Factors affecting the longevity of clover roots following shoot excision and its implications for managing N cycling in arable cropping systems

Controlled environment experiments were conducted to investigate factors that affect the longevity of clover roots after permanent shoot excision and its implications for managing N cycling in arable cropping systems. The hypothesis tested was that root longevity is related to the initial soluble sugar concentration of the tissue and its rate of depletion after defoliation. Red clover plants were grown in either sand or soil (depending on the experiment) for eight weeks before the shoot was excised at the crown. Root cell viability and concentrations of soluble sugars, starch, amino acids and soluble phenols were determined at regular intervals for up to seven weeks after defoliation. The effects of mechanical damage to the roots, shading of the shoot prior to defoliation, soil temperature, microbial inoculation and nature of the root growth substrate were investigated. Root longevity, defined as the time taken for more than 80% of root cells to lose viability after shoot excision, varied from two weeks to over seven weeks between treatments. Soluble sugar and starch concentrations declined after shoot excision. Treatments affected the initial concentration and rate of depletion of soluble sugars, but had little effect on starch concentrations. Amino acid concentrations increased temporarily after shoot excision before declining; in most cases the decline coincided with the loss of cell viability. Pooling data from the different experiments indicated a threshold bulk tissue sugar concentration of 24mgg DW⁻¹ below which cell viability declined. There was a significant positive relationship (r² 0.93) between the initial sugar concentration and root longevity when roots were left undisturbed after shoot excision. When roots were disturbed and cut into fragments, the rate of sugar depletion was accelerated and root longevity reduced compared to undisturbed roots. The results suggest that tillage to damage the root system would be a more effective method of enhancing the rate of root senescence, and by inference the early release of N, than defoliation. The data can be used to refine models of nutrient cycling in arable systems to account for the period of root senescence prior to post-mortem decomposition.