Roots biomass

T. Katterer, A-C. Hansson, and O. Andren, Wheat root biomass and nitrogen dynamics—effects of daily irrigation and fertili.sation. Plant Soil 151 21 (1993). 

Cairns M, Brown S, Helmer E, Baumgardner G. Root biomass allocation in the worlds upland forests. Oecologia, 1997. Ill pp. 1-11. 

Below-ground biomass is typically estimated from the root to shoot ratio (Johnson et al. 2006 Bolinder et al. 2007). Extreme care must be used when using published root to shoot ratios because different scientists define root to shoot ratios differently. For example, Johnson et al. (2006) defined root to shoot ratios for com (Zea mays) as the ratio between root biomass and total above-ground biomass (grain, stover, and cob), whereas Amos and Walters (2006) defined this value as the ratio between root biomass and com stover. In addition, a standardized root to shoot ratio has not been used in maintenance calculations. For example, Barber (1978) used a value of 0.17 for com, Huggins et al. (1998) used a value of 0.53, and Larson et al. (1972) did not consider roots. 

Fig. 8.6 The relationship between initial SOC and the net C balance (stover + roots - maintenance requirement), and relative amount of carbon supplied by the roots (maintenance requirement -root biomass carbon) tilled and no-tilled system. Calculations were based on tilled and no-tilled systems with maintenance requirements of 16% and 10% of the SOC, a 11,270 kg grain ha-1 (180 bu acre-1), a harvest index of 0.5, a root to shoot ratio of 0.55, and that non-harvested com stover contained 0.4 g carbon(g plant)-1...

Carbon turnover in production fields can be determined, using non-isotopic techniques, by combining historical soil samples, current soil samples, and whole field yield monitor data. Sensitivity analysis of such data shows that the amount of above-ground biomass that could be harvested decreases with root to shoot ratio (Table 8.1). For example, if root biomass is ignored, analysis suggests that only 20-30% of the above-ground biomass can be harvested, whereas if the root to shoot ratio is 1.0, then between 40% and 70% of the residue could be harvested. 

These findings broadly agree with experimental observations. Measured rates of CH4 oxidation in the rice rhizosphere range widely from 5 to 90% of the CH4 transported (Holzapfel-Pschom et al 1985 Epp and Chanton, 1993 van der Gon and Neue, 1996). This agrees with the model. Rates of O2 flow through rice roots to the rhizosphere are of the order of a few mmol 02m (soil surface) h (Section 6.4), which is sufficient to account for the rates of oxidation calculated with the model. Measured differences in emissions between rice cultivars are largely due to differences in root biomass (Lu et al., 1999) the effects of differences in root porosity are smaller (Aulakh et al., 2001a,b). 

Vohnik M et al.. The inoculation with Oidiodendron mains and Phialocephala fortinii alters phosphorus and nitrogen uptake, foliar C N ratio and root biomass distribution in Rhododendron cv. Azurro, Symbiosis 40 87—96, 2005. 

Berish, C. 1982. Root biomass and surface area in three successional tropical forests. Canadian Journal of Forest Research 12 699-704. 

A comparison of these data with those of an Amazonian forest shows that the aerial biomass of the trees of a cerrado sensu stricto in central Brazil may be only 8 to 22% of that of an Amazonian forest, and the basal area only 10 to 26% (Table 5.3). This difference in biomass reflects directly on the nutrient pools in the biomass. A comparison of the data reported by Klinge et al. (1995) for the aboveground biomass and nutrient stock in two inundation forests in the Ilha de Marchantaria with the data for a cerrado sensu stricto from central Brazil (Silva 1990) illustrates how nutritionally poor the cerrado is in quantitative terms. The proportions of stock of essential nutrients in the tree biomass of cerrado are 7 to 16% for P, 1.7 to 4.6% for K, 0.83 to 3.09% for Ca, and 3.5 to 7.4% for Mg. Thus Ca, K, and Mg seem to be much more deficient in the cerrados than P. We have no corresponding data for the stock of nutrients in the root biomass of trees for comparison among the two ecosystems. This comparison is only illustrative of two specific sites. Estimates of aboveground biomass for the Amazonian forests may vary... 

Mg ha i) in a cerrado sensu stricto. In the campo limpo, which is devoid of trees, the aerial biomass (5 5 Mg ha i) amounted to only 34% of the root biomass (16.3 Mg ha ) in the top 2 m of soil. This proportion was 31% in the campo sujo with its sparse distribution of trees and 47.9 to 53 3 in the cerrado sensu stricto. Thus the different cerrado vegetation types showed a higher root/shoot ratio than many tropical forests. 

Mobilization of water from the soil is closely related to root depth and root density in each layer of soil. Fine roots of active B. brizantha pastures, established in deeply weathered clayey soils in eastern Amazonia, reach depths of 8 m or more (Nepstad et al. 1994). In abandoned pastures (50% B. humidicola and P. maximum cover and 50% invading shrubs and small trees), fine roots ( < 1 mm in diameter) were found at depths of 12 m (Nepstad et al. 1994). Fine-root biomass in the superficial soil layers of an active pasture in Paragominas, eastern Amazonia, was 3 times higher than that found in an adjacent primary forest area. Fine root biomass in the active pasture decreased by a factor of 100 between the surface and 6 m depth. In an abandoned pasture area, the distribution pattern of fine-root biomass was similar to that observed in the deeper soil layers of the forest ecosystem. This pattern is associated with the fine roots of the existing dicotyledonous invading species. 

The availability of scarce plant nutrients in deep soil is only relevant to our understanding of secondary forest nutrient acquisition if plants are able to absorb these nutrients with deeply penetrating root systems. Enticing evidence for such an extensive approach to nutrient acquisition is supported by the distribution of fine-root biomass (diameter = 0-1 mm) in the secondary forest, as compared to that in the neighboring mature forest and active cattle pasture (Fig. 9.2). Fine root biomass to 6 m depth is virtually identical in mature and secondary forests, and is > 10 times greater at depth than in the active cattle pasture. Hence, after 16 years of recovery, some of the trees, lianas and palms of the secondary forest had re-established root systems to at least 6 m depth. 

But are there sufficient fine roots at depth to absorb significant amounts of soil nutrients In both mature and secondary forests, there is a hundred-fold decline in fine-root biomass from the soil surface to 6 m depth, which means that fine root length density at 6 m is less than 1 cm of root per 100 cm3 of soil. Moreover, the few roots that occur at depth are concentrated in patches of soft soil that comprise approximately 1% of the soil volume, and that show no nutrient enrichment (Carvalheiro and Nepstad 1996). Given the low mobility of phosphorus in the soil, it is unlikely that such a sparse, patchy root system could absorb substantial amounts of this scarce nutrient. 

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