Quantifying the contribution of above - and below ground N in legumes to soil N pools and subsequent maize in an improved fallow system using in situ 15N labelling techniques in western Kenya

A field experiment was undertaken in western Kenya to (i) determine the amount of legume below ground biomass N at a given time without physically disturbing the soil, (ii) study 15N recovery by maize from decomposing labelled roots of legume fallows and assess interactions occurring when mixed with the above ground fallow biomass and (iii) determine the distribution of excess 15N in water stable aggregates, free organic matter and the clay + silt fraction. One year old Sesbania sesban, Calliandra calothyrsus, Senna spectabilis, Cajanus cajan, Crotalaria grahamiana and Tephrosia vogelii were enriched in situ with labelled ammonium sulphate [(15NH4)2SO4] through stem injection techniques. The total above ground dry matter (DM) of the legumes varied between 589 g tree1 for Senna and 1572 g tree1 for Calliandra. The shoot to root ratio ranged from 1.2 for Senna to 3.1 for Calliandra. Total above ground N ranged from 8.6 to 23.1 g tree1 and was highest for Calliandra and lowest for Senna. Cajanus, Calliandra, Crotalaria and Tephrosia exhibited similar proportions of 15N enrichment in the leaves and wood, but Senna allocated more 15N to wood as compared to leaves. Total above ground 15N recovery ranged from 49% for Crotalaria to 69% for Tephrosia indicating that 27 to 50% is potentially allocated to the roots. About 2.5 to 7.4% of 15N is potentially recovered from roots less than 5 mm. A large proportion (39%) of below ground N became protected in water stable meso- and macroaggregates while around 20% was associated with the clay-silt sized fraction. This contrasts with the fate of 15N labelled foliage litter from which only 31% was found in aggregates, but a larger proportion remained in the free organic fraction, suggesting an important role of roots in aggregate formation. 15N recoveries of below ground N in the catch crop were small suggesting that these inputs play a more important role in soil structure rather than in nutrient supply.

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