Acid phosphatase activity in the rhizosphere: how a cereal and a legume create local hotspots
Résumé
Phosphorus is a poorly mobile, essential nutrient in terrestrial ecosystems. Phosphorus can be of greater local availability in the rhizosphere where greater biological activity can increase enzymatic activities and induce chemical changes such as to pH. However, many rhizosphere processes occur at the very fine-scale (millimetric) while sampling is difficult to do at less than the cm-scale. Using rhizoboxes with 2D access to the soil profile, we mapped and monitored acid phosphatase activity in situ in chickpea and durum wheat, sole and intercropped, as well as in bare soil rhizoboxes over 5 weeks, from small plants to post seed set for the legume. The soil originated from the nill treatment of a long-term fertilizer trial that had not received phosphorus application over 40 years, and thus had low phosphorus availability. We also monitored pH at a very fine scale several times a week using novel noninvasive optical sensors (optodes) to understand fluctuations as well as development of pH shifts in the soil over time and with plant root passage. Greater acid phosphatase activity was frequently observed near roots, especially chickpea roots, some root apices, and root nodules. We also observed heterogeneity of hotspots of acid phosphatase activity within the soil and investigated the size and frequency of these hotspots using geostatistics. We combine these results with data on pH and inorganic phosphorus to try to understand how rhizosphere processes impact local phosphorus availability and how these processes may shift when different plant species interact with each other.