Nitrogen Cycling in Agroforestry Systems of Sub-humid Zimbabwe: Closing the loop

Keywords: improved fallows, biological N 2 -fixation, nitrogen cycling, nitrate leaching, oxide emissions, N mineralization -immobilization, granitic sandsThis thesis focuses on nitrogen: its acquisition in cropping systems through biological N 2 -fixation and subsoil capture, its release by legume prunings and litter, its use by maize and its loss through leaching and as nitrous oxide gas. The context of the study is improved fallows using leguminous trees/shrubs on a sandy clay loam soil under sub-humid conditions inZimbabwe.Two-year legume fallows of Sesbania Sesban, Acacia angustissima and Cajanuscajan were evaluated for their residual N effects on two subsequent maize crops under minimum and conventional tillage management. The proportion of N 2 -fixed in litter was 56, 55, 84 and 58 % for Acacia, Sesbania , Cajanus, and cowpea, respectively, resulting in inputs of biologically fixed N of 122, 84, 97 and 28 kg N ha -1 . Maize growth following the legumes for two subsequent cropping seasons was in most cases not directly related to the N inputs due to pest infestation and drought. On a sandy soil, these legumes adapted poorly and did not improve N cycling.Soil samples for mineral N determination in profiles were taken at fallow termination and every two weeks during maize cropping with an auger in 0.2 m sections to 1.2 m depth. Pre-season NH 4+ amounts were > 12 kg N ha -1 in the 0-0.2 m layer for treatments that had a large litter layer. There was a flush of NO 3- -N in the Sesbania and Acacia plots with the first rains. Topsoil NO 3- had increased to >29 kg N ha -1 by the time of establishing the maize crop. NO 3- -N amounts decreased rapidly within three weeks of maize planting to 9 and 11 kg N ha -1 for the Sesbania and Acacia plots, respectively. Total NO 3- -N leaching losses from the 0-0.4mlayer ranged from 29-40 kg ha -1 for Sesbania and Acacia plots within two weeks when 104 mm rainfall was received to an already fully recharged soil profile. NO 3- -N t