Non-labile soil ¹⁵Nitrogen retention beneath three tree species in a tropical plantation

Soil organic matter is the largest sink for N additions to forests. Species composition may affect soil N retention by altering the amount or proportion of added N stored in non-labile organic pools. We measured ¹⁵N tracer retention in labile and non-labile pools of surface (0-20 cm) mineral soils, 7 yr after the tracer was applied to a 9 yr. old Puerto Rican tree plantation with replicated stands of three species (two N-fixers, one Eucalyptus, Euc). Laboratory incubations (13 mo) with repeated leaching separated total soil N into labile (inorganic N leached) and non-labile (total N minus leached N) pools, and a labile C treatment tested linkages between C availability and N retention. We hypothesized that species composition would alter the amount and proportion of recovered tracer N in non-labile organic matter. Surface soils contained 45% of the tracer, but the amount retained in labile and non-labile pools was similar among species. In contrast, the proportion of recovered tracer in non-labile pools was greater in soils beneath N-fixers (75%) than Euc (62%). Labile C additions increased the size of the non-labile tracer N pool. We conclude that tree species composition may affect long-term soil N retention by altering the proportion of N in slow-turnover, non-labile pools. Plants may also alter soil N retention by renewing labile C pools; a continuous supply of labile C increased the transfer of ¹⁵N into non-labile organic matter.