Estimating C and N rhizodeposition of peas and oats

Wichern, Florian

kassel university press, ISBN: 978-3-89958-275-8, 2007, 164 Pages

URN: urn:nbn:de:0002-2759

Zugl.: Kassel, Univ., Diss. 2006

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Content: The objective of the present work was to quantify the rhizodeposition of C and N simultaneously in situ under field conditions and to estimate its turnover and transfer into different soil compartments.

Two greenhouse pot experiments were conducted aiming at evaluating the cotton wick method for its suitability of simultaneous in situ application of sugar-urea mixtures as possible 13C and 15N-carriers into the stem of peas (Pisum sativum L.) and oats (Avena sativa L.). Different concentrations of cane sugar, glucose and urea solutions and mixtures thereof were applied to the plants. It was examined whether the wick method or the applied solutions had any effect on plants and the soil microbial biomass.

In two column-experiments under outdoor conditions C and N rhizodeposition, defined as root-derived C or N present in the soil after removal of roots and root fragments, was quantified during different growth stages of peas and oats. Plants were in situ labelled with a 13C-glucose-15N-urea mixture using the cotton wick method. Root derived 13C and 15N was detected in the soil microbial biomass, the extractable C pool, the soil inorganic N pool and in the remaining rootlets.
Sugar-urea mixtures were successful transferred into the plant. The solution uptake was driven by the transpiration stream. Plant development, below-ground, above-ground and total plant dry matter, as well as the soil microbial biomass were not affected by the method or the solution applied. Therefore, glucose-urea mixtures can be used as 13C and 15N-carriers, making in situ field investigation of C and N dynamics easier. In the column experiments under outdoor conditions, below-ground plant biomass was successfully labelled using 13C-glucose-15N-urea mixtures. Moreover, a high isotope recovery was achieved.

The estimated amounts of net C and N rhizodeposition under outdoor conditions, were higher than previous reports with results mostly obtained from pot experiments. Rhizodeposition ranged from 8 to 58% of recovered plant C and 5 to 71% of total plant N. The proportion of total plant C and N derived from rhizodeposition decreased during plant growth. Alike the majority of roots, most of the rhizodeposition was present at 0-30 cm soil depth.

Depending on the growth phase, 1 to 31% of C and N rhizodeposition was recovered in the microbial biomass and 4 to 40% of the N derived from Rhizodeposition in the inorganic N pool. Most of the rhizodeposition was present in rootlets and in other pools not further differentiated. Hence, a major part of the unrecovered C and N was probably immobilised in microbial residues. A higher proportion of the microbial biomass was derived from rhizodeposition in peas in comparison with oats. Additionally, the proportion of inorganic N derived from rhizodeposition was slightly higher in peas. Furthermore, the C-to-N ratio of the rhizodeposits and the roots was higher in oats in comparison with peas, indicating that rhizodeposits of peas were more easily available to soil microorganisms than those of oats.

Rhizodeposition of peas and oats represented a significant easily available C input into the soil which fuelled the turnover processes in the rhizosphere and therefore influence nutrient availability. Rhizodeposition N as a percentage of total plant N in oats and peas were in the same range, but amounts of N taken up by peas were higher and therefore also the amounts of N rhizodeposition. Hence, rhizodeposition of N represents a significant N pool contributing to the higher N availability after peas and has to be taken into consideration for N balances and estimations of N2-fixation. Moreover, differences in the turnover of N rhizodeposits and roots determine the N availability after the two crops.

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