A positive valorization way of sludge compost: recycling it to turfgrass instead of partial conventional substrate

ABSTRACT

Sludge is known to be a suitable amendment material to substrate used for horticultural products because of its soil-like physical and nutritional characteristics. Replacing a portion of substrate used for turfgrass with sludge has been shown to have positive effects, such as improved biomass productivity, root length and density as well as persistence of vegetation cover. Although the effects of adding sludge compost to turfgrass substrate have been widely investigated, the characteristics of commercial qualitative indicators of the generated turfgrass, substrate and sod products have been reported separately or incompletely. Therefore, this study was conducted to assess the effects of recycling composted sludge to turfgrass on commercial qualitative indicators. The results revealed positive and negative effects of sludge compost on the investigated parameters, with some factors showing positive responses to addition of sludge compost to a threshold, above which negative responses were observed. For example, water-holding porosity, biomass and chlorophyll content increased in response to sludge amendment, but increased salinity and weed density, as well as decreased tensile strength and intactness were also observed. Overall, sludge compost in moderate dose is a suitable material for replacement of partial turfgrass substrate, but excessive dose results in adverse effects.

KEYWORDS:

• Compost
• sludge
• sod
• substrate
• turfgrass

Introduction

It is well-known that sludge, especially aerobically composted sludge, is a good amendment material for horticultural substrate because its physical and nutritional characteristics are similar to soil. So far, accumulative studies indicate that recycling of sludge to turfgrass instead of partial substrate has positive effects on turfgrass growth (Stan et al. 2009; Bai et al. 2013), such as improved grass vegetation productivity (Linde & Hepner 2005; Singh & Agrawal 2008; Fuentes et al. 2010; Kitczak et al. 2012), increased dry matter accumulation of above-ground parts (Dede & Ozdemir 2015), enhanced root length and density (Chang et al. 2014), expanded persistence of vegetation cover (Brown & Gorres 2011), improved leaf color (Tesfamariam et al. 2009; Bilgili et al. 2011) and increased clipping yield (Nektarios et al. 2011). Strengthened disease suppression because of increasing microbial activity in the presence of composted sludge has also been reported (Stan et al. 2009).

However, the substitution of composted sludge for turfgrass substrate can also have detrimental effects. For instance, depressed seedling emergence and plant growth were observed when the proportion of sludge to substrates exceeded 40% (Cheng et al. 2007). In addition, Celebi et al. (2011) showed that plant height, green grass yield and plant-covered area values were reduced when high doses of sludge were used during the primary application period. Moreover, mass evaluation to sludge added sod revealed that incremental proportion of sludge compost to substrate led to a linear reduction in sod mass and decreased tensile strength of sod (Backes et al. 2013). Certainly, similar to other reuse methods, recycling to turfgrass is also puzzled by heavy metal resulted from sludge. Although no risk to food chain, there remains potential injury to physiological metabolism, biomass and plant growth of turfgrass (Hua et al. 2008; Wang et al. 2008; Wolejko et al. 2013). Salinity can also be problematic because of high salt content in sludge, which suppresses the establishment of plant seedlings (Cai et al. 2010; Liu et al. 2014).

After all, everything has both its pros and cons. It is coexistence on the advantage and disadvantage brought by recycling of sludge to replace turfgrass substrate. Although many studies have investigated the effects of adding sludge compost to turfgrass substrate, the characteristics of the generated turfgrass, substrate and sod products have been reported separately or incompletely. As a result, qualitative indicators of horticultural products, such as grass-leaf color, tiller number, time required for establishment, green maintenance period, sod intactness, tensile strength, and even transplantation performance, should be integratedly evaluated. Therefore, the goal of this study was integrated assessment on the effects of recycling of sludge to turfgrass instead of partial substrate, with an emphasis on commercial qualitative indicators. Systemic and objective conclusion is look forward to being drawn out in the aid of overall results carrying out in condition of field-scale experiment.

Materials and methods

Sewage sludge was collected from a municipal wastewater treatment plant in Qinghuangdao (Hebei Province, China) and mixed with wheat straw at a ratio of 5:4 by volume. The resulting mixture was then composted in a static forced aeration device for 28 days, after which the finished compost was placed in a storehouse and allowed to age for 5 months. The conventional turfgrass substrate was made of loam and organic fertilizer at a ratio of 4:1 by volume. The physico-chemical properties of the sludge compost and conventional substrate were shown in Table 1.

Table 1. Main physico-chemical property of the sludge compost and conventional turfgrass substrate.

Indicator	Sludge compost	Conventional turfgrass substrate
Organic matter (%)	28.0	5.6
Total nitrogen content (%)	4.6	1.8
Total phosphorus content (%)	2.7	1.1
Total potassium content (%)	1.9	0.6
Moisture (%)	48.5	36.6
Maturity (%)	88	–
Total porosity (%)	0.61	0.55
Water-holding porosity (%)	0.46	0.42
Electrical conductibility (mS/cm)	0.45	0.26
Bulk density (g/cm^3)	0.95	1.01

The experimental site, which is located in Shunyi District, northeast Beijing, China, is a sod production base of the Chinese Seed Group. The grass species used in the experiment was tall fescue (Festuca arundinacea). The rate of seeding was standardized at 50 g/m², and a close-spaced drill was employed to complete seeding. Fifteen different treatment units (one treatment unit consisted of three parallel plots, each with an area of up to 0.2 hectares, total treated area nearly nine hectares) that received different doses of sludge compost (0 [i.e. conventional substrate], 30, 60, 90 and 120 Mg/ha) were randomly designed and distributed in the field, around which there was a separation belt with width of 50 cm for every treatment unit to minimize the interaction between different treatments.

Substrate physical properties including water-holding porosity, total porosity, electrical conductibility and bulk density were monitored and analyzed for 80 days after amendment of turfgrass substrate with composted sludge according to previous methods (Cai et al. 2010; Liu et al. 2014). Indicators, including establishment time, concomitant weed density and tiller number, were counted up to do a statistic once formation of turfgrass after four months seeding, growth and daily maintenance (Onofri et al. 2010). The plant height and root length were measured when the turfgrass was grown for 3 months. The fresh weight per seedling, leaf width and chlorophyll content were measured in the laboratory after turfgrass was sampled. The sod weight, sod tensile strength and sod intactness were analyzed after turfgrass was cut to sod product (width × length × thickness, 0.4 m × 1.5 m × 8 cm) using a portable tensiometer and netting analysis tool respectively. The green maintenance period after transplanting and root regeneration ability were measured until the sod was transported to and transplanted in the fixed planting site after a one week adaptive period. The proportion of greening recovery in the following year was measured in a randomly selected area of 12 m² in a fixed planting site in April of the following year.

All results and data were analyzed by ANOVA using SPSS v13.0. Mean values followed by a different letter are significantly at a significance of P < .05 based on ANOVA (Tables 2–5).

Results

Effect of amendment of composted sludge on physical properties of substrate

As shown in Table 2, the bulk densities of substrate tended to decrease with increased sludge addition; however, significant differences were only observed between sludge amendments and conventional substrate (P < .05). There were no obvious differences among the doses of sludge applied (30, 60, 90 and 120 Mg/ha). These findings were, to some extent, associated with the loose structure of sludge compost. The EC value was attributed to the abundance of soluble salt in the solid material. As the sludge compost amendment doses increased, EC values also increased, but these were not evident until the dose reached 120 Mg/ha. There were no significant differences in porosity among amendment doses of sludge compost and conventional substrate. However, water-holding porosity increased significantly in comparison to conventional substrate as the sludge compost dose increased from 60 to 120 Mg/ha (P < .05).

Table 2. Effect of amendment of sludge compost to turfgrass on physical properties of substrate at different doses.

Dose of composted sludge added to substrate (Mg/ha)	Bulk density (g/cm^3)	EC (mS/cm)	Total porosity (%)	Water-holding porosity (%)
120	0.94 ± 0.07 a	0.29 ± 0.23 a	0.63 ± 0.03 a	0.49 ± 0.02 a
90	0.96 ± 0.04 a	0.27 ± 0.13 ab	0.63 ± 0.02 a	0.48 ± 0.06 a
60	0.95 ± 0.03 a	0.22 ± 0.08 b	0.65 ± 0.03 a	0.46 ± 0.02 a
30	0.98 ± 0.05 ab	0.23 ± 0.06 b	0.64 ± 0.04 a	0.43 ± 0.03 ab
Conventional substrate	1.01 ± 0.02 b	0.23 ± 0.03 b	0.64 ± 0.02 a	0.39 ± 0.03 b

Effect of amendment of composted sludge on biomass related profiles of turfgrass

During the production of turfgrass, fresh weight per seedling, plant height, root length and tiller number were selected to represent different tissue biomasses of total plants, stems, roots and leaves, respectively. As shown in Table 3, all indicators mentioned above showed gradual increases as sludge doses increased from 30 to 120 Mg/ha. However, when compared with conventional substrate, treatment with sludge compost at doses of 30–60 Mg/ha led to reductions in all of the aforementioned indicators except for tiller number.

Table 3. Effect of addition of sludge compost instead of partial substrate on biomass indicators of turfgrass at different doses.

Dose of composted sludge added to substrate (Mg/ha)	Fresh weight per seedling (g)	Plant height (cm)	Root length (cm)	Tiller number (piece)
120	5.51 ± 0.42 a	7.98 ± 0.35 a	7.86 ± 0.49 a	6.3 ± 0.9 a
90	5.22 ± 0.36 b	7.67 ± 0.28 b	7.42 ± 0.59 b	5.8 ± 1.1 ab
60	5.08 ± 0.28 c	7.58 ± 0.52 b	7.55 ± 0.35 ab	4.7 ± 1.2 b
30	5.03 ± 0.38 c	7.52 ± 0.31 b	7.18 ± 0.27 c	4.5 ± 1.2 b
Conventional substrate	5.42 ± 0.31 ab	7.87 ± 0.24 a	7.84 ± 0.36 a	5.1 ± 1.8 ab

Effect of amendment with composted sludge on turfgrass establishment

Leaf width and chlorophyll content are very important because sod purchasers prefer wider and darker green leaves. As shown in Table 4, leaf width increased as composted sludge dose increased from 30 to 120 Mg/ha, indicating that sludge compost improved leaf width. However, no significant differences in chlorophyll were observed among doses of 30, 60, 90 and 120 Mg/ha, but all treatments resulted in higher chlorophyll levels relative to conventional substrate. Concomitant weeds comprise an adverse problem of turfgrass and sod because of the need to manually remove them, which results in a direct price reduction of sod. There was a gradual increase in weed density as the dose of sludge compost changed from 30 to 120 Mg/ha, and all compost treatments showed significantly higher weed density relative to the control (P < .05). Establishment time reflects the speed of turfgrass development. There was an obvious delay in development observed between the 120 Mg/ha group and the control, 30 and 60 Mg/ha (P < .05) groups, but the 120 and 90 Mg/ha groups showed similar values, implying that incremental addition dose of sludge compost retard the speed of turfgrass development.

Table 4. Effect of amendment with different doses of sludge compost in place of substrate on turfgrass establishment.

Dose of composted sludge added to substrate (Mg/ha)	Leaf width (cm)	Chlorophyll content (mg/g)	Concomitant weed density (seedling/m^2)	Establishment time (day)
120	0.38 ± 0.05 a	1.13 ± 0.06 a	283.5 ± 15.8 a	67.7 ± 2.4 a
90	0.35 ± 0.02 ab	1.15 ± 0.06 a	262.4 ± 10.5 ab	63.5 ± 3.6 ab
60	0.32 ± 0.03 abc	1.11 ± 0.07 ab	234.8 ± 12.4 bc	61.6 ± 3.2 bc
30	0.30 ± 0.05 bc	1.08 ± 0.05 ab	228.2 ± 18.9 c	60.8 ± 4.1 bc
Conventional substrate	0.29 ± 0.04 c	1.04 ± 0.04 b	163.9 ± 21.4 d	58.2 ± 2.1 c

Effect of amendment with composted sludge on sod product related profiles

Sod weight is partially relative to bulk density, and also reflects the costs associated with moving from field to a fixed planting site. As shown in Table 5, as the does of sludge compost increased, the sod weight gradually decreased. Tensile strength and intactness are generally accepted as visible indicators of sod tolerance during moving, transporting, loading and unloading. Changes in tensile strength showed similar tendencies as sod weight, indicating that sod tensile strength decreased as the dose of sludge compost increased from 30 to 120 Mg/ha. Sod intactness was also affected by the addition of composted sludge, with that observed at 120 Mg/ha being obviously lower than the values observed at 30, 60 and 90 Mg/ha (P < .05). However, the sod intactness values of all treatment groups were lower than conventional substrate (P < .05). Taken together, these two indicators demonstrated that replacement of a portion of the substrate with sludge compost is disadvantageous for integrality keeping during sod transporting and moving.

Table 5. Effect of amendment with different doses of sludge compost on sod product related profiles.

Dose of composted sludge added to substrate (Mg/ha)	Sod weight (kg)	Sod tensile strength (N)	Sod intactness (%)
120	10.7 ± 0.4 a	15.7 ± 3.4 a	86.8 ± 0.5 a
90	11.2 ± 0.6 ab	19.6 ± 2.6 ab	88.5 ± 0.6 b
60	11.7 ± 0.5 b	22.7 ± 3.5 bc	88.8 ± 0.7 b
30	12.4 ± 0.4 bc	24.6 ± 1.7 c	89.2 ± 0.5 b
Conventional substrate	13.2 ± 0.6 c	26.5 ± 2.2 c	91.2 ± 0.4 c

Effect of amendment with composted sludge on turfgrass transplanting related indicators

When sod is produced, it is moved to another fixed planting site for second regeneration. During this process, greening maintaining period, root regeneration and proportion of greening recovery in the coming year are generally regarded as key indicators of sod quality. As shown in Table 6, sod treated with doses of 30 and 60 Mg/ha did not differ significantly from that produced using conventional substrate, but doses of 90 and 120 Mg/ha notably lengthened the greening maintenance period (P < .05). Root length after transplanting also presented a similar pattern, with significantly higher values being observed in the 120 Mg/ha group than other groups. The proportion of greening recovery in the coming year significantly improved relative to the control until a dose of 120 Mg/ha was reached, which was just higher than that of dose of 30 Mg/ha. Taken together, these results indicate that amendment of substrate with sludge compost improved the quality of turfgrass transplanting.

Table 6. Effect of amendment with different doses of sludge compost on turfgrass transplanting related indicators.

Dose of composted sludge amendment to substrate (Mg/ha)	Greening maintaining period (day)	Root regeneration ability (cm)	Proportion of greening recovery in coming year (%)
120	163.8 ± 1.8 a	9.05 ± 0.84 a	56.4 ± 5.6 a
90	161.3 ± 2.2 a	8.29 ± 0.63 ab	52.2 ± 9.6 ab
60	153.7 ± 2.8 b	7.93 ± 0.42 b	49.4 ± 5.7 ab
30	156.4 ± 3.2 b	7.62 ± 0.56 b	41.8 ± 6.5 c
Conventional substrate	154.5 ± 1.8 b	7.87 ± 0.89 b	50.9 ± 7.4 b

Discussion

Recycling of sludge to nutrient source of landscaping is a reasonable option of sludge reuse, including flower planting, container seedling, roadside green, restoration of abandoned mine-land and turfgrass establishment. The recycling way takes full advantage of phyto-nutrient in sludge without food chain pollution risk. The advantages of composted sludge as a partial substitute for horticultural substrates such as peat soil or cover soil are widely proved. However, studies conducted to date have only investigated the effects of sludge amendments on some partial or separate indices, while assessment of integrated qualitative properties of turfgrass development and sod production, as well as transplanting to fixed planting sites have yet to be conducted.

The effects of sludge compost on the physical properties of substrate have previously been reported. Johnson et al. (2006) and Cheng et al. (2007) found that sludge compost application to substrate at 90 Mg/ha significantly reduced bulk density, which agreed with the findings in the present study that amendment with sludge compost at levels exceeding 60 Mg/ha led to reduced bulk density (Table 2). Electrical conductivity is indicative of the soluble salts level, and high EC values may cause physiological injury to plants (Cheng et al. 2007; Singh & Agrawal 2008; Liu et al. 2014). Hueso-Gonzalez et al. (2014) found that accumulating sludge amendments increased EC in amended soil. Backes et al. (2013) also found a negative linear relationship between the sludge amendment dose to sod substrate and base saturation. Taken together, these findings indicate that abundant sludge amendment will introduce excess salt to substrate. In this study, EC increased remarkably until the dose of sludge compost reached 120 Mg/ha, which was in accordance with the results of previous studies. Total porosity and water-holding porosity are indicative of the ventilation and water retention of substrate (Stan et al. 2009; Nektarios et al. 2011). As shown in Table 2, once the dose of sludge compost exceeded 60 Mg/ha, the water-holding porosity increased significantly. This phenomenon implied that amendment of sludge compost improved the capacity to supply sufficient water for the roots to absorb.

Chang et al. (2014) reported that biosolids increased root length density by 23% compared with the control. Biomass and grass yield have also been reported to improve in response to sludge compost amendment (Bilgili et al. 2011; Nektarios et al. 2011). Grass height was found to be low in the first year, and then it increased with time (Celebi et al. 2011). Fresh weight per seedling, plant height, root length and tiller number are indicative of turfgrass biomass performance, including total, stem, root and leaf biomass, respectively, during the period of field growth. As shown in Table 3, 60 Mg/ha was the threshold dose of reductive effect. In other words, sludge amendment with less than 60 Mg/ha resulted in turfgrass biomass that was poorer than that of conventional substrate. The fact that these indicators acted in concert implied that the amendment dose of composted sludge had a cumulative effect until the dose of sludge compost amendment reached a certain level.

Leaf width, chlorophyll content, weed density and establishment time are important indicators of turfgrass establishment. Indeed, these indices, especially chlorophyll content, are used to determine sod price. Moreover, weed density is closely associated with labor costs because they must be removed manually. The results obtained by Bilgili et al. (2011) and Cheng et al. (2007) showed that turf color and leaf chlorophyll obviously improved in response to sludge amendment. In this study, the chlorophyll content increased significantly when the dose of sludge exceeded 90 Mg/ha. Weed concerns are usually ignored in assessment of quality following sludge amendment of turfgrass substrate (Waletzke & Backes 2006); however, the results of the present study indicate that this problem should receive increased attention because amendment with low doses of sludge increased weed density. Turfgrass establishment time determines whether sod can be sold in a timely manner or not; therefore, a longer establishment time is a disadvantage. Based on the results of this study, amendment with sludge at levels greater than 60 Mg/ha significantly reduced establishment time. These findings are similar to those reported by Tesfamariam et al. (2009), who found that amendment with sludge at doses up to 67 Mg/ha significantly improved turfgrass establishment rate. This delayed effect is attributed, at least in part, to the ability of sludge compost to behave as a slow-release organic fertilizer with a retardant growth response (Guertal & Green 2012).

After being cut to sod, the weight of turfgrass with substrate, tensile strength and intactness of sod was comparatively assessed. A gradual decrease in tensile strength and intactness were observed, which was not surprising. Indeed, Tesfamariam et al. (2009) found that the ability of sod to remain intact improved as sludge application dose increased to 33 Mg/ha, but deteriorated at higher doses. Backes et al. (2013) observed that the dose of sludge compost provided a linear increase in sod tensile strength at 36 and 48 Mg/ha, but reduced it at higher doses. These findings are coincident with the conclusion that the addition of sludge compost is not beneficial to sod remaining intact during handling and transport. Accordingly, if too much sludge compost is added to the substrate, the sod will have to be re-cut and substituted for deteriorated one.

Sod is usually in the fate of being transplanted to a fixed site for re-growth. Therefore, greening maintaining period, root regeneration ability and the proportion of greening recovery in coming year are three key indicators, reflecting transplanting quality of sod. From the results of this study, when the dose of sludge compost was incremental to 90 or 120 Mg/ha, the remarkable improvement was observed, indicating that high doses of sludge compost to substrate ranged from 90 to 120 Mg/ha, is beneficial to transplanting performance of sod. So far, this study case is first report on sludge compost amendment making better the transplanting quality of sod.

In summary, composted sludge is a suitable material for substituting partial turfgrass substrate. Additionally, assessment of integrated qualitative properties of substrate improvement, turfgrass growth and establishment, sod quality and transplanting performance showed moderate doses of sludge compost are beneficial, but excessive doses result in adverse effects.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

Hong-tao Liu is associate professor in Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences.

Sheng-kui Cheng and Mei Lei are professor in Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences.

Jun-xing Yang is assistant professor in Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences.

Lin-juan Zhang is associate professor in Shanghai Institute of Applied Physics, Chinese Academy of Sciences.

Additional information

Funding

This study was financially supported by Instrument Developing Project of the Chinese Academy of Sciences [YZ201605], National Key Research and Development Program of China [2016YFD0501401; 2016YFE0113100], Beijing Nova Program [Z121109002512061] and Beijing Nova Program Interdisciplinary Cooperation Project [Z161100004916029].