Review of estimation of plant rhizodeposition and their contribution to soil organic matter formation

The methods used for estimating rhizodeposition of plants (carbon (C) deposition of living roots), and the results obtained for different plant species are reviewed. Three tracer techniques using C isotopes to quantify rhizodeposition are discussed: pulse labelling, continuous labelling, and natural 13C abundance. Only the tracer methods provided adequate results for the whole rhizodeposition. The differences in the below-ground C translocation pattern between cereals and grasses are discussed. Cereals (wheat and barley) transfer 20 – 30% of total assimilated C into the soil. Half of this amount is subsequently found in the roots and about one-third in CO2 evolved from the soil by root respiration and microbial utilization of root-borne organic substances. The remaining part of below-ground translocated C is incorporated into the soil microorganisms and soil organic matter (SOM). The portion of assimilated C allocated below the ground by cereals decreases during growth and by increasing N fertilization. Pasture plants translocated about 30 – 50% of assimilates below-ground, and their translocation patterns were similar to those of crop plants. On average, the total C amounts translocated into the soil by cereals and pasture plants are approximately the same (1.5 Mg C ha−1; calculated for the productivity of about 6 Mg grain yield), when the same growth period is considered. However, during one vegetation period the cereals and grasses allocated beneath the ground about 1.5 and 2.2 Mg C ha−1, respectively. Finally, a simple approach is suggested for a rough calculation of C input into the soil and for root-derived CO2 efflux from the soil. Contribution of C4 carbon (from maize and some C4 grasses) to turnover of SOM (C3 soils) estimated by natural 13C abundance is reviewed. In average for Ap horizons, the portion of maize derived carbon increases of about 0.98% of SOM content per year. Factors influencing the contribution of maize-derived carbon to soil organic carbon are discussed. The contribution of maize derived carbon decreases with soil depth, without fertilization, after removal of above ground biomass, and with soil tillage.