Abdi, Zeila Dubow2019-01-162019-01-162018-09http://ir-library.ku.ac.ke/handle/123456789/18867A Thesis Submitted in Fulfillment of the Requirements for the Award of the Degree of Doctor of Philosophy in Integrated Soil Fertility Management in the School of Agriculture and Enterprise Development of Kenyatta University, September 2018High dependence on rain-fed agriculture, low and erratic rainfall, inadequate water and soil conservation techniques and declining soil fertility expose resource-poor farmers to low agricultural productivity in tropical regions. A study was conducted in 2012 at Kirege Primary School in Tharaka Nithi County to evaluate the effects of soil tillage, soil surface management and nitrogen fertilisation on rain-fed maize yields and properties of Humic Nitisols. Eight treatment combinations (T1M0N1, T1M0N0, T1M1N1, T1M1N0, T0M0N1, T0M0N0, T0M1N1 and T0M1N0) were laid out in randomised complete block design and replicated thrice. Data on grain and stover yields, mineral N, soil water content and soil organic carbon content were collected over six cropping seasons. Physical fractions of soil organic matter were evaluated down to 0-80 cm depths. T1M1N1 and T1M0N1 treatment combinations significantly had the highest maize yields in three out of six seasons (SR2013, LR2014 and SR2014), ranging from a low of 1.9 Mg ha-1 in SR2013 to a high of 4.9 Mg ha-1 in LR2015. T1M1N1 combination was superior for stover yields in four out of six seasons, ranging from a low of 2.7 Mg ha-1 in SR2013 to a high of 7.9 Mg ha-1 in LR2015. T0M0N1 combination had highest water use efficiency for biomass production (WUEB), with the highest water use efficiencies noted in LR2014 at 13.3 kg ha-1 mm-1 and short rains 2014 at 9.6 kg ha-1 mm-1 .Significantly higher WUEGY of 9.6 kg ha-1 mm-1 were recorded in T0M0N1 in short rains 2014 and 3.4 kg ha-1 mm-1 in T1M0N1 in long rains 2015. Generally, the NO3-N concentrations in all treatments decreased with increase in soil depth from 0-20 and 20-40, after which the concentrations increased from 40-60 and 60- 80 cm depths. There were significant differences in the amount and distribution of soil nitrate-N at the depths of 60-80 cm at 10 WAP, with higher net N mineralisation in minimum tillage treatments than in conventional tillage treatments. The highest soil water content in the study conserved in the minimum tillage with mulch retained at 0 kg of N fertiliser while minimum tillage treatments had higher crop evapotranspiration. Overall, soil carbon content decreased with increase in soil depth. In all the three class size aggregates, the highest increases in soil carbon were noted in the 0-20 cm depths, with the highest overall increases in both soil meso-aggregates and macro-aggregates between 2012 and 2015 observed in T1M1N1 combination. There was enhanced soil carbon in the 0-20 cm depth in all the three aggregate fractions in treatments with conventional tillage combinations, with increases of 3.5 t C ha-1 in micro-aggregates, 2.2 t C ha-1 in meso-aggregates and 3.7 t C ha-1 in macro-aggregates. While more rains fall during the SR seasons, the LR seasons were found to be more important, with higher grain and stover yields in SR than LR. Retention of mulch on-farm was found to be crucial for soil water conservation at soil depths below 50 cm and enhancing mineralisation of nitrogen, showing a clear potential for seasonal water buffering. This underscores the possibility of adapting to erratic climate regimes. This research has confirmed the importance of conventional tillage practices in ensuring better maize yields for farmers and promoting soil C content in smallholder maize-based farming systems conducted in Humic Nitisols and that in the long-term the formation of stable micro-aggregates in smallholder farms can be useful carbon sinks.enThesis