Effect of sewage sludge and digestate from anaerobic fermentation as soil additives on the nutritional status and accumulation of non-essential elements in plants with different nutrition strategies

Abstract

We explored how sewage sludge and digestate as a soil amendment impact the availability of non-essential elements to plants with different nutrition strategies. Four species were cultivated on soil or soil amended with sewage sludge and digestate: the specialized Ni hyper-accumulator Alyssum murale, the P-efficient Mn accumulator Lupinus albus, the P-inefficient metal accumulator Fagopyrum esculentum and the excluder species Carthamus tinctorius. Shoot accumulation of essential (P, Mg, Fe, Mn, Zn, Ni) and non-essential elements (As, Cd, Pb, Ge, Ga, rare earth elements: REE) was evaluated together with changes of labile element concentrations in soil. Shoot metal(loid) contents decreased in the order F. esculentum > C. tinctorius, L. albus > A. murale. Both treatments improved micronutrient supply in all species; however, shoot P only increased in sewage sludge-treated plants. Both treatments increased the concentrations of labile As, Ge, Ga and REE in soil, while labile Cd was exclusively higher when sewage sludge was added. The digestate treatment did not increase soil–plant transfer of the elements, except Ga in F. esculentum. However, the addition of sewage sludge increased shoot Cd contents in all species. Moreover, F. esculentum responded with a higher accumulation of Ga and L. albus accumulated more As and REE, while shoot accumulation remained unchanged in other species. Our findings suggest species-specific differences in the utilization of labile element pools that most likely derive from altered rhizosphere chemistry, where processes related to element acquisition depend on the plant's nutritional status.