Nitrogen cycling assessment in a hedgerow intercropping system using ¹⁵N enrichment

Abstract

Nitrogen (N) cycling was determined in monocultures of Sorghumbicolor (L.) Moench and alley cropped sorghum with Acaciasaligna (Labill.) H. Wendl. in semiarid Northern Kenya. N inputthrough biological N₂ fixation of the acacia, N transfer from thelegume to the intercrop and losses of applied N through harvest and leachingwere estimated using ¹⁵N enrichment. The biological N₂fixation and N transfer estimates clearly demonstrated the limitations of¹⁵N enrichment techniques in field experiments showing even highertransfer than actually fixed N. Therefore, N transfer in the hedgerowintercropping system could not be determined by the ¹⁵N dilutionmethodology. The ¹⁵N balance approach, however, yielded reliableresults even 1.5 years after ¹⁵N application. 74 to 88% of theapplied ¹⁵N was recovered after three cropping cycles, most of it inthe soil (0–1.2 m). Only about 10% of the¹⁵N was taken up by the above-ground vegetation of both monocultureand agroforestry. The trees took up more of the applied ¹⁵N(8.4%) than the sorghum (1.3%) in the agroforestry system,indicating nutrient competition between tree and crop. Leaching losses below 1.2m depth were low in this semi-arid environment with 3 and 6%of the applied ¹⁵N in the monoculture and agroforestry system,respectively. ¹⁵N losses from leaching were 2.5 times higher in thealley than under the tree row. Incorporating the leguminous tree into thesorghum cropping system had no effect on total leaching and total uptake ofapplied ¹⁵N in above-ground biomass.