NITROGEN DYNAMICS OF TROPICAL AGROFORESTRY AND ANNUAL CROPPING SYSTEMS

The relative importance of the processes of SOM (maintenance of active soil organic matter) and SYNCHRONY (timing of release of organically- bound nutrients to coincide with crop demand) were assessed for their contribution to the maintenance of crop nitrogen availability in alley cropping. Alley cropping is a system of agroforestry where trees and crops are intercropped, the former being periodically pruned to produc e mulch. Two maize alley cropping treatments, with Erythrina poeppigia na and with Gliricidia sepium, were compared to sole-cropped maize in an 8 yr old experiment at CATIE in Costa Rica. Maize productivity, mai ze N uptake, and N release from mulch and crop residue decomposition w ere measured each month during one cropping cycle. The effects of chan ges in active soil organic matter (SOM) on available N were assessed b y measuring field N mineralization and the size of the microbial N poo l through the cropping season. Two sub-treatments were introduced to a ssess the contribution of a current mulch application to maize N uptak e (1) removing the mulch, and (2) applying N-15 labelled mulch. Monthl y sampling of N-15 in the mulch, microbial biomass, and maize allowed assessment of the SYNCHRONY of mulch N release and crop uptake. Maize biomass and maize N content, N release from mulch and residue decompos ition, and N mineralization were all higher in the alley crop than the sole crop by 2.2-, 2.8-, 5.0- and 2.1-fold respectively. Soil microbi al N was not significantly different between treatments, but increased by 80% during the cropping season. Maize grown in the alley crop with the mulch removed contained only 3-15% less N at maturity. Similarly N-15` labelled mulch only contributed about 10% of crop N. The percent age contribution of mulch N-15 to the maize declined from 13-14% 30 da ys after planting to 8-11% 100 days after planting. Total recovery of mulch N by the maize was only about 10 kg ha-1 and almost all of this was taken up by 60 days after planting. The contribution of mulch N to weed N content declined from 15-24% 7 weeks after mulch application t o 2-6% 9 months after application. Mulch N contributed only 3-5% of th e microbial N pool at 40 days and this fell to zero by 105 days. The h igher rates of N mineralization under the alley crop compared to rates under the sole crop led to faster establishment of the maize in the a lley crop and maintained higher rates of N accumulation thereafter. Th ese higher rates of N mineralization resulted from the build up of rea dily-mineralizable organic N in the soil over the 7 yrs of tree mulch application. The size of the microbial N pool was not to be related to nitrogen availability nor organic residue inputs. Mulch N released du ring a cropping season accounted for about 15% of the increase in N up take by maize. Transfer of mulch N to the crop may have been restricte d by the low incorporation of mulch N into the microbial biomass. The long-term build-up of the SOM reserve of mineralizable organic N was m ore important than the SYNCHRONY of mulch N release and crop uptake in determining the substantially higher productivity and N uptake in the alley crop compared to the sole crop