Conflictos de uso de suelo año 2006

Although some research has been conducted in the different HRZs of Australia to

understand the impact of waterlogging on wheat and barley growth under field conditions (Robertson et al. 2009; Setter & Waters 2003; Setter et al. 2009), most work in Tasmania has been conducted under controlled or glasshouse conditions (Colmer et al. 2001; Falakboland et al. 2017; Malik et al. 2002; Pang et al. 2007; Pang et al. 2004). The field study conducted corroborated other studies and literature (Acuña et al. 2011), indicating that waterlogging is one the major abiotic constraints to cereal production in the region. The research also explored the potential of CRFs in mitigating the adverse effects of waterlogging and their use in broadacre cropping compared with conventional urea. The CRF improved wheat and barley growth under different irrigation regimes, however, the trend was not statistically different compared with conventional urea.Furthermore, the

research also investigated the uptake of NH4+ and NO3- ions along the root axis under

hypoxia; highlighting the significant variations between the different root zones and need for understanding the processes and mechanisms involved in the up-regulation of N uptake during hypoxia as these can be exploited by plant breeders to develop waterlogging

tolerant and nutrient-use efficient crops.

All in all, the findings are experimental evidence that N fertiliser application can alleviate the adverse effects of waterlogging in cereals. The timing of N application and source are important. Applying the full amount of the required fertiliser at sowing helps plants to withstand the adverse effects of transient and intermittent waterlogging through enhanced vegetative growth. Top-dressing urea after waterlogging and relieve of waterlogging stress enhances plant growth during recovery. Using CRFs may improve cereal growth and NUE through sustained N supply though there might be no significant yield advantage over conventional urea to warrant investmentdepending on the prevailing fertiliser price, which is highly variable. With the increased availablity of information and the different types of enhanced-efficient fertilisers, cereal producers can make informed decisions on fertiliser use given the current harsh climate change effects. Furthermore,the findings highlight the dynamic process of N uptake during hypoxia, which is characterised by significant

variations over relatively short distances and periods of time. Hypoxic conditions also seem to trigger mechanisms that mediate N uptake resulting in higher uptake rates. The genetic variability in tolerance to waterlogging observed between different cultivars and genotypes can be exploited by breeders in their efforts to breed for more resource-use efficient crops.

In response to the key hypotheses this research sought to investigate, the following deductions are proposed:

(a) Sensitivity to waterlogging varies between wheat and barley and within individual genotypes;

(b) Timing of N fertiliser application and source of applied N are crucial in alleviating the adverse effects of waterlogging in cereals;

(c) Controlled-release fertilisers can improve yield and NUE in cereals under waterlogged conditions though might not be significantly different from conventional urea and;

(d) Temporal and spatial variations occur in NH4+ and NO3- uptake along the root axis

and there is preference for NH4+ over NO3- during hypoxia.

Knowledge gaps that still warrant future studies include:

(a) The evaluation of different enhanced-efficient fertilisers (EEFs) for their potential in broadacre cropping. There are different forms of EEFs available on the global market for instance: nitrification inhibitors (NIs), polymer-coated fertilizers (PCFs)/CRFs and urease inhibitors (UIs) (Chen et al. 2008). While the potential of different CRFs for example: Agrocote/Agromaster, Environmentally Smart

Nitrogen (ESN) and Meister has been evaluated with promising potential to increase crop yields and improve NUE, the potential of nitrification inhibitors particularly in the HRZs prone to waterlogging remains inconclusive. Yet these inhibitors are said to be effective in flood prone areas or waterlogged soils where N loss due to leaching and denitrification is a common occurrence (Akiyama et al. 2010; Chen et al. 2008). This could be achieved through controlled studies, field experiments and simulation modelling, testing different scenarios.

(b) The evaluation of different CRFs and nitrification inhibitors with different longevity or release periods and release rates to assess the optimal time for N

fertilisers is dependent on the fertilizer formulation, crop type, soil climate and management factors (Chen et al. 2008). For instance, Shoji et al. (2001) observed that ~20% of the CRF N can remain within the fertilizer pellets at harvest if a fertilizer with the appropriate formulation or release rate is not selected.

Furthermore, the development of better CRFs is vital in the improvement of the efficacy of these fertilizers and improvement of NUE. The research findings have shown that CRFs can improve crop growth and NUE under different irrigation regimes. However, understanding the underlying processes and mechanisms influencing the efficacy of these fertilisers under different farming practices is paramount for the development of better CRFs.

(c) Unravelling the molecular and physiological mechanisms responsible for inducing N uptake during hypoxia. The study findings showed that under hypoxic

conditions, there is an increase in N uptake, which could be attributed to an upregulation of the mechanisms mediating N uptake resulting in higher N uptake rates. The HATS has been suggested to be activated under N limiting conditions (Feng et al. 2011) and the expression of AtNRT2.1, AtNRT2.2, AtNRT2.4 and

AtNRT2.5 also occurs in N deprived roots (Kotur & Glass 2015), however, it remains to be investigated, if these same processes or mechanisms are responsible for N uptake under hypoxic conditions where there is a combination of limited N and O2 availability.