Effects of litter quality and microarthropods on N dynamics and retention of exogenous ¹⁵N in decomposing litter

Summary

Surface additions of (¹⁵NH₄)₂SO₄ were used to measure the immobilization and subsequent movement of exogenous N added to two litter types of contrasting quality (Cornus florida and Quercus prinus). Litterbaskets were used to measure the litter mass loss and N dynamics and to follow the movement of the ¹⁵N label through litter, F layer, and soil pools. Half of the litterbaskets of each species were treated with naphthalene to reduce microarthropod densities. The faster decomposing C. florida litter maintained a higher excess atom % ¹⁵N, and a greater relative concentration of the labeled input (μg ¹⁵N g⁻¹) than did Q. prinus litter. In both litter types the excess atom % ¹⁵N, relative concentration (μg ¹⁵N g⁻¹), and absolute amount of label recovered in the litter declined over time. This occurred during a period of net accumulation of total litter N, implying simultaneous release of the initial input and immobilization of N from other sources. The concentration of ¹⁵N in the soil increased over time, while the F layer apparently acted as an intermediary in the transfer of ¹⁵N from litter to soil. Naphthalene effectively reduced microarthropod numbers in all horizons of the litterbaskets and significantly reduced the decay rates of Q. prinus, but not C. florida litter. Naphthalene did not appear to affect total N dynamics in the litter. However, with all horizons taken together, the naphthalene-treated litterbaskets retained more total ¹⁵N than the control litterbaskets. Naphthalene also changed the vertical distribution of ¹⁵N within litterbaskets, so that the litter retained less of the ¹⁵N-labeled input and the F layer and soil horizons retained more of the labeled input than in control litterbaskets. Our major conclusions are: (1) the N pool of decomposing litter is dynamic, with simultaneous N release and immobilization activating N turnover even during the net accumulation phase; (2) litter quality is an important determinant of immobilization and retention of exogenous N inputs and, therefore, turnover of the litter N pool; and (3) microarthropod activity can significantly affect the incorporation and retention of exogenous N inputs in decomposing litter, although these changes are apparently not reflected in net N accumulation or release during the 1st year of decomposition. However, the naphthalene may have affected microbially mediated N dynamics and this possibility needs to be considered in interpreting the results.