Nitrogen dynamics of tropical agroforestry and annual cropping systems

Abstract

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 produce mulch. Two maize alley cropping treatments, with Erythrina poeppigiana and with Gliricidia sepium, were compared to sole-cropped maize in an 8 yr old experiment at CATIE in Costa Rica. Maize productivity, maize N uptake, and N release from mulch and crop residue decomposition were measured each month during one cropping cycle. The effects of changes in active soil organic matter (SOM) on available N were assessed by measuring field N mineralization and the size of the microbial N pool through the cropping season. Two sub-treatments were introduced to assess the contribution of a current mulch application to maize N uptake (1) removing the mulch, and (2) applying 15N labelled mulch. Monthly sampling of 15N 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 decomposition, 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 microbial 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 15N labelled mulch only contributed about 10% of crop N. The percentage contribution of mulch 15N to the maize declined from 13–14% 30 days 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 to 2–6% 9 months after application. Mulch N contributed only 3–5% of the microbial N pool at 40 days and this fell to zero by 105 days. The higher rates of N mineralization under the alley crop compared to rates under the sole crop led to faster establishment of the maize in the alley crop and maintained higher rates of N accumulation thereafter. These higher rates of N mineralization resulted from the build up of readily-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 during a cropping season accounted for about 15% of the increase in N uptake by maize. Transfer of mulch N to the crop may have been restricted 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 more 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.