Excessive application of fertilizer
has caused the release of nitrogen (N) from agricultural fields to aquatic
systems. Leaching of nutrients from soils may deplete soil fertility,
accelerate soil acidification, and most importantly impose a threat to
environmental health (Ozacar 2003; Laird et
al. 2010). It is therefore very important to develop effective technologies
to hold N in soils.
Biochar is a carbonaceous material
produced during the pyrolysis of biomass (Hussain et al. 2017). An option to reduce N leaching could be the
application of biochar to soils (El-Naggar et
al. 2019). The adsorption of inorganic N on biochars has been paid much
attention. Biochar displayed strong adsorption ability of NH4+
in several studies (Dempster et al.
2012; Yao et al. 2012). Placing
biochar into soil has also been shown to inhibit the leaching of NH4+
under fertilization condition (Ding et
al. 2010; Dempster et al. 2012).
However, some studies showed that biochar could reduce NO3-
leaching (Dempster et al. 2012; Yao et al. 2012), while increased leaching
of NO3- with biochar has also been reported somewhere
(Laird et al. 2010; Kameyama et al. 2012). Moreover, some literatures indicated that significant biochar
NO3- adsorption only occurred at relatively higher
pyrolysis temperatures (>600oC) (Dempster et al. 2012; Kameyama et al.
2012). The statements for biochar NH4+ adsorption were consistent, but contradictory for NO3-
adsorption.
As a matter of fact, biochar
surface properties are likely to be gradually altered during environmental
exposure. In soils, due to its strong affinity for organic matter and inorganic
mineral ions, biochar is likely to undergo
a series of biogeochemical reactions and physical processes that will result in
the alteration of its properties with time (aging) (Cheng et al. 2008; Farrell et al.
2013; Ren et al. 2016; Sarma et al. 2018). This implies that these
changes have potential effects for altering biochar adsorption capacity of
inorganic N and the physicochemical properties of biochar amended soils.
However, most of the existing laboratory and field studies focused on freshly
produced biochars, and there is a little information on the study of aged
biochar and its impact on inorganic N adsorption and leaching from calcareous
clay soil.
On the other hand, in most of the studies,
artificial aging methods were used to examine the effects of simulation aging
processes on biochar properties (Liu et
al. 2013; Qian and Chen 2014). However, biochar aging occurred under
natural ambient conditions is ordinary and dramatic during biochar production,
storage, transportation to farmland, and final agricultural application because
of the frequent rainfall, illumination, and aeration. Therefore, the spontaneous aging
process was carried out in our study to prepare the corresponding aged
biochars. The objective of the study was to explore the effects of fresh and
aged maize straw-derived biochars on NH4+ and NO3-
adsorption in aqueous solution and leaching from calcareous clay soil.
Materials and Methods
Soil
The soil was collected from the
surface layer (0–20 cm) of a farmland in the suburb of Jinzhong, Shanxi
Province, China, locating at the southeastern margin of Loess Plateau. The
detailed information about the soil was showed in our previous paper (Wang et al. 2018).
Biochar
Fresh
biochar production: Maize (Zea mays L.) straw was collected from
the same area as the soil in harvesting season. The straw was air-dried at room
temperature and ground to pass a 10-mesh sieve. The ground straw was then
placed in a quartz boat of the tube furnace (SK-G10123K, Tianjin Zhonghuan Lab
Furnace Co., Ltd., Tianjin, China). The pyrolysis temperature was raised to 400oC
(approximately 5oC/min) and 600oC (approximately 10oC/min)
respectively with a N2 flow rate of 150 ml/min. The final
temperature was maintained for 2 h, and then the biochars were allowed to cool
to room temperature and ground to pass through a 2-mm sieve, which were hereafter referred to as the fresh
biochars (F400 and F600).
Biochar aging incubation: The fresh biochars (F400 and
F600) were incubated in open vessels at room temperature (25 ± 2oC)
in the dark and at 77.7% water content by weight. The water evaporation from
the open vessels was compensated by weight using deionized water every day.
After 50 d incubation, the biochar samples were air-dried and mixed thoroughly
to storage, which were defined as the
aged biochars and referred to as A400 and A600,
respectively.
Biochar characteristic
determination: To explore the effects of aging
incubation on biochar properties, the fresh and aged biochar characteristics
were determined. The pH was determined by shaking the glass bottle contained
1.0 g of biochar with 15 mL deionized water fiercely by hand for 1 min, and
then after 30-min settlement, the supernatant was measured with an electrode
(Mettler Toledo Delta 320). The amount of acidic/alkaline oxygen-containing
functional groups of biochar was determined by Boehm titration method (Boehm
1994). The specific surface area, total pore volume, and mean pore size of the
biochar were evaluated using the Sorption Analyzer (Quadrasorb SI, America
Quantachrome) N2 adsorption Brunauer-Emmet-Teller (BET) technique.
Biochar adsorption
capacity of ammonium and nitrate in aqueous solution
Batch adsorption experiment was conducted in 250 mL conical flask with stopper. About 1.0 g of fresh or aged
maize straw-derived biochar was added into the vessels and mixed with 100 mL
100 mg N/l NH4+ or NO3- solution.
The vessels without either biochar or nutrient elements were also included as
experimental controls. The mixtures were shaken at 170 rpm in a mechanical
shaker for 240 min at a constant temperature of 25 ± 0.5oC. The
inorganic N concentrations of the suspension at the time zero were presumed as
100 mg/L. Subsequently, at the time points 1, 5, 20, 40, 60, 90, 150, and 240
min after incubation the homogenous suspensions were collected from the conical
flasks. And then the collected samples were filtered through 0.45 µm cellulose
acetate membrane filters. The concentrations of NH4+-N
and NO3--N in the supernatants were measured using the colormetric method (APHA et al. 2012), using a dual beam UV/VIS spectrophotometer (UV-5100B,
Shanghai Metash Instruments Co., Ltd., Shanghai, China). Inorganic N adsorbed
by biochar was calculated based on the initial and final aqueous
concentrations. All the experiments were carried out in triplicate.
Ammonium
and nitrate leaching from soil column amended with biochar
All the fresh and aged maize straw-derived biochars (F400, F600, A400, and
A600) were selected to study their effects on inorganic N retention and
transportation in the calcareous clay soil. Soil columns were made of
plexiglass cylinders measuring 30 cm in height and 10 cm in diameter. A screw
cap with a hole in the middle allowing water drainage was fitted to the bottom
end of the column. Then, the screw cap was
covered with some quartz sand with the thickness of 3 cm (soaked in 2.0 mol/l H2SO4
overnight and washed with deionized water), which was enclosed by nylon net (70
µm pore size) to prevent quartz sand loss. The calcareous clay soil with (F400,
F600, A400, and A600; 2% mass fractions
by dry weight) or without biochars was packed into the column with the depth of
40 cm (0–15 cm: homogeneous soil-biochar mixture; 15–40 cm: original soil
sample) and the bulk density of 1.20 g cm-3 (equal to the field soil
after ploughed). An appropriate filter-paper was placed on the surface of the
soil in each column to homodisperse the
source water and to retard water evaporation. Altogether, the experiment
consisted of five treatments with three replicates (n = 15).
Initially, the columns were flushed with deionized
water to reach a saturated state to precondition the soil-biochar mixture,
which lasted for a week. We compensated
water loss due to evaporation by adding the appropriate amount of deionized
water through weighing every day. Then a
nutrient solution containing 0.8999 g NH4Cl (equal to 300 kg N/ha)
was sprayed onto the surface of the soil.
Thereafter, the columns were leached with 300 mL deionized water by dripping,
which lasted for 3 h. The leachate from
each column was collected in 400 mL polyethylene bottles for about 6 h after
the start of leaching. The bottles had a cap with a small hole drilled through
it that allowed the drain tube to be inserted into the bottle so that
evaporative water loss was minimized. We called this process a leaching event.
The leaching events were repeated for 16 times with an interval of 3 days in
the first 30 days and 2 days in the later 10 days. Between adjacent leaching
events when the soil was idle, the loss of water through evaporation was
compensated by adding deionized water through the weighing method. The leaching
experiment was carried out at room temperature (25 ± 2oC).
All the leachate samples were immediately filtered
through 0.45 µm cellulose acetate membrane filters for further analyses. The
volume of the leachate collected from
each column was determined gravimetrically (assuming the density is 1.0 g/mL).
The NH4+-N and NO3--N
concentrations in leachate samples were measured using the same method as
described above.
Column layered soil sample analyses
Immediately after the 16 leaching events, the topsoil (0–2 cm) and the
layered soil samples of 3–7, 8–12, 18–22, 23–27, 28–32, and 38–42 cm in the
column, which were defined as 0, 5, 10, 20, 25, 30, and 40 cm depth soil
samples, were collected with a spade. The potential ammonia-oxidation rate of
the layered soil samples was measured using a modified chlorate inhibition
method (Kurola et al. 2005). The
dilution-plate method was used to quantify the amount of ammonia-oxidizing
bacteria (AOB) in the layered soil sample
(Lin 2010; Wang et al. 2018).
Data analysis
The variance between any triplicate measurements
in this study was smaller than 5%, and the average value ± standard deviation
was reported. The graphing was performed using OriginPro 8.0 software.
Results
Physicochemical properties of maize
straw-derived biochars before and after aging
All the fresh and aged maize straw-derived biochars prepared at 400 and
600oC (designated as F400, F600, A400,
and A600, respectively) were alkaline
(10.3–11.7). The pH of the biochars prepared at 600oC (11.7 and 10.5
for the fresh and aged biochar, respectively) was much higher than that
prepared at 400oC (10.6 and 10.3). After 50 d aging incubation, the
aged biochars showed decreased pH (400oC: 0.3 units; 600oC:
1.2 units) and mean pore size (400oC: 75.1% of the fresh biochar;
600oC: 29.6% of the fresh biochar), but
increased carboxyl amount (400oC: 0.031 mmol/g; 600oC:
0.102 mmol/g) and specific surface area (400oC: 2.42 times of the
fresh biochar; 600oC: 15.3 times of the fresh biochar) (Table 1).
Ammonium and nitrate adsorption
kinetics of fresh and aged maize straw-derived biochars
Ammonium: NH4+-N adsorption amount of the four kinds of
biochars increased sharply in the first 5 min, and then increased slowly until
to 40 min, then reached to be stable. The biochar prepared at 400oC
showed relatively stronger NH4+ adsorption capacity than
that of the biochar prepared at 600oC either for the fresh or aged
biochars. Moreover, the aged biochar showed relatively stronger NH4+
adsorption capacity than fresh ones either for the 400 or 600oC
produced biochars. Therefore, the order of the NH4+
adsorption capacity of the biochars was as follows A400 > A600/F400 >
F600 (existing some counterchanges between F400 and A600). The result may be
attributed to the more carboxyl of biochar prepared at 400oC than
600oC (fresh biochars: 0.182 ± 0.068 vs. 0.086 ± 0.009 mmol/g; aged biochars:
0.213 ± 0.152 vs. 0.188 ± 0.018
mmol/g) and biochar aging process increased carboxyl amount (400oC:
0.182 ± 0.068 to 0.213 ± 0.152 mmol/g; 600oC: 0.086 ± 0.009 to 0.188
± 0.018 mmol/g) and specific surface area (400oC: 2.42 to 5.85 m2/g;
600oC: 1.33 to 20.4 m2/g). Particularly, A400 showed the
maximum balance adsorption amount of NH4+-N (4.20 mg NH4+-N/g
biochar) (Fig. 1).
Nitrate: Being different from NH4+, NO3--N
adsorption amount of the four kinds of biochars increased sharply in the first
5 min, and then increased slowly until to 20 min, then decreased slowly with
fluctuations and reached to be stable. However, the order of the NO3-
adsorption capacity of the biochars was the same as NH4+,
A400 > A600/F400 > F600 (existing some counterchanges between F400 and
A600). A400 also showed the maximum balance adsorption amount of NO3--N
(0.56 mg NO3--N/g biochar) (Fig. 2). Nonetheless, though
NO3- adsorption capacity of the four kinds of biochars
showed the similar trend as NH4+, the variations were
significantly large, indicating the instability of biochar NO3-
adsorption process. Moreover, the adsorption amount of NH4+-N
of the four kinds of biochars was 7.50–10.6 times more than that of NO3--N.
Table 1: The physicochemical property of
the fresh and aged maize straw-derived biochars
|
|
F400 |
F600 |
A400 |
A600 |
|
pH (H2O)* |
10.6 ± 0.04 |
11.7 ± 0.06 |
10.3 ± 0.04 |
10.5 ± 0.06 |
|
Carboxyl (mmol/g) |
0.182 ± 0.068 |
0.086 ± 0.009 |
0.213 ± 0.152 |
0.188 ± 0.018 |
|
Carbonyl (mmol/g) |
0.222 ± 0.108 |
0.239 ± 0.004 |
0.207 ± 0.092 |
0.206 ± 0.009 |
|
Phenolic hydroxyl
(mmol/g) |
0.538 ± 0.138 |
0.625 ± 0.005 |
0.517 ± 0.085 |
0.342 ± 0.006 |
|
Total acidic
oxygen-containing functional group (mmol/g) |
0.942 ± 0.038 |
0.950 ± 0.018 |
0.937 ± 0.048 |
0.736 ± 0.015 |
|
Total alkaline
oxygen-containing functional group (mmol/g) |
1.065 ± 0.020 |
1.216 ± 0.004 |
0.935 ± 0.025 |
0.945 ± 0.011 |
|
Specific surface area (m2/g) |
2.42 |
1.33 |
5.85 |
20.4 |
|
Total pore volume (cm3/g) |
0.0107 |
0.0064 |
0.0194 |
0.029 |
|
Mean pore size (nm) |
17.7 |
19.1 |
13.3 |
5.66 |
*: Biochar: water = 1
g: 15 mL; F400/F600: fresh biochar prepared at 400/600℃; A400/A600: aged
biochar prepared at 400/600℃
Effects
of fresh and aged maize straw-derived biochars on ammonium and nitrate leaching
from soil column
Ammonium: During the 16 leaching events, the
mass of leached NH4+-N peaked at the fourth, seventh,
sixth, ninth, and ninth leaching for the control, F400, F600, A400, and A600
treatment, respectively. All the biochars postponed NH4+
leaching peak occurring compared with control. Particularly, A400 and A600
showed much stronger capacity than F400 and F600 to delay the leaching peak
occurring. Compared with control, the maximum of leached NH4+-N
was reduced by 19.4, 17.1, 28.1 and 26.0% for F400, F600, A400, and A600
treatments, respectively; the cumulative mass of leached NH4+-N
was reduced by 20.9, 26.2, 24.3 and 24.5% for F400, F600, A400, and A600
treatments, respectively. Remarkably, maize straw-derived biochar incorporation
inhibited NH4+ leaching from calcareous clay soil and
postponed the leaching peak occurring, especially for the aged biochars (Fig.
3).
Nitrate: Like NH4+, both the fresh and aged biochars
inhibited NO3- leaching during the column leaching
experiment. However, being different with NH4+, NO3--N
leaching peak of A400 and A600 treatments occurred at the same leaching event
as control (the sixth), while NO3--N leaching peak of
F400 and F600 treatments was even advanced (the fifth). Compared with control,
the maximum of leached NO3--N
was reduced by 22.0, 20.5, 27.1 and 24.3% for F400, F600, A400, and A600
treatments, respectively. The results indicated that both the fresh and aged
biochars inhibited NO3- leaching. However, all the four
kinds of biochars incorporation did not postpone NO3--N
leaching peak occurring (Fig. 4).
%20912-918_files/image002.png)
Fig. 1: The ammonium adsorption abilities
of the fresh and aged maize straw-derived biochars
Profiles of potential
ammonia-oxidation rate and AOB amount in soil column after
leaching
Potential
ammonia-oxidation rate: In the control soil column, the potential ammonia-oxidation rate (PAR)
varied in the range of 41.4–62.2 nmol N/g DW/h,
with the minimum occurred at the 20-cm depth and the maximum occurred at the
surface soil. In the fresh biochar incorporated soil column, the PAR decreased
significantly (P
< 0.05) from the surface layer until to the 20 cm depth (F400: 121.6 to
29.9 nmol N/g DW/h; F600: 125.5 to 29.0
nmol N/g DW/h). However, there was no
significant (P > 0.05) difference
in the deep layer soil samples (below 20 cm depth). Nonetheless, in the aged
biochar incorporated soil column, the PAR decreased sharply and significantly (P < 0.05) from the surface until to
the deep layer (A400, 146.5 to 6.35 nmol N/g DW/h;
A600, 146.3 to 10.1 nmol N/g DW/h) (Fig.
5).
The PARs were always in the order of control
> F400/F600 > A400/A600 in the subsurface and deep layers of the soil
column. While in the surface layer, the situation was reversed. This phenomenon
may be due to biochars’ retention capacity of NH4+ (as
the substrate of ammonia-oxidizing microbes) in the surface soil, and the aged
biochars possessing relatively stronger
capacity of retarding NH4+ transportation into the deep
layers of soil column than fresh ones.
%20912-918_files/image004.png)
Fig. 2: The nitrate adsorption abilities
of the fresh and aged maize straw-derived biochars
%20912-918_files/image006.png)
Fig. 3: The variation of the mass of
leached NH4+-N during the 16 leaching events
AOB amount: The profile of ammonia-oxidizing bacteria (AOB) amount in soil column was
in the same pattern as PAR for all the five treatments. In the control, the AOB
amount varied in the range of 3.43×105 – 4.82×105
individual/g DW, with the minimum
occurred at 20 cm depth and the maximum occurred at the surface layer. In the
fresh biochar incorporated soil column, the AOB amount decreased significantly
(P < 0.05) from the surface to the
20 cm depth (F400: 1.08×106 to 2.54×105 individual/g DW; F600: 1.11×106 to 2.44×105
individual/g DW). However, there was no
significant (P > 0.05) difference
in the deep layer soil samples (below 20 cm depth) (F400: varied in the range
of 2.25×105 to 2.48×105 individual/g DW; F600: varied in the range of 2.18×105
to 2.31×105 individual/g DW).
In the aged biochar incorporated soil column, the AOB amount decreased sharply
and significantly (P < 0.05) from
the surface until to the deep layer (A400: 1.16×106 to 4.85×104
individual/g DW;
%20912-918_files/image008.png)
Fig. 4: The variation of the mass of
leached NO3--N during the 16 leaching events
%20912-918_files/image010.png)
Fig. 5: The profile of potential
ammonia-oxidation rate of the layered soil samples in columns after leaching
A600: 1.11×106 to 7.16×104 individual/g DW) (Fig. 6).
The reason for the distribution pattern of AOB
amount in the soil columns incorporated with and without fresh/aged biochars
may result from the distribution
characteristic of NH4+, which is the substrate of AOB.
And the discrepant profile of NH4+ in the layered soil
samples in columns after leaching was attributed to the different soil amended
materials (the fresh or aged maize straw-derived biochars).
%20912-918_files/image012.png)
Fig. 6: The profile of AOB amount of the
layered soil samples in columns after leaching
Discussion
The spontaneous aging process
occurring at room temperature and certain water condition decreased the pH and
mean pore size of the maize straw-derived biochars, but increased the carboxyl
amount and specific surface area. Moreover, the aging process also enhanced the
biochar adsorption ability of NH4+ and NO3-
in aqueous solution, especially for the biochar prepared at 400oC.
Furthermore, the aged biochars showed relatively stronger ability to retard NH4+
transportation into the deep layers of soil column compared with the fresh
biochars.
Aging
typically results in the increased amount of carboxyl functional group on the
biochar surface (Lin et al. 2012;
Sorrenti et al. 2016), which
coincides with a decrease of biochar pH. In agreement with our study, Olivier
(2011) showed that the acidic oxygen-containing functional group content of
biochar increased after oxidized by H2O2, promoting more
surface acid sites formation on biochar surface. Similar trends were also
observed in other studies of field-weathered biochars (Joseph et al. 2010; Jones et al. 2012) and laboratory-aged biochar (Yao et al. 2010). However, both carbonyl and phenolic hydroxyl
functional groups on the biochar surface were decreased. Correspondingly,
Wiedner et al. (2015) also indicated
that the phenolic groups of three composted biochars decreased up to 22%.
Moreover, aging influences the cation exchange capacity (CEC) of biochar-amended soil
(Steiner et al. 2007; Major et al. 2010), which may influence the
adsorption of inorganic N in soil.
For both NH4+ and NO3-,
the aged maize straw-derived biochar showed stronger adsorption capacity than
fresh biochar, especially for the biochar prepared at 400oC. In
accordance with our study, Singh et al.
(2010) also suggested that the N sorption capacity of biochar increases over
time due to surface oxidation. The reason for this phenomenon would be aging
caused increases in both CEC and anion exchange capacity (AEC), and specific
surface area, so they are thus likely to adsorb
large amounts of NH4+ and NO3-
(Bakshi et al. 2016). However, other study also showed that the AEC of biochar
decreases rapidly on their oxidation (Cheng et
al. 2008) or did not change significantly with aging (Lawrinenko et al. 2016). The inconsistent effect of
aging on AEC of biochar may come from the different
feedstock, charring temperature and techniques during biochar production.
Both the fresh and aged maize straw-derived
biochar inhibited NH4+ and NO3-
leaching from the calcareous soil column compared with control in our study.
However, A400 and A600 showed relatively stronger retention ability than F400
and F600 to postpone inorganic N leaching peak occurring, especially for NH4+
(Fig. 3 and 4). The reason for this phenomenon may be the enhanced adsorption
ability of biochar after aging as shown
in our study (Fig. 1 and 2). Consistent with our study, Singh et al. (2010) speculated that the
increased effectiveness of wood or poultry manure biochars in reducing NH4+
leaching over time was due to increased adsorption capacity of biochars through
oxidative reactions on the biochar surfaces with aging.
Furthermore, the profiles of the potential
ammonia-oxidation rate (PAR) and ammonia-oxidizing bacteria (AOB) amount of the
layered soil samples in columns at the end of the leaching experiment in our
study also indicated that the aged biochars showed relatively stronger ability
to hold NH4+, retarding NH4+
transportation into the deep layers of soil column (Fig. 5 and 6). In the surface layer of soil column (0–2 cm), both PAR and AOB
amount of the aged biochar-amended soil were much larger than that of fresh biochar
amended. Given that NH4+ is the substrate of AOB and the
reactant of nitrification process, the results indicated that the aged biochars
possessed relatively stronger NH4+ holding capacity.
Moreover, in the subsurface and deep layers of soil
column (below the depth of 5 cm), both PAR and AOB amount of the aged biochar
treatments were much smaller than that of fresh biochar amended, and the
difference became larger and larger as depth increasing,
indicating the aged biochar retarded NH4+ transportation
into the deep layers of soil column.
During the leaching experiment, both the fresh and
aged biochars postponed NH4+ leaching peak occurring,
while NO3- leaching peak of the aged biochars treatment
occurred at the same leaching event as control, and the fresh biochars
treatment even brought the NO3- leaching peak forward
compared with control (Fig. 3–4). The obvious different performance of NH4+
and NO3- leaching from the calcareous soil columns
amended with biochars in our study may result
from the different biochar adsorption abilities of NH4+
and NO3-. Our study showed that the adsorption ability of
NH4+ of either the fresh or aged biochars was 7.50–10.6
times more than that of NO3-. Concurrently, Kameyama et al. (2012) also concluded that NO3-
was only weakly adsorbed onto biochar compared with NH4+,
and it could be desorbed by water infiltration.
Conclusion
Aging process decreased the pH and mean pore size but increased the
carboxyl amount and specific surface area of the maize straw-derived biochars.
The aged biochar prepared at 400oC showed the strongest adsorption
ability on both NH4+ and NO3-,
while the fresh biochar prepared at 600oC showed the weakest
adsorption ability on inorganic N.
All the four kinds of biochars inhibited the
leaching of NH4+ and NO3- from the
calcareous soil column compared with control. Moreover, the aged biochars
showed much stronger ability than fresh biochars to delay the leaching peak
occurring. The distribution profiles of potential ammonia-oxidation rate and
AOB amount in soil column indicated that the aged biochars possessed relatively
stronger ability to retain NH4+ transportation into the
deep layers of soil column compared with
the fresh biochars. In all, the spontaneous aging
process enhanced the inorganic N adsorption capacity and retention ability
of maize straw-derived biochars.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 41503074).
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