Evaluation on maturity and stability of organic fertilisers in semi-arid Ethiopian Rift Valley

Study area and organic fertilisers tested

Most of the Tebo and Geldia seasonal rivers catchments (the study area; coordinate system, UTM WGS 1984, 528600, 973100 –541700, 951600; Fig. 1) are located in the semi-arid Ethiopian Rift Valley. The catchment areas are categorised into two sub-areas in terms of major maize growing areas in Ethiopia: mid-altitude dry (1000–1600 m a.s.l.; annual rainfall 800–1000 mm) and mid-altitude moist (1600–1800 m a.s.l.; annual rainfall 1000–1250 mm) sub-areas¹². These two categories cover 63% of the total maize growing area in Ethiopia. The major crops in the mid-altitude dry sub-area of the catchments area are sorghum, tef (Eragrostis tef), and maize, whereas those in the mid-altitude moist sub-area are wheat, tef, and maize¹².

Most households in the semi-arid Ethiopian Rift Valley hold continuously cropped maize fields (locally referred to as aradas²⁶), which acquire fertility from the regular input of OFs, such as compost (locally, kosi) or household wastes²⁷. Kosi is made from a variety of locally available organic materials, such as various types of animal dung, kitchen ash, crop residue, and feed refusals. These compost materials are piled up in the corners of house-yards for several months to a few years for decomposition²⁷. Household wastes are substantially a variety of organic materials themselves that also comprise kosi. It is mainly a housewife who collects these organic materials through house-yard sweeping and dumps directly on the arada that adjoins the homestead every few days. It has been since the beginning of the 2000s when the district agricultural office began giving fast compost training to farmers²⁸.

Compost maturity and stability tests

Microorganisms break down the chemical bonds of organic materials in the presence of oxygen and moisture, giving off heat. Rynk et al.²⁹ found that maintaining a stable temperature of > 62.8 °C within the compost pile, for more than three consecutive days had been effective for the destruction of most human pathogens, insect larvae, and weed seeds within the compost pile. Monitoring the volume of compost pile during the biodegradation process is another physical test to evaluate compost stability^30,31. Thus, monitoring compost pile temperature (self-heating test) and volume can be used as a simple and rapid method for assessing compost maturity or stability³².

The pH values of compost usually increase during the early stage of composting generally to above 8, caused by the release of ammonia, and then decrease slowly but steadily as ammonium (NH₄⁺) is nitrified to approach neutral values as compost matures³³. However, numerous studies have demonstrated that pH trends and final values over the composting are highly dependent on feedstock materials³².

C:N ratio generally decreases throughout the composting process due to the C losses¹³; however, the wide variability in feedstocks leads to variability in the final C:N ratios in different composts, making it difficult to place an absolute limit on C:N ratio that will be applicable to all feedstocks³². The pH values and C:N ratio of compost are useful in compost maturity evaluation if initial and final values are compared and if it is monitored in conjunction with other parameters for compost maturity³².

Sánchez-Monedero et al.³⁴, which analysed the evolution of the different forms of N during the composting of different feedstocks, found that the greatest concentration of NH₄⁺ coincided with the most intense period of OM degradation, whereas the highest concentrations of nitrate (NO₃⁻) were always produced at the end of maturation. They concluded that NO₃⁻ to NH₄⁺ ratio is a clear indicator of the compost stability. Wichuk and McCartney³² recommended that, because the ratio varied in mature composts, monitoring the ratio several times throughout the different stages of the composting process, rather than relying on the final value alone. NO₃⁻:NH₄⁺ ratio can be an effective indicator to evaluate the stability of kosi, fast compost, and household wastes, which have the same organic materials in common but are the products being in different phases of the composting process.

Respirometry (CO₂ evolution rate or O₂ uptake rate) has been widely used to evaluate the microbial activity and therefore, the stability of a compost sample⁹. The equipment for respirometric assays based on CO₂ evolution is generally simple and easy to use; however, the main disadvantage of these methods is that they are unable to distinguish between CO₂ produced aerobically from that produced anaerobically⁹. Respirometric assays based on O₂ uptake are the most accepted methods for determining the biological activity of material; however, their main disadvantage is that they need more expensive and troublesome instrumentation and more skilled labour⁹. Respirometric assays are not without flaws; nevertheless, many researchers recommended using either of the two respirometric assays or self-heating (in combination with a plant bioassay) to evaluate compost stability³².

Gómez-Brandón et al.¹³ compared several parameters and found that the change in dissolved organic carbon (DOC) with composting time gave a good indication of stability. However, the determination of DOC content requires expensive laboratory instruments such as absorption spectrophotometer. Instead, Wu et al.¹⁰ and Gómez-Brandón et al.¹³ found a significant correlation between DOC and microbial respiration. They referred to the way of evaluating compost stability based on CO₂ evolution and assessing compost maturity based on phytotoxicity bioassay (seed germination). Compost maturity is generally determined by phytotoxicity bioassay³².

To date, no stand-alone method exists to assess compost maturity, mainly because of the wide variety of composting feedstocks and management practices³⁵. A more thorough evaluation of both the stability and maturity states of compost could be obtained using a combination of tests³². An appropriate field test method would need to be rapid and sufficiently straightforward for operators to use³².

Considering these, (1) monitoring pile temperature and volume changes and (2) determinations of pH, OM, total N, and C:N ratio (total organic C to total N ratio) over the composting process; (3) determination of the final NO₃⁻:NH₄⁺ ratio; (4) CO₂ evolution test; and (5) phytotoxicity bioassay were combined to evaluate stability and maturity of the OFs, i.e., kosi, fast compost, and household wastes, in this study. Besides, because weed proliferation in arada fields is the primary cause of maize yield decline³⁶, (6) a weed seed germination test was conducted.

Monitoring of physical and chemical changes in compost piles

Five farmers who participated in the fast compost training from each of the two sub-areas were requested to make kosi and fast compost. The temperatures and volumes of the kosi and compost piles were monitored only in the mid-altitude dry sub-area over 90 days from the commencement day when the OF feedstock (organic materials) had been piled up (the kosi pile prepared was referred to as “test-kosi pile”). This test was conducted once in each 2014 and 2015. To ease the pile volume measurement, the organic materials collected from the farmers’ backyards were piled up in a rectangular wooden frame (1 m in width, 1 m in length, 1.5 m in depth; Fig. 1). Fast compost was made following the technical guidance of MoARD²⁸: each 20-cm-deep layer of the (1) maize and sorghum stalks, (2) animal dungs, and (3) tef residue and feed refusals were piled up in turn until it reached the top of the pile. The total depth of each material layer was arranged to be the same between (1), (2), and (3). (4) Ash (0.5 kg m⁻²) was sprinkled over each layer of the (1) and (3). Some humic soil (1–2 cm deep) was spread on top of each layer. Water was regularly added to keep the pile moist. Once every 21 days, all the organic materials in the piles were turned over to mix the materials. This process was repeated to make fast compost ready in 3 months²⁸. The same varieties of the organic materials were used for the kosi feedstock, but those compositions and proportions were decided by the individual farmer. The fast compost pile had a ceiling so that rainfall did not enter inside, whereas the test-kosi pile was rainfed (Fig. 1). Daily rainfall was measured by a simple rain-gauge installed near the test-kosi piles.

Farmers in Eastern and Southern Africa carry OFs from their kraals (cattle parking lot) and cattle sheds to the field and integrate it into their fields by ploughing operations carried out a couple of weeks later³⁷. Farmers in the semi-arid Ethiopian Rift Valley plough maize fields 3–4 times and tef fields 4–5 times before seeding³⁶. For both the crops, many farmers integrate the applied OFs into the soil at the ploughing time implemented immediately before the seeding or at the previous ploughing time. For maize, this period corresponds to the beginning of the rainy season from late-May to the beginning of June³⁶. As soon as they carry kosi to their fields, they begin the next kosi making. Thus, the test-kosi and fast compost samples were begun to prepare on 4th June in 2014 and 2nd June in 2015.

Daily temperatures were measured at randomly selected five points in the test-kosi and fast compost piles by a temperature probe (SINWA digital thermometer H1). Those mean values were designated as the daily temperature. The depths of the piles were measured every 10 days over the monitoring period, which were converted into volume. Similarly, pile pH (HORIBA portable pH meter D-210P) and total N and OM contents (loss-on-ignition method; ignition temperature 500({}^{o}c), overnight) of the piles were determined.

Only for pH, it was measured at the 2nd day of the monitoring together with the regular measurement made every 10 days, including the 1st day of the monitoring. Total organic C was estimated from the OM content determined³⁸, which was used to determine C:N ratio.

Total N in the sample was determined by the following on-site proximate analysis methods³⁹ (Table S1 in Supplementary Information online): after 1.0 g (dry matter) of the sample was placed in a 500-mL tall beaker and 8 mL of sulphuric acid was added, 4 mL of 35% hydrogen peroxide was added twice, which was capped with a dish. After a vigorous chemical reaction was settled, the tall beaker was heated for 5 min. After the beaker was cooled down, 2 mL of hydrogen peroxide was added, and then heated for 3 min; this operation was repeated six times. The solution was transferred to a volumetric flask, and water was filled to the marked line of 100 mL. Because Reflectquant ammonium test (0.2–7.0 mg L⁻¹ NH₄⁺) requires a test solution regulated in pH 4–13, after 29 mL of water was added to 1 mL of the solution, 0.4 g of calcium hydroxide was added, which was stirred hard. The filtrate was reacted with a Reflectquant ammonium test, and NH₄⁺ was determined with an RQFlex in a thermostat bath kept at 30 °C. A standard solution for NH₄⁺ (3.0 μg mL⁻¹) was simultaneously determined to correct determined NH₄⁺ in the sample. Corrected NH₄⁺ in the sample (X) was converted to total N (Y) using the equation³⁹, Y = 0.830 X (Table S1 in Supplementary Information online).

In the village in the mid-altitude dry sub-area where the sample piles were established, abundant pumice flow deposits were observed in soils; this was so in the humic soil added to the compost pile. The weight of pumice in the samples collected from the test-kosi and fast compost piles, if any, was measured, which was deducted from the crude ash mass measured after combustion.

Weed seeds germination test

A weed germination test was conducted in 2015 as follow: a fast compost sample and test-kosi sample were collected from the each of the 5 fast compost and 5 test-kosi piles set in the two sub-areas in 90 days of the monitoring period. Thus, 10 fast compost samples and 10 test-kosi samples were prepared. Besides, 5 kosi and 5 household wastes samples were collected from 5 farmers’ backyards in both the sub-areas (a kosi sample collected from farmers’ backyard was referred to as a farmer-kosi sample). In collecting a household wastes sample from a farmer’s backyard, approximately a 10 g sample was collected from each of the five places in the backyard, mixed to make it a composite sample. From each of the 10 fast compost, 10 test-kosi, 10 farmer-kosi, and 10 household wastes samples, 8 samples were collected to prepare 80 fast compost, 80 test-kosi, 80 farmer-kosi, and 80 household wastes samples. A filter paper was placed on a petri dish 9 cm in diameter, on which a sample was spread. The sample in the petri dish was uniformly watered and placed in the constant temperature room (kept at 25 °C) in Melkassa Agricultural Research Center for 10 days. Species of the plants germinated were identified at the National herbarium of Ethiopia. The 10 fast compost, 10 test-kosi, 10 farmer-kosi, and 10 household wastes samples tested in the weed germination test were also used for the NO₃⁻:NH₄⁺ ratio determination test, CO₂ evolution test, and phytotoxicity bioassay.

NO₃:NH₄⁺ ratio determination

NH₄⁺ and NO₃⁻ in the 10 fast compost samples, 10 test-kosi samples, and 10 farmer-kosi samples were determined, from which NO₃⁻:NH₄⁺ ratios were calculated.

Tanahashi et al.⁴⁰ found that cattle and swine manures contained the fraction of NH₄⁺ that cannot be extracted by potassium chloride (ammonium magnesium phosphate; MAP). They examined 59 cattle manures (26 dairy, 28 beef, and 5 dairy and beef mix) and 52 swine manures made by various production methods for an appropriate extraction method of NH₄⁺ containing MAP⁴⁰. As a result, they found that inorganic N containing MAP extracted by 0.5 mol L⁻¹ hydrochloric acid in the condition of the 1–10 ratio of dry manure weight (g) and extract volume (mL) had the strongest relationship (R² = 0.851, including some outliers) with inorganic N available in the culture soil used for laboratory incubations (30 °C, 4 weeks). Thus, this study used hydrochloric acid to extract inorganic N from the OFs. After 0.5 mol L⁻¹ hydrochloric acid solution (100 mL) was added to 10 g of the sample, the solution was stirred by a mixer for 2 min to make an extract. The extract was diluted, if necessary, and was reacted with Reflectquant ammonium test (measuring range of 0.2–7.0 mg L⁻¹ NH₄⁺) and Reflectquant nitrate test (5–225 mg L⁻¹ NO₃⁻) to determine NH₄⁺ and NO₃⁻ contents with an RQFlex, respectively (Table S1 in Supplementary Information online).

CO₂ evolution test

Using a simple respirometric instrument (Fig. 2)⁴¹, the 10 fast compost samples, 10 test-kosi samples, 10 farmer-kosi samples, and 10 household wastes samples were incubated at 30 °C for 21 h in the glass flask (Fig. 2), and CO₂ produced was determined. A 0.5 g air-dried sample was mixed with an air-dried 10 g arada soil collected from the mid-altitude dry sub-area and water (60% soil water saturation). A small container that contained a 2 g sodium hydroxide (carbon dioxide absorbent) was placed in the flask. The volume of water sucked by the measuring pipette was measured. From the volume of water measured, the volume of water measured at the control treatment (only the culture soil) was subtracted to obtain the volume of CO₂ produced. CO₂ evolved is soluble in aqueous solutions, and the solubility is pH-dependent⁹. Thus, the original pH of each sample was determined. Calcisols⁴² (Endopetric Hypercalcic Calcisol; clay loam) were typical soils in the arada fields in the mid-altitude dry sub-area²⁶.

Phytotoxicity bioassay (garden cress germination test)

For the 10 fast compost, 10 test-kosi, 10 farmer-kosi, 10 household wastes, and 10 arada soil samples, phytotoxicity bioassays were conducted.

In a garden cress germination test, the presence of phytotoxic substances is usually determined by a garden cress germination index (GI) that is calculated by the following equation:

where G_t = mean germination for treatment, G_c = mean germination for distilled water control, L_t = mean radicle length for treatment, and L_c = mean radicle length for distilled water control. The germination index was rated as follows⁴³: 1.0–0.8, no inhibition of plant growth; 0.8–0.6, mild inhibition; 0.6–0.4, strong inhibition; < 0.4, severe inhibition. In this study, GI values for the OF treatments (the OFs mixed with the soil) were compared to those for the soil treatment (the soil only) and a phosphate buffer solution control. The same arada soil used in the CO₂ evolution test was used.

The mean application rate of kosi N to arada fields in the semi-arid Ethiopian Rift Valley was 67 kg ha⁻¹ year⁻¹ DM²⁷. In this study, consideration was given to mix the OF sample with the culture soil used for incubation at the same application rate of kosi N. The weight of the OF sample mixed with the culture soil was determined based on the bulk densities of the arada soil at 0–10 cm depth from where the culture soil was collected and total N in the OF sample. Another consideration was given to put the electric conductivity (EC) and pH of the extracts from the OF treatments, the soil treatment, and the control treatment on the same level⁴⁴.

For the OF treatment, after the OF sample mixed with the soil was incubated in a pot at room temperature (22–27 °C) and 60% saturation with water for 1 day, the soil solution was strained through a No. 2 filter (pore size, 8 µm). To the filtrate, 2 mol L⁻¹ phosphate buffer solution (1 mol L⁻¹ each of the dipotassium hydrogen phosphate and potassium dihydrogen phosphate was contained) was added to adjust the EC of the filtrate to 4.0 dS m⁻¹ (the pH becomes 6.8–7.2 then⁴⁴).

The control treatment was prepared by diluting the phosphate buffer solution to make the EC at the same level as the OF extract and soil extract. Two filter papers were placed on a petri dish 6 cm in diameter, on which 3 mL of the OF extract, soil extract, or the phosphate buffer solution corrected (the control) was evenly dropped. Garden cress (Lepidium sativum, L.) 16 seeds were placed on it, which was kept in the dark at room temperatures (23–27 °C) for 96 h (4 days). Percentage seed germination and radicle lengths were measured for each petri dish to determine the germination index^43,44.

Statistical analysis

Multiple comparison tests (the Bonferroni correction) were conducted to detect differences in the mean values of (1) NH₄⁺ and NO₃⁻ concentrations and NH₄⁺:NO₃⁻ ratio and (2) original pH and the volume of CO₂ produced between each pair of the four OFs (farmer-kosi, test-kosi, fast compost, and household wastes). The same statistical tests were conducted for (3) the germination index between each pair of the four OF treatments and the soil treatment. SPSS ver. 20 (IBM) was used for the statistical analysis.

Source: Ecology - nature.com