The response of soil quality and carbon cycling to changes in agricultural systems and management practices. Assessing, predicting, and mapping

Abstract

Changes in agricultural systems, for example from conventional to organic, have the potential to alter a range of ecosystem functions and services, affecting soil quality (SQ) aspects including carbon (C) storage in agricultural soils. Yet, the effects of agricultural systems will not be consistent across agricultural soils, instead likely varying with management practices. Different management practices, such as grazing regime (non-grazed vs. grazed), proportions of temporary grass-clover leys in crop rotations (ley time proportion), crop rotation schemes (conventional vs. organic) and fertilisation sources (mineral vs. compost), bring about changes in inputs and outputs of soil organic matter (SOM), soil biodiversity, nutrient cycling, C distribution within SOM pools, molecular composition of SOM and consequently affect SQ as well as soil organic C stocks (SOC) and stability. In this thesis, the effect of changing the agricultural system from conventional to organic on SQ (using individual and integrated soil quality indicator approaches), SOC stocks (in situ and spatially-mapping), and the distribution of soil C among SOM fractions are investigated in a commercial split farm (~50% of the farm area under each system), with fields differing in terms of grazing regimes and with varying ley time proportions. Impacts of conventional vs. organic crop rotation schemes and mineral vs. compost fertility sources are assessed for SOM composition and SOC stocks and stability over time using a long-term experimental trial. A mechanistic model is used to validate empirical measurements of SOC stocks and to predict long-term effects of each treatment as well as other hypothetical scenarios. The farm-scale study generated the first direct comparison between the conventional and organic system under the same mixed farming system in the north-east of England, UK. The results reflect existing knowledge on the advantages of organic vs. conventional systems on SQ and indicated no major differences in SOC stocks between both systems. However, it also showed that in mixed farming systems, i.e. where arable and grazed livestock are present in a rotation, and with an increased ley time proportion, SQ and SOC stocks can be enhanced regardless of the agricultural system. The increased SOC stock appears to be related to increases in labile C of SOM pools, indicating that it might be susceptible to losses. Yet, simulations predicted that the use of mixed farming and/or increasing ley time proportions in crop rotations can result in accumulation of SOC in the long-term and thus they might be useful strategies to mitigate losses of SOC stocks in arable rotations. The results also suggested that future digital soil mapping studies should include agricultural system and management practice information as potential explanatory covariates, particularly for regionalscale mapping of SOC across farm enterprises. The results from the long-term experimental ii trial further emphasised that combining organic crop rotation and compost fertilisation can lead to SOC accumulation over time and improve its stabilisation across the whole soil profile (0- 0.60 m). Specifically, the organic rotation favoured SOC stability in subsoil layers (0.30-0.60 m), while compost fertilisation played an important role in the top 0-0.30 m. These results are confirmed by the higher relative weight loss and ion intensity for CO2 (m/z 44) at higher temperature levels (350-750 °C), and the observed higher relative abundance of products that are more resistant to degradation, e.g. n-Alkenes, aromatics, and polyaromatics. Nevertheless, simulations revealed that increases in SOC stocks (0-0.20 m depth) in the long-term are dependent on both the organic fertilisation inputs as well as crop choice in the rotation. Ultimately, the results from this thesis can contribute to ongoing efforts to attain a more sustainable agriculture sector, which, at least in part, depend on changes in agricultural systems and management practices.