ABSTRACT

A study was conducted at Umuahia North LGA, Abia State to ascertain the variability of structural properties and organic carbon storage of an Ultisol under selected landuse types. The area lies within lat. 5°291N to 5°3 1 'N and long. 7°30'E to 7°32'E and the soil is of hapludult. Four agricultural landuse types {continuously cultivated land (CC), forest land (FL), 3 — year grass fallow land (GL) and oil palm plantation (OP)] were selected for the study. Two sets of experiments were conducted. Experiment one was characterization of the respective landuse types while experiment two involved the evaluation of the variability in soil structural properties and organic carbon storage across the landuse types. In experiment one, three replicates of auger and core soil samples were collected from each landuse type, and this gave a total of twelve observational units. In experiment two, nine representative pits of 100cm depth were dug in each landuse type. The pits were georeferenced and delineated into five depths of 20 cm intervals. Soil samples were collected from each depth of the pit. Therefore, forty - five auger and core soil samples were collected from each landuse type. Thus, a total of one hundred and eighty (180) soil samples each for auger and core (4 landuse types x 5 depths x 9 replicates) were collected across the landuse types. Data collection for analysis of spatial variability was done using grid system and the analysis was by kriging approach. The samples were prepared and analyzed in the laboratory. The data obtained were subjected to statistical analyses using Genstat application, SPSS and GIS software packages. The results obtained from experiment 1 showed that soil physico-chemical properties varied across the landuse types. Forest land (FL) had the best rating in pH (5.60), TN (0.18 g/kg), avail. P (45.07 mg/kg), Mg (2.87 cmol/kg), K (0.22 cmol/kg) and CEC (7.91 cmol/kg) while OP showed the best rating in OC (2.11 g/kg), Ca (3.73 cmol/kg), EA (0.75 cmol/kg), and % BS (89.90 %). The highest sand fraction and low clay content were observed in FL and GL while CC and OP had relatively high clay content. The CC had the poorest rating in all the parameters measured except for BD which was observed to be highest under GL. The results obtained in experiment 2 revealed that there was significant interaction (P 0.05) of landuse type and depth in influencing soil structural properties and OC storage. At 0 — 20 cm depth, the highest values of dispersion ratio (DR) (29.48 %) and clay dispersion index (CDI) (42.33 %) were obtained under FL and GL, respectively, while the lowest value of DR (19.41 %) and CDI (25.00 %) were under CC. The highest values of aggregated silt + clay (ASC) (19.98 %) and clay flocculation index (CFI) (75.00 %) were obtained under CC while the lowest values of ASC (10.11 %) and CFI (57.67 %) were obtained under FL and GL, respectively. The highest value of mean weight diameter (MWD) (1.33 mm) at 0 — 20 cm depth was obtained under OP while the lowest (0.86 mm) was under GL. Organic carbon was highest (51.92 tha') under OP but lowest (22.98 tha') under CC. The highest values of hydraulic conductivity (Ksat), total porosity (PT) and macroporosity (PM) across the depth of 0-80 cm were observed in CC, followed by FL while GL had the lowest values. The highest bulk density (BD) was observed in GL while the lowest was in CC. The best microaggregate stability was observed at CC followed by OP while FL showed the worst stability of microaggregates in water. The OP had the highest values of MWD followed by GL, while CC had the lowest values across the depths. The order of OC storage was OP> FL > GL> CC. The results further revealed that these specific parameters varied significantly (P 0.05) with depths. There was significant decrease in OC storage, MWD, and water conductivity properties with depth in all the landuse types while clay content, microaggregate stability indices, BD, and water retention characteristics significantly increased with depth in all the landuse types. The results of the regression analysis showed that the rate of change (b = regression coefficient) in Ksat and BD for any unit change in OC was highest under CC (b = 0.21 and 0.02, respectively) and lowest under OP for Ksat (b = 0.06) and FL for BD (b = x, I - 0.005). It also revealed that the influence of OC on microaggregate stability (R2= coefficient of determination) was highest under FL (R2= 98.50 %, 67.800 % and 99.00 % for CDI, DR and CFI, respectively) and lowest under CC (R2= 67.30 % and 70.10 % for CDI and CFI, respectively) and OP (R2= 5.10 % for DR). There was a negative linear relationship between OC and DR under CC, while they had positive linear relationship in other landuse types. The highest influence of OC on MWD was observed under CC (R2= 97.40 %) and the lowest was observed under GL (R2= 52.6 %). The influence of OC on water retention characteristics was highest under CC (R2= 95.50 %, 94.50 % and 95.50 % for field capacity (FC), available water capacity (AWC) and permanent wilting point (PWP), respectively) while the lowest influence was observed under OP (R2= 81.30 %, 80.40 % and 81.20 % for FC, AWC and PWP, respectively). The results of the correlation analysis revealed that there was significant (P 0.05) positive relationship among OC, MWD, Ksat, PT (total porosity) and PM (macro porosity) while there was a significant negative relationship between OC and BD, clay content,CFI, ASC and water retention characteristics in all the landuse types. There was significant (P 0.05) positive relationship between clay content and microaggregate stability indices, clay content and BD as well as clay content and water retention properties in all the landuse types. Results of the spatial analysis showed that there was spatial variability in soil structural properties and organic carbon storage across the various landuse types. Areas with high concentration of OC were dominant in OP while areas with low concentration of OC were dominant under CC. The highest spatial variability in Ksat and PT was observed under GL and OP while the highest variability in CFI was observed under FL. Areas with relatively high CDI dominated the soils under FL and OP while areas with relatively low CDI dominated soils under CC and GL. Soils under FL and OP were dominated by areas with relatively high DR compared to CC and GL. Soils under FL showed high variability in ASC compared to the other landuse types, and had a high dominance of areas with relatively low ASC. There was less spatial variability in MWD under FL and CC than OP and GL. Areas with high MWD dominated the soil under FL. Generally, landuse significantly influenced soil structural properties and SOC. There was also a significant interaction of landuse and depth in influencing soil structural properties and SOC. There was also a remarkable spatial variability in soil structural properties and SOC within each landuse type. The continuous cultivation of soils will significantly reduce OC storage. Soils with high OC content are likely to have low clay content, and this will reduce the BD. Macroaggregate stability and water transmission properties of soils can be improved by improving the soil organic carbon content, but will weaken microaggregate stability. However, in a sandy soil, high OC can help in improving water retention characteristics. Pulverization of soils by tillage using simple farm tool can reduce BD and improve water conductivity properties at the pulverized zones of the soil whereas the use of heavy machineries in tillage operation will induce high soil BD as observed under GL. The dispersion of microaggregates may be aggravated with increase in OC, and this may have grave implication on sandy soils. The spatial variability in organic carbon storage and structural properties of soils indicates that attention be giving to areas with critical soil conditions so as to effectively manage the soils while saving time, labor and cost of managing the entire land.