Considerable progress has been made during the last decade towards understanding and quantifying the input and turnover of plant carbon in the rhizosphere. This was made possible by the development (partially by the authors) and combination of appropriate new methods, such as: –homogeneous labelling of whole plants with 14C –distinction between root and microbial respiration –separation of soil zones of known distances from the roots –determination of microbial soil biomass. These methods were applied to study the following aspects: –release of organic plant carbon into the soil by growing roots –utilization of this plant carbon by the microbial biomass in the rhizosphere –related influence on the turnover of soil organic matter, and –spatial range of such root influence in the soil. About 19% of the total photosynthetic production of the investigated plants was released into the rhizosphere as organic material. Most of this (15%) was transformed by the rhizosphere microorganisms into CO2, while only a small fraction (4%) remained in the soil, mainly as microbial cells (2.5%). As a result, microbial rhizosphere biomass increased considerably. Relative to the organic C‐input, however, the incorporation of root derived carbon by the microbial biomass was remarkably low (13%). Along with the increase in microbial rhizosphere biomass, the presence of plant roots also enhanced the decomposition of soil organic matter and affected soil aggregate stability. Root carbon and root influences were even detected up to 20 mm away from the roots. This may be partially attributed to the contribution of root derived volatiles. Accordingly, both the actual volume of the rhizosphere and its metabolic significance is greater than what has so far been assumed. Possible interactions involving root, soil and microbial carbon are discussed.
Recent progress in methods enables a better understanding of the turnover of P in the rhizosphere. Examples of this progress are the separation of soil layers differing in proximity to the roots, improved methods for extraction and fractionation of soil P, application of 32P isotope dilution analysis to follow P fluxes between various fractions and direct determination of microbially bound P and of root phosphatases. These methods were combined to investigate the following aspects –labile P pools, the P fluxes between these pools and their contribution to the P supply to growing maize roots –the role of microbial biomass in these interactions and the partition of mobilized P between plants and microorganisms –modifications of sorption and transport of P in the rhizosphere –plant availability of native and added organic phosphates, and the relative significance of root and soil phosphatases. There is a significant transformation of P in the rhizosphere with a corresponding redistribution among fractions of different plant availability. About 9% of the inorganic 32P added to soil were incorporated within 2 weeks into microbial and organic fractions. The transfer of P from non‐exchangeable forms exceeded the depletion of the exchangeable P by a factor of 5. About 53% of the mobilized P originated from inorganic, the remaining 47% from organic fractions. Of the mobilized P 80% was taken up by the plants and 20% was found in the microbial biomass. Up to 90% of the P in the rhizosphere soil solution was organic with a maximum just outside the root zone. Soluble inositol hexaphosphate modified the sorption of inorganic P, thus shifting its equilibrium solution concentration. The phosphatase activity of the roots is considerable. Both root phosphatase activity and the utilization of inositol hexaphosphate depend on the P supply and nutritional status of plants with regard to P. It is concluded that the rhizosphere is a key site of P transformation with a significant mobilization of P from the non‐exchangeable inorganic and organic fractions. Organic P fractions not only play a significant role as a P source but also modify important soil parameters related to the sorption and transport of P in the rhizosphere.
MATERIALS AND METHODS Seedlings of Viiafaba were grown for four weeks at two different light intensities (55 and 105 watts per square meter) in a saline (50 millimlar NaCI) and nonsaline nutrient solution. NaCi salinity depressed growth and restricted protein formation, CO2 assimilation, and especially the incorporation of photosynthates into the lipid fraction. Conversion of photosynthates in leaves was much more affected by salinity than was photosynthate turnover in roots. The detrimental effect of NaCi salinity on growth, protein formation, and CO2 assimilation was greater under low than under high light conditions. Plants of the high light intensity treatment were more capable of excluding Na+ and Cl and accumulating nutrient cation species (Ca2 , K+, Mg2e) than plants grown under low light intensity. It is suggested that the improved ionic status provided better conditions for protein synthesis, CO2 assimilation, and especially for the conversion of photosynthates into lipids.Saline conditions depress the growth rate of many plant species. Salinity also influences various biochemical processes such as the assimilation of N, protein synthesis, and CO2 assimilation (1, 4, 14, 15, During the first week of cultivation the solution was diluted to fifth and in the second week to half strength. In addition to these nutrients, NaCl also was added at two concentrations: I mM, control treatment; and 50 mm, salinization treatment. A concentration of 1 mm NaCl was chosen for the control treatment, because under field conditions some Na+ and Cl-always is present in the soil solution. Such a low NaCl level has no salinity effect. Plants in both of these treatments were exposed to two light intensities: 55 w/m2 and 105 w/m2. The experiment thus consisted of a 2 x 2 factorial design. Each treatment comprised four 5 x 1 pots with five plants per pot. NaCl was added gradually after the first week ofgrowth. On each day 10 mmol NaCl/l was introduced into the nutrient solution until the final level of 50 mm. The pH of the nutrient solutions ranged from 5.2 to 5.8. The nutrient solutions were continuously aerated and were completely renewed every fourth day. The plants were grown in growth chambers supplying the following environmental conditions: 14 h light, day temperature 24 C, night temperature 14 C, air humidity 70%.Twenty-seven days after germination one plant was harvested from each pot (four replications per treatment) and divided into leaves, stems, and roots. These were weighed, cut, and then subdivided. One subsample was oven-dried for the determination of the dry matter content. In this subsample the minerals were analyzed. In the other subsample the N fractions were analyzed according to a technique described by Helal et al. (4). As the effect of salinity on the N fractions did not differ much for leaves, stems, and roots, the N contents were recalculated on a total plant matter basis (Table II). The effect of salinity on the mineral content was especially evident in the leaves and for this reason only ...
Vortrag bei der gemeinsamen Sitzung der Kommissionen 1. Ill und IV der DBG am 9. Oktober 1990 in Braunschweig Z. Pflanzenerndhr. Bodenk.. 154,403407 (1991) BVCH Verlagsgesellschaft mbH. W-6940 Weinheim. 1991 0044-3263/91/0612W3 S 3.50 + .25m
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