Tillage, Crop Residue and Inorganic Nitrogen Effects on Crop Yields, Soil Carbon and Nitrogen in Kirimari Ward in Embu County, Kenya

Abstract

Integration of crop residues, inorganic N and appropriate tillage system is an important strategy for improving soil fertility, enhancing crop yields and reducing food insecurity. Such integration under zero tillage systems could enhance soil structural development and nutrient management hence mitigating against soil impoverishment through nitrogen and carbon losses. Crop residues combined with inorganic N in zero tillage results to a controlled immobilization-mineralization process which could avert nitrogen leaching while concurrently improving carbon stock within the macroaggregates relative to conventional tillage systems. However, despite zero tillage being promoted as a promising nutrient management strategy, there is little documentation on the effects of applying stover residues and inorganic N on maize yields, soil mineral N, carbon and aggregate fractions in Embu. The objectives of the study were to; (i) assess the effects of applying different rates of residue and inorganic N on maize productivity in conventional relative to zero tillage systems, (ii) examine the effects of applying different levels of residue, inorganic N and tillage on soil mineral N and (iii) assess the effects of residues, inorganic N and tillage on organic and active carbon fractions and soil aggregation. An on-station trial was set in a randomized complete block design replicated three times. The test crop was maize. Six treatments were laid in 6 × 4.5 m plots comprising of combinations of different rates of maize stover residues (0, 3 and 5 tons ha-1) and N in form of urea (0, 80, 120 kg ha-1) in conventional and zero tillage systems. The key variables assessed were; grain and stover yields, nitrate-N and ammonium-N concentration sampled at four depths (0-10, 10-30, 30-60 and 60-90 cm) during sowing, 8th leaf, 10th leaf and dent stage, soil carbon and aggregate fractionation at four depths at the end of the season. Analysis of variance was done using GenStat 14th edition, means separated using least significant difference (p≤0.05) and orthogonal contrasts between treatments run using R environment. Application of inorganic N as the only nutrient input resulted to higher (p≤0.01) grain yield than the control. Application of inorganic N as the only nutrient input increased (p≤0.02) phosphorus accumulation in maize grain in conventional system compared to the control. At 0-5 cm depth, application of similar (3 tons and 80 kg N ha-1) input rates increased macroaggregates yield in zero tillage system by 28.1% over that of conventional tillage system. Averaged over the different depths, incorporating 3 tons ha-1 residues caused 18.2 and 11.2% higher active and organic carbon, respectively, compared to surface residue application. Applying 5 tons of residues and 80 kg N ha-1 under zero tillage system has an increased potential of enhancing maize grain yield to similar levels as conventional systems. Besides, increasing the application period of 5 t ha-1 of residues with appropriate N rates could improve soil nutrient replenishment potential through macroaggregate build-up in the study area.