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Transformations and recovery of residue and fertilizer nitrogen15 in a sandy Lixisol of West Africa
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The fate of 15N-labeled plant residues from different cover-cropping systems and labeled inorganic N fertilizer in the organic, soil mineral, microbial biomass and soil organic matter (SOM) particle-size fractions was investigated in a sandy Lixisol. Plant residues were from mucuna (legume), lablab (legume), imperata (grass), maize (cereal) and mixtures of mucuna or lablab with imperata or maize, applied as a surface mulch. Inorganic N fertilizer was applied as 15N-(NH4)2SO4 at two rates (21 and 42 mg N kg–1 soil). Total N release from mucuna or lablab residues was 2–3 times higher than from the other residues, whereas imperata immobilized N throughout the study period. In contrast, 15N was mineralized from all the plant residues irrespective of the mineralization–immobilization pattern observed for total N. After 168 days, 69% of soil mineral N in mucuna- or lablab-mulched soils was derived from the added residues, representing 4–8% of residue N, whereas 9–30% of inorganic N was derived from imperata, maize and the mixed residues. At the end of the study, 4–19% of microbial biomass N was derived from the added residue/fertilizer-N, accounting for 1–3% of added residue-N. Averaged across treatments, particulate SOM fractions accounted for less than 1% of the total soil by weight but contained 20% of total soil C and 8% of soil N. Soils amended with mucuna or lablab incorporated more N in the 250–2000 μm SOM pool, whereas soil amended with imperata or the mixed residues incorporated similar proportions of labeled N in the 250–2000 μm and 53–250 μm fractions. In contrast, in soils receiving the maize or inorganic fertilizer-N treatments, higher proportions of labeled N were incorporated into the 53–250 μm than the 250–2000 μm fractions. The relationship between these differences in residue/fertilizer-N partitioning into different SOM particle-size fractions and soil productivity is discussed.