Rhizodeposition of maize: Short-term carbon budget and composition

The aim of this study was to assess differences in rhizodeposition quantity and composition from maize cropped on soil or on 1:1 (w/w) soil-sand mixture and distribution of recently assimilated C between roots, shoots, soil, soil solution, and CO2 from root respiration. Maize was labeled in (CO2)-C-14 atmosphere followed by subsequent simultaneous leaching and air flushing from soil. C-14 was traced after 7.5 h in roots and shoots, soil, soil solution, and soil-borne CO2. Rhizodeposits in the leachate of the first 2 h after labeling were identified by high-pressure liquid chromatography (HPLC) and pyrolysis-field ionization mass spectrometry (Py-FIMS). Leachate from soil-sand contained more C-14 than from soil (0.6% vs. 0.4%) and more HPLC-detectable carboxylates (4.36 vs. 2.69 mu M), especially acetate and lactate. This is either because of root response to lower nutrient concentrations in the soil-sand mixture or decreasing structural integrity of the root cells during the leaching process, or because carboxylates were more strongly sorbed to the soil compared to carbohydrates and amino acids. In contrast, Py-FIMS total ion intensity was more than 2 times higher in leachate from soil than from soil-sand, mainly due to signals from lignin monomers. HPLC-measured concentrations of total amino acids (1.33 mu M [soil] vs. 1.03 mu M [soil-sand]) and total carbohydrates (0.73 vs. 0.34 mu M) and (CO2)-C-14 from soil agreed with this pattern. Higher leachate concentrations from soil than from soil-sand for HPLC-measured carbohydrates and amino acids and for the sum of substances detected by Py-FIMS overcompensated the higher sorption in soil than in sand-soil. A parallel treatment with blow-out of the soil air but without leaching indicated that nearly all of the rhizodeposits in the treatment with leaching face decomposition to CO2. Simultaneous application of three methods-C-14-labeling and tracing, HPLC, and Py-FIMS-enabled us to present the budget of rhizodeposition (C-14) and to analyze individual carbohydrates, carboxylates, and amino acids (HPLC) and to scan all dissolved organic substances in soil solution (Py-FIMS) as dependent on nutrient status.