Abstract
Amendment of agricultural soils with municipal sewage sludge (SS) or biosolids may provide a valuable source of plant nutrients and organic matter. Utilization of SS in cultivation of oil- and biofuel-producing crops such as canola (Barassica napus) has not been adequately studied. The aim of this study was to compare the influence of biosolid application rates on biomass production of canola and on soil properties and nutrient availability measured both by conventional methods and a new technique involving plant root simulators (PRS probes). For this purpose, a greenhouse pot experiment was established in a Typic Xerfluvent (5 kg) from central Greece amended with various rates of SS [0 Mg ha(-1) (C) 20 Mg ha(-1) (SS1), 50 Mg ha(-1) (SS2), and 100 Mg ha(-1) (SS3)] and cultivated with canola. After a 2-month planting period, the whole plants and soils were removed from the pots. One kg of the soils was selected for analyses, and the rest of the soil was cultivated again for 2 more months with the same crop. Bioavailability of phosphorus (P) and potassium (K) was measured using the conventional methods (Olsen and ammonium acetate method respectively) as well as by PRS probes. Results showed that application of SS significantly increased canola biomass only in the treatment SS1 compared to the control in the first planting period. However, in the second period, SS application significantly increased canola biomass in the treatments SS2 and SS3 compared to the control and SS1. Soil pH decreased significantly from 7.9 in the control treatment to 6.9 in the treatment SS3. Furthermore, soil organic matter, nitrate nitrogen, and hot water-extractable boron were significantly increased with increasing SS application rate. Olsen P increased significantly with SS application in both planting periods but its increase was greater in the second planting period compared to the first one. Phosphorus-supplyrate (PSR) as assessed by the PRS probes increased significantly with SS application rate in both planting periods but it decreased significantly in the second planting period as compared to the first one. Plant tissue concentration P showed a trend similar to P supply rate (PSR). A strong relationship was recorded between PSR and Olsen P while plant tissues P concentration was strongly correlated with PSR and Olsen P (R-2 0.93*** and 0.87*** respectively), indicating that probe method estimates better bioavailability of P to the grown plants compared to the Olsen method. Application of SS decreased significantly available K and potassium-supply rate (KSR) compared to the control, especially in the second planting period. Potassium-supplying rate decreased significantly in the second period compared to the first one. No relationships were recorded among KSR, ammonium acetate K, and plant tissue K concentration.