Rhizosphere Soil
C. Rhizosphere Dimensions
amah (23), among others, regarded a zone of 1-2 mm around roots as the zone in which microbial growth was in~uenced by root exudates.
ssen (24) studied the effect of an elevated atmospheric the dist~bution of carbon and rhizode~osition for three ration of their data showed that the rhizosphere volume was in-
by a higher nitrogen supply through an increase in root m sphere volume per unit root m showed the opposite: at low e volume was twice that at high The amount of rhi~odeposition
of rhizosphere soil at high N was twice as high as that at portional to root density. An explanation for this phenomenon ce in root ~ o ~ h o l o g y - i . e . , low N plants have finer roots, capable of exploring more soil volume for limited n u ~ e n t s ,
the utilization efficiency of rhizodeposits. That is, use at low so that su~strates must move away over longer distances.
The total rhizosphere volume was equal for all three species.
it root mass increased in the order ~ o Z i ~ ~ ~ e ~ e ~ ~ e , A ca ovina. This means that ~ o l i ~ ~ has more root mass per unit phere soil. Again, differences in root m o ~ h o l o g y , with ~ o l i ~ ~ r roots than ~ e s t ~ c a , could explain this. The a ~ o u n t of rhizod~posit
osphere soil was higher in soil planted with ~ o Z i ~ ~ but less than could be expected on the basis of the root density ( i h a n , unpublished results).
The total rhizosphere volume was 15% higher at high CO,. This was caused by an increased mass of the root system, as rhizosphere volume per unit root mass
Characteristics of Rhizosphere and ~onrhizosphere Soil for Three Crass Species Grown in a Continuous I4C-CO2 Labelled Atmosphere After 8 Weeksa
Ratio of Ratio of
350 to 700 ppm C 0 2 low to high N supply
hizosphere soil (dry wt) 0.86
1.02 0.84
r h , ~ ~ ~ ~ h ~ r e / ~ rhizosphere soil 0.86
0.79 hizosphe~e soil per gram root
l4CbUlk per gram bulk soil
0.52 1.96 0.49 0.53 0.41
a ~ i z o d e ~ o s i t i o n is measured as I4C recovered f?om soil adhering to the roots or soil not adhering to the roots. Soil moisture content was constant in all treat~ents.
was equal for both COz treatments. Root morphology seems not affected by COz concentration. At high COz, more rhizodeposition per gram of rhizosphere soil was found. This could indicate an increased rhizodeposition at high COz (unpub- lished results). In another experiment with four grass species (Holcus lanatus, Anthoxmthurn odorcctum, Festuca ruhra. and Festuca ovina), these results were confirmed: at the lower N supply, larger rhizosphere volumes (ratio of low N to high N : 1.4) were recovered and less IJCIg of rhizosphere soil (ratio of low N to high N:0.6) was found. No differences between lower and higher N supply were found in the non-rhizosphere soil (Kuikman, unpublished results).
Clearly, the concentration of exudates and rhizodeposits depends on soil nutritional status and on plant species; this may affect the microbial utilization and subsequent turnover of rhizodeposits in soil.
111. CARBON DYNAMICS IN THE RHIZOSPHERE
A. Input Rates of Rhizodeposition
Input rates of organic C into the soil system are hard to quantify, particularly for natural ecosystems and to a lesser extent for agricultural ecosystems. Whereas quantity and quality of carbon inputs via litter fall and plant residues after harvest might be directly measurable, inputs via roots and rhizodeposition are more diffi- cult to assess.
The quantification of gross root production, rhizodeposition, microbial as- similation, and the production of organic materials in soil has made increasing progress ever since stable ( I T ) and radioactive ("C) carbon isotopes have been used (see Chap. 12). Measurements of soil organic matter dynamics without these isotopes are difficult due to the large amount present as compared to the smaller rates of input.
Several authors have applied in situ pulse labeling of plants (grasses and crops) with 1"C-C02 under field conditions with the objective of quantifying the gross annual fluxes of carbon (net assimilation, shoot and root turnover, and de- composition) in production grasslands and so assess the net input of carbon (total input minus root respiration minus microbial respiration on the basis of rhizode- position and soil organic matter) and carbon fixation in soil under ambient cli- matic conditions in the field.
Recently, pulse labeling has frequently been applied to determine the fate of carbon in crops such as barley and wheat and the losses from roots and subsequent microbial transformations. In general, the results indicate that 15-25% of the net '"C assimilation is transferred to the roots and that there are seasonal differences in the distribution of assimilated carbon. Meharg and Killham (25) measured the C distribution in perennial ryegrass (L. perenne). At 8 days after the pulse with
I4C, 26% of the label was recovered in shoots, 7% in roots, and 1% in soil; the remainder was returned to the atmosphere as CO2 (25). In field studies, the annual below-ground C transfer in wheat and spring barley has been estimated to be 475-1765 kg C ha" year" and 583-1652 kg C ha" year", respectively (26).
Swinnen (27) estimated that more carbon was transferred to the roots by wheat than by barley and gave a range of 1500-2300 kg C ha-' year-!. Total rhizode- position constituted 29-50% of the carbon translocated below ground and was
1 S-2.1 times the amount of C in roots left at crop harvest (28). Microbial respira- tion of rhizodeposits was higher under conventional than under integrated man- agement. Of the annual root growth, about 50% had decayed by the end of the growing period (28-30). Several authors have shown, by pulse-labeling wheat and barley plants at four to five stages during growth, that the distribution of carbon changes during the growing season. Also, about 10% of total crop carbon fixation was released within the soil during the first 24 h after labeling (31).
Rattray et al. (18) used "C pulse labeling to determine the distribution of C between L. perenne and its associated rhizosphere microcosm. The translocation of I4C to below ground was already detectable 30 min after the pulse application and reached the maximal value within 3 h. Then, the microbial biomass accounted for 74% of the total IJC rhizosphere pool and, after 24 h, about 30% of the assimi- lated I4C had been translocated below ground. Partitioning of recent assimilates changed with increasing COz concentration in the atmosphere. The proportion of 14C translocated below ground almost doubled from 18% at ambient CO2 con- centration (450 ppm) to 34% at 750 ppm CO2 concentration (18).
B. Decomposition of Roots
Wedin et al. (32) analyzed changes in the stable C isotope composition (I3C) of above- and below-ground litter of C, and C4 grasses and monitored the short- term (4 years) accretion of soil organic matter in soils with a known isotopic composition. They concluded on the basis of the measured isotopic shifts that
14% of the total soil C was new C over the period of 4 years. This amount was estimated to equal 30% of net primary production (NPP) over these 4 years.
Kuikman (33) gives similar estimates on the basis of a field trial where pulse labeling was applied to study annual carbon fluxes and decomposition of roots in a managed grassland. The total stock of C was approximately 30000 kg C/ha in the top 30 cm; annual input due to rhizodepositions was 2200 kg/ha; and between 550 and 710 kg C of this was stored in soil per year. In addition to this below-ground input, 1550 kg C was gained from above-ground littering.
Applying the same turnover rates to the litter C input as in soil, 375-500 kg C of this C was stored in soil per year, which equals 3.1-4. l % of the total C stock in soil and represents a rapid turnover of the soil organic matter pool.