Composition variability of spent mushroom substrates during continuous cultivation, composting process and their effects on mineral nitrogen transformation in soil

Geoderma 307 (2017) 30–37

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Composition variability of spent mushroom substrates during continuous cultivation, composting process and their effects on mineral nitrogen transformation in soil

MARK

Zimo Lou, Yue Sun, Xiaoxin Zhou, Shams Ali Baig, Baolan Hu, Xinhua Xu⁎ Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, People's Republic of China

A R T I C L E I N F O

A B S T R A C T

Keywords: Spent mushroom substrates Compositional analysis Soil amendment Mineral nitrogen

Spent mushroom substrates (SMSs) are a typical by-product of mushroom production, which is rich in nutrient like nitrogen. The reuse of SMSs as soil amendments has become the focus of attention. Due to its nutrient content, SMSs could contribute to reduce the use of non-renewable resources, such as peat. Recently, a SMSs recycling strategy was adopted in an edible mushroom industry Ltd., China. The waste materials experienced a continuous cultivation, composting process which sequentially includes the cultivation of Pleurotus eryngii, the cultivation of Pleurotus ostreatus, the composting process, and were finally used as soil amendments. The main objective of this study was to investigate the composition variability of SMSs during continuous cultivation, composting process and their effects on mineral nitrogen transformation in amended soil. The components analysis suggested the relative moisture and polysaccharide content of SMSs decreased by 33.6% and 17.1%, respectively, during the continuous cultivation, while protein increased by 29.5%. Moreover, relative humic acid content of SMSs increased by 18.6% during composting process while biodegradable matter (BDM), polysaccharide and protein decreased by 38.6%, 79.4% and 50.0%, respectively. The results of a 42-day incubation experiment suggested that addition of spent mushroom composts (SMCs) can significantly enhance the mineral nitrogen contained in soil. With the amendment of SMCs, 39.4% of input nitrogen was converted into mineral nitrogen within day 42. Combined application of SMCs/urea was considered to be a better strategy, since it provided soil with more mineral nitrogen than SMCs alone throughout the whole incubation.

1. Introduction China is one of the most important mushroom-producing countries contributing over 80% of the global total output of mushrooms and yearly production > 20 million tons (Li, 2012; Phan and Sabaratnam, 2012).As a by-product of mushroom industry, spent mushroom substrates (SMSs) have also shown a dramatic increase over the years (Kapu et al., 2012). About 5 kg of SMSs are produced from each kilogram of mushrooms (Paredes et al., 2009) and the quantity of SMSs generated by mushroom production in China was 13.2 million tons per year in 2010 (Gao et al., 2015). Traditionally, incineration has been applied for the final disposal of abandoned SMSs, which could cause a series of environmental problems including air pollution. Thus, it is necessary to adopt new techniques for the beneficial use of SMSs in value-added applications. SMSs have potential uses in many fields, such as soil-less growing medium (Medina et al., 2009; Ribas et al., 2009), soil and water bioremediation (García-Delgado et al., 2013; Jordan et al., 2008; Lau et al.,

⁎

Corresponding author. E-mail address: [email protected] (X. Xu).

http://dx.doi.org/10.1016/j.geoderma.2017.07.033 Received 8 August 2016; Received in revised form 4 July 2017; Accepted 26 July 2017 0016-7061/ © 2017 Elsevier B.V. All rights reserved.

2003; Li et al., 2012), energy feedstocks (Finney et al., 2009), animal feeds (Li et al., 2001) and organic amendments (Courtney and Mullen, 2008; Paula et al., 2017). However, most of these applications are not viable, and are unable to completely solve the disposal problem of SMSs; only agricultural use is an economically and ecologically acceptable way (Paredes et al., 2016). Considering the nutrient-rich residues contained in SMSs, mushroom production companies in China have already established a continuous cultivation which means to reuse the original spent substrates as the medium of another mushroom. Moreover, the residual wastes after harvesting can be composted and used as soil amendments. So far, the whole recycling of SMSs has been realized. Qingyuan, a county located in the southwestern part of Zhejiang Province, is one of the main cultivation areas of edible mushroom in China. Being considered as one of the largest species of edible mushroom, Pleurotus eryngii (also known as king trumpet mushroom) is widely cultivated in Qingyuan. In order to recycle SMSs, local edible mushroom industries tend to reuse the SMSs of Pleurotus eryngii (SMSs-

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PE) as cultivation medium of another common edible mushroom, Pleurotus ostreatus. Subsequently, the collected residues (SMSs-PO) of mature Pleurotus ostreatus are composted to make spent mushroom composts (SMCs), which can be used as soil amendments. The recycling process of SMSs has been defined as a continuous cultivation, composting process. As reported, the use of compost for soil amendment is a promising agricultural practice environmentally and economically viable (Cicatelli et al., 2014). However, the nutrients contained in SMSs are not available for plants immediately (Curtis and Suess, 2006). This is particularly the case for nitrogen (Hackett, 2015). Maher et al. (2000) has reported that only 10.8% of the total N in SMCs was present in the plant available forms of nitrate and ammonium while Bercher and Pakula (2014) reported that mineral N contained < 10% of total N in SMSs. All the literatures mentioned above suggest that nitrogen content contained in SMCs is slowly released to soil through mineralization (Stewart et al., 2000). Although some studies have discussed the nitrogen fractions in SMSs or properties of SMSs/SMCs-amended soil, few researchers have focused on the release process of mineral nitrogen from SMSs/SMCs to soil. In other words, the mineralization of nitrogen in SMSs/SMCs needs to be thoroughly evaluated since it is the main process regulating nitrogen availability. Hence, in this study, we first analyzed the physico-chemical properties of SMSs at different stages of the continuous cultivation, composting process (including the prepared cultivation mediums as PCMs, the reused cultivation medium as SMSs-PE, the raw composting material as SMSs-PO, and the product of compost as SMCs). Moreover, an indoor incubation experiment was conducted under field conditions to evaluate variations of mineral N transformation in soil amended with SMSs at different stages. Soils amended with traditional nitrogen fertilizers including urea and ammonium sulfate were set up as control groups in this work.

Table 1 Physicochemical characteristics of selected soils. Soils Sampling site pH (1:2.5, w/v) SOM (%) CEC (cmol/kg) Texture (%) Sand Silt Clay

Hangzhou, Zhejiang Province of South China (30°18′20″N, 120°4′21″E) 6.37 3.5 12.8 24.2 60.4 15.4

prior to experiments (Tong and Xu, 2012; Zhao et al., 2014). 2.2. Component analysis of SMSs/SMCs Generally, several parameters related to nutritional ingredients of SMSs/SMCs were measured to evaluate the changes that occurred during the continuous cultivation, composting process. Moisture content was determined through an oven-drying method. The pH values were measured by extracting SMSs/SMCs in distilled water with a ratio of 1:5 (w/v), using a Mettler Toledo pH meter. Electrical conductivity (EC) and total dissolved solids (TDS) were analyzed through a water extract (in the ratio of 1:5, w/v) using conductivity meter (DDBJ-350, LeiCi, China). Prior to chemical analysis, all the materials were pulverized to pass through a 0.15-mm sieve. Total carbon, hydrogen and nitrogen contents were assessed using a CHN elemental analyzer (Flash EA 1112, ThermoFinnigan), while the C/N ratio was calculated subsequently. Protein component was assessed using Coomassie Brilliant Blue method (Sedmak and Grossberg, 1977). Polysaccharide component was obtained by using phenol‑sulfuric acid method, as described elsewhere (Masuko et al., 2005). Ash content was measured based on China National Standards (GB/T 12532-2008 determination of ash content in edible mushroom). Cellulose, hemicellulose and lignin contents were analyzed according to the Van-Soest method recorded in Agricultural Handbook (No. 379). After NDF (Neutral detergent fiber), ADF (acid detergent fiber), and ADL (acid detergent lignin) were all determined; the cellulose content was estimated as ADF-ADL, the hemicellulose was calculated as NDF-ADF, while the lignin content was equal to the value of ADL. Contents of total humic acid and BDM (biodegradable matter) were detected by the methods described by Bao (2000) and Zhao et al. (2002), respectively.

2. Materials and methods 2.1. Materials The prepared cultivation medium (PCMs), SMSs of Pleurotus eryngii (SMSs-PE), SMSs of Pleurotus ostreatus (SMSs-PO) and SMCs, used in the present study were generously provided by an edible mushroom industry Ltd., located in Qingyuan county (Zhejiang Province, China). The detailed information about the PCMs was recorded in a Chinese patent (patent number: CN201310312985.7, Wu et al., 2013). The formulation of PCMs contains sweet potatoes (5–10%), sawdusts (30–40%), wheat brans (10–20%), bagasses (5–15%), straws (10–30%), calcium carbonates (5–8%) and etc. After harvesting fruit bodies of Pleurotus eryngii, residual mediums were collected as SMSs-PE. Afterwards, SMSs-PE (93%) was combined with sawdust (5%), CaSO4 (2%), mineral salt (0.5%) to produce mediums for Pleurotus ostreatu growing. Similarly, the residual mediums of Pleurotus ostreatu were collected as SMSs-PO. Finally, the SMSs-PO was supplemented with CaSO4 (2%), mineral salt (0.5%), small amounts of lime water (to neutralized the organic acid during composting) and conducted with a combined static, heap-turning composting process to obtain the mature compost, which was collected as SMCs. As shown in Fig. S1(c) and (d), mixed materials were built into static heaps. Static piles were periodic turning to ensure adequate aeration using turning machines. Additionally, forced ventilation was not used in this case. The composting process was completed in three-months. Considering land-use and soil properties, the loamy soil samples used in the incubation experiment were collected from Hangzhou, Zhejiang Province, China (30°18′20″N, 120°4′21″E), which served as a typical subtropical cropland. The main background values of soils were presented in Table 1. The soil samples were collected from the top layer in soil profile, as described by other researchers (Zhao et al., 2014; Tong and Xu, 2012) (0–20 cm). According to published works, all the fresh materials and soil were air dried and pulverized through a 2-mm sieve

2.3. Characterization of substrate samples FTIR (Prestige-21 Japan) analyses were performed to characterize the changes of chemical functional groups with a wavelength range from 4000 to 400 cm− 1. In order to describe the morphological changes in the samples, a scanning electron microscopy (SEM) (FEIquanta 200F, Netherland) was performed under acceleration voltage of 30 kv with a largest magnification of 10 K. 2.4. Incubation experiment To evaluate the mineral N transformation in soil, incubation experiments were designed under simulated field conditions. A series of 250 mL beakers were prepared for incubation experiments. Each beaker was filled with different treatments, which contains 600 g of soil and corresponding amount of nitrogen source (Table 2). A constant concentration value of 400 mg N/kg was added into treatments with different nitrogen nutrient sources, according to the application rates of nitrogen fertilizer in actual field (Ju et al., 2009). As typical forms of N fertilizer, urea and ammonium sulfate were both chosen for chemical fertilizers. Based on the total content of nitrogen measured in Table 3, detailed dosages of SMSs-PE, SMSs-PO and SMCs were adjusted and 31

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3. Result and discussion

Table 2 Details of the treatments in incubation experiments. Treatments

Dosage

Abbreviation

0 mg N/kg addition 400 mg N/kg ammonium sulfate

– 1.13 g ammonium sulfate 0.52 g urea 9.41 g SMSs-PE 13.68 g SMSs-PO 21.00 g SMCs 4.7 g SMSs-PE + 0.26 g urea 6.84 g SMSs-PO + 0.26 g urea 10.50 g SMCs + 0.26 g urea 4.70 g SMSs-PE + 0.57 g urea 6.84 g SMSs-PO + 0.57 g urea 10.50 g SMCs + 0.57 g urea

CK AS

400 mg N/kg urea 400 mg N/kg SMSs-PE 400 mg N/kg SMSs-PO 400 mg N/kg SMCs 200 mg N/kg SMSs-PE + 200 mg N/kg urea 200 mg N/kg SMSs-PO + 200 mg N/kg urea 200 mg N/kg SMCs + 200 mg N/kg urea 200 mg N/kg SMSs-PE + 200 mg N/kg ammonium sulfate 200 mg N/kg SMSs-PO + 200 mg N/kg ammonium sulfate 200 mg N/kg SMCs + 200 mg N/kg ammonium sulfate

3.1. Compositional changes of SMSs/SMCs Table 3 presented the parameters related to nutritional ingredients and results revealed compositional variations among different substrates. As mentioned above, the continuous cultivation, composting process of substrates could be divided into 3 stages. Firstly, slight changes appeared between PCMs and SMSs-PE during the growth of Pleurotus eryngii. In contrast, the chemical compositions of SMSs-PE and SMSs-PO were observed to be different from each other during the growth of Pleurotus ostreatus. Finally, noteworthy chemical changes were discovered between SMSs-PO and SMCs, throughout the composting process. The moisture contents of all the substrates followed the order of PCMs > SMSs-PE > SMSs-PO (Table 2). That could be ascribed to the consumption of moisture for the development of fruit bodies. Actually, the relative humidity surroundings during the growth of Pleurotus eryngii and Pleurotus ostreatus have been adjusted to their optimum conditions. Since Pleurotus eryngii needs about 90–95% RH (Zhang et al., 2014) and Pleurotus ostreatus needs about 80–85% RH (Thongklang and Luangharn, 2016), the moisture content of SMSs-PE (56.1%) was higher than that of SMSs-PO (42.3%). The gap between PCMs and SMSs-PO suggested that moisture content decreased by 33.6% during continuous cultivation. The consumption of carbohydrates was another indicator of cultivation. Polysaccharide levels dropped by 17.1% between PCMs and SMSs-PO, which could be interpreted as the metabolism of both mushrooms. Moreover, the slight increase of pH value between SMSsPO and SMSs-PE could be attributed to the addition of CaSO4 and salts before inoculation. The relatively stable contents of TDS, and EC level reflected that the water soluble salts showed negligible differences during continuous cultivation. Recent studies revealed that Pleurotus ostreatus could degrade lignocellulosic substrates (Sánchez, 2010). Similarly, in this case the percentage of lignin and cellulose contained in SMSs-PO decreased by 3.2% and 5.6% when compared to SMSs-PE, respectively. Such decreases in cellulose in respect to lignin seem to be the case not only in spent substrates but in other lignocellulosic substrates as well (Koutrotsios et al., 2014). The protein content in the substrates showed an increasing trend from PCMs (17.3 mg/g) to SMSsPE (18.6 mg/g), then to SMSs-PO (22.4 mg/g) in this case. This phenomena could be ascribed to the fact that the fruit formation would give rise to an increase in protein. As reported by Li et al. (2001), residual mycelial biomass could increase the protein content of spent mushroom substrates. The comparative analysis of SMSs-PO and SMCs demonstrated chemical changes during composting process. Based on the elemental analysis results presented in Table 3, O/C atomic ratios were found to be higher in SMCs than that of SMSs-PO, while the opposite results were obtained for H/C atomic ratios. The outcome could be ascribed to the accumulation of aromatic compounds and the loss of carbohydrate, as reported elsewhere (Ko et al., 2005). During composting, the organic compounds were decomposed into stable products, which could be used as nutrient sources (Huang et al., 2006). Thus, the BDM, polysaccharide and protein contents of SMCs were found to decrease by 38.6%, 79.4% and 50.0%, respectively, as compared to SMSs-PO. In addition, the humic acid contents of SMCs was 18.6% higher than that of SMSs-PO, which confirmed that biodegradable organic matter was converted into humic substances throughout composting. Moreover, the decomposition of protein also led to the N release in the form of NH3, which increased pH from 6.2 (SMSs-PO) to 7.2 (SMCs). According to the reported studies, a final C/N ratio < 20 indicates the maturity of the compost (Khalil et al., 2008). In this study, the C/N ratio of 13.6 was observed for SMCs after a three-months composting process and thus fully matured.

U SMSs-PE SMSs-PO SMCs SMSs-PE/U SMSs-PO/U SMCs/U SMSs-PE/AS SMSs-PO/AS SMCs/AS

Table 3 Compositional changes of SMSs/SMCs during the continuous cultivation, composting process. Parameters

PCMs

SMSs-PE

SMSs-PO

SMCs

C (%) H (%) O (%) N (%) C/N H/C (atomic ratio) O/C (atomic ratio) Ash (%) Protein (mg/g) Polysaccharide (mg/g) Moisture (%) pH (1:5, w/v) EC value (1:5, w/v)(mS/cm) TDS(mg/l) Lignin (%) Cellulose (%) Hemiscellulose (%) BDM (%) Total humic acid (%)

35.1 5.2 28.5 2.6 13.5 1.78 0.60 14.1 17.3 11.7 63.7 5.6 4.1 21 5.7 20.0 9.0 – –

37.1 5.2 34.0 2.6 14.6 1.68 0.68 11.0 18.6 7.3 56.1 5.8 4.4 21.8 14.8 34.7 7.1 39.8 –

32.1 4.4 32.0 1.7 18.4 1.64 0.75 14.3 22.4 9.7 42.3 6.2 4.5 22.5 11.6 29.1 7.3 32.4 11.8

15.3 2.1 43.2 1.1 13.6 1.64 2.10 12.7 11.2 2.0 41.6 7.2 3.7 18.2 12.1 29.8 7.0 19.9 14.0

presented in Table 2. The incubation procedure was conducted in the dark, as reported elsewhere. (Zhao et al., 2014; Tong and Xu, 2012; Song et al., 2014). During the incubation period, all the treatments were cultivated inside a shaking incubator (TS-211GZ, Tensuc, China) aerobically at 25 °C for six weeks. Deionized water was added to soil for maintaining the soil moisture at 60% of field water-holding capacity (Novak et al., 2009). The loss of water due to the evaporation was supplemented by an appropriate amount of water every three days (Zhao et al., 2014; Koutrotsios et al., 2014). Soil samples from each treatment were taken on day 0, 1, 2, 5, 7, 14, 21, 28, 42 for analyses. Each samples were replicate three times. Based on the Chinese standards of environment protection (HJ 634-2012), NH4+-N and NO3−-N in soil were first extracted with 1 M KCl with a ratio of 1:5 (w/v), and then measured by the indophenol blue method (Dorich and Nelson, 1983; Medina et al., 2012) and UV-spectroscopy (Sempere et al., 1993; Medina et al., 2012), respectively. The pH value was measured on a 1:2.5 soil:water (w/v) ratio (Chinese standards of agriculture, NY-T 1377-2007). The statistically significant difference tests were conducted using the Duncan's multiple range test program from SPSS 20.0.

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Fig. 3. Dynamics of (a) NH4+-N, (b) NO3−-N and (c) mineral N in soil with/without amendment of SMCs. Fig. 1. FTIR spectra of SMSs/SMCs samples during the continuous cultivation, composting process.

polysaccharides appeared around 1029 cm− 1 (Tsui and Juang, 2010). The absorption signal at 1321 cm− 1 firstly increased from PCMs to SMSs-PE, and finally decreased from SMSs-PO to SMCs, implying the transfer of nitrogen from substrates to mushroom fruits (Li et al., 2001) among continuous cultivation and consumption of organic nitrogen among composting process. As the maturation continues, peaks at 1029 cm− 1 further decreased, indicating that materials underwent progressive polysaccharides consumption and became stable (Li et al., 2001). The vibration of band at 3330 cm− 1 could be ascribed to existence of hydrogen bonding in cellulose (Abidi et al., 2014). With the degradation of lignocellulose, the gentle peaks lose their intensities from SMSs-PO to SMCs. In addition, the weakened strengths at 2920 cm− 1 vibration could be attributed to the loss of specific

3.2. Characterizations of SMSs/SMCs Fig. 1 presents the FTIR spectra of different samples. It was observed that cellulose spectra were more prominent in all the samples, which was consistent with the study reported earlier (Cao and Tan, 2002). Numerous absorption bands were recorded around 3330 cm− 1, including the peaks attributed to the eOH functional group and eNH group (Toptas et al., 2014). Aliphatic CeH stretch was noticed around 2920 cm− 1, and the C]O bonds of eCOOH group at 1645 cm− 1. A narrow band appeared at 1321 cm− 1 could be assigned to the CeN bonds in amides or amines (Lau et al., 2003). Moreover, CeO stretch of

Fig. 2. SEM analysis of (a) PCMs (b) SMSs-PE (c) SMSs-PO (d) SMCs.

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noteworthy to find that the SMCs sample became more compacted than SMSs-PO, suggesting the humification of raw materials. As an evidence, the BDM contents of SMCs was decreased by 38.6% when compared to SMSs-PO, while the total humic acid contained in material increased from 11.8% to 14.0%.

Table 4 Summary of mineral nitrogen contents in soil with amendment of urea/ammonium sulfate/SMCs after 42-day incubation. Treatmentsa

NH4+-N (mg N/kg)

NO3−-N (mg N/kg)

Mineral-N (NH4+-N+ NO3−-N, mg N/kg)

CKb Uc ASd SMCse

2.13 ± 0.22 c 26.03 ± 3.74 a 20.37 ± 0.54 b 3.27 ± 2.60 c

27.82 ± 3.71 c 105.17 ± 10.55 b 115.46 ± 5.30 b 154.08 ± 23.58 a

29.95 ± 3.63 b 131.20 ± 5.10 a 135.83 ± 5.10 a 157.35 ± 24.10 a

3.3. Evaluation of mineral nitrogen transformation in SMCs-amended soil 3.3.1. Mineral N transformation in soil with the addition of SMCs The relation between carbon to nitrogen ratio (C/N) and available nitrogen has been discussed in a previous study (Chalhoub et al., 2013). A C/N ratio > 15 could limit N availability in soil due to the use of mineral N to build microorganism protein (Gutser et al., 2005; Amlinger et al., 2003). The composition of C/N ratio were measured and followed in the sequence of SMSs-PO (18.4) > SMSs-PE (14.6) > SMCs (13.6) during the continuous cultivation, composting process. It suggested that the SMCs could contribute more available N than SMSs-PE and SMSs-PO in soil. The addition of SMCs increased the initial mineral N content in soil from 15.74 mg N/kg (incubation day 0) to 64.00 mg N/kg (incubation day 0), as shown in Fig. 3. Over the incubation time, the mineral N of control treatment rose slightly with a minimum value of 14.62 mg N/kg in day 1 and reached to 29.95 mg N/kg in day 42. After a 42-day

a Mean values in columns followed by the same letter do not differ significantly (P < 0.05) between the treatments. b CK refers to treatment amended with no addition of nitrogen source (Table 2). c U refers to treatment amended with urea as nitrogen source (Table 2). d AS refers to treatment amended with ammonium sulfate as nitrogen source (Table 2). e SMCs refers to treatment amended with SMCs as nitrogen source (Table 2).

constituents of cell walls, such as pectin (Singh et al., 2009). In order to describe the morphological changes of samples, the SEM images of sieved PCMs, SMSs-PE, SMSs-PO and SMCs were depicted in Fig. 2. It can be noticed that all the samples showed irregular surface. However, Fig. 2(b and c) demonstrated a series of pores on the fibrous structures as the result of degradation and consumption of lignin, cellulose and hemiscellulose (Yan and Wang, 2013). In Fig. 2(d), it was

Fig. 4. Variations of the NH4+-N in the soil amended with different nitrogen sources during the 6 weeks of incubation (a) SMSs-PE/U addition; (b) SMSs-PO/U addition; (c) SMCs/U addition; (d) SMSs-PE/AS addition; (e) SMSs-PO/AS addition; (f) SMCs/AS addition.

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Fig. 5. Variations of the NO3−-N in the soil amended with different nitrogen sources during the 6 weeks of incubation (a) SMSs-PE/U addition; (b) SMSs-PO/U addition; (c) SMCs/U addition; (d) SMSs-PE/AS addition; (e) SMSs-PO/AS addition; (f) SMCs/AS addition.

2014). In day 42, the net NO3− accumulation in U and AS treatments reached 105.17 mg N/kg and 115.46 mg N/kg, respectively, thus could account for 26% and 29% of the original N. As shown in Table S4, the ANOVA analysis suggested that there was a significant difference in mineral N content among SMCs and U or AS treatment in day 42. Therefore, SMCs was considered to be a better N-available source when compared to ammonium sulfate and urea, with a higher mineral N content of 157.35 mg N/kg. The dynamics of mineral N transformations in soils amended with SMCs and chemical fertilizers were presented separately in Fig. 4(c and f), and Fig. 5(c and f). Due to hydrolysis of urea, the NH4+-N in U treatment continuously increased to a peak value of 43.50 mg N/kg at day 7 (Fig. 4). Subsequently, the maximum value then declined to a steady value around 26.03 mg N/kg at day 42 due to nitrification. Accordingly, NO3−-N content of U treatment was observed to increase during the first week of incubation and then remained stable after day 14 (Fig. 5c). Circumstances were somewhat different in AS treatment. Owing to the fact that all the input N elements existed as ammonium nitrogen in (NH4)2SO4, the NH4+-N contents of AS treatment were much higher (~ 372.20 mg N/kg) than urea and SMCs treatments at the beginning (day 0) (Fig. 4f). As a result of soil nitrification, NO3−-N contents in AS treatment kept rising during the whole incubation (Fig. 5f). It was apparent that SMCs showed a potential to be prolonged

incubation, the mineral N of SMCs treatment increased significantly from 29.95 mg N/kg (CK) to 157.35 mg N/kg (P < 0.05, as shown in Table S4). This result suggested that the incorporation of SMCs could notably enhance the mineral N in soil. Fig. 3 presents the dynamics of mineral N in soil. It could be observed in Fig. 3a that the NH4+ concentration in SMCs-added soil increased to a peak at approximately day 5 and then declined to control level at about day 21, which suggested that the mineralization of SMCs occurred in the first week after the application of SMCs. Fig. 3b shows the growth with respect to NO3− concentration. In response to the mineralization period, NO3− concentration in soil rose slightly in the first 7 days. Nevertheless, the curve shows apparent rising trend for the rest of incubation, implying the positive effects on soil nitrification with amendment of SMCs. As a result, the net NO3− accumulation in SMCs treatment reached to 154.08 mg N/kg, which accounted for 38.5% of the original N (400 mg N/kg) in soil. 3.3.2. Comparisons between soils amended with SMCs and chemical fertilizer Table 4 shows the distribution of mineral N species of SMCs/chemical fertilizer treatments after 42 day incubation. It was noticed that the addition of urea or ammonium sulfate tended to fluctuate the nitrification of soil to various extents, as mentioned earlier (Zhao et al., 35

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N released source when compared to chemical fertilizers.

Acknowledgements

3.3.3. Combined application of SMCs and chemical fertilizer The blending of compost increased immobilization of urea-N in soils (Han et al., 2004). In this study, the combined application of SMCs and chemical fertilizer was also studied in the incubation experiment, as shown in Figs. 4 and 5. Fig. 4c presented the comparative analysis results of NH4-N+ content among U, SMCs and SMCs/U treatments. The NH4-N+ content contained in soil followed an order of U (26.03 mg N/ kg) > SMCs/U (5.47 mg N/kg) ~ SMCs (3.27 mg N/kg) after 42 days incubation. The ANOVA analysis suggested that there was no significant difference between SMCs/U and SMCs treatment in day 42. However, NO3−-N concentrations of SMCs/U in day 42 was found to be much higher than U, as shown in Fig. 6c. Similarly, the mineral N value of SMCs/U group throughout the 42-day incubation was higher than that of SMCs, which indicated that the combined use of SMCs/U could supply more available N to soil (Table 4S). In order to clarify the importance of composting, SMSs-PE/U and SMSs-PO/U treatments were also conducted for comparison. As shown in Table S2, NH4+-N of both SMSs/U and SMCs/U combined treatments showed no significant difference from day 21 to day 42, which suggested that NH4+-N content in soil reached a stable level. On the contrary, NO3−-N of SMCs/U increased dramatically (P < 0.05) when compared to SMSs-PE/U and SMSs-PO/U after day 21 (Table S3), suggesting that combination of SMCs/U could have a lasting impact on soil. Comparative analysis in Table S4 demonstrated that mineral nitrogen content of SMCs/U it was significantly higher than that of SMSs/U after 42 days incubation, since the mineralization peak postponed. The phenomenon was consistent with other studies. At first, nitrogen contained in urea would be immobilized when combined with compost (Han et al., 2004). Subsequently, the immobilized nitrogen was converted into microbial biomass nitrogen, which could be easily mineralized (Aoyama and Nozawa, 1993). In contrast, Table 4S also presented that combined application of ammonium sulfate and SMCs in soil obtained a poor mineral N content of 39.56 mg N/kg after 42 days incubation. As shown in Table S1, pH value of AS, SMCs and SMCs/AS treatment ranged from 5.64 to 6.07, 7.20 to 7.67, and 7.15 to 7.47, respectively. As a consequence of higher pH, N in ammonium sulfate would be transformed into ammonia under alkaline condition. Thus, the mineral N in AS/ SMCs treatment was severely limited. In sum, the combined use of SMCs and urea could provide more mineral N than single use, while the combination of SMCs and ammonium sulfate was not considered to be a good strategy.

The authors acknowledge for the financial support from the Major Science and Technology Program for Water Pollution Control and Treatment (2014ZX07101-012-04). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.geoderma.2017.07.033. References Abidi, N., Cabrales, L., Haigler, C.H., 2014. Changes in the cell wall and cellulose content of developing cotton fibers investigated by FTIR spectroscopy. Carbohydr. Polym. 100, 9–16. Amlinger, F., Götz, B., Dreher, P., Geszti, J., Weissteiner, C., 2003. Nitrogen in biowaste and yard waste compost: dynamics of mobilisation and availability—a review. Eur. J. Soil Biol. 39, 107–116. Aoyama, M., Nozawa, T., 1993. Microbial biomass nitrogen and mineralization-immobilization processes of nitrogen in soils incubated with various organic materials. Soil Sci. Plant Nutr. 39, 23–32. Bao, S.D., 2000. Soil Agro-Chemistry Analysis. Chinese Agriculture Press, Beijing (in Chinese). 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4. Conclusion In summary, recycling of SMSs during continuous cultivation, composting process can be classified into continuous cultivation of Pleurotus eryngii, Pleurotus ostreatus and composting of SMSs. Component analysis suggested that moisture and polysaccharide content decreased, but protein increased during continuous cultivation; while BDM, polysaccharide and protein decreased, but humic acid increased during composting process. The mature materials named SMCs could significantly enhance the mineral nitrogen contained in soil. Soil amended with 400 mg N/kg SMCs had enhanced mineral nitrogen of 157.35 mg N/kg when compared with control (29.95 mg N/kg), after 42 days incubation. Comparison among different treatments suggested that SMCs had a more prolonged effect on mineral nitrogen than chemical fertilizers. Combined application of SMCs/urea was considered to be a better strategy than SMCs independently, with more mineral nitrogen over a longer time. It would be worthwhile in future studies to focus on the release of gases during composting of SMSs and application of SMCs, since the possible production of NH3, N2 and N2O would cause N loss during composting (Mahimairaja et al., 1994; Leytem et al., 2011) and N2O would act as a greenhouse gas with strong radiative forcing. 36

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