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 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 N ₂ O-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 N ₂ O fluxes, despite very high soil moisture conditions. N ₂ O-N emissions were greatest for Sesbania plots with only 0.3 kg ha ^-1 lost in 56 days.

The effects of improved fallows on rainfall partitioning and associated soil loss were investigated using simulated rainfall at 35 mm h ^-1 . Immediately after fallow clearance, infiltration rates were greater than water application rate for the Acacia and natural fallow treatments, but steady state infiltration rates were 24 mm h ^-1 in Sesban and 5 mm h ^-1 in continuous maize. The estimated runoff losses after 30 minutes of rainfall were 44% from continuous maize compared with 22% from Sesbania and none from Acacia and natural fallow plots. After one post-fallow crop, water infiltration was still greater than 35 mm h ^-1 in the Acacia plots. Steady state infiltration rates after 30 minutes of rainfall were 8 and 5 mm h ^-1 for Sesbania and continuous maize systems, respectively. Planted tree fallows increase infiltration rates, but the effect markedly decrease after one year of maize cropping in non-coppicing tree fallows.

In a field litterbag decomposition experiment, the course of the decomposition could be adequately described by the function Y = (Y0-Q)e ^-kt + Q, and the relative decomposition constants for Sesbania and Acacia litters were 0.053 and 0.039 d ^-1 , respectively. Under laboratory incubation conditions, N mineralized from fresh Sesbania prunings was 55% after 120 days compared with 27% only for the Sesbania litter. During the same period, fresh prunings of Acacia released only 12 % of the added N while Acacia litter released 9 %. N mineralization from Acacia fresh prunings was depressed by the high protein binding capacity of its polyphenols. Except for Sesbania litter, the rest of the senesced legume materials showed N immobilization up to 60 days. 

The study showed that Sesbania, Acacia and Cajanus produced useful amounts of biomass on soils of at least loamy texture and improved N cycling significantly, but adapted poorly in lighter soils. Under conditions of severe texture constraints, mucuna showed robustness but has the disadvantage of having no direct food value. Gaseous N losses after incorporating legume materials were small, and the reduction of NO ₃^- leaching is the single largest challenge to increased N recovery in the highly porous soils.