Effects of maize (Zea mays L.) and sorghum (Sorghum bicolor (L.) Moench) residue management on soil carbon sequestration, nutrient cycling and spinach (Spinacia oleracea) yield.

Abstract

The increasing threat of climate change and declining soil quality presents significant challenges to global food security and crop production. This necessitates the adoption of agricultural practices that mitigate climate change while improving soil quality and crop yields. This study aimed to investigate the effects of maize (Zea mays L.) and sorghum (Sorghum bicolor L.) residue management, on soil carbon storage, nutrient cycling and spinach (Spinacia oleracea) yield. Contrasting cultivars of maize (SC701 and R201) and sorghum (AS8 and PAN8816) were grown at Ukulinga Research Farm of the University of KwaZulu-Natal. The aboveground biomass was harvested at maturity and ground. Half of the ground biomass was retained as raw feedstock, while the other half was pyrolyzed to produce biochar types at 350 and 650oC in a muffle furnace and at ~400°C in a field kiln. The feedstocks and biochars were characterized for proximate and elemental composition, chemical properties, structural characteristics and functional groups. An incubation study was conducted to assess the effects of crop type, cultivar and conversion of the residues to biochar on CO2 emissions, nitrogen (N) and phosphorus (P) mineralization and soil pH. Treatments included feedstock and biochar produced at 350°C and 650°C, applied at a rate equivalent to 10 t C ha-1 in 100 g of soil and were replicated three times with a control (soil only). The CO2 emissions were periodically measured for 120 days. A separate incubation experiment with identical treatments was set up with enough replication to allow for destructive sampling to analyze N and P mineralization and soil pH periodically for 120 days. A field experiment was conducted during the summer and winter seasons of 2023 at the same research farm to evaluate the effects of crop type, cultivar and conversion to biochar of residues on CO2 emissions, soil carbon pools, soil fertility and spinach yield. In the summer, biochar produced in the kiln was applied at 5 t C ha-1 to 15 m2 plots. In winter, the plots were split into two, where one half received repeated applications to assess cumulative effects, while the other half was left untreated to evaluate residual effects. Spinach seedlings were planted and grown for 12 weeks, with weekly CO2 emissions measured using a CO2 auto-analyzer. At the end of each season, spinach was harvested and soil samples were collected to analyze soil carbon pools, chemical properties and enzyme activities. The characterization study revealed that biochars produced at 650oC exhibited enhanced stability, with a reduction in labile functional groups and an increase in fixed carbon content, making them highly suitable for carbon sequestration. Additionally, biochars produced at this temperature demonstrated improved pH and ash content, making them effective forameliorating acidic soils and enhancing nutrient retention. The incubation study showed that cultivar-specific differences and pyrolysis temperatures significantly influenced CO2 emissions, N and P mineralization and soil pH than broad crop type. Feedstock treatments resulted in the highest CO2 emissions, whereas biochars, particularly those produced at 650oC consistently emitted lower CO2 emissions and increased nutrient availability compared to feedstock and biochars produced at 350oC. Biochars from cultivars R201 and AS8 were identified as more effective for carbon storage, while those from SC701 and PAN8816 enhanced N availability, making them ideal for improving soil fertility. Field experiments revealed that CO2 emissions, soil carbon pools, soil chemical properties, enzyme activities and spinach biomass yield parameters were more strongly influenced by cultivar-specific differences than by crop type. Biochar consistently outperformed feedstock in improving soil carbon pools, chemical properties, enzyme activities (urease and alkaline phosphomonoesterase activity) and spinach biomass yields, particularly in the summer season. In contrast, feedstock applications were more effective in enhancing other enzyme activities (βglucosidase and acid phosphomonoesterase activity). Biochars from cultivars SC701 and PAN8816 improved soil pH, nutrient availability and spinach biomass yields, while feedstocks increased micronutrient tissue concentrations. In the first season, biochar treatments significantly increased SOC pools compared to the control. Among maize cultivars, the application of biochar from R201 cultivar (M-R201-B) resulted in the highest SOC content (20.54 g kg-1), giving a 276% increase over the control, while the application of SC701 feedstock (6.37 g kg-1) yielded the lowest SOC content. Similarly in sorghum cultivars biochar from cultivar AS8 resulted in the highest (19.50 g kg-1) while feedstock of cultivar AS8 gave the lowest SOC content (6.11 g kg-1). Biochar applications also enhanced POC and POXC pools with biochars from R201 and PAN8816 cultivars showing the highest contents of these pools amongst the treatments. Additionally, biochar treatments enhanced MBC with SC701 and PAN8816 biochars exhibiting higher concentration while feedstock treatments resulted in lower MBC concentration. Cumulative biochar applications significantly increased soil carbon pools while reducing CO2 emissions compared to the control. Moreover, the residual effects resulted in increased carbon pools compared to the control. These findings highlight the importance of crop cultivar selection and pyrolysis conditions in influencing the characteristics of biochar types and maximizing the benefits of applications of these materials from maize and sorghum. They provide valuable insights into sustainable residue management practices, demonstrating their potential to mitigate climate change through carbon storage while improving soil quality and crop yields.