Humic substances nutrients

This chapter focuses on the effects of humic substances present at the rhizo-sphere on plant growth and nutrient uptake. The main structural features of humic substances, their nutritional function, and the capacity to interact with plant metabolism are also presented. 

Humic substances have been proved to stimulate plant growth and nutrient accumulation (for review see Refs. 4,49, and 50). Various studies performed on excised roots or whole plants show that usually the uptake of cationic and anionic... 

A schematic repre.sentation of the possible interaction between humic substances and mechanisms of nutrient acquisition located at the plasma membrane is shown in Fig. 2. 

Senesi N, Loffredo E (1994) Influence of soil humic substances and herbicides on the growth of pea (Pisum sativum L.) in nutrient solution. J Plant Nutr 17 493-500... 

The increased growth response documented in almost all the solarization studies is mainly due to the above-cited higher levels of macronutrients or the improved uptake of micronutrients solubilized by humic substances (Chen and Aviad 1990 Chen et al. 1991). As a consequence of the enhancing effect of solarization on soil nutrients, Flores et al. (2007) suggested the application of low rates of mineral fertilizers before heating soil, in order to avoid an increased vegetative growth of the plants at the expense of crop yield. 

Variation among Sampling Sites. Dissolved humic substance samples from seven end-member environments were isolated for study. Autochthonous inputs to DOM were expected to dominate in Big Soda Lake and in Island Lake, which is a groundwater-sustained eutrophic lake in the sandhills of western Nebraska. Allochthonous inputs to DOM from a swamp environment predominate in the Suwannee River. They also dominate in the Calcasieu River in western Louisiana, but the proportion of swampland is much lower there. The Temi River is a tropical blackwater tributary of the Orinoco River in Venezuela, where allochthonous inputs dominate. The entire Sagavanirktok River basin is located north of the tree line on the North Slope of Alaska a mixture of allochthonous and autochthonous inputs was expected for the various rivers and lakes in this basin. Lastly, Hidden Lake Creek, which is the outlet of Hidden Lake on the Kenai Peninsula of Alaska, was sampled to determine if nutrient inputs from decaying salmon were contributing to primary production and autochthonous inputs to DOM. 

Qualls, R. G., B. L. Haines, and W. T. Swank. 1991. Fluxes of dissolved organic nutrients and humic substances in a deciduous forest. Ecology 72 254-266. 

Jansson, M. 1998. Nutrient limitation and bacteria-phytoplankton interactions in humic lakes. In Aquatic Humic Substances (D. O. Hessen and L. J. Tranvik, Eds.), pp. 177-195. Springer-Verlag, Berlin. 

In many freshwater systems, humic substances account for most of the DOM pool (Thurman, 1986). This material provides a reservoir of slowly metabolized nutrients that coexist and interact with a labile pool of rapidly consumed monomers and polymers (Tranvik, 1992, 1998 Sondergaard and Middleboe, 1995 Munster and De Haan, 1998 see Chapter 19). The literature clearly illustrates the amphitrophic nature of humic DOM it acts variously as a significant energy source for bacteria (Tranvik and Hofle, 1987 Tulonen et al., 1992 Moran and Hodson, 1994 Volk et al., 1997 Jannson, 1998) and as an inhibiter of growth and metabolic activity (Moran and Hodson, 1990 Tranvik, 1992 Foreman et al., 1998). This range of... 

Jones, R. I. 1998. Phytoplankton, primary production and nutrient cycling. In Aquatic Humic Substances (D. O Hessen and L. J. Tranvik, Eds.), pp. 145-176. Springer-Verlag, Berlin. Kirchman, D. L. 2000. Uptake and regeneration of inorganic nutrients by marine heterotrophic bacterial. In Microbial Ecology of the Oceans (D. L. Kirchman, Ed.), pp. 261-288. Wiley, New York. 

The positive effects of humic substances often occur when the plants are grown in nutrient solution under anexic conditions in thin film isolators. This shows that the beneficial effects are due to the humic substances per se, rather than being mediated by microbial breakdown products. 

A beneficial effect of humic substances on the nutrient uptake and contents of plants has been reported for the major inorganic elements, such as nitrogen, phosphorus, potassium (Mylonas and McCants, 1980), and sulfur (Guminski, 1968). In addition, the uptake and contents of nutrients such as calcium, magnesium (Mylonas and McCants, 1980), sodium (Vaughan and McDonald, 1976) and copper (Rauthan and Schnitzer, 1981) are also enhanced by humic substances. Most of these reports on the effects of HS on the nutrient contents of plants are purely descriptive, and little attempt has been made to elucidate the mechanisms of the action of the humic material. The authors proposed both an indirect and a direct effect of HS on plant nutrition. In the former case HS may, for example, chelate a cation, thus changing... 

Also, Schmidt et al. (2005) found a significant increase in root hair density by working with Arabidopsis thaliana, which were treated with water extractable humic substances (WEHS), suggesting that these substances induce a nutrient acquisition response that favors the uptake of nutrients via an increase in the absorptive surface area. Furthermore, a phenotypical analysis of an array of mutants harbouring defects in root epidermal patterning revealed that root hair density of the ttg and gl2 mutants, defective in cell specification, was significantly modified, indicating an effect at/or downstream of the determination of the cells. 

Schmidt, W., Cesco, S., Santi, S., Pinton, R., and Varanini, Z. (2005). Water-extractable humic substances as nutrient acquisition signals for root hairs development in Arabidopsis. In Rizosphere 2004—Perspectives and Challenges,Hartmann, A., Schmid, M., Wenzel, W., and Hinnsinger, P., eds., GSF-Berich, Neuherberg, p. 71. 

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