Nitrogen dynamics of tropical agroforestry and annual cropping systems

TLDR: 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.

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 15 N labelled mulch. Monthly sampling of 15 N 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 15 N labelled mulch only contributed about 10% of crop N. The percentage contribution of mulch 15 N 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.