Deciphering the rhizobium inoculation effect on spatial distribution of phosphatase activity in the rhizosphere of alfalfa under copper stress

Abstract

Legume–rhizobium symbiosis plays an important role in agriculture and ecological restoration. However, the regulatory mechanisms of rhizobium in alleviating heavy metal stress through the biochemical response of plant-soil system is limited. In this study, alfalfa was inoculated with a copper (Cu)-resistant rhizobium, and its effect on plant growth and the spatial distribution of phosphatase in the rhizosphere under Cu stress was assessed. Our results showed that rhizobium inoculation alleviated Cu-induced growth inhibition, and increased the nitrogen and phosphorus content in alfalfa tissues. Moreover, inoculated plants had a higher Cu uptake than non-inoculated plants, with a much higher increase in the roots than in the shoots; thus, inoculation with rhizobium was shown to decrease the transfer coefficient and promote Cu phytostabilization. The zymograms illustrated that the distribution of phosphatase activities was associated with the presence of roots. Compared with the non-inoculated treatment, the rhizobium inoculation increased the hotspot areas of phosphatase by 26.1% and 39.3% at the Cu 0 and Cu 800 treatments, respectively. In addition, the available phosphorus in the soil showed negative correlations with soil phosphatase activity (p < 0.05). The model of partial least squares path modelling (PLS-PM) indicated that soil Cu content directly influenced the hotspot areas of phosphatase activities in the rhizosphere and explained most (86%) of the variation. Thus, the enzymatic hotspots were concluded to mainly be affected by the Cu content of soil, and the phosphatase activities in the rhizosphere were mainly regulated by the ratio of nitrogen to phosphorus. These findings provide a basis for the spatio-temporal dynamics of biogeochemical reactions in the rhizosphere of polluted soils.