Influence of dissimilarities in temporal and spatial N-uptake patterns on¹⁵N-based estimates of fixation and transfer of N in ryegrass-clover mixtures

Abstract

The temporal N-uptake patterns of white clover (Trifolium repens L.) mixed with perennial ryegrass (Lolium perenne L.) and of red clover (Trifolium pratense L.) mixed with Italian ryegrass (Lolium multiflorum Lam.) were determined in successive harvests of herbage within the growth cycles of a ley established near Zürich (Switzerland). Rooting patterns were examined by injecting¹⁵N-fertilizer at soil depths ranging from 10 to 40 cm. The results were analyzed to determine the effect of variations in time and depth of N-uptake on the¹⁵N-based measurement of N from symbiosis (N_sym) and N from transfer (N_trans).

Grasses in mixture appeared to have deeper rooting systems than grass monocultures, which led to an overestimation of N transfer from white clover to perennial ryegrass if¹⁵N was spread on the soil surface.

White clover generally lagged behind grass in soil N- uptake. Soil N-uptake of red clover slowed down before that of the grass because % N_sym almost reached 100% during the second half of each growth cycle. However, the effect of these dissimilarities on the seasonal average of %N_sym did not exceed 2%.

It is concluded that at the observed high levels of N₂ fixation, failure to account for the N-uptake patterns of the test and reference crops only slightly affected the estimates of % N_sym and % N_trans, and did not invalidate the observed differences between species.