Fertilizer Management Strategy to Reduce Global Warming Potential and Improve Soil Fertility in a Nitisol in Southwestern Ethiopia ^†

Abstract

Proper fertilizer management and applications could effectively reduce global warming potential (GWP) and improving soil fertility under cereal production. However, the effect of soil fertilizer management practices on GWP and soil fertility is less understood in the agricultural soils of Ethiopia. The present study evaluated the effects of fertilizer application strategy on GWP, and soil fertility in a Nitisol. Both field and laboratory incubation experiments were conducted with the same treatments. Seven treatments (Cont: no input; 100 min: 100% mineral fertilizer, 80 min: 80% mineral fertilizer + 1.4 t ha⁻¹ compost; 60 min: 60% mineral fertilizer + 2.8 t ha⁻¹ compost; 50 min: 50% mineral fertilizer + 3.5 t ha⁻¹ compost; 30 min: 30% mineral fertilizer + 4.9 t ha⁻¹ compost, and 100 comp: 100% compost) with four replications were applied on maize crop (Zea mays L. Bako hybrid 661) for two consecutive growing seasons. The laboratory incubation experiment was also carried out with two moisture levels (40% and 75% of water-filled pore space) to simulate the seasonal rainfall pattern. GWP was calculated by summing up the quantified gas emissions of nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄). The laboratory result for GWP shown that soil fertilized with mineral fertilizer alone was significantly (p < 0.05) increased by 27–34% of the average GWP compared to combined fertilizer treatments in the soil at a moisture level of 75%. From the field experiment, most plant nutrients were significantly increased in combined fertilizer treatments compared to sole mineral fertilizer application. For example, carbon, calcium, magnesium and potassium were increased by 26.2–39.8%, 73.2–168.8%, 146.6–251.5% and 47–99% respectively in combined fertilizer compared to 100 min treatment. The study revealed that combining 30 or 50 kg N ha⁻¹ of mineral fertilizer with biowaste compost (4.9 or 3.5 t ha⁻¹) would be a suitable combination to mitigate the GWP and improve soil quality in smallholder farming systems, due to a slow release of N during decomposition into the soil compared to mineral fertilizer alone. However, to evaluate GWP under the field conditions, future investigations are recommended.

1. Introduction

Agricultural soil is one of the source for greenhouse gas (GHG) emissions as the result of the excessive application of N fertilizer, animal manure, and decomposition of organic material [1]. The major GHGs, which contribute to global warming potential (GWP), are carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄). The average amounts of GHG emitted from agricultural soils are estimated to be 14% of the total global GHG emissions and thereby accelerate GWP [2,3].

Based on the global average value Ethiopia is contributing an insignificant amount of GHGs (0.03%) emissions as a global warming potential, from all agricultural activities [4,5,6]. GHGs emitted from fertilized crop fields are not well investigated and understood in Ethiopia. Currently the amount of N fertilizer (46 kg N ha⁻¹) applying in the crop field has a non-significant effect on the GHGs emissions when we compare the data with other developed countries application rates (169 kg N ha⁻¹) [7]. However, in the future, Ethiopia aimed to increase the application of mineral fertilizer per hectare from 65 kg in 2010 to 247 kg in 2030 [6]. Studies suggested combining organic and mineral fertilizers application is a viable option to reduce N losses through N₂O emission and their impact on GWP [5,8]. Combined uses of organic and mineral fertilizer have been widely used as a means for soil fertility improvement. Economically it is affordable for poor farmers, and environmentally suitable. However, the effect of soil fertilizer management practices on GWP and soil quality is less understood in the agricultural soils of Ethiopia. The present study evaluated the effects of combined application of biowaste compost and mineral fertilizers on GWP, and soil fertility in a Nitisol.

2. Material and Method

The two-year field experiment was conducted at the research station of Jimma University in Southwestern Ethiopia with latitude, 7°42′ N; longitude 36°49′ E. The research site is characterized as humid tropical climate with a minimum of 13 °C and a maximum of 28 °C [9]. The annual maximum and minimum precipitation in the area is around 1200 and 2400 mm, respectively. The soil was Nitisol with silty clay loam texture, pH of 4.98, organic carbon content of 2.4%, and total N of 0.22%.

The laboratory incubation experiment was carried out at the University of Rostock, Germany with the same fertilizer treatments as in the field experiment in four replications. Gas samples from the headspace of the sealed jars were collected by 60 mL syringes, transferred to evacuated vials, and the gas concentrations of N₂O, CO₂ and CH₄ were measured with a gas chromatograph (GC-2014, Shimadzu). Gas emissions (g ha⁻¹ day⁻¹) were calculated by following Comeau et al. [10] equation for soil heterotrophic respiration assessment using minimally disturbed soil microcosm cores. The GWP was determined for fertilizer rate and type using the following equation.

$$\mathrm{GWP}={\mathrm{N}}_2\mathrm{O}\times 298+{\mathrm{CO}}_2+{\mathrm{CH}}_4\times 25$$

where GWP is global warming potential (kg CO₂ eq. ha⁻¹); N₂O is the amount of N₂O (kg ha⁻¹); CO₂ is the amount of CO₂ (kg ha⁻¹); CH₄ is the amount of CH₄ (kg ha⁻¹); 298, and 25 is GWP coefficients to convert N₂O and CH₄, respectively, to CO₂ equivalents.

In the beginning, before the treatment application, and after harvest, composite soil samples were collected from the surface layer (0–20 cm) was analyzed following the standard laboratory procedures for each soil parameter.

One-way analysis of variance (ANOVA) was used to determine the effect of different fertilizer types on GWP and soil parameters. The mean values were determined by using the Tukey multiple-comparison test by using SPSS (22.0 version).

3. Result and Discussion

3.1. Fertilizer Management Effect on Global Warming Potential

The result for GWP revealed significantly different (p < 0.05) values among the treatments (Figure 1). The soil amended with 100 min was significantly (p < 0.05) increased by 27%, 30.4% and 34% of the average GWP values compared to 80 min, 50 min, and 30 min treatments, respectively, in wet soil. This may be attributed to the slow release of mineral N by microbial activity and low contribution to GHGs emissions compared to 100 min treatment. The soil amended with 100 comp was significantly reduced by 62.7% of GWP compared to the 100 min treatment in wet soil. In the 40% WFPS condition, the 100 min treatment significantly (p < 0.05) increased GWP by 34% and 51% in comparison to the 100 comp treatment and the Cont, respectively. With increasing ratios of mineral fertilizer, the GWP values were increased in both soil moisture conditions. However, in a dry soil condition, there was no significant difference among 100 min treatment and combined fertilizer treatments (Figure 1). The study is in opposite with the Bharali et al. [11] finding, who reported higher GWP (887.4 kg CO₂ eq. ha⁻¹) in combined application of NPK with Sesbania aculeate compared to NPK alone applied filed (540.6 kg CO₂ eq. ha⁻¹). The justified reason by the author was application of NPK together with organic source enhanced the emission of CH₄ by providing additional C substrate compared to NPK alone in the rice field. This might be not true in our case since the soil moisture in the current experiment was lower compared to the previous study to enhance methanogenesis. In this study the main GHGs created GWP amount variation among the treatments were due to N₂O and CO₂ than CH₄. The influence of fertilizer type is clearly observed on the emissions of N₂O and CO₂ compared to Ch₄. This indicates that the contribution of CH₄ gas to GWP is not only influenced by fertilizer type but also soil moisture [9]. This study provides very useful insights for policymakers to design appropriate nutrient management strategies to reduce GWP from cereal production systems. In the current cereal production system, the contribution of fertilizer application to GHG emissions and GWP are very low due to the application of low rates. However, in the future, the amount of mineral fertilizer application will increase from about 65 kg ha⁻¹ in 2010 to about 250 kg ha⁻¹ in 2030 [12] to increase grain yield for the ever-increasing population in Ethiopia. This situation will be expected to contribute significantly to GWP. To minimize the future impact of fertilizer application on GWP without reducing the crop production in the country, applying the combined fertilizer at a rate of 30 or 50 kg N ha⁻¹ of mineral fertilizer with biowaste compost (4.9 or 3.5 t ha^–1) is a viable option, for smallholder farmers [9].

3.2. Effect of Combined Fertilizer on Soil Fertility

From the field experiment, most plant nutrients were significantly increased in combined fertilizer treatments compared to sole mineral fertilizer application. For example, carbon, calcium, magnesium, and potassium were increased by 26.2–39.8%, 73.2–168.8%, 146.6–251.5%, and 47–99%, respectively, in combined fertilizer (30 min and 50 min) in comparison to 100 min treatment (

Table 1. Soil minerals (mg kg⁻¹ soil) and chemical properties (0–20 cm) after harvest in different treatments (2018–2020) (mean ± SD).

Soil Parameters	Treatment
	Cont	100 min	80 min	60 min	50 min	30 min	100 comp
Fe	46.5 ± 2.8 ab	52 ± 2 ab	65 ± 3.1 ab	58.9 ± 3.5 ab	48.2 ± 2.8 ab	13.5 ± 2.1 b	3.7 ± 0.6 a
Ca	266.1 ± 21 b	162.6 ± 30 a	350.3 ± 11.6 d	356.2 ± 21.2 d	281.7 ± 34.1 b	437.1 ± 38.9 c	121.7± 7.3 a
Mg	30.4 ± 2.3 abd	16.3 ± 2.9 a	47.7 ± 6.5 cd	42.9 ± 9.7 d	57.3 ± 10.5 c	40.2 ± 3.3 bd	20.5 ± 1.4 a
K	27.9 ± 3.6 a	61.7 ± 12.2 e	90.5 ± 6.3 bc	122.8 ± 11.2 d	108.6 ± 1.6 d	101.5 ± 7.9 c	75 ± 5.2 be
N	227.5 ± 7.1 a	332.5 ± 26.3 bd	315 ± 23.8 ad	371 ± 21.6 b	335.3 ± 12.8 bd	350 ± 18.3 bd	285 ± 17.3 a
P	0.3 ± 0.01 a	0.2 ± 0.05 a	−0.14 ± 0.1 a	0.4 ± 0.1 a	0.07 ± 0.01 a	−0.02 ±0.01 a	0.6 ± 0.1 a
S	45 ± 12 a	−2.2 ± 1.02 a	20 ± 8.02 a	10 ± 2.1 a	235 ± 55.1 b	30 ± 4.1 a	15 ± 2.8 a
C	2375 ± 95.7 a	2575 ± 359.4 a	2975 ± 596.5 ac	3250 ± 70.2 bc	3600 ± 81.7 b	3475 ± 221.7 bc	3875 ± 170.8 b
Zn	8.4 ± 1.2 ab	7.9 ± 2.1 ab	18.2 ± 1.5 c	10.1 ± 0.9 abc	7.5 ± 1.3 ab	12 ± 0.4 bc	2.8 ± 0.4 a
Mn	158 ± 16.8 a	182.5 ± 17.3 a	407.6 ± 64.3 b	238.3 ± 2.3 c	181.4 ± 12.7 ac	38.5 ± 1.8 b	179 ± 2 a
pH	0.04 ± 0.01 a	0.14 ± 0.01 bc	0.16 ± 0.04 bc	0.17 ± 0.03 bc	0.15 ± 0.06 bc	0.20 ± 0.01 b	0.09 ± 0.01 ac
Ec	0.2 ± 0.004 a	0.2 ± 0.01 a	0.17 ± 0.02 a	0.20 ± 0.01 a	0.17 ± 0.01 a	0.16 ± 0.01 a	0.18 ± 0.01 a
CEC	0.06 ± 0.006 a	0.1 ± 0.001 d	0.13 ± 0.006 c	0.10 ± 0.01 b	0.07 ± 0.004 ab	0.09 ± 0.01 bd	0.07 ± 0.003 ad

Means in the same letters are not significantly different at 5% level of significance; and means in the different letters are significant at (p < 0.05) by using Tukey-test.

). The value for zinc was significantly higher in 80 min compared to Cont, 100 comp, 50 min, and 100 min treatments. For the pH value, there was no significant difference between combined fertilizer and mineral alone. The field with 80 min exhibited significantly higher electrical conductivity (Ec) value in comparison with other treatments. There were no significant differences between other treatments for Ec values.

The significant increment of organic C in the combined fertilizer could be associated with the positive effects of biowaste compost application on organic C storage. The results are also supported by Ogundijo et al. [13] who reported the application of 10 t ha^–1 poultry manure with (120 kg ha^–1 NPK) significantly increased organic C amount over sole fertilizer application (120 kg ha^–1 NPK). The highest value of total N was recorded in treatment with combined fertilizer and the lowest value was recorded in the Cont (Table 1). The possible reason for the highest value of total N could be the slow release of mineral N reduces the losses of N by gaseous (N₂O) form in combined fertilizer than sole mineral. It indicates the positive influence of the integration of compost with mineral fertilizer on total N. The main result of the research finding is supported by the previous studies [14,15], namely that observed higher N value in the combined fertilizer field in comparison with mineral fertilizer received field. Nitrogen is the most susceptible plant nutrient and is greatly required by plants [16]. The result suggests that compost supplement with mineral fertilizer can increase the concentrations of macro and micronutrients in the soil and this could enhance productivity of soils.

4. Conclusions

The application of mineral fertilizer alone increases GWP in Nitisol in wet condition in comparison to combined application. Most plant nutrients and some chemical soil properties were significantly increased in combined fertilizer application than sole fertilization. We recommend that 30 or 50 kg N ha⁻¹ of mineral fertilizer in combination with compost of 4.9 and 3.5 t ha^–1 be applied in Nitisol to reduce GWP with increased soil resilience in smallholder farming systems in Ethiopia. In addition, future investigation would be recommended on-farm and on-stations in the field to consider different factors, such as plants. Since, in this study, we only consider two moisture levels, one temperature, and one soil type, other factors should be investigated in the future at field conditions.