Short-term effects of raw rice straw and its derived biochar on greenhouse gas emission in five typical soils in China

Abstract

A short-term study was conducted to investigate the greenhouse gas emissions in five typical soils under two crop residue management practices: raw rice straw (Oryza sativa L., cv) and its derived biochar application. Rice straw and its derived biochar (two biochars, produced at 350 and 500°C and referred to as BC350 and BC500, respectively) were incubated with the soils at a 5% (weight/weight) rate and under 70% water holding capacity for 28 d. Incorporation of BC500 into soils reduced carbon dioxide (CO₂) and nitrous oxide (N₂O) emission in all five soils by 4−40% and 62−98%, respectively, compared to the untreated soils, whereas methane (CH₄) emission was elevated by up to about 2 times. Contrary to the biochars, direct return of the straw to soil reduced CH₄ emission by 22−69%, whereas CO₂ increased by 4 to 34 times. For N₂O emission, return of rice straw to soil reduced it by over 80% in two soils, while it increased by up to 14 times in other three soils. When all three greenhouse gases were normalized on the CO₂ basis, the global warming potential in all treatments followed the order of straw > BC350 > control > BC500 in all five soils. The results indicated that turning rice straw into biochar followed by its incorporation into soil was an effective measure for reducing soil greenhouse gas emission, and the effectiveness increased with increasing biochar production temperature, whereas direct return of straw to soil enhanced soil greenhouse gas emissions.

Key words:

• biochar
• crop residue management
• greenhouse gas emission
• rice straw
• soil

INTRODUCTION

In the past decades, China has produced about 630 Mt of crop straws annually (H. Liu et al. 2008). The common management of these crop residues is in situ burning in the field, which is considered as a convenient and economical method for farmers to deal with them (Lu et al. 2010). However, the burning of crop residues reduces the possibility of carbon (C) sequestration in the soil and meanwhile enhances the release of some greenhouse gases [GHGs, e.g., carbon dioxide (CO₂)] (Pathak et al. 2006; Yan et al. 2006).

One of the effective ways to resolve the problem of burning is direct return of straw to soil, which can increase the organic C input and thereby probably sequester C in the soil (Y. Singh et al. 2004). However, most straw C is labile and readily mineralized to CO₂ in a few years (Lehmann et al. 2006). Straw returned to soil also tends to stimulate methane (CH₄) emission (Zou et al. 2005), which would offset part of the C sequestration quantity. Turning straw into biochar under low temperature (< 700°C) and limited oxygen conditions, followed by its return to soil, has recently proven a promising approach for long-term C sequestration in soil (Lehmann and Joseph 2009). Biochar usually possesses strong aromatic-C structure and it is more biologically and chemically recalcitrant than many other forms of organic matter in soil (Keiluweit et al. 2010; B.P. Singh et al. 2010). Such a great recalcitrance makes the biochar undergo a much slower decomposition with turnover rate of hundreds to thousands of years (Lehmann et al. 2006; Fowles 2007). Moreover, some studies showed that biochar can also reduce soil CH₄ and/or nitrous oxide (N₂O) emissions (Yanai et al. 2007; Castaldi et al. 2011), although this was not true in all situations (Kim et al. 2011; Knoblauch et al. 2011). The variability of the observations could be related to different biochar and soil properties (Shneour 1966; Spokas and Reicosky 2009) and the relation needs to be further investigated.

Hence, in this study, we investigated GHG emissions in five typical soils in China under two crop residue management practices: raw rice straw (Oryza sativa L., cv.) and its derived biochar application. The five soils were collected from different locations in China varying in climate and soil fertility conditions. A 28-d incubation time was selected since a significant change in soil GHGs emissions from organic amendments often occurs within a short term (Yanai et al. 2007; Novak et al. 2010; Jones et al. 2011; Cheng et al. 2012; Nelissen et al. 2012). The specific objectives of this study are (1) to quantify GHG emission characteristics related to different treatments in the five soils, (2) to elucidate effects of different amendments and soil properties on the GHG emissions and (3) to prove biochar returned to soil is a better crop residue management practice than straw returned to soil for net GHG mitigation potential.

MATERIALS AND METHODS

Rice straw, biochar, and soils

Rice straw was collected from Changshu Agro-ecological Experimental Station, Chinese Academy of Science, China. The biochar was obtained from the rice straw using a typical slow pyrolysis process (Cao and Harris 2010). Briefly, the straw was air-dried, ground to less than 2 mm and put into a lab-scale stainless steel pyrolysis reactor. On the reactor top, there were two small vents (Ф1 cm) equipped with a loose lid which allowed the release of gases during the heating treatment and prevented the inflow of air during the cooling period. After the air in the reactor was replaced completely using pure molecular nitrogen (N₂), it was heated in a muffle furnace (SX2-12-10, China) at a speed of approximately 20°C min^–1 and held at 350 or 500°C for 4 h. The solid residue in the reactor was C-rich and designated as biochar (Cao and Harris 2010). The biochars produced at 350 and 500°C were referred to as BC350 and BC500, respectively. The biochar was then ground to pass through a 1.0-mm sieve for future characterization and amendment. Selected physical and chemical properties of the straw and biochars are given in Table 1.

Table 1 Physico-chemical properties of rice (Oryza sativa L., cv.) straw, biochars (BC350 and BC500), and five typical soils (YT, AS, SY, CS, and FQ).

	Straw	BC350	BC500	YT	AS	SY	CS	FQ
pH (H2O)	/^a	9.50	10.5	5.05	8.04	7.90	6.31	7.77
C (g kg^–1)	379	529	572	5.14	14.3	5.80	26.6	16.3
N (g kg^–1)	10.5	10.7	15.8	0.06	-^b	0.20	1.81	0.20
C/N	42.1	57.7	42.2	99.9	/	33.8	17.1	95.1
Ash (g kg^–1)^c	128	286	332	/	/	/	/	/
SA (m^2g^–1)	/	3.62	34.3	/	/	/	/	/
PV(cm^3g^–1)	/	0.02	0.06	/	/	/	/	/
APD(nm)	/	10.8	6.42	/	/	/	/	/
DOC (g kg^–1)	/	4.69	0.77	0.62	0.64	0.64	1.00	0.70
Sand (%)	/	/	/	25.0	38.5	51.7	17.0	18.1
Silt (%)	/	/	/	54.2	55.5	40.7	71.7	71.3
Clay (%)	/	/	/	20.8	6.0	7.6	11.3	10.6

^a /, not determined; ^b, below detection limit; ^c, heating at 900°C.YT, Yingtan; AS, Ansai; SY, Songyuan; CS, Changshu; FQ, Fengqiu; H₂O, water; C, carbon; N, nitrogen; SA, BET-N₂ surface area; PV, pore volume; APD, average pore diameter; DOC, dissolved organic C.

Five typical soil samples were collected from eastern, western, southern, northern, and central parts of China, respectively, based on climate and soil fertility conditions, i.e., Changshu (31°33′ N, 120°42′ E) in Jiangsu Province, Ansai (36°51′ N, 109°19′ E) in Shaanxi Province, Yingtan (28°12′ N, 117°0′ E) in Jiangxi Province, Songyuan (45°11′ N, 124°49′ E) in Jilin Province, and Fengqiu (34°35′ N, 115°34′ E) in Henan Province. For simplicity, the five soil samples were accordingly abbreviated as CS, AS, YT, SY and FQ, respectively. The five soils were classified as Gleyi-Stagnic Anthrosols, Loessi-Orthic Primosols, Argi-Udic Ferrosols, Hapli-Udic Isohumosols and Ochri-Aquic Cambosols, respectively, based on pedogenesis and soil taxonomy according to Z.T. Gong et al. (2006), and Typical Endoaquepts, Ustochnept, Plinthudults, Pachic Udic Haploborolls and Aquic Ustochrepts, respectively, according to USDA Soil Survey Staff (1994). All soil samples were collected from the cultivated layers (0–20 cm), air-dried, crushed, and passed through a 2-mm sieve. Soil properties are given in Table 1.

Soil treatment and gas sampling

The four treatments were established with three replicates: Soil, Soil + Straw, Soil + BC350 and Soil + BC500. Specifically, 50 g of dried soil was first placed in a polyvinyl chloride (PVC) plastics column container (60 mm height, 50 mm diameter) and distilled water was added to achieve a moisture content of 40% water holding capacity (WHC). The soil was then incubated at 25 ± 1°C in the dark for 7 d to stabilize microbial activity (J.Y. Wang et al. 2011). After 7 d of pre-incubation, the soil was completely mixed with straw, BC350 or BC500 at a ratio of 5% amendment [weight/weight (w/w), dry basis] and water content in all amendments was then kept to 70% WHC. The incubation lasted for 28 d.

The CO₂, CH₄, and N₂O gases were collected at 1, 2, 3, 5, 7, 14 and 28 d using the closed chamber method (Yanai et al. 2007). A clean column-PVC lid (150 mm height and 70 mm diameter, 577 mL) was used as a gas-tight chamber. Initially, approximately 5 mL of gas was drawn from the headspace of the chamber using a gas-tight syringe as the initial value. After incubation with closed chamber lid for 2 h, the gas in the chamber headspace was collected as the final value. The real GHGs content change was obtained from the difference between the final value and the initial value. After gas sampling, the soil incubation containers were flushed with ambient air and kept open until the next sampling.

Analytical methods

Specific surface area and pore size distribution of biochars were determined using a BET-N₂ SA analyzer (JW-BK222, Jwgb, China). The pH of soil and biochar was measured using pH detector (EUTECH pH 510, USA) with a solid to water ratio of 1: 2.5 [weight/volume (w/v)] for soil and 1:15 (w/v) for biochar. The ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^–-N) in the biochar were extracted with distilled water (solid to water ratio of 1:200, w/v) for 16 h in a shaker at room temperature, and those in the soil were extracted with 2 mol L^–1 potassium chloride (KCl) solution (solid to water ratio of 1:10 w/v) for 1 h in a shaker at room temperature. The suspensions were then filtered through a 0.45-μm membrane filter for determination of NH₄⁺-N and NO₃^–-N using an ultraviolet spectrophotometer, and dissolved organic carbon (DOC) was measured using a multi N/C 2100 TOC (Germany) analyzer (W. Gong et al. 2009). The concentrations of C and nitrogen (N) in soil and biochar were determined using an elemental analyzer (Vario EL III, Elementar, Germany). Soil texture was measured with a laser particle characterization analyzer (Berkman Coulter, Los Angeles, USA).

The concentrations of CO₂, CH₄, and N₂O were simultaneously detected by a gas chromatograph (Agilent 7890A, USA) equipped with two detectors: a hydrogen flame ionization detector applied to detect CO₂ and CH₄ and an electron capture detector applied to detect N₂O. More details on conditions and parameters of the gas chromatograph can be found in J.Y. Wang et al. (2011).

Calculation of greenhouse gas emission andglobal warming potential

The emission rates of CO₂, CH₄ and N₂O were calculated using the following equation:

(1)

where F is emission rate (mg CO₂-C kg^–1 h^–1, μg CH₄-C kg^–1 h^–1, and μg N₂O-N kg^–1 h^–1), ρ is the density of CO₂-C, CH₄-C or N₂O-N under standard state (CO₂-C and CH₄-C normalized, 0.536 kg m^–3; N₂O-N 1.25 kg m^–3); dc/dt is the change in concentrations of the gases per unit time (mg kg^–1 h^–1 or μg kg^–1 h^–1), V is the headspace of the apparatus (m³), T is incubation temperature (°C), and W is weight of soil (kg).

The cumulative emission amounts of the three gases in the 28-d incubation were calculated based on the following equation:

(2)

where S is the CO₂, CH₄ or N₂O cumulative emission(mg CO₂-C kg^–1, μg CH₄-C kg^–1 or μg N₂O-N kg^–1), F_i is described above in equation (1), t_i is incubation days when gas sample is collected.

The global warming potential (GWP) value was calculated from all three GHGs in mg CO₂ equivalent per kilogram soil (IPCC 2007). The default GWP of CH₄ and N₂O in a 100-yr time frame were regarded as 25 and 298 respectively, while the GWP value for CO₂ was taken as 1 (IPCC 2007).

Statistical analysis

Difference between the treatment and soil type was analyzed with two-way analysis of variance (ANOVA). Difference among the treatments was analyzed with one-way ANOVA. All tests of significance (P < 0.05) were conducted with Tukey-Kramer highly significant difference (HSD) test. Statistical analyses were performed using SPSS 17.0 (SPSS software for Windows 7, Version 17.0, 2010).

RESULTS

Characteristics of straw, biochar, and soils

Selected properties of rice straw, straw-derived biochar, and soils are shown in Table 1. Both BC350 and BC500 were alkaline, and BC500 pH was about 1 unit higher compared to BC350. By turning rice straw into biochar, the C concentration was increased by 40% for BC350 and 51% for BC500. The C concentration of BC500 was higher than that of BC350, while DOC of BC500 was much lower than that of BC350. The surface area and pore volume of BC500 were much higher than that of BC350, but conversely for the average pore diameter.

Properties of the five soils varied greatly (Table 1). Soil pH ranged from acid to alkaline (5.05–8.04), but they were much lower than those of the biochars. The CS and FQ soils contained higher C, N and DOC compared with the YT, AS and SY soils. For particle distribution, the CS and FQ soils had higher silt content (> 71%) than the other three (YT, AS and SY) soils (< 55%).

GHGs emissions from soil influenced by straw or biochar addition

The emission dynamics of CO₂, CH₄, and N₂O gases are shown in Fig. 1–3. All three gases emissions showed a similar tendency among the five soils; i.e., the greatest change happened in the first 7 d and remained stable in the latter 21 d. Nevertheless, extent of the effects of different amendments and soil types on the three gases were variable.

Figure 1 Dynamics of soil carbon dioxide (CO₂) emission from five soils (a, b, c, d and e stand for soil from YT, Yingtan; AS, Ansai; SY, Songyuan; CS, Changshu; and FQ, Fengqiu respectively) with different treatments and 28 d incubation. Soil, Soil + Straw, Soil + BC350, and Soil + BC500 stand for soil only, soil + 5% rice (Oryza sativa L., cv.) straw, soil + 5% straw biochar produced at 350°C, and soil + 5% straw biochar produced at 500°C, respectively. C, carbon.

Figure 2 Dynamics of soil methane (CH₄) emission from five soils (a, b, c, d and e stand for soil from YT, Yingtan; AS, Ansai; SY, Songyuan; CS, Changshu; and FQ, Fengqiu respectively) with different treatments and 28 d incubation. Soil, Soil + Straw, Soil + BC350, and Soil + BC500 stand for soil only, soil + 5% rice (Oryza sativa L., cv.) straw, soil + 5% straw biochar produced at 350°C, and soil + 5% straw biochar produced at 500°C, respectively. C, carbon.

Figure 3 Dynamics of soil nitrous oxide (N₂O) emission from five soils (a, b, c, d and e stand for soil from YT, Yingtan; AS, Ansai; SY, Songyuan; CS, Changshu; and FQ, Fengqiu respectively) with different treatments and 28 d incubation. Soil, Soil + Straw, Soil + BC350, and Soil + BC500 stand for soil only, soil + 5% rice (Oryza sativa L., cv.) straw, soil + 5% straw biochar produced at 350°C, and soil + 5% straw biochar produced at 500°C, respectively. N, nitrogen.

CO₂ emission reached a peak at 7 d and then decreased steadily in most soils, except in the CS soil that had the highest emission at 14 d (Fig. 1). The cumulative CO₂ emission during the incubation period significantly (p < 0.05) changed from 141 ± 26 to 8298 ± 565 mg C kg^–1 with different treatments and different soils (Fig. 4a). Compared to the control soil, straw significantly increased the CO₂ emissions (p < 0.05) in all five soils by about 4 to 34 times and BC350 elevated the CO₂ emissions (p < 0.05) from 78 to 289% except for the YT soil. However, BC500 suppressed the CO₂ emission from 4 to 40% except for the YT soil. Overall, the CO₂ cumulative emission with rice straw addition was higher than that with biochar addition, following the order of Soil + Straw > Soil + BC350 > Soil > Soil + BC500.

Figure 4 Cumulative emissions of (A) carbon dioxide (CO₂), (B) methane (CH₄), and (C) nitrous oxide (N₂O) from five soils (YT, Yingtan; AS, Ansai; SY, Songyuan; CS, Changshu; and FQ, Fengqiu) with different treatments and 28 d incubation. Soil, Soil + Straw, Soil + BC350, and Soil + BC500 stand for soil only, soil + 5% rice (Oryza sativa L., cv.) straw, soil + 5% straw biochar produced at 350°C, and soil + 5% straw biochar produced at 500°C, respectively. C, carbon; N, nitrogen.

The CH₄ release dynamic was not as clear as that of CO₂ for all amendments, with great variation in the first week (Fig. 2). The CH₄ emission increased with the addition of BC500 while straw and BC350 reduced CH₄ emission over the incubation period, which was contrary to the CO₂ emission (Fig. 1). The cumulative CH₄ emission during the 28-d incubation in the BC350 and straw amendment soils was reduced by 22–69% and 10–49%, respectively, compared to the control soil, while BC500 returned to soils elevated the CH₄ emission by up to about 2 times (Fig. 4b).

Like CH₄, the N₂O release dynamic had a great variation in the first week (Fig. 3). Straw returned to YT and SY soils significantly (p < 0.05) suppressed N₂O emissions by 80% and 81%, respectively, while the emissions increased by up to 14 times in the other three soils (Fig. 4c). BC350 addition to YT, SY, and CS soils inhibited the N₂O emissions by up to 85%, while the N₂O emissions increased in AS and FQ soils by 30 and 66%, respectively. However, BC500 addition to all five soils significantly (p < 0.05) reduced N₂O emissions by 62–98%.

Contribution of straw or biochar addition to GWP

As shown above, the effects of straw and biochar on the GHGs emissions were highly variable with the amendments, gas types and soil types. However, when all three greenhouse gases were normalized on a CO₂ basis, the overall GWP from the five soils followed the order of CS > YT > SY ≈ FQ > AS (Table 3). Compared to the control soil, straw addition significantly (p < 0.05) increased GWP by about 4 to 31 times, while BC500 addition to all five soils consistently decreased GWP by up to 45%, although BC350 elevated GWP by up to 2.6 times which was still much lower than that of returned straw (Table 3).

Table 2 The pH, ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^–-N)_, and dissolved organic carbon (DOC) in the different treatments with 28 d incubation

Soil	Treatment	pH	NH4^+-N (mg kg^-1)	NO3^--N (mg kg^-1)	DOC (g kg^-1)
YT	Soil	5.17 ± 0.04b	1.15 ± 0.35c	7.40 ± 2.8c	0.92 ± 0.05b
	Soil + Straw	4.77 ± 0.10c	0.90 ± 0.19c	41.4 ± 6.2a	1.15 ± 0.03a
	Soil + BC350	4.90 ± 0.03c	2.04 ± 0.22b	29.1 ± 2.0b	0.82 ± 0.03b
	Soil + BC500	5.64 ± 0.16a	4.79 ± 0.47a	0.26 ± 0.00d	0.81 ± 0.04b
AS	Soil	8.27 ± 0.17b	0.38 ± 0.09b	17.9 ± 8.6a	0.99 ± 0.10b
	Soil + Straw	7.36 ± 0.05c	1.47 ± 0.26a	31.1 ± 1.3a	1.14 ± 0.17a
	Soil + BC350	8.01 ± 0.08b	1.10 ± 0.39ab	16.9 ± 2.4a	0.96 ± 0.12b
	Soil + BC500	8.56 ± 0.03a	0.62 ± 0.26b	21.3 ± 11.9a	0.83 ± 0.12c
SY	Soil	7.90 ± 0.17a	1.30 ± 0.52a	35.3 ± 14.0a	0.89 ± 0.10b
	Soil + Straw	7.33 ± 0.09b	1.81 ± 1.12a	30.9 ± 11.7a	1.28 ± 0.13a
	Soil + BC350	7.50 ± 0.02b	1.30 ± 0.28a	37.9 ± 6.8a	0.88 ± 0.11b
	Soil + BC500	7.98 ± 0.10a	1.13 ± 0.41a	50.0 ± 11.0a	0.78 ± 0.12b
CS	Soil	6.43 ± 0.11b	5.80 ± 0.22a	99.27 ± 2.3a	0.99 ± 0.07b
	Soil + Straw	6.56 ± 0.19b	1.83 ± 0.30b	16.9 ± 1.7c	1.42 ± 0.10a
	Soil + BC350	6.48 ± 0.10b	6.23 ± 0.86a	58.4 ± 6.1b	1.03 ± 0.07b
	Soil + BC500	7.29 ± 0.02a	2.73 ± 0.74b	84.4 ± 11.9a	1.01 ± 0.10b
FQ	Soil	7.95 ± 0.06b	0.97 ± 0.34a	96.9 ± 21.5a	0.71 ± 0.13c
	Soil + Straw	7.36 ± 0.02d	1.64 ± 0.29a	23.7 ± 1.8b	1.26 ± 0.20a
	Soil + BC350	7.65 ± 0.10c	1.84 ± 0.16a	78.6 ± 7.6a	0.95 ± 0.14b
	Soil + BC500	8.13 ± 0.02a	1.66 ± 0.64a	75.7 ± 11.3a	0.71 ± 0.14c

Different letters for each parameter indicate significant difference [p < 0.05, one-way analysis of variance (ANOVA)] between the treatments in one soil.Soil, soil only; Soil + Straw, soil + 5% rice (Oryza sativa L., cv.) straw; Soil + BC350, soil + 5% BC350; Soil + BC500, soil + 5% BC500; YT, Yingtan; AS, Ansai; SY, Songyuan; CS, Changshu; FQ, Fengqiu.

Table 3 Total carbon dioxide (CO₂) equivalent emission [global warming potential (GWP), mg CO₂ kg^–1 soil] following different treatments with 28 d incubation

Soil	Treatment	GWP (mg CO2 kg^–1 soil)
YT	Soil	1179 ± 149b
	Soil + Straw	5499 ± 173a
	Soil + BC350	1099 ± 51b
	Soil + BC500	1104 ± 66b
AS	Soil	200 ± 38c
	Soil + Straw	6327 ± 21a
	Soil + BC350	721 ± 20b
	Soil + BC500	152 ± 26c
SY	Soil	588 ± 67c
	Soil + Straw	5701 ± 216a
	Soil + BC350	1252 ± 83b
	Soil + BC500	440 ± 14d
CS	Soil	2203 ± 788b
	Soil + Straw	9596 ± 703a
	Soil + BC350	2549 ± 524b
	Soil + BC500	1208 ± 59c
FQ	Soil	599 ± 53c
	Soil + Straw	5404 ± 527a
	Soil + BC350	1051 ± 151b
	Soil + BC500	344 ± 69d

Different letters for each parameter indicate significant difference [p < 0.05, one-way analysis of variance (ANOVA)] between the treatments in one soil.Soil, soil only; Soil + Straw, soil + 5% rice (Oryza sativa L., cv.) straw; Soil + BC350, soil + 5% BC350; Soil + BC500, soil + 5% BC500; YT, Yingtan; AS, Ansai; SY, Songyuan; CS, Changshu; FQ, Fengqiu.

Effect of different treatments on soils properties after incubation

In order to obtain the relationship of the GHG emissions between soils and amendments, selected physico-chemical properties of control soils and amended soils including pH, NH₄⁺-N, NO₃^–-N, and DOC were assessed after the 28-d incubation. BC500 addition increased soil pH, while both straw and BC350 addition reduced soil pH, except for CS soil, compared to control soil (Table 2). Cumulative CH₄ and CO₂ emission rates of all soils except CS soil were significantly (p < 0.05) correlated with pH change. Moreover, there was also a significantly (p < 0.01) negative correlation between N₂O cumulative emission rate and pH for AS and FQ soils (Table 4).

Table 4 The correlation coefficients between cumulative emission of greenhouse gases (GHGs) and characteristics of five type soils following different treatments after 28 d incubation.

Soil	property	CH4	CO2	N2O
YT	pH	0.94**	–0.60*	0.24
	DOC	–0.37	0.84**	–0.22
	NO3-N	/	/	–0.58*
AS	pH	0.71**	–0.92**	–0.71**
	DOC	–0.43	0.64*	0.50
	NO3-N	/	/	0.27
SY	pH	0.70*	–0.78**	0.34
	DOC	–0.43	0.94**	–0.08
	NO3-N	/	/	–0.31
CS	pH	0.66*	–0.30	–0.53
	DOC	–0.51	0.95**	0.55
	NO3-N	/	/	–0.36
FQ	pH	0.69*	–0.85**	–0.83**
	DOC	–0.58*	0.92**	0.93**
	NO3-N	/	/	–0.88**

/, not analyzed; *, p < 0.05; **, p < 0.01.CH₄, methane; CO₂; carbon dioxide; N₂O, nitrous oxide; DOC, dissolved organic carbon; NO₃-N, nitrate nitrogen; YT, Yingtan; AS, Ansai; SY, Songyuan; CS, Changshu; FQ, Fengqiu.

The DOC concentrations in the treatments of the five soils changed smoothly after incubation (Table 2). Briefly, straw returned to soils was significantly greater in DOC change than the other treatments. Compared to the control soil, BC350 and BC500 additions reduced the DOC concentrations for YT, AS and SY soils, and the opposite results were found in the other two soils. Cumulative CO₂ emission rates of the five soils were significantly (p < 0.01) positively correlated with DOC change. Moreover, for FQ soil, there was significantly (p < 0.05) negative correlation between DOC change and CH₄ emission rate, while significantly (p < 0.01) positive correlation existed between that and N₂O cumulative emission rate (Table 4).

The effects of different treatments in five soils on inorganic N (NH₄⁺-N and NO₃^–-N) content changed differently (Table 2). Briefly, no significant (p > 0.05) differences of NH₄⁺-N content were found in different treatments for SY and FQ soils. Biochar addition to soil significantly (p < 0.05) increased NH₄⁺-N content for YT soil. There were no significant (p > 0.05) differences in NO₃^–-N content among all treatments of AS and SY soils; however, significant (p < 0.05) differences between treatments were found in YT soil from 0.26mg kg^–1 to 41.4 mg kg^–1. Compared to the control soil, straw, BC350 and BC500 additions reduced the NO₃^–-N content in CS and FQ soils (Table 2). The cumulative N₂O emission rate of the FQ soil was significantly (p < 0.01) correlated with NO₃-N change (Table 4).

DISCUSSTION

Elevated CO₂ emissions were observed from the soils with raw straw and BC350 return, which was probably due to the fact that straw and BC350 were rich in DOC and thus more readily degradable via microbial activity (Lehmann et al. 2006). All the treated soils with straw addition showed higher initial CO₂ emissions, probably resulting from the rapid mineralization of the fresh straw biomass (Smith et al. 2010). The highest emission in the CS soil was probably due to its highest DOC content (Table 1). Contrary to the straw and BC350 treatments, BC500 addition inhibited CO₂ emission in all five soils, though the emission did not reach a significant (p < 0.05) level except for in the FQ soil, and emission rate remained stable during the incubation period (Fig. 1). BC500 had a strong alkalinity with pH > 10 (Table 1). After the BC500 was returned to soil, the soil pH increased (Table 2), which may allow the produced CO₂ to be trapped in the soil solution as carbonates, resulting in a reduction of CO₂ emissions. Our observations agreed with previous work showing that application of biochar to soil can inhibit soil CO₂ release (Spokas et al. 2009; Y.X. Liu et al. 2011; J.Y. Wang et al. 2011). Compared to the other four soils, both BC350 and BC500 had less effect on CO₂ emissions from the YT soil (Fig. 4a), probably due to its acidity (Table 2). A liming experiment also showed that acidic soils were less favorable for CO₂ production (Mørkved et al. 2007). Less CO₂ emission in the biochar-treated soil may be also a result of the condensed aromatic nature of biochar that is difficult for microorganisms to degrade/mineralize and is therefore more stable in soil than non-charred straw (Lehmann et al. 2006). Bruun et al. (2012) indicated that only 3–12% of added biochar-C had been emitted as CO₂ within 2 months of soil incubation, while the straw feedstock lost more than 50% of its initial C content. Knoblauch et al. (2011) showed that only 4–9% of rice husk biochar was mineralized to CO₂ within 3 yr of soil incubation, compared to 31–100% of the rice husk mineralized to CO₂. The present study indicated that the C loss as CO₂ in both BC350 and BC500 biochars only accounts for less than 4% of the initial C content within 28 d of soil incubation, while the raw straw lost 23–37% of its initial C content as CO₂ emission (Fig. 4a). Moreover, BC500 derived at higher pyrolysis temperature had lower DOC than BC350 (Table 1), resulting in less CO₂ emission than BC350 (Fig. 4a). Similar results for the biochar temperature effect were obtained in other works. For example, Pereira et al. (2011) found that low-temperature (400°C) biochar induced a relatively higher CO₂ efflux than did high-temperature (500°C) biochar.

Note that what was detected in our experiment was the net result of CH₄ oxidation and CH₄ production. Wide variations in CH₄ emission from soils with biochar amendment have been reported in the literature (Kim et al. 2011). Some studies indicated that biochar incorporation into soil reduced CH₄ emission since biochar could improve soil aeration and decease the anaerobic condition in soils, which could decrease CH₄ production (inhibition methanogens activity), and/or biochar could increase CH₄ oxidation (promoting methanotroph activity) or enhance the adsorption of CH₄ in soil based on the high porosity and the large surface area (Lehmann et al. 2011; Scheer et al. 2011). However, other studies argued that incorporation of biochar into soil could increase CH₄ emissions, probably due to the improved soil water holding capability, high DOC content, pH and metals which may potentially inhibit CH₄ oxidation and/or promote CH₄ production (Zhang et al. 2010; Knoblauch et al. 2011). In this study, there was significant positive correlation between CH₄ emission and pH for all five soils (Table 4). Return of straw and BC350 reduced soil pH (Table 2), resulting in decline of CH₄ emission, compared to the control soil (Fig. 4b). However, the BC 500 had high pH (Table 1), which increased soil alkalinity (Table 2), especially in the acidic YT and CS soils, enhancing CH₄ production (Mer and Roger 2001). For the FQ soil, there was significant negative correlation between the CH₄ emission and DOC (Table 4), the same conclusion as reported by Ström et al. (2003). In our experiment, 70% WHC and such a short-term incubation (28 d) was probably not the best condition for CH₄ production, but it may have improved CH₄ oxidation. The net result showed that the DOC was mineralized in terms of CO₂. As a result, the negative correlation between CH₄ production and DOC was observed for the FQ soil.

The cumulative N₂O emissions from CS and FQ soils were higher than those of the other three soils (Fig. 4c), probably due to the high NO₃^–-N and DOC in these two soils (Table 2), which may favor soil denitrification, generating more N₂O. Impressively, BC500 addition to all five soils significantly (p < 0.05) reduced N₂O emissions by 62–98%, which supported the idea that biochar returned to soil was a great effective measure for straw management to mitigate N₂O emissions from cropland (B.P. Singh et al. 2010; Bruun et al. 2011). Soil pH, DOC, and NO₃^–N contents were key factors influencing N₂O emission in the biochar-amended soils (Weier and Gilliam 1986; Granli and Bøckman 1995). Application of biochar, especially BC500, into soil significantly reduced N₂O emission, in accordance with other reports (B.P. Singh et al. 2010; Castaldi et al. 2011). It can probably be attributed to increase of soil pH induced by biochar (Table 2), which promoted N₂O convert to N₂ (Clough et al. 2010). In addition, biochar had high surface area and pore volume (Table 1) which might adsorb N₂O in soil air and water (Bagreev et al. 2001), and metal oxides on or near biochar surfaces may catalyze the reduction of N₂O to N₂ (Lehmann and Joseph 2009). BC350 returned to YT, SY and CS soils would inhibit N₂O emissions, while the increase in AS and FQ soil can probably be attributed to pH, DOC or NO₃^–-N change (Table 2). For example, there was a significant negative correlation between N₂O emission and pH for AS and FQ soils (Table 4). Moreover, there was a significant negative correlation between N₂O emission and NO₃^–-N for FQ soil (Table 4) probably due to high NO₃^–-N content that could inhibit N₂O production and/or stimulate N₂O reduction (Ellis et al. 1996; Luo et al. 1996). In addition, the N₂O production gene and N₂O consuming gene change was probably one potential cause (C. Wang et al. 2013).

The biochar type and application rate can also affect the GHGs emissions. Spokas et al. (2009) reported sawdust biochar amendment reduced CO₂ and N₂O production and inhibited ambient CH₄ oxidation for all application rates from 2% to 60%. O’Toole et al. (2013) observed wheat (Triticum sp.) straw biochar application to soil at 5% and 10% increased CO₂ emission, while the emission decreased at 1%, compared to the control soil. Van Zwieten et al. (2010) indicated a biosolid biochar application rate of 5% caused less CO₂ and N₂O emission than control soil. Our work showed a similar result, i.e., CO₂ and N₂O emissions were reduced at a 5% rate of rice straw biochar application.

As discussed above, effects of straw and biochar on GHGs emissions were highly variable with the amendments, gas types and soil types. However, when all three greenhouse gases were normalized on a CO₂ basis, the overall GWP from the five soils followed the order of CS > YT > SY ≈ FQ > AS, and that for all treatments decreased as straw > BC350 > BC500 (Table 3).

CONCLUSIONS

The present study demonstrated the different GHG emission characteristics with two crop residue management practices: raw rice straw and its derived biochar application. The amendment of BC500, which was produced at 500°C, a relatively higher temperature, significantly reduced soil CO₂ and N₂O emissions in all five soils, though it increased CH₄ emission. However, the GWP of soil with BC500 application was much lower than that of straw and even lower than that of BC350 which was produced at 350°C, a relatively low temperature. The GWP for all treatments followed the order of straw > BC350 > BC500, and that from the five soils decreased as CS > YT > SY ≈ FQ > AS. Comparing the two crop residue management practices, converting straw into biochar followed by its application to soil showed an effective method for global warming mitigation, and the effectiveness increased with biochar production temperature. It should be pointed out that the effects observed in this study were only obtained in the laboratory experiments with one straw and its biochar amendment during a short time of 28 d incubation. The long-term effects and field-scale trials with other straws and their derived biochar amendments need to be assessed.

ACKNOWLEDGMENTS

This work was supported in part by the National Natural Science Foundation of China (No. 21077072, 21107070), the Shanghai Pujiang Talent Project (11PJ1404600), The University Innovation Project (AE1600004), and the Colleges and Universities Excellent Young Talents Fund Project of Anhui Province (No. 2012SQRL149).