchange point of soil colloids, thus increasing the CEC (Lehmann, 2009).
In Fig. 5, ESP decreased more significantly when adding biochar compared with the furfural as the incubation time went on.But at the beginning of the experiment, ESP is increased.At the end of the test, ESP of T 2 and C 2 decreased to 51 % and 43 % of their initial ESP, respectively.On one hand, biochar has a high concentration of exchange Ca 2+ / Mg 2+ to replace Na + for soil colloidal absorption (Hu and Wang, 1987;Lashari et al., 2013), thus decreasing the exchange Na + in the soil.Table 1 represents that biochar contains 3 times higher exchange Ca 2+ than soil.On the other hand, biochar with loose and porous texture can increase the total porosity of soil (Lehmann and Joseph, 2009), thus losing more exchange Na + during rainfall and reducing the ESP.

Variation of AP
A significant increase of AP content in alkaline soil was observed by adding furfural and biochar (Fig. 6).On one hand, both furfural and biochar can lower the soil acidity due to their lower pH value, which is accompanied by a significant increase of AP (Devau et al., 2011).On the other hand, furfural and biochar have higher AP content.The AP content in furfural is about 40 times that in soil (Table 1).Therefore, the application of furfural can increase the AP content in soil directly.The AP content was increased by 2-5 times by adding 5 % biochar and 4-6 times by adding 5 % furfural.This indicated the better performance of furfural compared   C2: soil added with 5 % biochar; 13

Table Figure Captions
Table 1 The physical and chemical properties of materials in this study Table 2 Change of inorganic N and available P in leaching solution with biochar in AP growth.According to Table 1, during the carbonization of furfural, the AP content decreased although TP content increased, indicating the occurrence of phosphorus immobilization during the carbonization of furfural (Parvage et al., 2013).This corresponds to the significant increase of exchange Ca 2+ during the carbonization.Therefore, the significant increase of exchange Ca 2+ during the carbonization leads to the reduction of AP content (Tunesi et al., 1999).4 -N decreased to less than 2 mg kg −1 .This may be caused by the gradual decrease of organic nitrogen that is easy to be mineralized (Stanford and Epstein, 1974;Powers, 1990;Wennman and Kätterer, 2006), increased ammonia volatilization in soil due to the increased soil pH value (Dancer et al., 1973;Xu, 2012Xu, , 2013) ) and NH + 4 -N losses caused by leaching.Significant decrease of NO − 3 -N concentration was observed after two artificial rainfall events (Fig. 8).Particularly, NO − 3 -N concentration of five groups decreased by more than 95 %, indicating the easy leaching losses of NO − 3 N (Delgado, 2002).It can be seen from Figs. 7 and 8 that furfural and biochar did not increase the inorganic nitrogen in soil (P > 0.05).Singh (2010) also reported similar results which may be caused by lower inorganic nitrogen content in biochar.

Impact of furfural and biochar on soil leachate
It can be known from Table 2 that        crease of furfural and biochar dosages.During the second leaching process, no significant difference of NH + 4 -N concentration in all five treatments was observed.With the increase of leaching processes, the NH + 4 -N and NO − 3 -N concentrations in leachate from all treatments decreased significantly.The NO − 3 -N concentrations in leachate from C 1 decreased from 14.85 mg L −1 of the first leaching process to 0.6 mg L −1 .The involvement of furfural and biochar did not reduce the leaching losses of inorganic nitrogen.Soluble salts, such as NO − 3 -N and NH + 4 -N, will become dissolved and lost during rainfall or irrigation.
During the two leaching processes in this test, AP concentration in leachate from C 1 and C 2 increased, which goes on continuously with the enhanced biochar dosage.This may be caused by the poor AP retaining capacity of biochar, thus making AP easy to lose through leaching.However, the AP concentration in leachate from T 1 and T 2 decreased with the increase of furfural dosage, indicating its better retaining capacity of AP compared to biochar.

Conclusions
A short-term (56 d) incubation experiment was conducted to compare the amendments of furfural and its biochar on the properties of saline soil.The results showed that both furfural and biochar can improve the fertility of studied soil because the reduced pH, increased soil organic carbon (SOC) content and cation exchange capacity (CEC), and enhanced the available phosphorus (AP) will increase plant growth in this soil.Compared with biochar, furfural was more pronounced in decreasing soil pH and improving phosphorus availability; 5 % biochar can increase the AP content by 2-5 times, while 5 % furfural can increase the AP content by 4-6 times.This is possibly related to the lower pH value of furfural.In addition, biochar increases more SOC content and greatly decreases exchangeable sodium percentage (ESP) of the soil.
4.Lower soil pH is beneficial for the dissolution and activation

Fig. 6 Figure 6 .
Fig.6Effects of the added furfural and biochar on soil available P contents.CK; C1: soil added with 2.5% biochar; T1: soil added with 2.5% furfural; C2: soil added with 5% biochar; T2: soil added with 5% furfural.The vertical lines are means (n=3) ±standard error, different letter, for the same sampling date, represent significant differences (Tukey post hoc test, p <0.05) Figure6.Effects of the added furfural and biochar on soil available P contents.

Table 1 .
The physical and chemical properties of materials in this study.

Table Figure Captions 1 Table 1 The physical and chemical properties of materials in this study 2 Table 2 Change of inorganic N and available P in leaching solution 3 Fig. 1
Effects of the added furfural and biochar on soil pH.Effects of the added furfural and biochar on soil organic matter contents.

Table Figure Captions 1 Table 1 The physical and chemical properties of materials in this study 2 Table 2 Change of inorganic N and available P in leaching solution 3 Fig. 1
Effects of the added furfural and biochar on soil pH.

ons chemical properties of materials in this study ic N and available P in leaching solution
The vertical lines are means (n=3) ±standard error, different letters 18 furfural and biochar on soil organic matter contents.CK; C1: soil T1: soil added with 2.5% furfural; C2: soil added with 5% biochar; urfural.The vertical lines are means (n=3) ±standard error, different C2: soil added with 5 %

biochar; re Captions ysical and chemical properties of materials in this study e of inorganic N and available P in leaching solution
f the added furfural and biochar on soil organic matter contents.CK; C1: soil biochar; T1: soil added with 2.5% furfural; C2: soil added with 5% biochar; d with 5% furfural.The vertical lines are means (n=3) ±standard error, different t significant differences (Tukey post hoc test, p <0.05) f the added furfural and biochar on soil CEC.CK; C1: soil added with 2.5% soil added with 2.5% furfural; C2: soil added with 5% biochar; T2: soil furfural.The vertical lines are means (n=3) ±standard error, different letters ficant differences (Tukey post hoc test, p <0.05)

Table Figure Captions 1 Table 1 The physical and chemical properties of materials in this study 2 Table 2 Change of inorganic N and available P in leaching solution 3 Fig. 1
Effects of the added furfural and biochar on soil pH.Effects of the added furfural and biochar on soil EC.CK; C1: soil added with 2.5% 8 Effects of the added furfural and biochar on soil organic matter contents.CK; C1: soil 12 added with 2.5% biochar; T1: soil added with 2.5% furfural; C2: soil added with 5% biochar; Effects of the added furfural and biochar on soil CEC.CK; C1: soil added with 2.5% 16 Effects of the added furfural and biochar on soil exchange ESP.CK; C1: soil 20 Effects of the added furfural and biochar on soil available P contents.CK; C1: soil 24 Effects of the added furfural and biochar on soil NH 4 Effects of the added furfural and biochar on soil NO 3

Table Figure Captions 1 Table 1 The physical and chemical properties of materials in this study 2 Table 2 Change of inorganic N and available P in leaching solution 3 Fig. 1
Effects of the added furfural and biochar on soil pH.CK; C1: soil added with 2.5% 4 biochar; T 1 : soil added with 2.5% furfural; C2: soil added with 5% biochar; T 2 : soil 5 added with 5% furfural.The vertical lines are means (n=3) ±standard error, different letters 6 represent significant differences (Tukey post hoc test, p <0.05) 7 Effects of the added furfural and biochar on soil EC.CK; C1: soil added with 2.5% 8 biochar; T1: soil added with 2.5% furfural; C2: soil added with 5% biochar; T2: soil 9 added with 5% furfural.The vertical lines are means (n=3) ±standard error, different letters 10 represent significant differences (Tukey post hoc test, p <0.05) 11

Table 2 .
Change of inorganic N and available P in leaching solution.CK (control); C1 (soil added with 2.5 % biochar); T 1 (soil added with 2.5 % furfural); C2 (soil added with 5 % biochar); T 2 (soil added with 5 % furfural).This is possibly related to the porous structure of biochar and its higher exchange Ca 2+ content.In general, furfural and biochar exhibited a different effect depending on the property: furfural was more effective in decreasing pH and increasing available P, whereas biochar played a more important role in increasing SOC and reducing ESP of saline soil.