Can plant biostimulants improve microbial activity in soil?

The Fertiliser Problem part 5

There are a number of strategies where biostimulants can help the fertiliser problem.

Our previous articles gave insight to the fertiliser problem we are facing globally and demonstrated the nitrogen cycle. We introduced the need for biostimulants as a way to enable plants to better use the nutrients available to them, therefore allowing producers to use less fertilisers , the effect of biostimulants on plant roots and how they can reduce the nitrogen requirement.

So what other strategies show that biostimulants can make a difference?

The following article looks at the effect of biostimulants on the microbial activity in soil.

Improved soil microbial activity leading to better nutrient availability.

Plant growth promoting rhizobacteria (PGPR) or plant growth promoting fungi such as arbuscular mycorrhiza fungi (AMF) and Trichoderma spp. may induce plant growth by improving the availability of nutrients such as N, P and Fe. A variety of symbionts such as Azorhizobium, Allorhizobium, Bradyrhizobium, Mezorhizobium, Rhizobium, and Sinorhizobium and non-symbiotic nitrogen-fixing bacteria such as Azospirillum, Azotobacter, Bacillus and Klebsiella sp. are now being used worldwide to enhance the availability of different nutrients including N, P and micronutrients, thus enhancing plant productivity under both conventional and organic farming systems.

As with nitrogen, several PGPR can enhance the solubility of phosphates, resulting in an increased availability of phosphate in the soil that can be assimilated by the plant (Canbolat et al., 2006; Liao et al., 2008). Different bacterial/fungal strains belonging to the genera Aspergillus, Bacillus, Flavobacterium, Micrococcus and Pseudomonas have been reported to be active in the solubilization of inorganic phosphate compounds (i.e., dicalcium and tricalcium phosphate and rock phosphate).

In addition to P, some PGPR strains e.g. Bacillus. mucilaginosus and B. megaterium, were able to improve the availability of K and also to release K from its immobile forms in the soil (Han and Lee, 2005). Furthermore, through producing siderophores, PGPR, as well as Trichoderma atroviride, can enhance iron solubility and hence uptake and translocation by plant (Lugtenberg and Kamilova, 2009; Colla et al., 2015).

Mycorrhizal symbioses and bacterial symbionts:

Most terrestrial plant species form arbuscular mycorrhizas (AM) which play an important role in plant nutrition by providing access to soil derived nutrients from sources not necessarily otherwise accessible to roots (Smith and Read 1997).

AM fungal associations are important in the phosphorous nutrition of a wide range of plants (Smith & Read, 1997).  A large proportion of the nitrogen requirement of plants in low nitrogen environments can also be taken up as NH4 + through mycorrhiza (George et al. 1995). There is also evidence to suggest arbuscular mycorrhizal associations in wheat and carrot may enhance the uptake of organic nitrogen (glycine and glutamate) (Hawkins et al., 2000). However, for both scenarios, the overall net contribution to the plant N budget is likely be minor in a N rich agricultural soil.

Although usually considered important for P uptake, arbuscular mycorrhizal fungi (AMF) can also increase uptake of other nutrients including Zn, NH4 , NO3 , Cu, K, and others. Such improvements in plant nutrition are of particular significance in soils of low nutrient status (Hetrick 1991; Menge 1983), and where the distribution of soil nutrients is heterogeneous (Cavagnaro et al. 2005; Hetrick 1991). AM are especially important with respect to the uptake of relatively immobile nutrients (e.g. P, Zn, NH4 , etc) that form depletion zones around roots (Tinker and Nye 2000). Improvements in the nutrition of plants colonised by AM fungi (AMF) can be attributed to uptake of nutrients via the mycorrhizal pathway, and/or to indirect effects brought about by morphological and physiological changes in roots due to colonisation by AMF. AMF may also influence nutrient availability via their effects on soil physicochemical properties (e.g. pH; Li and Christie 2001) and microbial communities. The most widely studied benefit to plants of forming AM is that of improved mineral nutrient acquisition, particularly P (Smith and Read 1997). While large amounts (up to 100%) of plant P can be supplied via the mycorrhizal pathway (Smith et al. 2004), AMF also have an important role to play in uptake of other nutrients. It has been demonstrated that up to 60% of a plant Cu, 25% N, 25% Zn and 10% K can be delivered by the external hyphae of AMF (Marschner and Dell 1994). However, our knowledge of mechanisms underlying uptake of these nutrients, and their provision to plants by AMF, lags behind that of P. This represents a significant and increasingly recognized knowledge gap.

The biostimulant action of two strains of Trichoderma (T. virens GV41 or T. harzianum T22), under suboptimal, optimal, and supraoptimal levels of N in two leafy vegetables: lettuce and rocket was investigated by Fiorentino et al. who reported that T. virens GV41 improved Nitrogen Use Efficiency (NUE) of lettuce and favoured the uptake of N present in the soil of both leafy vegetables. The beneficial effect of microbial-based biostimulants was species-dependent with more pronounced effects recorded on lettuce. The findings also demonstrated that Trichoderma inoculation strongly modulated the composition of eukaryotic populations in the rhizosphere, by exerting different effects with a suboptimal N treatment compared to N fertilized treatments. In addition to beneficial fungi, bacterial inoculants could also improve the availability of nutrients and their utilization by plants. The use of multiple microbial inoculants (bacteria + fungi) containing Agrobacterium, Azotobacter, Azospirillum, Bacillus, Pseudomonas, Streptomyces, Trichoderma, and R. irregularis was found to be effective for wheat production when compared to using commercial mineral and chemical fertilizers applied at the recommended level for on-farm use in south-western Australia in soils moderately deficient in N and P.

Zinc solubilization by PGPR is relatively a newer approach. A research group from Pakistan. screened zinc solubilizing rhizobacteria isolated from wheat and sugarcane and analyzed their effects on wheat (Kamran et al.). The authors reported the potential of Pantoea, Enterobacter cloacae, and especially Pseudomonas fragi to be used as microbial-based biostimulant to overcome zinc deficiency under low input scenario.

Micronutrient uptake: Amino acids can chelate metals such as Fe, Zn, Mn, Cu, making them more readily absorbable through the roots and leaves via specific transporters, such as lysine histidine transporter 1 (LHT1), amino acid permease 1 (AAP1) and AAP5 (Ghasemi et al., 2012; Jie et al., 2008). In nature, plants often secrete specific nonprotein amino acids known as phytosiderophores from their roots into the soil to improve micronutrient availability (Dakora and Phillips, 2002; Kinnersley, 1993). For this reason, many micronutrient foliar sprays (for example, Koksal et al., 1999; Rodríguez-Lucena et al., 2010) and hydroponic solutions (for example, Ghasemi et al., 2012) contain amino acid mixtures.


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