Parámetros que caracterizan el relieve de la cuenca

2.5.1

Liming to overcome soil acidity and Al toxicity

Liming is one of the solutions for improving plant growth by enhancing soil physical, chemical, and biological properties and consequently it improves crop yield and nitrogen use efficiency. Sufficient quantities of lime are needed to increase the soil pH to at least 5.8, for aluminium sensitive species such as lucerne (Edmeades et al., 1983). However, the beneficial effect of liming depends highly on the sub-soil pH and related Al content. Soil acidity indices like pH, base saturation, and aluminum saturation are used as the basis for liming acid soils. Furthermore, economic considerations are also important criteria in determining the quantity of lime applied to acid soils (Li et al., 1997).

Overcoming soil acidity and Al toxicity with lime and fertiliser is possible by surface application (Grewal, 2010; Kearney et al., 2010). However, this can be uneconomic or difficult where aluminium is present throughout the soil profile (Moir and Moot, 2014) as amelioration of the surface layer will not allow the plant roots to penetrate the acid layer and reach critical water and nutrient supplies below it. Therefore, screening for more Al tolerant species plus lime application is usually the most effective strategy to improve legume production on acid soils.

Mullen et al. (2006), evaluated the effect of soil acidity and liming on lucerne and following crops in central-western New South Wales. Dry matter production of lucerne was increased by lime applied at rates up to 2 t/ha. They reported that the application of lime increased soil pHCa from 4.3 to 5.8 and,

at 2 t/ha or higher, decreased exchangeable Al in the surface layer from 14.4% ECEC to < 0.5% ECEC about 2 years after liming. In their field experiment the effects of lime persisted up to 7 years post lime application and the soil pH of the limed treatments were maintained with no clear decline in the effect of the lime. Their results support the application of lime to improve the productivity of lucerne and subsequent crops, even when the soil is acidic to depths below the cultivation layer. In contrast, Moir 14

and Moot (2010), conducted a two year field experiment in Lees Valley to evaluate lucerne yield responses to two forms of lime. Soil exchangeable aluminium plummeted to low levels but lucerne yields were not influenced by lime rates or soil aluminium. They reported 1.2 t/ha of lucerne DM yield when 4 t lime/ha was surface applied at the Lees Valley site. This result highlights the difficulty of lucerne persistence in the Lees Valley experimental site. They also reported unclear effects of lime form on soil exchangeable aluminium.

Moir and Moot (2014) determined the effects of historic liming on soil pH at three high country locations, and in turn, on exchangeable Al, in two surface soil horizons Their results highlighted the problem of higher soil-exchangeable plant-available aluminium levels in Brown soils, which are common to South Island high country. They reported strong relationships between soil pH and exchangeable plant-available Al for these soils. Liming had a strong medium-term effect on soil pH at all three sites, especially in the 0-7.5 cm soil horizon. Soil pH increased an average of 0.16 units/t lime applied. However, Al levels of 3.0 mg/kg or above are likely to decrease the dry matter production of several plant species (Wheeler et al., 1992).

2.5.2

Inoculation with efficient rhizobia

Several previous studies have shown that inoculation of legumes with Rhizobium hasthe potential to increase production by fixing biological N2 (Cheng et al., 2004). However, all strains of Sinorhizobium meliloti do not, stimulate plant growth to a similar extent in a given lucerne cultivar. A strain inducing superior performance in one cultivar may produce a suboptimal response in another which indicates that they must be matched carefully for optimum N2 fixation (Hartel and Bouton, 1989). The approach

to inoculant development traditionally consisted of isolation, testing and selection of single strains with desired properties, such as high nitrogen fixation efficiency in symbiosis with selected host plants. In cases where indigenous rhizobia capable of nodulating the host were already present in soil but not efficient, the capacity of the inoculant to outcompete those strains became important. The selection of strains is still hampered by the fact that, even though the genes encoding nodulation and nitrogen fixation in rhizobia are well-known (Correa and Barneix, 1997), the genetic basis for symbiotic effectiveness, competitiveness and tolerance to environmental stress factors is largely unknown. Indigenous rhizobia are those found naturally in the soil of a given locality and great diversity occurs in most soils especially where compatible legumes are grown and the soil is fertile (Zengeni et al., 2006). Rhizobia are diverse at species and strain levels. One soil may contain various species and various strains within a species (Duodu et al., 2009), while similar isolates may be found in distant places (Brockwell et al., 1995a). A challenge for agriculture is to match rhizobia and legume crops for optimal performance either by having plant genotypes adapted to local rhizobial populations or by inoculation with effective strains adapted to prevailing environmental conditions and with competitive

ability against local, less effective strains. Although indigenous rhizobia may pose challenges of competition to inoculated strains, they are an important resource that must be preserved by integrated soil fertility approaches. Continual isolation and characterization to identify new isolates offers the opportunity of improving BNF with minor limits geographically to the areas of use. It is important to continually isolate higher nitrogen-fixing isolates to be used as inoculant strains from the wide diversity of indigenous rhizobia. There have also been reports of inoculant strains losing their symbiotic properties (Versalovic et al., 1991b). A wide diversity of isolates ensures a sustainable source of replacement strains and may be developed into strains for commercial use (De Bruijn, 1992). Previous studies have tried to take advantage of BNF symbiosis by inoculating legumes with more effective nitrogen fixing strains to improve crop growth without addition of chemical nitrogen fertilisers (Duodu et al., 2009).