Variations in Plant Nutrient Allocation, Physiological Development Patterns and Soil Structural Characteristics as Influenced by Zero Tillage Systems in the Central Highlands of Kenya

Abstract

The global food requirements are projected to rise above the current demand. Surface application of residues in zero tillage results to nitrogen immobilization, posing nutrient use efficiency challenges, while tillage and crop residue removal destroys soil structure. Little is documented on how integrated use of crop residues and inorganic nitrogen under zero tillage could affect plant nutrient allocation, physiological development, and soil structural improvement relative to conventional tillage systems. A study was conducted to: (i) assess how nitrogen and phosphorus accumulation in maize grain and stover are affected by application of different rates of residue and inorganic nitrogen in conventional relative to zero tillage systems, (ii) examine how maize development is influenced by application of different levels of residue, inorganic nitrogen and tillage and (iii) assess how application of residues, inorganic nitrogen and tillage influence soil aggregate stability. An on-station trial was set in a randomized complete block design replicated three times during the 2015 short rain season. Six treatments were laid, comprising a combination of different rates of maize stover residues (0, 3 and 5 tons/ha) and nitrogen as urea (0, 80, 120 kg/ha), in conventional relative to zero tillage systems. Soil was sampled to assess nitrate- nitrogen concentration at four depths, namely, at sowing, 8th leaf, 10th leaf and dent stage, soil carbon and aggregates at the four depths at the end of the season. Minidisc infiltrometer of 0.25 radius was used monthly to assess treatment effects on soil hydraulic conductivity and leaf chlorophyll recorded every fortnight from maize topdressing to tasselling using SPAD-502 meter. Analysis of variance was done using GenStat analysis software 14th edition, means separated using least significant difference (P≤0.05). Residue application in conventional tillage increased nitrogen (56%) and phosphorus (29%) allocation in maize grain compared to when equivalent rates of inputs were applied in zero tillage while increasing residue quantity from 3 to 5 t/ha increased grain phosphorus allocation by 24% (P≤0.05). When equal rates of inputs were applied in zero and conventional tillage, the latter had taller (12.3%) plants (P≤0.05) but with similar leaf area index and chlorophyll content as those of zero tillage. At 0-5 cm, the large macro aggregates were affected by depth and treatment × depth interaction (P≤0.01) with zero tillage+5R+80N having 41% higher large macro aggregates than zero tillage+3R+80N. The ability of zero tillage+5R+80N treatment to increase grain phosphorous allocation, moderate leaf nitrogen levels through maize vegetative stages and produce higher macro aggregate proportions prompts its consideration as a best nutrient management zero tillage strategy for central highlands of Kenya. Its feasibility under mixed farming system characterized by stiff competition for stover with the livestock component requires a further study.