Managing plant diversity and soil-root-biota interactions for improved nutrient efficiency

Improving the efficiency of nitrogen use in Irish agriculture to maintain agricultural productivity whilst reducing the amount of N lost to the environment is a major challenge. This thesis assessed the full potential for exploitation of natural agro-biodiversity, plant-soil-microbiome interactions and inorganic fertiliser alternatives for improved nitrogen use efficiency (NUE) in Irish agriculture. In Chapter 3, plant diversity and plant-soil-soil biota interactions were examined by measuring the root traits, soil structure and abundances of soil microbes and N-cycling microbial communities in monocultures and mixtures of 6 grassland species (grasses, legumes and herbs). Root traits associated with high potential N uptake and biological N2 fixation were identified in the multispecies swards, however, there was no change in the abundance of soil bacteria or fungi gene abundances or microbial N-cycling gene abundances compared to grass monocultures. Biostimulants potentially improve NUE but there has been limited synthesis of their application to grasslands. This research gap was addressed in a systematic scoping review in Chapter 4. Based on the literature review, in Chapter 5, four commercial biostimulants were selected for further study in a greenhouse trial and two of these products were selected for testing in field conditions. In a field trial, the selected biostimulants were applied to a grass monoculture and a 6-species multispecies sward. Although the seaweed extract biostimulant promoted grass yields under greenhouse conditions, there were no biostimulant effects on yield, N-uptake or soil microbial gene abundance under field conditions. From the results of the experimental trials and literature review in this thesis it is evident that the effectiveness of biostimulants on grasslands is highly variable. The short- and long-term effects of slurry (a commonly used alternative to inorganic N fertiliser) application on earthworm mortality was investigated in Chapter 6. In the short-term, splash plate slurry applications caused greater earthworm surfacing in a grassland than low emission slurry spreading methods, however, the number of earthworms surfaced after 2 hours was very small compared to the total earthworm population. In the long term, there was no negative effect of repeated slurry application to grassland or arable fields on earthworm population size or biomass. This thesis examined a range of management practices that have the potential to improve NUE efficiency in Irish agriculture and influence plant-soil-soil biota interactions which are intrinsically linked to N-cycling in agroecosystems. Further research could promote reliable use of biostimulants for improving grassland NUE, improve understanding of the growth and N use of diverse root systems in multispecies swards, and identify slurry management practices which reduce earthworm mortality at field scale.

2026-01-23 JG: Author's and other handwritten signatures removed for GDPR compliance