«William F. Brinton1, Andreas Tränkner 2 1. Woods End Research Laboratory, Box 297, Mt Vernon ME 04352 USA 2. AUC-GmbH, Augustastr. 9, D-51379 BONN ...»
Compost Maturity as Expressed by Phytotoxicity and Volatile
William F. Brinton1, Andreas Tränkner 2
1. Woods End Research Laboratory, Box 297, Mt Vernon ME 04352 USA
2. AUC-GmbH, Augustastr. 9, D-51379 BONN Germany
We examine a database of 899 samples from our routine analyses of composts across USA which vary from active
(Frischkompost) to cured (Fertigkompost) product. Of these samples, a subset of 69 exhibiting some phytotoxicity
traits were examined using plant assays. About 15% of samples were moderately plant toxic and 2.5% exhibited extreme growth suppression. To determine maturity we evaluate wheat and cress seedling growth in a peat:compost blend and compare to CO2-evolution and volatile organic acid (VOA) content. Compost samples showed a range of concentration between 72 and 88,737 mg/kg and 0.01 to 0.81%/day for VOA and CO2-C, respectively. Both wheat and cress yield showed signiﬁcant negative correlations with maturity expressed as VOA concentration or CO2-rate, supporting the hypothesis that mature composts are more beneﬁcial for plants. Wheat was more sensitive to VOA than was cress which exhibited greater sensitivity to conductivity. Critical levels of VOA in compost which result in observable growth depression when applied in seedling trials are approximately 5,000 mg/kg. Critical respiration rates associated with observable depression are about 0.4% CO2-C loss/day. The relationship of a variety of factors to observed maturity is discussed.
2.1. VOA in Composts Maturity is a qualitative trait of compost that may be expressed in a number of ways and is generally held to be relevant to plant performance. Volatile organic acids (VOA) have been cited to be responsible for phytotoxicity (plant growth suppression) when immature compost is used for growing plants (Manios et al., 1989). However, the production of VOA in compost has not been well studied, nor is it known how phytotoxicity attributable to VOA may relate to other factors such as CO2-evolution or salt content (Brinton, 1998).
The presence in compost of volatile organic acids may be recognized by an unpleasant “garbage” or spoiled-food odour. VOA may be observed in organic materials undergoing rapid decompostion with oxygen deprivation The volatile acids which normally occur in composts are short-chain fatty acids of C2 to C6 in length, with formic acid the shortest and butyric and iso-valeric acid Page 1 among the largest. It is not well known how the different volatile acids affect plant growth although there is evidence the effects vary with carbon-length and volatility index(Lechner, 1994).
We previously reported a survey of 712 compost samples where 26% had appreciable VOA above 5,000mg/kg at some point during composting while 6% had VOA above 20,000mg/kg. The VOA correlated negatively with age of compost and were highest in the ﬁrst 20-35 days. VOA levels appeared to be controlled by a complex, dynamic relationship of carbon biodegradability, pH and porosity. The microbial dynamics which are involved are as yet poorly understood, but it is clear that anoxia (no-oxygen) conditions may not be required to produce signiﬁcant VOA.
Prior work in plant growing media reveal that VOA levels as low as 500mg/kg can exert a phytotoxic inﬂuence on plant seedlings (Lynch, 1977). The principal deleterious effects on plants appear to be in the nature of root suppression but nutrient-ion leakage has also been implicated (Lynch, 1977; Lee, 1977). Plants exposed to high VOA do not necessarily die, but may remain static for long periods of time. Thus, rapid identiﬁcation and response to problem composts is made more difﬁcult, and the resulting economic losses can be very large. Basing plant growth evaluation on maturity standards would be a way to avoid later damage.
2.2. VOA in Compost and Resulting Phytotoxicity The presence in composts of volatile organic acids is certainly well established but poorly understood (Manios, et al. 1989). VOA are metabolic by-products of anaerobic respiration and are breakdown products of grease and fats in raw wastes (Henefeld-Fourrier,1980). The composting process includes many events in which episodic oxygen depletion occurs both at macro- and micro-pore levels. Such oxygen depletion may result in temporary production of copious amounts of short-chain carbon compounds. In a previous paper we discussed odorant and microbial aspects of VOA production (Brinton, 1998).
VOA may also enter compost from source materials. VOA are found in waste water (Kawamura et al., 1985; Anselme, et al., 1985) and may be present in large concentrations in biosolids (Howgrave et al., 1991; Rains et al., 1973) and raw manures (Hoshika, 1982; Zahn et al., 1997). Freshly composted sewage sludge has already been reported to contain high levels of organic acids, primarily acetic acid, that inhibit plant growth (Devleeschauwer et al., 1981). Food scraps from households also contain copious amounts of VOA and one study showed that storage of food waste in plastic bags instead of paper increases VOA production (Detroit, 1996).
Phytotoxicity has been previously associated with immaturity of compost (Ianotti et al., 1994) and depletion of organic acids is correlated with improved plant performance (Herrmann et al., 1993).
Many MSW composts have been associated with high levels of phytotoxicity associated with organic acids (de Bertoldi, 1992, 1993). Phytotoxicity of immature composts has been positively correlated with organic acid content (Logsdon, 1989; Manios et al., 1989; Evans and Brinton, 1997).
3. MATERIALS AND METHODS
3.1. Sample Collection Compost samples have been obtained from selected compost facilities, including several reporting horticultural problems from their customers. About 7 facilities had compost samples reported by authorities for plant damage to horticultural centers and resultant economic damage with associated legal actions. Samples obtained from a distance were collected is non-sterile, plastic 4-liter containers and were shipped via 2-day service to the laboratory in chilled coolers. Samples involved in the investigations contained source ingredients including biosolids, yard trimmings, manures and food scraps. The age of composts ranged from 3 days to 925 days.
3.2. VOA and CO2-Evolution Analysis VOA were determined by total distillation and individual compounds by chromatography. For distillation, samples were water extracted and distilled in H2SO4 at pH1.8 and the resulting distillate titrated to a standard endpoint (SWMM, 1994). For testing individual volatile organic acids in the C2-C9 range, we extracted composts with weak alkaline solution, ﬁltered and diluted prior to injection into a liquid-ion chromatograph with a PRP-X300 base-anion exchange column equipped with macroporous co-poly(styrene-divinylbenzene) and single-gradient eluent (Lee, 1984; Bevilacqua and Califano, 1989). CO2 evolution rate was determined on 40 g samples after 1-day of equilibration after sampling with an incubation temperature of 32C. CO2 is trapped in a NaOH-barium chloride solution and titrated to endpoint against HCl.
3.3. Phytotoxicity Analysis Plant phytotoxicity was determined on one of two ways. To examine the effects of VOA on rootlets we utilized Hoagland nutrient solution in 1/2 liter containers to which were added extracts of compost and standardized VOA concentrations of acetic, butyric and propionic acids. In a growth stud, we employ a 1:3 (v/v) blend of test compost in limed sphagnum peat (pH= 6.2) and grow garden cress and wheat to 7 days at which point they are counted and cut and weighed. Plant growth is reported as percent germination and as fresh weight percent of Pro-Mix BX control (Premier Peat, LTD. Québec).
3.4. Preliminary Investigations: Bacteriology of VOA Production The phenomenon of VOA production is recognized in farming since crop residues may result in oxygen deprivation when plowed in. The resultant VOA may trigger temporary phytotoxicity (Lynch, 1977). In composting, similar oxygen deprivation factors are involved and VOA is produced at high rates (Liao et al., 1994, Van Durme et al., 1992; Lechner 1993).
It should be recognized that volatile organic acids are regularly and temporarily produced in normal composts. Fermentative respiration occurs when oxygen is low but not necessarily absent.
Under semi-aerobic conditions, a wide variety of microbes utilize compounds other than oxygen as terminal electron acceptors for respiration (Brock and Madigan, 1991). The levels of O2 which Page 3 favor production are not precisely known; nevertheless they are clearly less than the oxygen content frequently cited as ideal for composting. Facultative anaerobes will grow aerobically during regular composting yet will produce VOA soon after a period of oxygen stress sets in. Furthermore, several aerotolerant anaerobes such as lactobacillus grow in the presence of oxygen and will produce VOA.
In compost, oxygen supply is clearly episodic. This reﬂects the diversity of the microbial background (Brinton & Droffner, 1994). Thusly, there exists a shunting of metabolic by-products between aerobes and facultative anaerobes in composts. This results in a dynamic self-regulatory mechanism (see Figure 1, after Brinton 1998).
We have previously observed a moderate correlation (r = 0.61) between substrate CO2-respiration rate and the production of VOA in diverse composts. This suggests that the issue of phytotoxicity may be closely tied to maturity in general. Thus, the way is open to attempt a broader deﬁnition based on laboratory attributes.
4.1. Selection of Samples from Surveyed Compost Facilities In a previous study we examined volatile organic acid content of 712 composts from 340 compost facilities (Brinton, 1998). This data indicated a potential for plant phytotoxicity in many comPage 4 posts as 149 samples exhibited VOA greater than 5,000 mg/kg, with a mean concentration of 4,385 mg/kg (Brinton, 1998). In order to further develop the study process, we identiﬁed problem facilities from within this group. These are deﬁned as compost producers receiving complaints about poor growth performance. In this case, 69 out the total of 340 were selected for further study. Compost from these facilities was subjected to wheat and cress seedling assays in addition to standard laboratory characterization.
4.2. Laboratory Data Set For the selected compost samples that we examine more closely for growth properties, we developed a analysis database. This and a correlation matrix are seen as follows (See Table 1, Table 2).
Based on this data, the range was slightly greater but the mean similar to the previous survey of 712 composts where our average was 4,385 mg/kg with a SD of 7,298.
4.3. VOA in Compost vs. Age and Treatment In this project no attempt was made to distinguish age of material although all composts surveyed were considered by the producer to be ﬁnished.. Previously, we found that age and material do affect observed VOA content and VOA declines on average 1,000 mg/kg per week (Brinton, 1998). We have previously observed that VOA may be highly signiﬁcantly correlated with respiration activity of composts as determined by CO2 evolution rate. High respiration is felt to be an indicator for immaturity, as elsewhere we have demonstrated a close correlation with the Dewar self-heating test (Brinton et al., 1995). VOA levels therefore appear to dependent partly on compost conditions and age of the process. Thus, each compost possess a unique capability to become phytotoxic.
4.4. Correlation Regress of Observed Traits In the following table (Table 2) we compute correlations between variables. Of the most signiﬁcant are negative correlations between VOA and yield of wheat seedlings and salt and yield of cress. All traits of VOA and CO2 respiration are negatively correlated with observed plant yield properties, however, wheat indicates VOA levels more predictably than does cress. Cress yield is
a. Values of correlation coefﬁcient, r, that are signiﬁcant at p = 0.001
4.5. Volatile Organic Acids and CO2-Evolution Rate Since volatile organic acids play an important role in affecting the quality of plant growth, it was considered important to evaluate VOA levels based on CO2 evolution rate, a determinant of compost maturity. Using previous data, and combining with current results, we produce the following graph (Figure 2). Of the observed variability in VOA for all 899 samples, 34% is explained directly by the CO2-rate (r=0.58, p ≤ 0.001). Our subset of 69 samples, identiﬁed separately in the graph (see Figure 2, boxes) exhibit a steeper slope in correlations with observed CO2-rate (r = 0.352; p = 0.003). In other words, the 69 samples we examine produced more VOA per unit of respiration than the average of the whole set.
VOA and CO2-rate are dynamic properties and will depend on relative factors such as porosity and moisture content. Therefore, high CO2 rate alone may not explain VOA levels, and this explains the variability in the data. Indeed, since high VOA often leads to a depression of pH, and CO2 is pH dependent, a “negative feedback” dynamic is certainly probable. Because of the many related factors, it is only possible to postulate a cycle in which a variety of factors inﬂuence each other simultaneously during composting (Brinton, 1998).
4.6. CO2 Evolution Rate versus Cress Growth Properties By blending compost at a standard dilution into limed-peat we produce growing conditions that are potentially optimal in regard to porosity, salt and nutrients. Considering the relationship we describe of VOA concentration to CO2 rate, and the relative ease with which CO2 evolution can be measured in the lab, it becomes interesting to examine more closely data for cress yield and com
post CO2-rate. We enlarged the sample set from 69 to 155 composts for which we had data on cress and CO2 levels. The correlation of this group increased from r = 0.254 (Table 2) to become r = 0.37 (p=0.001). By logarithmically transforming CO2-rate, we produce a highly signiﬁcant simple correlation coefﬁcient of r= 0.53, p 0.0001.