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Biochar Effect on Water Evaporation and Hydraulic Conductivity in Sandy Soil
Pedosphere 26(2): 265–272, 2016 doi:10.1016/S1002-0160(15)60041-8 ISSN 1002-0160/CN 32-1315/P c 2016 Soil Science Society of China ⃝ Published by Elsevier B.V. and Science Press
Biochar Effect on Water Evaporation and Hydraulic Conductivity in Sandy Soil ZHANG Jun, CHEN Qun and YOU Changfu Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084 (China) (Received January 12, 2015; revised November 13, 2015)
ABSTRACT Biochar, as a kind of soil amendment, has important effects on soil water retention. In this research, 4 different kinds of biochars were used to investigate their influences on hydraulic properties and water evaporation in a sandy soil from Hebei Province, China. Biochar had strong absorption ability in the sandy soil. The ratio of water content in the biochar to that in the sandy soil was less than the corresponding ratio of porosity. Because of the different hydraulic properties between the sandy soil and the biochar, the saturated hydraulic conductivity of the sandy soil gradually decreased with the increasing biochar addition. The biochar with larger pore volume and average pore diameter had better water retention. More water was retained in the sandy soil when the biochar was added in a single layer, but not when the biochar was uniformly mixed with soil. Particle size of the added biochar had a significant influence on the hydraulic properties of the mixture of sand and biochar. Grinding the biochar into powder destroyed the pore structure, which simultaneously reduced the water absorption ability and hydraulic conductivity of the biochar. For this reason, adding biochar powder to the sandy soil would not decrease the water evaporation loss of the soil itself. Key Words:
pore structure, pore volume, porosity, soil water retention, water holding capacity
Citation: Zhang J, Chen Q, You C F. 2016. Biochar effect on water evaporation and hydraulic conductivity in sandy soil. Pedosphere. 26(2): 265–272.
INTRODUCTION There are large areas in China where plants and crops are hard to grow due to the shortage of water. Improper agricultural activities and severe drought due to climate change result in further soil degradation, i.e., sandification and desertification, which permanently increases the evaporation water loss and decreases the soil water retention in these lands. According to the 4th round of national desertification and sandification monitoring carried out by the State Forestry Administration of China from 2005 to the end of 2009, the desertified land area of China was 2 623 700 km2 and the sandified land area 1 731 100 km2 (SFA PRC, 2011). The harsh situation of soil degradation imposes serious threats to nationwide ecological security. It is vital to make urgent efforts to mitigate or reverse the effects of desertification. One of the rigorous measures is the improvements of water retention in degraded soil and thus the soil fertility. For this purpose, sustainable agricultural practices could be beneficial. Traditionally, China is an agricultural country. Plentiful biomass resource is produced ∗ Corresponding
author. E-mail: [email protected].
from agricultural residues and livestock manure in rural area. Most of these biomass feedstocks are currently burned inefficiently (Zhang et al., 2007; Wu et al., 2008; Sun et al., 2014). This is not only a waste of energy, but also a disaster to the local environment. One way to sustainably exploit the biomass resource from agricultural sector is to pyrolyse these agricultural wastes and to put the products (i.e., biochar) in soils. As the solid product of biomass pyrolysis, biochar has been well considered as a soil amendment changing the soil water retention (Gaunt and Lehmann, 2008; Kinney et al., 2012; Novak and Watts, 2013). The basic idea in this work is to use biochar in degraded or sandy soils for improving water evaporation loss and soil fertility and preventing desertification. Biochar-type substances were discovered in Amazonian Dark Earths (Lehmann and Joseph, 2009). The biochar was added to soil as the result of forest fires, slash-and-burn agriculture and commercial production (Tryon, 1948; Wardle et al., 2008). In such areas, the soil had better water retention compared with that without biochar addition (Beck et al., 2011). Besides, the biochar has high level of resistance to be minera-
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lized to CO2 (German, 2003; Fowles, 2007; Steiner et al., 2007). Because of the stability of biochar, the cost to improve soil water retention by the addition of biochar could be relatively low (Lehmann, 2007a, b; Asai et al., 2009). As a result of these discoveries, more and more researchers have turned their attention to biochar. When biochar was added in the soil, there was a significant increase in water retention capacity of the soil (Glaser et al., 2002; Lehmann et al., 2003; Beck et al., 2011; Eastman, 2011; Karhu et al., 2011; Baronti et al., 2014; Tammeorg et al., 2014). However, there are many factors that will influence the effect of the biochar on soil water retention and seem to exist an optimum application rate of biochar because of water repellence (Piccolo et al., 1996). Soil types could also be a factor changing the effect of biochar on soil water retention. In some studies, biochar has a better water retention capacity in sandy soils than in other soil types (Tryon, 1948; Abel et al., 2013). Moreover, the effect of biochar on soil water retention also depends on its properties. Some characteristics of biochar have been studied in recent literatures (Novak et al., 2012). However, further work still need to fully understand the effect of biochar on water retention. In our previous research, biochars were proved to have strong water holding capacity (WHC) (Zhang and You, 2013). Moreover, the relationships between the WHC of the biochars and the surface area, total pore volume, and porosity structure were analyzed. The total pore volume played a more important role than the surface area or surface functional groups in WHC for the chars with a relatively large pore size. Although the addition of biochar to soil has been found to increase soil water retention, few studies have been made on the mechanism of biochar’s effect on soil water evaporation and on how to maximize the water retention of the biochar (Sohi et al., 2009). In general, biochar could increase the water content in soil when applied to soils through two ways. One way is that it can increase the soil’s water retention by retaining water in its pores by capillary force and reduce the mobility of the water (Tryon, 1948; Pietik¨ainen et al., 2000; Karhu et al., 2011). Because of the high porosity, biochar was thought to have much stronger water holding capacity than the soil (Pietik¨ainen et al., 2000; DeLuca et al., 2009). The other way was by changing the hydraulic properties of the soil (Asai et al., 2009; Karhu et al., 2011). The biochar added to the soil could improve soil aggregation by binding to other soil constituents (Piccolo et al., 1996; Herath et al., 2013; Soinne et al., 2014). Moreover, the different pore size distribution between the biochar and the soil will also change the
hydraulic conductivity of the soil. In this research, a further experiment was done to study the biochar’s effect on sandy soil water evaporation. Specifically, it was conducted to analyze the relationship between the characteristic of biochar and soil water retention capacity, elaborate the mechanism of biochar’s effect on soil water evaporation, and help to choose the right biochar and application method. It would play an important role in guiding the use of biochar as a kind of soil amendment in decreasing soil water evaporation. MATERIALS AND METHODS Biochar and sandy soil samples Four biochars were prepared from pyrolysis of pine (Pinus sylvestris var. mongolica Litv) and poplar (Populus davidiana) at 450 and 550 ◦ C, respetively. In this research, the poplar biochars were labeled as Y450 and Y550, while the pine biochars were labeled as S450 and S550. It has been thought that pore size distribution in biochar particles is dependent on both raw materials and pyrolysis temperatures. The particle size of the biochar was about 5–8 mm in diameter after pyrolysis. The critical characteristics and pore size distribution of the biochars were given in the former research (Zhang and You, 2013). The sandy soil sample used in this study was collected from a desert in Hebei Province, China (40◦ 30′ N, 115◦ 70′ E). The total annual precipitation in that region is about 370 mm. The soil is almost sandy, with the sand content of 995 g kg−1 and silt content of 5 g kg−1 , and is alkaline with a pH of 8.6. The average particle size of the sandy soil was 0.28 mm (Mastersizer 2000, Malvern Instruments Ltd., UK) and the porosity was 0.44. The bulk density was 1.50 g cm−3 , and the real density was 2.66 g cm−3 . Water holding capacity A water balance experiment was carried out to investigate the water transport between the sandy soil and the biochar particles when they were mixed together. Under certain water contents of the sandy soil and the biochar, the net water transfer between them will approach zero, i.e., the steady state is reached. The experiment was done in the device shown in Fig. 1. The mixure of biochar Y550 and sandy soil was put in the column on the right. The container on the left was used to provide a constant water pressure for the soil column. Under this condition, the water content of the soil in the column would keep at a steady state. This design offered the convenience that the water distribu-
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tion along the height of the column could give different water balance situations through one batch of tests. Each water test continued for 48 h to make sure that the water distribution in the soil column reached a steady condition. After the experiment, biochar and sandy soil were sampled every 3 cm. The water contents of biochar and sandy soil in each layer were measured respectively.
Fig. 2 Device for measurement of soil saturated hydraulic conductivity. H is the water head and L is the height of soil coulumn.
Fig. 1 Device for analyzing the water holding capacity of biochar and sandy soil.
Saturated hydraulic conductivity The saturated hydraulic conductivity (Ks ) of sandy soil was measured by the device shown in Fig. 2. Different amounts of Y550 biochar were added into the sandy soil column by different methods, i.e., by uniformly mixing or in one layer separately, in order to study the influence of the biochar on soil saturated hydraulic conductivity. The constant-head method was employed to determine Ks . The water height in the left container remained constant during the tests. This allowed water to move through the sandy soil column on the right under a steady state head condition. The quantity of water (Q) flowing through the soil column was measured over a period of time (t). With the height (L) and cross-sectional area (A) of the soil column, according to Darcy’s law (Swartzendruber, 2005), the saturated hydraulic conductivity in the sandy soil column could be calculated by Eq. 1: Ks =
QL AtH
(1)
where H is the water head, as shown in Fig. 2. Sandy soil water evaporation Plastic containers with a diameter of 11 cm and a height of 19 cm were used for sandy soil water evapo-
ration experiment. Before the experiment, the biochar samples and the sandy soil were oven-dried at 105 ◦ C for 24 h. The experimental conditions of 9 test groups are shown in Table I. According to our previous work (Zhang and You, 2013), the water holding capacity of the biochar was decided by its total pore volume and pore structure. Different biochars had very different apparent densities. Therefore, the volume basis was used in this research. For each group, 100 mL of the amendment (biochar samples or glass balls) were added to 2 000 g of sandy soil in different ways, i.e., uniformly mixing or in a single layer at different heights. In other words, the differences between each group were the added material and the mixing methods, as shown in Fig. 3. The volume of the added material was calculated from the mass divided by the apparent density of the biochar or glass balls. Data from both groups 7 and 8 were taken as reference. As both the porosity of the biochar and the volume occupied by the biochars TABLE I Experimental conditions of each group for studying sandy soil water evaporation with the addition of different biochars Group
Added materiala)
Mixing method
1 2 3 4 5 6 7 8 9
Y450 S450 Y550 S550 Y550 Y550 Glass balls None Y550 powder
Uniformly mixed Uniformly mixed Uniformly mixed Uniformly mixed In the bottom In the middle Uniformly mixed
a) Y450
Uniformly mixed
and Y550 are the biochars prepared from pyrolysis of poplar at 450 and 550 ◦ C, respectively, and S450 and S550 from pyrolysis of pine at 450 and 550 ◦ C, respectively.
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Fig. 3 Mixing methods of biochar and sand, i.e., uniformly mixed (a), in the bottom (b), and in the middle (c), to study sandy soil water evaporation.
would influence soil water evaporation, group 7 with glass balls was used to clarify the volumetric effect on soil water evaporation. Two tests were carried out to study the soil water evaporation. During the first test, 100 mL of water was added to the soil mixture of groups 1–8. During the second test, an additional group 9 was included to study the influence of biochar particle size on soil water evaporation. 200 mL of water was added to the soil mixture of each group. After the addition of the water, all the groups were placed in the laboratory environment (temperature = 25 ◦ C, relativity humidity = 20%) for 14 d, letting the water evaporate in the open. The mass of each group was weighed every 24 h to measure the water loss. Both tests were done with 3 replicates to make sure that the results are reliable. Statistical analysis One-way analysis of variance (ANOVA) was used for analyzing the significance of the differences among the experimental results obtained from the sandy soil saturated hydraulic conductivity tests and soil water evaporation tests. SPSS statistics for Windows 18.0.0 (SPSS, Inc., Chicago, USA) was used for these statistical tests. RESULTS AND DISCUSSION Water holding capacity and saturated hydraulic conductivity The water content in the biochar was ca. 30% higher than that in the soil, possibly because of the strong absorption ability of the biochar (Table II). This result was expected because biochar had a higher porosity (0.87) than the sandy soil sample (0.44). Biochar could retain water in the pores with capillary forces, which is one of the important mechanisms explaining why biochar could improve the water holding capacity of soil (Tryon, 1948; Pietik¨ainen et al., 2000; Karhu
et al., 2011). As shown in Table II, the water content ratios between the biochar and the sandy soil ranged from 1.25 to 1.52. The differences among these ratio values appeared insignificant when compared to the large variations in the water content of the sandy soils. Previous studies (Zhang and You, 2013; Gray et al., 2014) showed that the water holding capacity of the biochar had a significant positive correlation with its pore volume. It is thus reasonable to deduce that the water content ratios in soil layers may have a close relationship with the ratio of biochar porosity to that of the soil. However, the water content ratios were much lower than the porosity ratio between the biochar and the sandy soil (1.98). The reason was that the porosity measured by the mercury porosimeter included some micropores, which could not absorb water under this condition. TABLE II Water contents of the sandy soil and biochar and the water content ratio between biochar and sandy soil Soil layer cm 0–3 3–6 6–9 9–12 12–15
Water content in soil 0.17 0.21 0.28 0.32 0.36
± ± ± ± ±
Water content in biochar
cm3 cm−3 0.22 0.02c 0.32 0.01b 0.35 0.01b 0.42 0.01a 0.46 0.02a) db)
± ± ± ± ±
0.04c 0.02b 0.14ab 0.04a 0.12a
Water content ratio 1.29 1.52 1.25 1.31 1.28
± standard deviations (n = 3). followed by the same letter(s) within each column are not significantly different at P < 0.05 by least significant difference test.
a) Means
b) Means
The addition of biochar could also alter the water flow behavior in the sandy soil mixture through the change in the bulk hydraulic properties. As shown in Table III, the saturated hydraulic conductivity of the sandy soil gradually decreased with the increasing ratios of added biochar. Compared to those in the cases where the sandy soil was mixed with the biochar uni-
BIOCHAR EFFECT ON SANDY SOIL WATER
formly, Ks values from the cases where the biochar was added in one layer were significantly lower (P < 0.05, Table III). The reason was that the water could flow through the sandy soil between the biochar particles when they mixed with each other uniformly. When the biochar was in one layer, however, the water had to flow through the biochar which had much lower saturated hydraulic conductivity. This process leads to an accumulation of water in that layer and more water TABLE III Saturated hydraulic conductivities of the sandy soils mixed by different methods with biochar Y550 (dry weight, DW) prepared from pyrolysis of poplar at 550 ◦ C Biochar Mixing method addition ratio g kg−1 DW 0 7
15
25
Saturated hydraulic conductivity
cm min−1 0.814 ± 0.012a) ab) Uniformly mixed 0.814 ± 0.012a Uniformly mixed (powder) 0.639 ± 0.012b In the middle 0.695 ± 0.009b Uniformly mixed 0.716 ± 0.012b Uniformly mixed (powder) 0.604 ± 0.012c In the middle 0.492 ± 0.009c Uniformly mixed 0.709 ± 0.012b Uniformly mixed (powder) 0.512 ± 0.012d In the middle 0.429 ± 0.009d
± standard deviations (n = 3). followed by the same letter within each mixing method are not significantly different among different biochar addition ratios at P < 0.05 by least significant difference test. a) Means
b) Means
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to be absorbed by biochar. Soil water evaporation The results of the sandy soil water evaporation in the first test are shown in Fig. 4. It can be seen that the water loss curves clearly showed two stages of evaporation process for the soils, i.e., constant-rate stage and falling-rate stage (Aydin et al., 2005). The curves of water loss vs. time were almost linear with an evaporation rate of about 15% of initial soil water content per day in the first 5 d. Then the evaporation rate decreased a bit in the next 2 d. By the end of the 7th d, the quantity of water left in the soils was approximately 20%–30% relative to the quantity of water initially added. Although the total water losses for groups 1, 4 and 5 were slightly higher than groups 7 and 8, the differences among the water losses due to evaporation for all the groups were insignificant. This implies that the added biochars had insignificantly (P > 0.05) effects on the water evaporation in the sandy soil. As mentioned above, the mixture was consisted mostly of sandy soil with a small fraction of biochar (ca. 10 g kg−1 ). When limited quantity of water was poured into the mixture, most of water was held by the sandy soil due to the existence of the residual saturation (Brooks and Corey, 1964). No free water was available to move among particles. Under this circumstance, the biochar did not show significant effect because it could not ab-
Fig. 4 Cumulative water evaporation loss of each test group by adding 100 mL of water to the sandy soil mixed with different biochars. See Table I for the detailed descriptions of groups 1–8. Vertical bars indicate standard deviations of the means (n = 3).
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sorb much water from the sandy soil. The water loss curves from the second test of water evaporation are shown in Fig. 5. Similar to the results illustrated in Fig. 4, this test showed clearly two stages for the water evaporation from the sandy soils. During the first 7 d, the curves showed a constant-rate stage. The cumulative water loss curve for each group was almost in a straight line and the differences of the water loss among the different groups were very small (P > 0.05). Because the evaporation was largely determined by environmental conditions in this stage, added biochar made no differences. Starting from the 8th d, the evaporation process turned to the fallingrate stage. Differences in water loss among the group 6, groups 1, 3 and 5, and groups 2, 4, 7, 8 and 9 became increasingly significant (P < 0.05). The reason is that the evaporation in this stage was mainly determined by the hydraulic properties of the soil, which were af-
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fected by adding biochar, as mentioned above. Comparison between the results of groups 7 and 8 showed that the volume occupied by the adding material only had a minor influence on the water loss. Comparing the results of group 1 with group 3 (or group 2 with group 4), the difference in soil water evaporation was insignificant (P > 0.05). The reason is that the biochars produced from the same raw material had the similar pore size distribution and water absorption ability (Zhang and You, 2013). Effects of the pine biochar on soil water evaporation were much smaller than those of the poplar biochar. This result was consistent with the previous research (Zhang and You, 2013). Pore structure is the most important factor influencing the water absorption capacity. The pore volume and the average pore diameter of the pine biochar are very small, so the pine biochar’s water retention was almost the same as the sandy soil. However, the
Fig. 5 Cumulative water evaporation loss of each test group added with 200 mL of water to the sandy soil mixed with different biochars. See Table I for the detailed descriptions of groups 1–9. Vertical bars indicate standard errors of the means (n = 3).
BIOCHAR EFFECT ON SANDY SOIL WATER
poplar biochar’s water retention was much better than that of the sandy soil. Moreover, the mixing method also had an important influence on the soil water evaporation. As shown in Fig. 5, group 6 had the least water loss since day 12 (P < 0.05) compared to groups 3 and 5, which had the same adding material but different mixing methods. This is because the biochar layer in the middle slowed down the water flow and then prolonged the evaporation process. The water loss curves of groups 3 and 9 showed clearly that particle size also had a very important influence on soil water evaporation. For the same kind of biochar, grinding the biochar into powder significantly decreased its water retention. The water retention of group 9 since day 11 was even worse (P < 0.05) than that of group 8, in which there was no biochar. As the biochar was ground into powder, its pore structure, especially the macrospores, was destroyed. This led to the decrease in biochar porosity and thus the water absorption ability (Zhang and You, 2013). Moreover, fine biochar particles filled into the voids among the sandy soil particles and thus the pores of the sandy soil were clogged by the biochar, which significantly reduced the hydraulic conductivity of the soil (Table III). This significantly reduced water retention of the sandy soil. CONCLUSIONS The biochar had a strong ability in adsorbing water compared with the sandy soil. The ratio of water content in the biochar to that in the sandy soil remained almost the same under the same conditions. The saturated hydraulic conductivity of the sandy soil gradually decreased with increasing ratios of biochar addition using the same mixing method. When the biochar layer (in the middle) divided the sandy soil into two parts, the saturated hydraulic conductivity of sandy soil would significantly be reduced. Grinding biochar into powders would also significantly reduce the saturated hydraulic conductivity of sandy soil under the same addition ratio and mixing method. The reason was that the pore structure of the biochar, especially the macropores, was destroyed. The biochars took insignificantly (P > 0.05) effects on the evaporation of water when the adding water was 100 mL. When adding 200 mL water, this effect was also influenced by the biochar type and mixing methods. REFERENCES Abel S, Peters A, Trinks S, Schonsky H, Facklam M, Wessolek
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Biochar Effect on Water Evaporation and Hydraulic Conductivity in Sandy Soil ZHANG Jun, CHEN Qun and YOU Changfu Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084 (China) (Received January 12, 2015; revised November 13, 2015)
ABSTRACT Biochar, as a kind of soil amendment, has important effects on soil water retention. In this research, 4 different kinds of biochars were used to investigate their influences on hydraulic properties and water evaporation in a sandy soil from Hebei Province, China. Biochar had strong absorption ability in the sandy soil. The ratio of water content in the biochar to that in the sandy soil was less than the corresponding ratio of porosity. Because of the different hydraulic properties between the sandy soil and the biochar, the saturated hydraulic conductivity of the sandy soil gradually decreased with the increasing biochar addition. The biochar with larger pore volume and average pore diameter had better water retention. More water was retained in the sandy soil when the biochar was added in a single layer, but not when the biochar was uniformly mixed with soil. Particle size of the added biochar had a significant influence on the hydraulic properties of the mixture of sand and biochar. Grinding the biochar into powder destroyed the pore structure, which simultaneously reduced the water absorption ability and hydraulic conductivity of the biochar. For this reason, adding biochar powder to the sandy soil would not decrease the water evaporation loss of the soil itself. Key Words:
pore structure, pore volume, porosity, soil water retention, water holding capacity
Citation: Zhang J, Chen Q, You C F. 2016. Biochar effect on water evaporation and hydraulic conductivity in sandy soil. Pedosphere. 26(2): 265–272.
INTRODUCTION There are large areas in China where plants and crops are hard to grow due to the shortage of water. Improper agricultural activities and severe drought due to climate change result in further soil degradation, i.e., sandification and desertification, which permanently increases the evaporation water loss and decreases the soil water retention in these lands. According to the 4th round of national desertification and sandification monitoring carried out by the State Forestry Administration of China from 2005 to the end of 2009, the desertified land area of China was 2 623 700 km2 and the sandified land area 1 731 100 km2 (SFA PRC, 2011). The harsh situation of soil degradation imposes serious threats to nationwide ecological security. It is vital to make urgent efforts to mitigate or reverse the effects of desertification. One of the rigorous measures is the improvements of water retention in degraded soil and thus the soil fertility. For this purpose, sustainable agricultural practices could be beneficial. Traditionally, China is an agricultural country. Plentiful biomass resource is produced ∗ Corresponding
author. E-mail: [email protected].
from agricultural residues and livestock manure in rural area. Most of these biomass feedstocks are currently burned inefficiently (Zhang et al., 2007; Wu et al., 2008; Sun et al., 2014). This is not only a waste of energy, but also a disaster to the local environment. One way to sustainably exploit the biomass resource from agricultural sector is to pyrolyse these agricultural wastes and to put the products (i.e., biochar) in soils. As the solid product of biomass pyrolysis, biochar has been well considered as a soil amendment changing the soil water retention (Gaunt and Lehmann, 2008; Kinney et al., 2012; Novak and Watts, 2013). The basic idea in this work is to use biochar in degraded or sandy soils for improving water evaporation loss and soil fertility and preventing desertification. Biochar-type substances were discovered in Amazonian Dark Earths (Lehmann and Joseph, 2009). The biochar was added to soil as the result of forest fires, slash-and-burn agriculture and commercial production (Tryon, 1948; Wardle et al., 2008). In such areas, the soil had better water retention compared with that without biochar addition (Beck et al., 2011). Besides, the biochar has high level of resistance to be minera-
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lized to CO2 (German, 2003; Fowles, 2007; Steiner et al., 2007). Because of the stability of biochar, the cost to improve soil water retention by the addition of biochar could be relatively low (Lehmann, 2007a, b; Asai et al., 2009). As a result of these discoveries, more and more researchers have turned their attention to biochar. When biochar was added in the soil, there was a significant increase in water retention capacity of the soil (Glaser et al., 2002; Lehmann et al., 2003; Beck et al., 2011; Eastman, 2011; Karhu et al., 2011; Baronti et al., 2014; Tammeorg et al., 2014). However, there are many factors that will influence the effect of the biochar on soil water retention and seem to exist an optimum application rate of biochar because of water repellence (Piccolo et al., 1996). Soil types could also be a factor changing the effect of biochar on soil water retention. In some studies, biochar has a better water retention capacity in sandy soils than in other soil types (Tryon, 1948; Abel et al., 2013). Moreover, the effect of biochar on soil water retention also depends on its properties. Some characteristics of biochar have been studied in recent literatures (Novak et al., 2012). However, further work still need to fully understand the effect of biochar on water retention. In our previous research, biochars were proved to have strong water holding capacity (WHC) (Zhang and You, 2013). Moreover, the relationships between the WHC of the biochars and the surface area, total pore volume, and porosity structure were analyzed. The total pore volume played a more important role than the surface area or surface functional groups in WHC for the chars with a relatively large pore size. Although the addition of biochar to soil has been found to increase soil water retention, few studies have been made on the mechanism of biochar’s effect on soil water evaporation and on how to maximize the water retention of the biochar (Sohi et al., 2009). In general, biochar could increase the water content in soil when applied to soils through two ways. One way is that it can increase the soil’s water retention by retaining water in its pores by capillary force and reduce the mobility of the water (Tryon, 1948; Pietik¨ainen et al., 2000; Karhu et al., 2011). Because of the high porosity, biochar was thought to have much stronger water holding capacity than the soil (Pietik¨ainen et al., 2000; DeLuca et al., 2009). The other way was by changing the hydraulic properties of the soil (Asai et al., 2009; Karhu et al., 2011). The biochar added to the soil could improve soil aggregation by binding to other soil constituents (Piccolo et al., 1996; Herath et al., 2013; Soinne et al., 2014). Moreover, the different pore size distribution between the biochar and the soil will also change the
hydraulic conductivity of the soil. In this research, a further experiment was done to study the biochar’s effect on sandy soil water evaporation. Specifically, it was conducted to analyze the relationship between the characteristic of biochar and soil water retention capacity, elaborate the mechanism of biochar’s effect on soil water evaporation, and help to choose the right biochar and application method. It would play an important role in guiding the use of biochar as a kind of soil amendment in decreasing soil water evaporation. MATERIALS AND METHODS Biochar and sandy soil samples Four biochars were prepared from pyrolysis of pine (Pinus sylvestris var. mongolica Litv) and poplar (Populus davidiana) at 450 and 550 ◦ C, respetively. In this research, the poplar biochars were labeled as Y450 and Y550, while the pine biochars were labeled as S450 and S550. It has been thought that pore size distribution in biochar particles is dependent on both raw materials and pyrolysis temperatures. The particle size of the biochar was about 5–8 mm in diameter after pyrolysis. The critical characteristics and pore size distribution of the biochars were given in the former research (Zhang and You, 2013). The sandy soil sample used in this study was collected from a desert in Hebei Province, China (40◦ 30′ N, 115◦ 70′ E). The total annual precipitation in that region is about 370 mm. The soil is almost sandy, with the sand content of 995 g kg−1 and silt content of 5 g kg−1 , and is alkaline with a pH of 8.6. The average particle size of the sandy soil was 0.28 mm (Mastersizer 2000, Malvern Instruments Ltd., UK) and the porosity was 0.44. The bulk density was 1.50 g cm−3 , and the real density was 2.66 g cm−3 . Water holding capacity A water balance experiment was carried out to investigate the water transport between the sandy soil and the biochar particles when they were mixed together. Under certain water contents of the sandy soil and the biochar, the net water transfer between them will approach zero, i.e., the steady state is reached. The experiment was done in the device shown in Fig. 1. The mixure of biochar Y550 and sandy soil was put in the column on the right. The container on the left was used to provide a constant water pressure for the soil column. Under this condition, the water content of the soil in the column would keep at a steady state. This design offered the convenience that the water distribu-
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tion along the height of the column could give different water balance situations through one batch of tests. Each water test continued for 48 h to make sure that the water distribution in the soil column reached a steady condition. After the experiment, biochar and sandy soil were sampled every 3 cm. The water contents of biochar and sandy soil in each layer were measured respectively.
Fig. 2 Device for measurement of soil saturated hydraulic conductivity. H is the water head and L is the height of soil coulumn.
Fig. 1 Device for analyzing the water holding capacity of biochar and sandy soil.
Saturated hydraulic conductivity The saturated hydraulic conductivity (Ks ) of sandy soil was measured by the device shown in Fig. 2. Different amounts of Y550 biochar were added into the sandy soil column by different methods, i.e., by uniformly mixing or in one layer separately, in order to study the influence of the biochar on soil saturated hydraulic conductivity. The constant-head method was employed to determine Ks . The water height in the left container remained constant during the tests. This allowed water to move through the sandy soil column on the right under a steady state head condition. The quantity of water (Q) flowing through the soil column was measured over a period of time (t). With the height (L) and cross-sectional area (A) of the soil column, according to Darcy’s law (Swartzendruber, 2005), the saturated hydraulic conductivity in the sandy soil column could be calculated by Eq. 1: Ks =
QL AtH
(1)
where H is the water head, as shown in Fig. 2. Sandy soil water evaporation Plastic containers with a diameter of 11 cm and a height of 19 cm were used for sandy soil water evapo-
ration experiment. Before the experiment, the biochar samples and the sandy soil were oven-dried at 105 ◦ C for 24 h. The experimental conditions of 9 test groups are shown in Table I. According to our previous work (Zhang and You, 2013), the water holding capacity of the biochar was decided by its total pore volume and pore structure. Different biochars had very different apparent densities. Therefore, the volume basis was used in this research. For each group, 100 mL of the amendment (biochar samples or glass balls) were added to 2 000 g of sandy soil in different ways, i.e., uniformly mixing or in a single layer at different heights. In other words, the differences between each group were the added material and the mixing methods, as shown in Fig. 3. The volume of the added material was calculated from the mass divided by the apparent density of the biochar or glass balls. Data from both groups 7 and 8 were taken as reference. As both the porosity of the biochar and the volume occupied by the biochars TABLE I Experimental conditions of each group for studying sandy soil water evaporation with the addition of different biochars Group
Added materiala)
Mixing method
1 2 3 4 5 6 7 8 9
Y450 S450 Y550 S550 Y550 Y550 Glass balls None Y550 powder
Uniformly mixed Uniformly mixed Uniformly mixed Uniformly mixed In the bottom In the middle Uniformly mixed
a) Y450
Uniformly mixed
and Y550 are the biochars prepared from pyrolysis of poplar at 450 and 550 ◦ C, respectively, and S450 and S550 from pyrolysis of pine at 450 and 550 ◦ C, respectively.
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Fig. 3 Mixing methods of biochar and sand, i.e., uniformly mixed (a), in the bottom (b), and in the middle (c), to study sandy soil water evaporation.
would influence soil water evaporation, group 7 with glass balls was used to clarify the volumetric effect on soil water evaporation. Two tests were carried out to study the soil water evaporation. During the first test, 100 mL of water was added to the soil mixture of groups 1–8. During the second test, an additional group 9 was included to study the influence of biochar particle size on soil water evaporation. 200 mL of water was added to the soil mixture of each group. After the addition of the water, all the groups were placed in the laboratory environment (temperature = 25 ◦ C, relativity humidity = 20%) for 14 d, letting the water evaporate in the open. The mass of each group was weighed every 24 h to measure the water loss. Both tests were done with 3 replicates to make sure that the results are reliable. Statistical analysis One-way analysis of variance (ANOVA) was used for analyzing the significance of the differences among the experimental results obtained from the sandy soil saturated hydraulic conductivity tests and soil water evaporation tests. SPSS statistics for Windows 18.0.0 (SPSS, Inc., Chicago, USA) was used for these statistical tests. RESULTS AND DISCUSSION Water holding capacity and saturated hydraulic conductivity The water content in the biochar was ca. 30% higher than that in the soil, possibly because of the strong absorption ability of the biochar (Table II). This result was expected because biochar had a higher porosity (0.87) than the sandy soil sample (0.44). Biochar could retain water in the pores with capillary forces, which is one of the important mechanisms explaining why biochar could improve the water holding capacity of soil (Tryon, 1948; Pietik¨ainen et al., 2000; Karhu
et al., 2011). As shown in Table II, the water content ratios between the biochar and the sandy soil ranged from 1.25 to 1.52. The differences among these ratio values appeared insignificant when compared to the large variations in the water content of the sandy soils. Previous studies (Zhang and You, 2013; Gray et al., 2014) showed that the water holding capacity of the biochar had a significant positive correlation with its pore volume. It is thus reasonable to deduce that the water content ratios in soil layers may have a close relationship with the ratio of biochar porosity to that of the soil. However, the water content ratios were much lower than the porosity ratio between the biochar and the sandy soil (1.98). The reason was that the porosity measured by the mercury porosimeter included some micropores, which could not absorb water under this condition. TABLE II Water contents of the sandy soil and biochar and the water content ratio between biochar and sandy soil Soil layer cm 0–3 3–6 6–9 9–12 12–15
Water content in soil 0.17 0.21 0.28 0.32 0.36
± ± ± ± ±
Water content in biochar
cm3 cm−3 0.22 0.02c 0.32 0.01b 0.35 0.01b 0.42 0.01a 0.46 0.02a) db)
± ± ± ± ±
0.04c 0.02b 0.14ab 0.04a 0.12a
Water content ratio 1.29 1.52 1.25 1.31 1.28
± standard deviations (n = 3). followed by the same letter(s) within each column are not significantly different at P < 0.05 by least significant difference test.
a) Means
b) Means
The addition of biochar could also alter the water flow behavior in the sandy soil mixture through the change in the bulk hydraulic properties. As shown in Table III, the saturated hydraulic conductivity of the sandy soil gradually decreased with the increasing ratios of added biochar. Compared to those in the cases where the sandy soil was mixed with the biochar uni-
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formly, Ks values from the cases where the biochar was added in one layer were significantly lower (P < 0.05, Table III). The reason was that the water could flow through the sandy soil between the biochar particles when they mixed with each other uniformly. When the biochar was in one layer, however, the water had to flow through the biochar which had much lower saturated hydraulic conductivity. This process leads to an accumulation of water in that layer and more water TABLE III Saturated hydraulic conductivities of the sandy soils mixed by different methods with biochar Y550 (dry weight, DW) prepared from pyrolysis of poplar at 550 ◦ C Biochar Mixing method addition ratio g kg−1 DW 0 7
15
25
Saturated hydraulic conductivity
cm min−1 0.814 ± 0.012a) ab) Uniformly mixed 0.814 ± 0.012a Uniformly mixed (powder) 0.639 ± 0.012b In the middle 0.695 ± 0.009b Uniformly mixed 0.716 ± 0.012b Uniformly mixed (powder) 0.604 ± 0.012c In the middle 0.492 ± 0.009c Uniformly mixed 0.709 ± 0.012b Uniformly mixed (powder) 0.512 ± 0.012d In the middle 0.429 ± 0.009d
± standard deviations (n = 3). followed by the same letter within each mixing method are not significantly different among different biochar addition ratios at P < 0.05 by least significant difference test. a) Means
b) Means
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to be absorbed by biochar. Soil water evaporation The results of the sandy soil water evaporation in the first test are shown in Fig. 4. It can be seen that the water loss curves clearly showed two stages of evaporation process for the soils, i.e., constant-rate stage and falling-rate stage (Aydin et al., 2005). The curves of water loss vs. time were almost linear with an evaporation rate of about 15% of initial soil water content per day in the first 5 d. Then the evaporation rate decreased a bit in the next 2 d. By the end of the 7th d, the quantity of water left in the soils was approximately 20%–30% relative to the quantity of water initially added. Although the total water losses for groups 1, 4 and 5 were slightly higher than groups 7 and 8, the differences among the water losses due to evaporation for all the groups were insignificant. This implies that the added biochars had insignificantly (P > 0.05) effects on the water evaporation in the sandy soil. As mentioned above, the mixture was consisted mostly of sandy soil with a small fraction of biochar (ca. 10 g kg−1 ). When limited quantity of water was poured into the mixture, most of water was held by the sandy soil due to the existence of the residual saturation (Brooks and Corey, 1964). No free water was available to move among particles. Under this circumstance, the biochar did not show significant effect because it could not ab-
Fig. 4 Cumulative water evaporation loss of each test group by adding 100 mL of water to the sandy soil mixed with different biochars. See Table I for the detailed descriptions of groups 1–8. Vertical bars indicate standard deviations of the means (n = 3).
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sorb much water from the sandy soil. The water loss curves from the second test of water evaporation are shown in Fig. 5. Similar to the results illustrated in Fig. 4, this test showed clearly two stages for the water evaporation from the sandy soils. During the first 7 d, the curves showed a constant-rate stage. The cumulative water loss curve for each group was almost in a straight line and the differences of the water loss among the different groups were very small (P > 0.05). Because the evaporation was largely determined by environmental conditions in this stage, added biochar made no differences. Starting from the 8th d, the evaporation process turned to the fallingrate stage. Differences in water loss among the group 6, groups 1, 3 and 5, and groups 2, 4, 7, 8 and 9 became increasingly significant (P < 0.05). The reason is that the evaporation in this stage was mainly determined by the hydraulic properties of the soil, which were af-
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fected by adding biochar, as mentioned above. Comparison between the results of groups 7 and 8 showed that the volume occupied by the adding material only had a minor influence on the water loss. Comparing the results of group 1 with group 3 (or group 2 with group 4), the difference in soil water evaporation was insignificant (P > 0.05). The reason is that the biochars produced from the same raw material had the similar pore size distribution and water absorption ability (Zhang and You, 2013). Effects of the pine biochar on soil water evaporation were much smaller than those of the poplar biochar. This result was consistent with the previous research (Zhang and You, 2013). Pore structure is the most important factor influencing the water absorption capacity. The pore volume and the average pore diameter of the pine biochar are very small, so the pine biochar’s water retention was almost the same as the sandy soil. However, the
Fig. 5 Cumulative water evaporation loss of each test group added with 200 mL of water to the sandy soil mixed with different biochars. See Table I for the detailed descriptions of groups 1–9. Vertical bars indicate standard errors of the means (n = 3).
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poplar biochar’s water retention was much better than that of the sandy soil. Moreover, the mixing method also had an important influence on the soil water evaporation. As shown in Fig. 5, group 6 had the least water loss since day 12 (P < 0.05) compared to groups 3 and 5, which had the same adding material but different mixing methods. This is because the biochar layer in the middle slowed down the water flow and then prolonged the evaporation process. The water loss curves of groups 3 and 9 showed clearly that particle size also had a very important influence on soil water evaporation. For the same kind of biochar, grinding the biochar into powder significantly decreased its water retention. The water retention of group 9 since day 11 was even worse (P < 0.05) than that of group 8, in which there was no biochar. As the biochar was ground into powder, its pore structure, especially the macrospores, was destroyed. This led to the decrease in biochar porosity and thus the water absorption ability (Zhang and You, 2013). Moreover, fine biochar particles filled into the voids among the sandy soil particles and thus the pores of the sandy soil were clogged by the biochar, which significantly reduced the hydraulic conductivity of the soil (Table III). This significantly reduced water retention of the sandy soil. CONCLUSIONS The biochar had a strong ability in adsorbing water compared with the sandy soil. The ratio of water content in the biochar to that in the sandy soil remained almost the same under the same conditions. The saturated hydraulic conductivity of the sandy soil gradually decreased with increasing ratios of biochar addition using the same mixing method. When the biochar layer (in the middle) divided the sandy soil into two parts, the saturated hydraulic conductivity of sandy soil would significantly be reduced. Grinding biochar into powders would also significantly reduce the saturated hydraulic conductivity of sandy soil under the same addition ratio and mixing method. The reason was that the pore structure of the biochar, especially the macropores, was destroyed. The biochars took insignificantly (P > 0.05) effects on the evaporation of water when the adding water was 100 mL. When adding 200 mL water, this effect was also influenced by the biochar type and mixing methods. REFERENCES Abel S, Peters A, Trinks S, Schonsky H, Facklam M, Wessolek
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