Long-Term Manure Application in Urban Gardens: Impacts on Soil Fertility, Mineral Composition, and Variability

Urban and peri-urban agriculture (UA) plays an increasingly important role in promoting sustainable urban development, providing socioeconomic, environmental, and educational benefits. However, UA is often linked to nutrient accumulation in soils since vegetable-growing areas typically receive substantial inputs of both organic and inorganic fertilizers. This study examines soil variability in two sections of an urban allotment garden subjected to long-term manure fertilization for 12 or 16 years, with application rates up to 10–12 kg m−2 yr−1. Surface soils were analyzed for organic and inorganic carbon, total-N, available-P and -K, pH, and elemental composition using portable X-ray fluorescence (pXRF). Prolonged manure incorporation substantially enhanced soil fertility, as evidenced by increases in soil organic carbon (up to 3.78%), total-N (up to 0.38%), available-K (up to 412 mg kg−1), and both total- and available-P (up to 2485 and 276 mg kg−1, respectively). Marked shifts in mineral composition were also detected, including significant increases in total Ca, inorganic C (as calcium carbonate), Sr, and S. Despite the high manure inputs, no accumulation of potentially toxic elements (PTEs) was observed. However, pronounced spatial heterogeneity emerged among individual plots, with coefficients of variation reaching 58% for S and 47% for Zn, reflecting differences in fertilization intensity and management practices. Portable X-ray fluorescence (pXRF) analysis proved highly effective for detecting soil compositional changes and adequate for predicting K and P availability, highlighting its value as a rapid diagnostic tool for precision agriculture. Overall, these findings demonstrate the agronomic benefits of long-term organic fertilization while emphasizing the need for careful management to avoid nutrient imbalances and ensure sustainable practices that minimize environmental risks.