Retention of nitrogen from artificial urine using pine bark and potato waste biochar and ash and the fertiliser value of the products.

Abstract

Inadequate sanitation in rural and peri-urban areas contributes to environmental pollution, while soil nutrient mining poses a serious threat to food security for resource-poor farmers. The situation is becoming increasingly severe, with over 90% of human excreta and wastewater being discharged into the environment with little or no treatment. Human urine, rich in nitrogen (N), phosphorus (P), and potassium (K), is a promising fertiliser alternative, but its high-water content and alkalinity hinder direct use. The increasing need for sustainable waste management and nutrient recycling has prompted interest in the reuse of human urine as a nutrient source in agriculture. Organic materials like potato waste and pine bark are abundant in South Africa and they offer a potential solution for stabilising urine nutrients. However, there is limited information on the use of these organic wastes and their biochar and ash types. The aim of this study was to determine the recovery of nitrogen by potato waste and pine bark and their biochar and ash types and the potential of the urine-based solid fertilisers to release the nutrients and improve crop productivity. The feedstock, ash and biochar types pyrolysed at 350 °C and 650 °C from cull potatoes (CP) and pine bark (PB) were used for recovery of nitrogen (N) from artificial human urine (AHU). The AHU was added to each medium (50 g) six times at 29 and 33 ml and dehydrated each time at 45 and 60 °C, respectively. Therefore, cumulative urine additions at days 0, 14, 21, and 28 were 0, 58, 116 and 174 ml for 45 °C (4.5 L/m²) and 0, 66, 132 and 198 ml for 60 °C (5.1 L/m²), with dehydration in between. In addition, pine bark feedstock, kiln biochar and ash were also treated with the AHU and dehydrated at the glasshouse (Min temp: 9 and max. temp: 57 °C). Samples of the urine-products were collected before each urine addition, and analysed for pH, total carbon (C), and nitrogen (N) and total N recovered from AHU was calculated. The final urine-based products were applied to soil from the 0-20 cm depth of a Westleigh (Acrisols) to study nutrient release patterns in the laboratory and crop growth under glasshouse conditions. A field trial was also carried out on the same soil. The urine-products were applied to soils at triple N (300 kg ha-1 ) and at recommended N (100 kg ha-1 ) rate for the incubation and spinach experiments. Destructive sampling was done at days 0, 7, 14, 21, 28, 42, and 56, with analysis of pH, ammonium- and nitrate-N and extractable P. The urine- products were applied to 2 kg of soil (0-20 cm depth) for spinach growth under a glasshouse. The pot trial ran for 84 days with plant sampling at 6 and 12 weeks and analysis for dry matter, nutrient uptake and tissue micronutrients. The residual soil samples were analysed for pH and extractable P. For the field trial, the PB- urine products were applied at the recommended N rate for spinach in the first season. The PB feedstock, biochar and ash that were not pre-treated with urine were also included as reference materials. In the second season, the plots were split into two, with half treated with the same treatment at the same rate (cumulative effects), while the other half did not receive additional treatment (residual effects). Spinach dry matter yield, nutrient uptake, and tissue micronutrients were determined. The different media from PB recovered more N than those from CP. The 60 °C temperature generally promoted rapid dehydration and consumed less energy than 45 °C. The most N was recovered on PB biochar pyrolysed at 650 °C with dehydration at 60 °C. The CP feedstock also recovered more N than CP ash and CP biochar, which showed negative N recovery. Results demonstrated that urine-PB ash treatment enhanced nitrate-N, while CP ash maintained high extractable phosphorus and pH. In glasshouse trials, PB-based products significantly improved spinach dry matter yield and nitrogen uptake, with performance often comparable to synthetic fertilisers. However, elevated sodium (Na) and manganese (Mn) concentrations in plant tissue were observed, necessitating monitoring to prevent potential toxicity. Field trial confirmed the agronomic viability of PB urine-based fertilisers, The pine bark urine treatments had higher dry matter yield and N uptake relative to the Pine bark un-enriched products, during the first season. Cumulative additions led to lower dry matter and nitrogen uptake in PB biochar-enriched urine (PB biochar-U), which was similar to the negative control and lower than PB-U. However, PB biochar-U showed higher residual effects than the negative control. Notably, repeated application did not exacerbate soil salinity due to effective sodium uptake by the initial crop. This research highlights the potential of urine-based fertilisers derived from PB and CP materials as sustainable alternatives to synthetic nitrogen inputs, offering a circular solution to nutrient management. These findings imply that urine-based products derived from pine bark and cull potatoes, particularly biochar and ash from pine bark, are effective in recovering nitrogen, releasing nutrients in the soil, and improving crop productivity. The potential of these products to show similar results to synthetic fertiliser in spinach yield and nutrient uptake under glasshouse and field conditions shows their potential as sustainable nitrogen sources. Moreover, less effects on soil salinity and the observed residual benefits suggest that these material can support long-term soil fertility in resource limited settings. This research offers practical solutions for nutrient recycling, closing sanitation-agriculture loops, and enhancing food security.