This study investigated the C mineralization and chemical modification of a typical tropical soil amended with regional compost of different stability. Compost samples were produced from coffee pulp and fruit and vegetable waste in a method of small heap composting and the samples were collected in three different phases of composting. Both the fresh waste and compost samples were analyzed for their phytotoxicity. These samples were added to a tropical Nitisol at the rate of 48 t ha?1 and a control was set up without amendment. The CO2-C respired was determined during 98 days of incubation and the incubated samples were taken at the start and end of incubation for molecular-chemical analysis by Pyrolysis-Field Ionization Mass Spectrometry (Py-FIMS). The fresh waste yielded a germination index (GI) < 26% indicating phytotoxicity but this disappeared in all the composts (GI > 100%). The CO2-C respired was best explained by a first order plus linear model. A soil amended with a compost taken at the thermophilic phase attained the lowest overall organic C loss. In general, the Py-FIMS revealed a significant enrichment of stable N-compounds during the incubation in all amended soils compared to the control. Furthermore, among the compost-soil mixtures Py-FIMS indicated significantly higher increases in the proportions of carbohydrates, peptides and phenols/lignin monomers at the expense of fatty acids and sterols in soil amended with composts from the thermophilic phase. Thermal volatilization curves of Py-FIMS indicated enrichments of stable N-compounds and peptides in compost amended soil. This was a result of enhanced decomposition and stabilization of decomposition products by physical protection through association with clay and soil aggregates. In summary, application of compost shortly after reaching the high temperature phase was shown to be more efficient in organic C sequestration in a clay-rich tropical agricultural soil than mature composts.
Organic waste recycling in agriculture through composting is increasing as an environmentally sustainable waste management strategy [
Coffee (Coffea arabica L.) is one of the abundant agricultural products generates expressive amounts of agricultural residues during processing [
Aerobic incubation experiments were used to measure organic matter decomposition or soil C sequestration potential in compost-fertilized, inorganic fertilized or non-amended soils [7,17]. Moreover, better understanding of the decomposition dynamics need fitting the resulting carbon dioxide (CO2) release data to different kinetic models used to describe the mineralization of organic waste. Incubation of different waste mixtures (prepared with sewage sludges, manures, city refuse, cotton waste, olive-mill wastewater, and sweet sorghum bagasse) after they had been composted for various periods with soil revealed decreases in CO2 evolution with longer composting duration [
Pyrolysis-field ionization mass spectrometry (PyFIMS) was used to characterize organic matter in sewage farm soils [
Therefore, the objectives of the study were (1) to investigate how the CO2-respiration from a tropical Nitisol was altered following the application of differently stabilized composts, and (2) to evaluate effects of the differently stabilized composts on the molecular composition of SOM and its short-term modification during an incubation experiment.
The overall objective was to find out which compost is favorable to improve the soil fertility by sequestration of C in the soil based on a low CO2-release to the atmosphere.
The soil was sampled from a coffee plantation farm located in the southwestern highlands of Ethiopia and beloged to the major Soil Unit “Nitosol” (FAO, 1998). The sampling area is situated 36˚36'E longitude and 7˚56'N latitude (
Composts were produced from 8 m3 fruit and vegetable waste (dominated by fruit), 1600 kg wet coffee pulp waste and 300 kg garden trimmings (dry leaves, and young tree branches) in a method of small heap composting. Heaps were piled in a bamboo box of dimensions 1.5 m (l) × 1.0 m (w) × 0.8 m (h). The platform used has been considered as pilot compost production unit by the Addis Ababa city council environmental protection authority, Ethiopia.
The regional climate is tropical, with average temperatures between 15˚C and 25˚C. Three piles were constructed for experimental purposes with three replicates each and monitored as treatment COM1 (composed of coffee pulp waste), COM2 (fruit and vegetable waste), and COM4 (coffee pulp waste with fruit and vegetable waste in which the ratio was 50:50 by volume). The same quantity of soil was added as additional source of microbial colonization and garden trimmings to improve the structure. The experiment lasted from November 2010 to February 2011. The piles were manually turned each week during the active phase which lasts approximately 13 to 17 days, and then in a 15-day-interval during the maturation phase. Depending on the situation the piles
were watered so as to maintain the moisture above 40%. Temperatures were measured daily for the first two week and in a 3-day-interval during the next period always in the early morning using digital thermometers (0.5 and 1.0 m in length) at 2 different points of the heaps (25 and 60 cm depth). The composting was considered to be finished when the temperature of the mixture remained stable and near ambient (about 21˚C).
Representative compost samples (about 1 kg) were taken by mixing nine subsamples from different levels of the section in the pile along the whole profile at different phases of the composting, day 8 (thermophilic phase = T), day 24/36 (mesophilic phase = M, depending on the duration each treatment lasted in this phase), and 89/119 (final compost = F, depending on the duration each treatment lasted in this phase). In this way composts of different degree of stability were obtained from three different composting piles. The samples were air dried and ground to pass through a 0.5-mm-sieve. The main properties of the composts are shown in
Raw coffee pulp waste (RCPW) and composts sampled at the end of the thermophilic phase (COM1-T, COM2-T, COM4-T), mesophilic phase (COM1-M, COM2-T, COM4- M) and final compost (COM1-F, COM2-F, COM4-F) were investigated for possible phytotoxic effects. A germination test was carried out using garden cress (Lepidium sativum) and radish (Raphanus sativus) seeds as suggested by [
10 g dry weight of sieved soil samples (<2 mm) were thoroughly mixed with organic amendment at desired application rates (48 t·ha−1) and placed in 100 ml incubation vessels in five replicates each. They were monitored as treatments S+RCPW, S+COM1-T, S+COM1-M, S+COM1-F, S+COM2-T, S+COM2-M, S+COM2-F, S+COM4-T, S+COM4-M and S+COM4-F. Soil controls were run without any amendment. Distilled water (1 to 5 ml) was added to the soil-compost-mixtures to keep the moisture at 60% of water-holding capacity. The incubation vessels were placed in a glass container containing 25 ml of 0.05 M NaOH and made air tight with a film plaster. To maintain sufficient O2 in the vessels they were opened several times in the first week and for two hours per day during the following weeks. Empty vessels were used as blanks. The CO2 evolved was measured by titration of the NaOH solution with 0.05 M HCl after the carbonate was precipitated by adding excess 0.05 M BaCl2 in 24-hour-intervals. The incubation was carried out in a temperature-controlled incubator at 25˚C for 98 days. Subsamples (2 g) were taken destructively from each treatment at the start (day 1) and end (day 98) of incubation for chemical and mass spectrometric analyses. Percent increase and decrease in soil organic C as a result of added composts and subsequent mineralization were computed using a mathematical formula:% increase Corg = (Ct/Cs ) × 100) − 100), % decrease Corg = (100 − (Ca/Cb) × 100) where Ct is total organic C of the compost-soil mixture, Cs is organic C of the control soil, Cb is organic C at the end of incubation, and Ca is organic C at the start of incubation.
The soil samples were analyzed for pH, EC in 0.01 M CaCl2 suspensions (1:2.5 w/v) and composting samples were analyzed for pH, EC in H2O suspensions (1:10 w/v). Whereas the concentrations of total organic carbon (Corg), total nitrogen (Nt) and total sulfur (St) were determined for both soil and compost samples using a CNS analyzer (Vario EL III; Elementar Analysensysteme, Hanau, Germany).
For Py-FIMS analyses composting pile COM4 was selected because this treatment reached stability in a shorter time period than the other treatments and high analyses costs restricted the number of samples that could be measured. About 3 mg of the air dried, ground and homogenized soil samples amended with differently stabilized COM4 were thermally degraded in the ion source (emitter: 4.7 kV, counter electrode −5.5 kV) of a double-focusing Finnigan MAT 95 mass spectrometer. All samples were heated in a vacuum of 10−4 Pa from 50˚C to 700˚C in temperature steps of 10˚C over a time period of 18 minutes. Between magnetic scans the emitter was flash-heated to avoid residues of pyrolysis products. About 65 magnetic scans were recorded for the mass range m/z 15 to 900.
Ion intensities were referred to 1 mg of the sample. For each of the single scans, the absolute and relative ion intensities of ten classes of compounds in the OM were calculated by summation of the ion intensities of indicator signals to obtain thermograms of their volatilization and averaged Py-FI mass spectra. This procedure was done for each three replicate measurements per soil sample and the results were averaged for statistical analyses.
The C-losses during the incubation experiment were fitted to mathematical models using the non-linear regression procedure. Means and standard errors were calculated for chemical and phytotoxicity parameters, CO2-C volatilization data and ion intensities from Py-FIMS. Comparisons between means of ion intensities of compound classes of different treatments were done by applying One Way ANOVA test (LSD mean comparison method were used). All statistics were computed using data analysis and graphic software (Origin 8.1 G).
In the experiment with garden cress the GI varied between 3% and 112% (
In all treatments the cumulative respiration curves showed two distinct phases for CO2-C evolution. An initial most intensive biological transformation phase was followed by a slower second phase (
In the short time of three weeks between 35% and 69% of the total C had been evolved. The cumulative
COM: compost; T: thermophilic phase; M: mesophilic phase; F: final stage; RCPW: row (fresh) coffee pulp waste; Corg: total organic carbon; Ntot: total nitrogen; C/N: carbon to nitrogen ratio; EC: electrical conductivity and standard errors in brackets.
CO2-C respired from the amended soils were varied between 5.6 g·C·(kg·soil)−1 and 10.6 g·C·(kg·soil)−1 at the end of the incubation (
The application of differently stabilized composts significantly affected the CO2-C release from soil (
In the Py-FIMS analyses of the control and soils amended with differently stabilized COM4 the thermograms of total ion intensity (TII) showed a reduction in intensity and a shift towards higher pyrolysis temperature during incubation. For instance the thermal evolution of molecules reached maximum intensities in the temperature 460˚C and 480˚C at start of the incubation and shifted to 490˚C and 510˚C at the end of incubation (see upper right inserts in
In the Py-FI mass spectra, a particular enrichment of N-compounds during the incubation in compost treated soil was reflected by m/z signals 67, 110 and 117 being more intense in Figures 3(b), (d) and (f) than in Figures 3(a), (c) and (e). This difference, however, was not shown in the control (Figures 3(g) and (h)). Comparison of mass spectra at the start and end incubation indicated increased proportions of carbohydrates (e.g. more intense m/z signals 84, 96, 110 in Figures 3(b), (d), (f) and (h) than in Figures 3(a), (c), (e) and (g)), lignin building blocks (e.g. more intense m/z signals 156, 168, 178, 192, 194, 202, 216 and 218 in Figures 3(b), (d), (f) and (h) than in Figures 3(a), (c), (e) and (g)). A decreased proportion of homologous series of free fatty acids and alkenes was indicated by m/z signals at 252, 266, 280, 294, 308 322, 336, and 350 (e.g. less intensive in Figures 3(a), (c), (e) and (g) than in Figures 3(b), (d), (f) and (h)) in all incubated treatments, but most pronounced in S+ COM4-T.
The TII values were significantly larger in all amended soils than in the control at start of the incubation (
The relative ion intensities (% TII) of compound classes indicated a significant (p < 0.05) enrichment of N-compounds during the incubation in all amended soils
(by 39.1% in S+COM4-T, by 28.6% in S+COM4-M and by 40.9% in S+COM4-F). However, in the control the proportions of N-compounds did not significantly change between the start and end of incubation (
The temperature-resolved volatilization curves for N-compounds and peptides showed a clear difference in thermal stability between S+COM4-T and the control during the incubation (
CARB: carbohydrates; PHLM: phenols/lignin monomers; LIPID: lipids; NCOMP: N-compounds; ALKY: alkylaromatics; LDIM: lignindimers; STER: sterols; PEPT: peptides; SUBE: suberin; FATTY: n-Fatty Acids (n-C16 to n-C34) and standard errors in brackets; S: soil; COM: compost; T: thermophilic phase; M: mesophilic phase; F: final stage; RCPW: row (fresh) coffee pulp waste.
The low GI of 3% with cress and 26% with radish measured for the RCPW indicates its high phytotoxcity [20,21]. Moreover, the germination delay observed in this treatments can be attributed to the high salt content (electrical conductivity = 6.73 mS·cm−1) of the sample [
The observed pattern in CO2-C release agreed with the model used by [
The variation in the CO2-C release among the soils amended with differently stabilized composts despite of similar initial Corg contents and C/N ratios indicated a C immobilization. This immobilization was stronger in the soil amended with COM-T than in the soil amended with COM-M. This confirms reference [
The larger TII in the compost-amended treatments than in the control at the start of incubation coincided with the Corg concentrations in the treatments. It is explained by the additional organic matter from the composts [25,26]. However, the larger TII in S+COM4-T than in any other treatment at the end of incubation proved selective preservation of compounds that were possibly mineralized in samples with longer composting time. The best explanation for such a selective protection is the binding of organic matter at clay surfaces, and a consecutive aggregation of clay-organic matter complexes [24,27,28].
The signal patterns of Py-FI mass spectra indicated the clearest impact of compost application by increasing proportions of N-compounds at the expense of fatty acids and sterols (
The temperature-resolved Py-FIMS showed that thermally stable carbohydrates, phenols/lignin monomers, and peptides were more enriched during the incubation of S+COM4-T than in any other compost amended treatment. This indicates that these compound classes were microbially synthesized during incubation or selectively preserved. The pronounced enrichment of phenols/lignin monomers agreed with [
Computing the percentage decrease of ion intensities in the temperature range < 400˚C (
The decrease in TII mainly in the temperature range below 400˚C (thermally labile compounds) in the compost amended soils only implied that a large portion of labile components of the compost material itself was transformed or degraded during the incubation process. This supports the idea that decomposition of the added composts in soil goes along with humification in the composts themselves [
1) The combination of laboratory incubation and PyFIMS for the first time provided compelling evidence for effects of the compost stability on C mineralization and the molecular composition SOM when composts were applied to a typical clay-rich tropical soil.
2) The combined results revealed that it would be sufficient to end up the composting of coffee pulp and fruit waste immediately after a short thermophilc phase as its application better conserved the C (a) during composting and (b) during transformations in the soil.
3) Application of compost from the early phase of composting may result in a quicker mineralization of biologically labile organic matter from the compost and a better enrichment of SOM with stable compounds of plant and microbial origin such as carbohydrates, phenols/lignin monomers and peptides at the expense of free fatty acids and sterols compared to mature compost. This may have considerable economic implications as composting of coffee pulp in heaps may take 6 to 8 months to reach the mature stage.
4) Therefore, the application of compost from the early composting phase can be recommended not only as a soil amendment but also as a measure to mitigate CO2-enrichment in the atmosphere. Forthcoming studies will be directed to disclose in more detail the stabilization mechanism of biologically labile, compost-derived organic molecules in tropical and other soils.
The ECPB-DAAD program provided a scholarship for B. Eshetu. The Mass Spectrometry Laboratory of the Rostock Soil Science group was supported by the “Exzellenzförderprogramm” of the Ministerium für Bildung, Wissenschaft und Kultur Mecklenburg-Western Pomerania, project UR 07 079. Furthermore, we thank Dr. R. Beese and Dipl.-Chem. Kai-Uwe Eckhardt, University of Rostock, for technical support and data analyses.