Comparative assessment of soil phosphate status, water eutrophication, and potentially toxic metal accumulation in Usagu agro-ecosystem

Abstract

The present study assessed (i) soil fertility, phosphate status, sorption capacity and saturation, (ii) accumulation and distribution of potentially toxic elements (PTEs) in soils and plants, and (iii) soil and surface water quality as they are influenced by current farming practices in Usangu agro ecosystem (UA) following increased agricultural intensification and urbanization. Soil, sediment, plant, and water sample were collected from ten irrigation schemes for plant nutrients, physico chemical soil properties, PTEs, water quality determination and assessment. The results indicated that 90% of studied soils had N, P, K, Mg and most micronutrients (Cu and Zn) in deficient levels compared to recommended levels. To ensure sustainable intensification of agricultural production, additional fertilization is recommended. The P soil sorption capacity (PSC), P saturation status (PSD) and its determinants (inherent P (PM3), AlM3, FeM3, and CaM3) which influence P availability, storage, and P losses to the environment determined in UA, were in a range (mg/kg) of: PM3 (14.9- 974.7), AlM3 (234.7-3789.4), FeM3 (456.9-2980.2), and CaM3 (234.7-973.3). Where estimated PSCM3 ranged 5.6-34.9 mmol/kg, indicating low to high status for P holding or fixation. Some soils had very low PSCM3 indicating a risk of P loss to environment. The estimated PSDM3 ranged from 0.01-17.6% and was less than 24%, indicating low P loss risk from farmlands to water bodies. Some of studied soils had PSDM3 above 15%, which is equivalent to 25% based on ammonium oxalate method, which can cause higher P loss risk. The spatial PTEs distribution in soils in paddy, maize farming, and conservation areas across contrasting land management schemes (Group I dominated by agricultural areas versus Group II dominated by residential and agricultural areas) varied significantly (P < 0.05) across groups and land use. Where total and bioavailable concentrations (in µg/kg) of some PTEs determined were; Cr (Group I = 1662 and Group II = 1307 (Total), Group I = 55.1 and Group II = 19.2 (bioavailable)) and Pb (Group I = 5272 and Group II = 6656 (Total), Group I = 1870 and Group II = 1730 (bioavailable)). Farming areas had higher PTEs concentrations than non-farming areas. Overall, concentrations of Fe (99.5%), As (87%), Se (66%) and Hg (12%) were above Tanzanian maximum permissible limits. Furthermore, water quality and eutrophication characterization in UA observed water pH ranging 4.9 to 6.8, where some were outside FAO acceptable range (6.5-8.4) for irrigation water. The NH4-N ranged 10.6- 70, NO3-N (8.4-33) and total N (19.1-104 mg/L), which increased as water moved from intakes to drainages. Whilst total P ranged 0.01-1.65 mg/L and increased from intake to drainages and in some sampling points exceeding 0.1 mg/L indicating water quality degradation.