Interactions between tree seedling roots and humus forms in the control of soil C and N cycling

The hypothesis that roots enhance soil-N turnover in humified soil organic 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 22 weeks, with (2) roots only, shoots excised, and (3) with roots removed and soils amended with different rates of glucose. Indices of C-flux and of soil available-C were derived and compared to plant-N uptake, extractable soil mineral-N, anaerobically mineralized soil-N, N bioavailability to Agrostis grass following harvest of seedlings, and to seedling fine root C-chemistry. Significant soil x species interactions were found for total soil-CO2 efflux, root-dependent CO2, soil available-C and microbial biomass. In all cases, roots were important contributors to C-cycling in the mull soil but not in the mor soil. C was more limiting in the mor than in the mull microbial community. Plant-N uptake and the mineral-N pool was greater in the mor soil, reflecting that soil's higher specific N-supplying capacity (N-mineralized:CO2). Seedlings decreased the mineral-N pool in both soils, but the presence of roots increased N-mineralization in the mull soil and decreased N-mineralization in the mor soil. Significant positive relationships were observed in the mull soil only between soil respiration and plant N uptake 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 respect to the autecology of birch. Soil respiration correlated with the non-polar extract content but not the lignin:N ratio of fine roots. Results suggest that root-released C in mull SOM is sufficient to relieve energy limitation to soil microbes and allow them to access appreciable amounts of soil-N, whereas ligninolytic activity, which may ultimately control soil-N turnover in mor SOM, is not increased by rhizodeposition.