INTERACTIONS BETWEEN TREE SEEDLING ROOTS AND HUMUS FORMS IN THE CONTROL OF SOIL C-CYCLING AND N-CYCLING

The hypothesis that roots enhance soil-N turnover in humified soil org anic matter (SOM) (mull) but not in lignified SOM (mor) was tested in a study involving the growth of eight species of tree seedlings on the two contrasting humus forms. After 12 and 22 weeks of seedling growth , soil-CO2 efflux was measured with (1) growing seedlings, and after 2 2 weeks, with (2) roots only, shoots excised, and (3) with roots remov ed and soils amended with different rates of glucose. Indices of C-flu x and of soil available-C were derived and compared to plant-N uptake, extractable soil mineral-N, anaerobically mineralized soil-N, N bioav ailability to Agrostis grass following harvest of seedlings, and to se edling fine root C-chemistry. Significant soil x species interactions were found for total soil-CO2 efflux, root-dependent CO2, soil availab le-e and microbial biomass. In all cases, roots were important contrib utors to C-cycling in the mull soil but not in the mor soil. C was mor e limiting in the mor than in the mull microbial community. Plant-N up take and the mineral-N pool was greater in the mor soil, reflecting th at soil's higher specific N-supplying capacity (N-mineralized:CO2). Se edlings decreased the mineral-N pool in both soils, but the presence o f roots increased N-mineralization in the mull soil and decreased N-mi neralization in the mor soil. Significant positive relationships were observed in the mull soil only between soil respiration and plant N up take at mid-season, and between soil respiration and N-mineralization at late-season. Birch root activity in the mull soil was greater than that of all other seedlings and this observation is discussed with res pect to the autecology of birch. Soil respiration correlated with the non-polar extract content but not the lignin:N ratio of fine roots. Re sults suggest that root-released C in mull SOM is sufficient to reliev e energy limitation to soil microbes and allow them to access apprecia ble amounts of soil-N, whereas ligninolytic activity, which may ultima tely control soil-N turnover in mor SOM, is not increased by rhizodepo sition.