Alley cropping with Senna siamea in South-western Nigeria: I. Recovery of N-15 labeled urea by the alley cropping system

Improved cropping systems with in-situ production of organic matter require the input of additional inorganic N to maintain crop production in a susta inable way. For proper management of this fertilizer-N, it is necessary to quantify how the applied fertilizer N is used by the various components of the system and by the system as a whole. The fate of a single application o f N-15 labeled urea-N through the different components (crop, hedgerow, sur face litter, soil profile up to 150 cm) of a Senna siamea alley cropping sy stem, intercropped with maize in the first and cowpea in the second season, was followed for a period of 1.5 years (1994-1995), equivalent to 2 maize and 1 cowpea crop. Special attention was given to the role of the particula te organic matter (POM) in the cycling of urea-N through the soil organic m atter (SOM). The maize crop recovered 26.5 and 1.7% of the applied urea-N a t harvest in 1994 and 1995, respectively. The cowpea pods recovered only 0. 7% of the applied urea-N at harvest. The highest proportion of applied urea -N recovered by the hedgerow occurred at 38 days after 1994 maize planting (DAP) (3.8%), while at later dates, recoveries of applied urea-N were alway s below 1%. This indicates that the Senna hedge is not a strong competitor for the applied urea-N during crop growth, i.e. while the Senna canopy is p runed at regular intervals. At 21 DAP, 12.7% of the applied urea-N was reco vered in the surface litter and this value dropped significantly to 1.6% at 107 DAP and remained below 1% up to 480 DAP. The top 10 cm of soil contain ed 21% of the applied urea-N at 21 DAP and this value dropped to 9% at 480 DAP. Significantly more urea-N was recovered in the top 10 cm of soil than in the deeper soil layers at all sampling times. At 21 DAP, 11% of the appl ied urea-N was recovered in the 120-150 cm layer. This fast movement of ure a-derived N to deep soil layers must have happened by preferential flow in macropores as the rainfall between urea application and the first sampling (74.2 mm) was not high enough to explain downward movement of N with the mo bile water. Significant linear relationships between the proportion of urea -N in the different soil layers (excluding 0-10 cm) and the anion exchange capacity (AEC) and silt+clay content of the respective layers were found at 67, 107, 347 and 480 DAP. The total N content of the POM fraction increase d significantly between 0 and 101 DAP from 127 to 171 mg N kg(-1) and decre ased to 92 mg N kg(-1) at 480 DAP. The highest recovery of applied urea-N i n the POM pool was measured at 101 DAP (3.6%) and this value decreased to 1 .8% at 480 DAP. The total recovery of applied urea-N was 81% at 21 DAP, and decreased to values varying between 53 and 60% up from 38 to 347 DAP. At 4 80 DAP, the recovery decreased further to 47%. The fast movement of a subst antial amount of urea-N may be responsible for this incomplete recovery, al ready at 21 DAP. Although the soil N status in the fertilized alley croppin g system appears to be favourable for plant growth, this may be short-lived in the absence of further urea additions, as the soil-derived maize uptake in 1995 was already significantly lower than in 1994, and as the labile PO M pool decreased significantly between the maize harvest in 1994 and 1995.