Soil fertility effects of repeated application of sewage sludge in two 30-year-old field experiments

Recirculation of plant nutrients from waste materials back to agriculture is necessary in a sustainable food production system. In this study we investigated the long-term effect of direct land application of sewage sludge (SS) on soil fertility on two Swedish farms, where field experiments were set up in 1981 with three rates of SS (0, 4 and 12 Mg dry matter ha^?1 every 4 years) and three rates of mineral fertilisation, in a factorial design with four replicates. At one site, SS application tended to increase crop yield over time but, when plant nutrients were not limiting, the effect was only significant for spring barley. We deduced that improved soil structure was the main driver for this fertility enhancement after SS application, as indicated by lower soil bulk density and higher soil carbon concentration. After 30 years, soil organic carbon stocks to 0.40 m depth differed by up to 17 Mg ha^?1 between treatments. According to carbon balance calculations, retention of carbon derived from SS ranged between 18 and 20% for the soil layers analysed at the two sites. Soluble phosphorus (P) increased with sludge application rate, but represented only around 2% of the expected residual P according to P-balance calculations. The fertiliser value of nitrogen was also low, with only 3–8% nitrogen use efficiency. Heavy metals such as copper, zinc and mercury showed moderate accumulation in the soil, but elevated levels of metals were not detected in crops, even at the highest dose of SS. These results show that SS represents a valuable resource for improving soil fertility in terms of soil organic matter and soil structure, but its efficiency for nutrient cycling is very low within the time frame considered in the study.