Non‐labile Soil 15Nitrogen Retention beneath Three Tree Species in a Tropical Plantation

Abstract

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.