Environ Sci Pollut Res DOI 10.1007/s11356-014-4067-1

RESEARCH ARTICLE

Short-term effects of rice straw biochar on sorption, emission, and transformation of soil NH4+-N Fan Yang & Xinde Cao & Bin Gao & Ling Zhao & Feiyue Li

Received: 4 August 2014 / Accepted: 30 December 2014 # Springer-Verlag Berlin Heidelberg 2015

Abstract Although previous work has explored and reported the influence of biochar on the fate and transformation of soil nitrogen (N), the governing mechanisms are still unclear. In this study, an incubation experiment was first conducted to investigate the overall fate of NH4+-N in two soils: GleyiStagnic Anthrosols (pH=6.31) and Argi-Udic Ferrosols (pH= 5.05) amended with rice straw biochar. In addition, batch sorption experiments were designed to explore the potential mechanisms of NH4+-N transformation in biochar-amended soils. Results showed that the KCl extractable NH4+-N concentrations in the amended Anthrosols and Ferrosols decreased by 9–35 and 5–22 %, respectively, compared to the unamended soils, but limited nitrification of NH4+-N into NO3−-N was observed in both soils. In Anthrosols, biochar increased NH4+-N sorption, but it decreased N biotransformation (mineralization, nitrification, and assimilation) into NO3−-N. It implies that the chemical sorption is a dominant process in the biochar-amended soil. As for Ferrosols, biochar seemed to have less effect on either NH4+ sorption or biotransformation. Biochar addition promoted NH3 emission in both soils due to the elevated pH, but the overall amount of the N emission losses were negligible.

Responsible editor: Zhihong Xu Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-4067-1) contains supplementary material, which is available to authorized users. F. Yang : X. Cao (*) : B. Gao : L. Zhao : F. Li School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China e-mail: [email protected] B. Gao Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA

Keywords Adsorption . Ammonia nitrogen . Biochar . Nitrification . NH3 emission

Introduction In the past few decades, approximately 6.3×108 t of crop residue was produced per year in China (Liu et al. 2008). Rice straw accounts for a majority of the crop residue and is commonly burned on site (Zeng et al. 2007), which causes the hazy weather and greenhouse gases emission (Pathak et al. 2006). One of the approaches to solve this problem is thermally converting these crop residues into biochar in an oxygen-limited environment. It has been proven that biochar can act as C-sink pool when added to soils, and meanwhile biochar can serve an amendment to improve soil quality (Dempster et al. 2011, Zimmerman et al. 2011). Because of its unique structure and surface characteristics, biochar shows a strong affiliation to various chemicals, and thus may also affect the transformation and fate of the chemicals in soils (Wang et al. 2012). An increasing number of studies have explored the effect of biochar amendment on NH4+-N transformation in soils (Clough and Condron 2010). For instance, Berglund et al. (2004) found that nitrification rates increased with charcoal amendments in laboratory incubations, but this result was not verified in field studies (Berglund et al. 2004). DeLuca et al. (2006) found that the field-collected charcoal significantly increased the nitrification potential, net nitrification, and gross nitrification in forest soils (1 % w/w), but had no effect on nitrification in grassland soils that have high nitrifier activity (DeLuca et al. 2006). By contrast, some studies found that fresh biochar inhibited the transformation of NH4+-N into NO3−-N in soils. For instance, Zheng et al. (2012) showed oak pellet biochar decreased cumulative extractable NO3−-N in fertilized soils by 8 %. Similar results were reported by

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Nelson et al. (2011) that biochar application increased NH4+-N concentrations (from 1.1 to 4.8 mg kg−1) in soils during the first 10 days, but consistently decreased NO3−N (from 5 to 10 mg kg−1) during the whole 56-day study. Using 15N pool dilution technique, Cheng et al. (2012) showed that wheat straw biochar in soils had no effect on the N transformation processes. Generally, biochar poses negatively charged surface (Cheng et al. 2008) and therefore, they usually have good sorption ability to cations, including NH4+. For instance, Yao et al. (2012) found that biochar derived from agricultural and forest residuals have strong sorption ability to aqueous NH4+. In addition, biochar produced from slow pyrolysis are alkaline (basic) and may affect N cycle in soils by changing the speciation of NH4+ (Chen et al. 2012). Previous studies have also shown that biochar likely favors the growth of microorganisms related to NH4+-N transformation by modifying the soil physical habitat to promote microbial activity and community structure (Dempster et al. 2011). All these factors may play important roles in controlling the fate and transformation of NH4+-N in biochar-amended soils. The overall objective of this work was to determine the governing mechanisms of NH4+-N transformation in biochar-amended soils. Biochar was produced from rice straw which occupies the majority of crop residues in China. A range of incubation and batch sorption experiments was conducted to quantify the effect of rice straw biochar on the fate and transformation of NH4+N in two types of soils. The specific objectives were as follows: (1) to measure the adsorption of NH4+ on the rice straw biochar, (2) to determine the effect of biochar amendment on NH3 emission in soils, and (3) to determine the effect of biochar amendment on NH4+-N transformation (microbial) in soils.

Two typical soil samples were collected from Changshu (31° 33′ N, 120° 42′ E), Jiangsu Province, and Yingtan (28° 12′ N, 117° 0′ E), Jiangxi Province in China. Based on their pedogenesis and taxonomy, the soils were classified as Gleyi-Stagnic Anthrosols and Argi-Udic Ferrosols, respectively. All soil samples were collected from the cultivated layers (0–20 cm), then dried, crushed, and passed through a 2-mm sieve. The pH values of soil and biochar were measured using pH detector (EUTECH pH 510, USA) with the solid to water ratio of 1:2.5 (w/v) for the soils and 1:20 (w/v) for the biochar. Ash content of the biochar was determined using the standard method (2001, 1986). Contents of C, H, and N in the soil and biochar were determined using an elemental analyzer (Vario EL III, Elementar, Germany). Compositions of other elements (such as Ca, Mg, Fe, and Al) in the biochar and the soils were measured using the USEPA method 3050B (USEPA 1986) with an inductively coupled plasma spectrometer (ICP-AES, ICAP6000 Radial, Thermo, English) (Zhao et al. 2013a). Cation exchange capacity (CEC) was determined according to a modified barium chloride compulsive exchange method (Lee et al. 2010). Specific surface area and pore size distribution of the biochar was determined using a BET-N2 SA analyzer (JW-BK222, Jwgb, China). Soluble NH4+-N and NO3−-N in the soils and biochar were extracted with 2 M KCl (solid to water ratio of 1:10 for soil and 1:200 for biochar, w/v, g mL−1) for 4 h in a shaker at 25 °C. The suspensions were then filtered through 0.45-μm membrane filters, and NH4+-N and NO3−-N concentrations were determined using spectrophotometric method (Mulvaney et al. 1997). All analyses were conducted in duplicate. Selected physical and chemical properties of the soils and the biochar are presented in Table 1.

Incubation experiment Materials and methods Biochar and soils Biochar was produced from rice straw through the slow pyrolysis (Zhao et al. 2013b). Briefly, the rice straw was air-dried and ground to less than 2 mm. The feedstock was then added into a lab-scale stainless steel pyrolysis reactor. The reactor was purged with pure N2, placed in a Muffle Furnace (SX212–10, China), and heated at a speed of approximately 20 °C min−1 until 500 °C. After maintaining the peak temperature for 4 h, the furnace was turned off and the reactor was naturally cooled down. The solid residue in the reactor was referred to as biochar, which was grounded and passed through a 1.0-mm sieve prior to use.

The incubation experiment was conducted in glass desiccators (186 mm in internal diameter and 325 mm in height). The desiccators were filled with 1-kg soil (Anthrosols or Ferrosols) and 200 mg kg−1 NH4+-N in the form of NH4Cl, and mixed with biochar at rates of 1 and 5 % (w/w). Three replicates were conducted for each treatment. During the whole experiment, the temperature was kept at 25±1 °C and soil moisture content was maintained at 70 % of maximum water-holding capacity (MWHA) by adding distilled water to compensate water loss every other day. The incubation lasted for 28 days, during which soil samples were collected on days 1, 3, 5, 7, 14, and 28 for the analysis of NH4+N and NO3−-N.

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Basic physicochemical properties of the biochar and the soils

pH C (g kg−1) N (g kg−1) Ash (g kg−1) C/N Al (g kg−1) Ca (g kg−1) Fe (g kg−1) Mg (g kg−1) DOC (g kg−1) Yield (%) NH4+-N (mg kg−1) NO3−-N (mg kg−1) CEC (cmol kg−1) Specific surface area (m2g−1) Average pore diameter (nm) Pore volume (cm3g−1)

Biochar

Anthrosols

Ferrosols

10.6 602 10.9 332 60.5 0.45 11.6 1.34 3.38 0.77 29.8 5.54 91.5 80.8 33.2 3.74 0.051

6.31 26.6 1.81 / 24.5 68.1 7.35 34.2 8.38 / / 13.9 50.3 15.0 / / /

5.05 5.14 0.40 / 143 4.80 0.073 25.0 0.29 / / 2.21 8.49 8.89 / / /

Sorption of NH4+ About 4 g of soil and 0.2 g of biochar samples were used as the sorbent in the sorption experiment with four different arrangements: (1) 4-g Anthrosols only, (2) 4-g Ferrosols only, (3) 0.2g biochar mixed with 4-g Anthrosols, and (4) 0.2-g biochar mixed with 4-g Ferrosols. Three replicates were used for each arrangement. The sorbents were added into 50-mL polypropylene tubes containing 40 mL NH4Cl solution with 0.02 M KCl as the background electrolyte. Two or three drops of chloroform were added into the tubes to inhibit microbes. The initial concentrations of the NH4+-N solutions used in the tubes were set as 10, 20, 40, 60, 80, 100, 120, and 140 mg L−1. It is worth noting that this concentration range was designed to simulate the adsorption environment in the real soils on the basis of the ratios among soils, biochar, and NH4+-N. The mixture was then agitated on a reciprocating shaker at a speed of 240 rpm at 25 °C for 20 h (this time was predetermined to be enough to reach sorption equilibrium). After that, solid and liquid phases were separated by centrifugation at 4000 rpm for 15 min and the solution was filtered through a 0.45-μm membrane filter for determining the NH4+N concentrations in the filtrates.

Biotransformation and emission of NH4+-N Part of the soils and biochar was fumigated by chloroform to kill the bacteria (Brookes et al. 1985), and an incubation experiment was used to determine the effect of biochar amendment on NH 3 emission and biotransformation

including mineralization, nitrification, and assimilation. The incubation experiment was designed with three comparisons: (1) Anthrosols vs. Ferrosols, (2) 5 % biochar vs. no biochar, and (3) sterilized vs. unsterilized. There were eight treatments (2×2×2) and three replicates were used for each treatment. Both the sterilized and unsterilized soils and soil-biochar mixtures were placed separately in 100-mL polypropylenetubes (40-mm inner diameter and 100-mm height). During the incubation, the condition was controlled at 25±1 °C and 70 % MWHC. NH4+-N and NO3−-N contents were measured after 14 days. To measure the emission, a 5-mL polypropylene tube with 3 mL 2 % boric acid solution was placed within the 100-mL polypropylene tube. The 2 % boric acid solutions were used to sorb the NH3 released from the four unsterilized incubation tubes. The incubation systems were airtight for 24 h at days 1, 3, 5, 7, and 14 before sampling. The NH4+-N concentrations in the boric acid solutions were measured to determine the cumulative NH3 emission during the 14-day incubation. In order to further demonstrate the adsorption of biochar on NH4+-N, biochar was separated from sterilized treatments after the incubation by blowing the soil-biochar mixture, as biochar had lower density than soil particles. The separated biochar was used to detect N content using an elemental analyzer (Vario EL III, Elementar, Germany). The sterilized treatments were chosen to eliminate the biotic disturbance. The net abiotic (i.e., sorption and emission) and biotic (i.e., mineralization, nitrification, and assimilation) NH4+-N changes in soils caused by biochar addition can be written as: ΔC abiotic ¼ C ck‐sterilized −C 5%‐sterilized ΔC biotic ¼ ðC initial −C 5%‐unsterilized Þ−ðC initial −C ck‐unsterilized Þ−ΔC abiotic

ð1Þ

Where ΔCabiotic (mg NH4+-N kg−1) and ΔCbiotic (mg NH4+N kg−1) are the 5 % biochar caused NH4+-N abiotic and biotic changes, respectively; Cck-sterilized (mg NH4+-N kg−1) and Cck−1 + + unsterilized (mg NH4 -N kg ) are NH4 -N levels in the nobiochar soils under sterilized and unsterilized conditions after 14-day incubation, respectively; C5%-sterilized (mg NH4+N kg−1) and C5%-unsterilized (mg NH4+-N kg−1) are the NH4+N levels of 5% biochar-amended soils under sterilized and unsterilized conditions after 14-day incubation, respectively. Cinitial (mg NH4+-N kg−1) is the initial NH4+-N amount of treatment at the start of incubation (original soil NH4+-N + applied 200 mg kg−1 NH4+-N). The cumulative NH3 emission during the 14-day incubation can be written as:  X  ð F iþ1 þ F i Þ  ðt iþ1 −t i Þ S¼ 2

ð2Þ

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Where S is the NH3 cumulative emission (μg NH3-N kg−1), Fi (μg NH3-N d−1 kg−1) is NH3 emission rate, and ti is incubation days. Statistical analysis The difference among the treatments was analyzed with one-way ANOVA. All tests of significance (P