Nitrogen cycling in agroforestry systems of sub- humid Zibambwe: closing the loop

This thesis focuses on nitrogen: its acquisition in cropping systems through biological N2 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 in Zimbabwe. Two-year legume fallows of Sesbania Sesban, Acacia angustissima and Cajanus cajan were evaluated for their residual N effects on two subsequent maize crops under minimum and conventional tillage management. The proportion of N2-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 NH4+ 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 NO3--N in the Sesbania and Acacia plots with the first rains. Topsoil NO3- had increased to >29 kg N ha-1 by the time of establishing the maize crop. NO3--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 NO3 --N leaching losses from the 0-0.4 m layer 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. NO3--N then increased below the 0.4 m depth during early season when the maize had not developed a sufficient root length density to effectively capture nutrients. Nitrous oxide emissions were small, with a peak of 12 g N2O-N ha-1 day-1 from Sesbania plots and near background fluxes in maize monoculture plots. The decrease of mineral N concentration in the topsoil resulted in reduced N2O fluxes, despite very high soil moisture conditions. N2O-N emissions were greatest for Sesbania plots with only 0.3 kg ha-1 lost in 56 days.

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