28 Research & Development Reports Soil compaction: Impact on root activity and implications for nutrient cycling Ian Bingham and Bob Rees C rop growth involves the net fixation of carbon dioxide by photosynthesis and the absorption from the soil of mineral nutrients such as nitrogen, phosphorus and sulphur for the synthesis of more complex carbon molecules. Some of these nutrients are removed from the field with the harvested product, but a significant proportion of each is returned to the soil (recycled) through the incorporation of straw and leaf litter, and the death and decay of roots. Nutrients may also be recycled by exudation of organic compounds from living roots. The deposition of organic matter is critical to soil biological processes as it fuels microbial activity, which in turn influences nutrient availability. Managing the system so that nutrients are made available to the following crop during the period of greatest demand is the key to improving the sustainability of crop production. Effective management requires a sound understanding of the mechanisms of nutrient cycling and the factors that influence it. Plant root systems are highly dynamic. In perennial species, individual roots often have a surprisingly short life-span. For example the average life-span of red clover roots can be as little as 14 days. Thus there is a steady turnover of roots during the season; as new roots are produced, others die and decay. In annual species such as cereal crops, root death tends to be more synchronous, with the majority occurring during and after flowering. Clearly root death has the potential to deposit large quantities of organic matter into the soil. However, not only is the quantity of organic material deposited of significance, but also its quality. The chemical composition of root tissue will determine the ease Figure 1. Line drawings of roots from (left to right) low compaction/high N; high compaction/high N; low compaction/low N; and high compaction/low N. Note the shorter main root and longer individual lateral (branch) roots under high compaction. For scale, the main root far left is 43 cm long. 28