ABSTRACT

A study was conducted at Umuahia North LGA, Abia State to ascertain the variability of structural properties and carbon storage of Ultisols under selected landuse types. Four agricultural landuse types (continuously cultivated land (CC), forest land (FL), grassland (GL) and oil palm plantation (OP)) were selected for the study. Two sets of samples were collected for the study. The first set was collected by delineating each landuse into three portions of approximately equal dimensions. Soil samples were randomly collected from the top soil of each portion and bulked. Thus, three bulked samples were obtained from the top soils of each landuse representing three replicates, and used for the characterization of each landuse under investigation. This set of samples gave a total of twelve (12) observational units (4 landuses x 3 replicates) and the layout was a randomized complete block design (RCBD). Another set of soil samples were collected from each of the landuse types for specific parameters (aggregate stability indices, bulk density, water retention characteristics, water conductivity properties and organic carbon storage). This was achieved by digging nine (9) replicates of 100cm depth mini-pits in each landuse type. Soils were sampled at every 20cm intervals of each pit. This was laid out as split plot experiment in RCBD with landuse type and depth as factors. Landuse type was the main plot factor while depth was the sub-plot factor. The four (4) levels of landuse type and five (5) levels of depth gave a total of twenty (20) treatment combinations. Hence, there were a total of one hundred and eighty (180) observational units. Similarly, core samples of soil were collected from the pits in each landuse type. The samples were prepared and sent for laboratory analyses. The data obtained was subjected to statistical analyses using appropriate statistical packages. The results obtained from the first set of samples showed that landuse types varied significantly (P ≤ 0.05) in the soil physico-chemical properties with forest land (FL) and OP having good qualities for all the parameters measured. Thus, FL had the best rating for pH (5.60), TN (0.18g/kg), avail. P (45.07mg/kg), Mg (2.87cmol/kg), K (0.22cmol/kg) and CEC (7.91cmol/kg) while OP showed the best quality in OC (2.11g/kg), Ca (3.73cmol/kg), EA (0.75cmol/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 lowest quality of all the parameters measured except for BD which was observed to be highest under GL. The results of the specific parameters obtained from the second set of samples revealed that there was significant interaction (P ≤ 0.05) of landuse type and depth in influencing soil structural properties and OC storage. At 0 – 20cm depth, the highest values of DR (29.48%) and 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 ASC (19.98%) and 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 MWD (1.33mm) at 0 - 20cm depth was obtained under OP while the lowest (0.86mm) was under GL. Organic carbon was highest (51.92ton/ha) under OP but lowest (22.98ton/ha) under CC.  The highest values of K_sat, PT and PM across the depth of 0-80cm were observed in CC, followed by FL while GL had the lowest values. The highest 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 lowest stability of microaggregates to 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 the 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 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 = - 0.005). it also revealed that the influence of OC on microaggregate stability was highest under FL (b = 0.44, – 0.12 and – 0.45 for CDI, DR and CFI, respectively) and lowest under CC (b = 0.24 and – 0.23 for CDI and CFI, respectively) and OP (b = 0.02 for DR). Whereas there was a negative linear relationship between OC and DR under CC, other landuse types showed positive linear relationship. The highest influence of OC on MWD was observed under CC (b = 0.016) and the lowest was observed under GL (b = 0.002). The influence of OC on water retention characteristics was highest under CC (b = - 0.62, - 0.22 and -0.40 for FC, AWC and PWP, respectively) while the lowest influence was observed under OP (b = - 0.16, -0.06 and – 0.10 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 and PM 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 K_sat 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 but much under 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. Oil palm plantation (OP) improved OC storage at the top soil followed by FL while CC significantly reduced OC storage. Soils with high OC content are likely to have low clay content, and this will reduce the BD. Organic carbon storage helped in improving macroaggregate stability while clay helped in improving microaggregate stability. High OC content greatly improved water conductivity properties while high clay content greatly improved water retention characteristics. However, in a sandy soil, high OC can help in improving water retention characteristics. Pulverization of soils by tillage using simple farm tools reduced BD and improved water conductivity properties at the pulverized zones of the soil whereas the use of heavy machineries in tillage operation induced 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. Intrinsic properties of soils such as texture greatly influenced soil structural and aggregate stability indices.