Estimating C and N rhizodeposition of peas and oats

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. 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. 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 and of the inorganic N pool was derived from rhizodeposition in peas in comparison with oats. 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.