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Investigating nitrogen transfer between plants in agricultural grassland by using a 15N stable isotope labelling approach

Bristol student theses: Doctoral ThesisDoctor of Philosophy (PhD)

Authors

  • Katrina Pears

Research units

Abstract

The world’s population is predicted to reach 9.5 billion by 2050. This will put increasing pressure on already stretched food supplies. Previously, food supply has been increased by the use of synthetic fertilisers, particularly the use of nitrogen (N). However, fertilisers provide an unsustainable source of N, due to high energy demands for production as well as over-application and inadequate matching of fertiliser application to crop demand (synchrony). One solution to this global problem is the use of legumes, such as white clover (Trifolium repens L.), which are capable of fixing atmospheric N2, N can then be supplied to an associated non-legume crop. To date, legume and non-legume cropping systems have seen little application due to a lack of understanding of the unique N-transfer pathway. Three major belowground pathways have been identified: plant exudation, legume decomposition and mycorrhizae associations. A better understanding of the different N-transfer pathways is needed to maximise the benefits of the association and to develop appropriate land-use management strategies, this is addressed by this research. The research has focused on developing and validating a method for introducing a 15N-label to white clover and following the N-transfer through the plant and soil systems into associated perennial ryegrass (Lolium perenne L.). The method developed comprised a split-root labelling technique, enabling CO(15NH2)2 to be injected into a sand-filled labelling compartment. This allowed substantial 15N enrichment to be achieved, facilitating the investigation of the routing and controls on N-transfer within an agricultural soil. Laboratory experiments revealed that under normal conditions N-transfer from clover to ryegrass, as a proportion of non-legume N derived from the transfer of legume root N (NdftR), provided on average 2.67% of N. However, similar amounts of N were transferred in the reverse direction (1.98%), showing evidence for bi-directional flow. Incorporation of clover shoots into ryegrass soil, significantly increased NdftR (9.34%), whilst, clover exudates are likely to represent about one-third of total N-transfer. Perturbing N-transfer through modifications to the soil biota was shown to increase N-transfer (sterilised soil > weevil addition > fungi addition), although not significantly. Application of compound-specific amino acid (AA) techniques enabled the investigation of whether different N-transfer pathways influenced the distribution of 15N-label within the pool of soil AAs, thereby assessing microbial N assimilation and routing of N. Overall, there was a very low percentage incorporation of the applied 15N-label into individual AAs, although the percentage depended on the individual experiment, with total incorporation into the soil protein pool ranging from 0.1 to 2.4%. The majority of experiments revealed preferential routing into glutamic acid due to its central role within AA biosynthesis, which was seen to be similar to those AAs with the closest biochemical proximity. A key achievement from this research was the development of a robust repeatable method which allows easy manipulation and the investigation of a range of different treatments on N-transfer from clover-to-ryegrass. New insights into the effect of plant stress through 15N leaf-labelling or clover shoot removal were observed, resulting in significant reductions in the concentrations of soil hydrolysable AAs, questioning the use of the commonly used leaf-labelling technique and the effects of defoliation on N cycling and ecosystem functioning. The results generated from studying different N-transfer pathways revealed the importance of decomposition in N-transfer, revealing the rapid decomposition and N release of clover shoot material. This finding is extremely useful in developing land-use management strategies, where incorporation of clover shoot residues into soil can provide sustainable amounts of N in the short-term, which can improve the synchrony between clover and ryegrass, potentially increasing productivity and sustainability.

Details

Original languageEnglish
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Award date6 Nov 2018

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