Changes to soil quality indicators following conversion to organic vegetable production (OF0401)

Summary

This is the final report of Defra project OF0401. The attached report document starts with an Executive Summary, from which this text is extracted.
The aim of this 1 year study was to examine how key functional indicators of soil quality are affected by contrasting organic and conventional management regimes. In particular, the project investigated the impact of contrasting fertility building regimes on soil quality, focussing on the initial 5-year period following conversion from conventional to organic production.
Five 0.8 ha areas at HRI-Wellesbourne were selected for study. These were: two organic vegetable rotations supporting contrasting fertility building regimes, an organic arable rotation, a grass-clover ley, and a conventionally managed cereal rotation. The organic areas had been converted from conventional cereal production 5 years prior to the start of the study. The conventional area was adjacent. A range of chemical, biological and physical attributes were determined.
There were differences between the organic and conventional management regimes in most chemical, biological and physical soil quality parameters. Contrasting organic management regimes had different effects on soil quality. Relative to organic vegetable and conventional arable management, the organic arable management rotation enhanced amounts of light fraction organic matter and labile N, with beneficial implications for long term nutrient retention and soil organic matter development. There was little difference in chemical quality between the organic vegetable and the conventional arable areas.
There was evidence that organic management promoted a microbial community that was distinct in composition and functional attributes to that in conventional soil. Relative to conventional management, areas under organic management had greatly increased inoculum of arbuscular mycorrhizal fungi, a larger proportion of 'active' relative to 'resting' biomass within the microbiota, increased metabolic diversity and a distinct microbial community metabolism. However, there was evidence that the productivity of newly converted organic systems could be limited by low inoculum and diversity of arbuscular mycorrhizal fungi inherited following conventional management.
The clearest effect on soil structure was with regard to the detrimental effects of vegetable production rather than to any benefit associated with organic management. Wheeling lines caused compaction that resulted in poor growth of subsequent cereal crops. However, it is likely that increased levels of organic matter may result in a soil better able to cope with damaging operations.
There were differences in the susceptibility of the chemical and biological quality parameters to change. These differences provide possibilities to use selected parameters as early indicators of the effects of management on soil quality. Furthermore, the results highlight the need, when investigating soil quality, to consider a wide variety of 'quality' analyses. Limited data sets, focussing on traditional measures of soil quality (e.g. total SOM and biomass-N), could lead to unsound conclusions regarding the effects of management on other functional aspects of soil quality.
There are opportunities to conduct further statistical analysis of our comprehensive data set in order to develop an index suitable for quantifying soil quality in organic systems. Such an index would be of generic value to rate soil quality in diverse agricultural systems. Further work is needed to determine the applicability and conclusions of our study to other soil types and organic management regimes. The work has highlighted fundamental shifts in microbial community structure and functioning following conversion from conventional to organic management. There is a need to characterise and quantify these changes. This will provide new groups of 'indicator' organisms which could be suitable for assessing changes to soil quality, and could also provide opportunities to manage soil microbial communities to improve the sustainability of organic and conventional farming.