Evidence for recycling of N from plants to soil during the growing season

In spite of the known below-ground biomass production of plant roots that concurrently introduce significant amounts of carbon and nitrogen into the soil, the effects of these inputs on N cycling in the soil–plant system are seldom considered. Here, we report on two field experiments carried out between 1995 and 1997 at the FAM Research Station Scheyern: (1) a N-turnover experiment to determine the N fluxes derived from ¹⁵N-labeled clover residues incorporated into the plough layer of defined plots, and (2) a root production experiment to assess the above (shoot) and below ground (gross and net root) biomass production of winter wheat in different fields, but nearby the ¹⁵N plots. An initial 50% decrease in soil organic ¹⁵N at 0–20-cm soil depth was recorded between fall, 1996 (incorporation of clover straw) and spring, 1997 (138 days after incorporation), which was then followed by a period of stability in ¹⁵N levels in the soil organic N until the harvest of winter wheat (286 days after incorporation). This stability may be explained in two ways: (a) actual stability of clover-derived ¹⁵N remaining in the second phase, e.g., due to recalcitrant compounds or microbial immobilization; or (b) apparent stability, e.g., because the actual mineralization of clover-derived ¹⁵N in the soil was compensated by secondary inputs of organic ¹⁵N (recycling). Further results showed that the first explanation was unlikely, as (1) between 138 and 286 days after clover incorporation, the mean ¹⁵N signature in soil mineral N was 2.1 at.%, indicating a persistent mineralization of clover residues; and (2) a decrease in soil microbial biomass ¹⁵N occurred in the second phase, indicating a continued N turnover in the soil. The amount of clover-derived ¹⁵N accumulated below the plough layer at 20–110-cm soil depth (11.5%) between early spring and the harvest of wheat also corroborated the return of mineralized ¹⁵N into the soil being due to the root N inputs by winter wheat. Based on the depth distribution of winter wheat net root biomass (root production experiment) and on soil organic ¹⁵N depth distribution (¹⁵N-turnover experiment), the root N input into soil was estimated to be 282 kg ha⁻¹, equivalent to 54% of total net N assimilation of winter wheat. Thus, the results of this study give substantial evidence for a N loop between soil and growing plants, whereby a part of the net mineralized N taken up by plants is continuously returned into the soil by their roots. The implications of this N loop for the interpretation of ¹⁵N experiments and for plant nutrition are discussed.