Soil Health Improvement Technologies to Enhance Drought and Nutrient Resilience in Urban Agroecosystems in Zimbabwe

Abstract

Soil degradation which is linked to poor soil organic matter management remains a significant barrier to sustained crop production in smallholder urban agriculture (UA) in sub-Saharan Africa (SSA). While organic nutrient resources are often used in UA to complement inorganic fertilisers in soil fertility management, they are usually scarce and of poor quality to provide optimum nutrients for crop uptake. Alternative soil nutrient management options are required. Aluminium-water treatment residual (Al-WTR), a by-product of the drinking water treatment process is an alternative organo-mineral resource that can be used to complement mineral and organic nutrient resources in urban agroecosystems. Although previous research has revealed the transformative effects of Al-WTR on soil physicochemical properties, there is still some inconsistency about the effects of Al-WTR on relations between plant yield and nutrients, particularly phosphorus (P). The aim of this study was to evaluate the impact of co-applying Al-WTR in combination with other organic nutrient resources (compost, cattle manure and maize stover) as ‘co-amendments’ on soil physical, biological, and chemical properties, P sorption and maize productivity in UA in Zimbabwe. The study employed field, greenhouse, and laboratory approaches to test different Al-WTR-based options for improved soil health. The main treatments included single amendments of Al-WTR, compost (C), cattle manure (CM), maize stover (MS) or their co-amendments as Al-WTR + CM, Al-WTR + MS or Al-WTR + C; an unamended control and standard NPK. A field experiment to determine the influence of Al-WTR co-amendments on soil organic carbon (SOC) and selected soil physical properties showed higher accumulation of SOC and lower soil bulk density; higher soil structural stability, water holding capacity and higher maize grain yields in the co-amendments compared to the unamended soils. The co-amendment of Al-WTR and cattle manure (Al-WTR + CM) accumulated higher (4.96 g. kg-1) concentration of SOC and the lowest (1.30 g cm-3) bulk density, whilst the unamended control recorded the least (4.55 g. kg-1) in SOC and the highest (1.35 g. cm-3) bulk density. The co-amendment, Al-WTR + CM also exhibited greater soil structural stability, recording an average of 121.64 g. kg-1 water-stable aggregates (WSA) and 0.17 mm in mean weighted diameter (MWD), equating to an increase of 393% (WSA) and 141% (MWD), relative to the unamended control. The co-amendment, Al-WTR + CM also resulted in increments of at least 0.02 cm3 cm-3 in readily available water, whilst also retaining > 10% more water at field capacity relative to the control. Both co-amendments, Al-WTR + CM and Al-WTR and maize stover (Al-WTR + MS) in turn yielded four times more maize grain yield compared to the unamended control. Results also showed a higher biological activity in the co-amendments, suggestive of a high turnover potential of the co-amendments in restoring soil health. The co-amendment of Al-WTR + CM attained the highest microbial biomass carbon (190 ± 1.14 mg C kg-1) and microbial biomass nitrogen (35.80 ± 0.51 mg N kg-1) at 6 weeks after planting maize, whereas the least (120 ± 1.58 mg C kg- 1 and 18.72 ± 0.35 mg N kg-1) were recorded for the unamended control. Soil basal respiration (CO2-C emission) was higher in Al-WTR + MS, which gave the highest CO2-C emission of 167 ± 3.44 mg CO2-C kg-1 soil. The unamended control on the other hand recorded a higher metabolic quotient, releasing > 0.10 mg CO2-C microbial C day-1 more, compared to the co-amendments, suggesting more available carbon in the co-amendments and therefore less microbial strain compared to the unamended soil. Results of a short-term greenhouse experiment to evaluate the benefits of applying Al-WTR in combination with compost and inorganic P fertiliser, on soil chemical properties, and maize (Zea mays L.) productivity and nutrient uptake showed higher (3.92 ± 0.16 g) maize shoot biomass at 5 weeks after emergence in the co-amendment of 10% C + 10% Al-WTR, significantly (p <0.05) out-yielding the unamended control which yielded 1.33 ± 0.17 g. The addition of inorganic P fertiliser to the co-amendment (10% C + 10% Al-WTR + P) further increased maize shoot yield by about six-fold (7.23 ± 0.07 g), showing the important role of inorganic P fertilisers in crop production. The co-amendment, 10% Al-WTR + 10% C + P increased maize uptake of the micronutrients Zinc (Zn), Copper (Cu) and Manganese (Mn) by 13.63-, 1.08- and 0.79- mg kg-1, respectively, compared with the single amendment of 10% C + P. The enhanced micronutrient uptake can potentially improve maize grain quality and subsequently human nutrition for the urban population in SSA. A laboratory experiment to understand P sorption characteristics of a sandy soil co-amended with different ratios of Al-WTR and compost under varying levels of pH, particle size and P concentration showed higher maximum P sorption in the single amendment of Al-WTR compared to the co-amendments. The co-amendments in turn showed a reduction in crop inorganic P fertiliser requirements by ranges of 30 - 70% in the co-amendments compared to the single amendment of Al-WTR. Overall, results from this study showed that Al-WTR co-amendments can be used to re-build soil heath, enhance maize productivity, and improve human nutrition in smallholder urban agro-systems of Southern Africa and partly contribute to the United Nations Sustainable Development Goals (UN SDGs) linked to both soil and human health.