It is difficult to measure apportioning of energy to roots because accurate measurement of belowground NPP entails a number of precautions, as previously noted. Over the last 2 decades, there have been only a few estimates of the carbon costs that have been exacted by the fungal or rhizobial symbiont upon its root partner. The following are two examples of the sorts of measurements that have proven informative.
Pate et al. (1979) compared partitioning and utilization of assimilated carbon and nitrogen, using nonnodulated, nitrate-fed, and nodulated, dinitrogen-fixing plants of white lupine, Lupinus albus L. Pate et al. (1979) calculated production and losses, and noted that not only were the nodulated root microflora more active, but there was also more new root growth under the stimulation of the nodule bacteria, which were acting as a greater root sink for translocated carbon.
Kucey and Paul (1982) measured two symbionts, an AM mycorrhiza and rhizobia in seedlings of fava beans, Vicia faba L. The bean seedlings were arranged experimentally as either mycorrhizal- or nonmycor-rhizal-infected, and also as nodulated or nonnodulated bean plants— four treatments in all. After inoculating or infecting the plants of choice, they then measured CO2 fixation rates, translocation of 14C-labeled photosynthate to roots, and nitrogen-15 fixed by the various plants. Kucey and Paul found gradually increasing amounts of labeled 14C translocated and/or evolved belowground, as a function of infection complexity (Table 2.4). In addition, they obtained useful information on root and shoot weight, and rates of respiration.
The nodules of fava beans utilized 6% of the carbon fixed by singly infected (rhizobial) plants, but twice that amount, or 12%, of the carbon used by the doubly infected plants, i.e., both mycorrhiza and rhizobia symbionts (Table 2.5). Interestingly, rates of CO2 fixation increased significantly with biotic complexity, but changes in root and shoot biomass, while opposite to that of CO2, were statistically insignificant.
Other studies, using real-time monitoring of 11C under laboratory conditions (Wang et al., 1989), found that mycorrhizal infection nearly doubled the "sink strength" of the roots, hence there was greater carbon flow of translocated photosynthate in studies of African Panicum
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