Influence of topography and vegetation cover on soil organic carbon stocks, soil loss, water balance and greenhouse gas fluxes in wooded grasslands of Laikipia County, Kenya

Abstract

Soil carbon stocks (SOCs), soil water balance and greenhouse gas fluxes measurements in wooded grassland are often done in a single assessment. This is oblivious of its heterogeneous nature and asymmetric distribution that characterizes wooded grassland and thence inaccurate results are captured. This information is essential, albeit lacking, for designing sustainable strategies important for management of the fragile wooded grassland ecosystems. The present study investigated topographical and vegetation cover types effects on SOCs, soil water balance (SWB) and greenhouse gas fluxes in wooded grasslands of Laikipia County, Kenya. This study was conducted during the short and long rainy seasons of 2016 in Ilmotiok group ranch of Laikipia County. Soil organic carbon stocks (SOCs), soil water balance (SWB) and greenhouse gas fluxes across different topographical positions and vegetation cover were quantified. The experimental design was a RCBD with a split plot layout. The main plots were topographical zones (TZ); mid slopes (MS), foot slope (FS) and toe slope (TS). The subplots were vegetation cover (VC) types: tree (T), grass (G) and bare (B). Sampling of soil was done at intervals of 10 cm to a depth of 50cm in a zigzag manner using a soil auger. The sampling was done along a transect line of 150m after every block of 50m forming three replicates. The sampled soil was analyzed for texture, bulk density (BD) and soil organic carbon. Runoff plots were set up across the TZ and VC types to monitor runoff (RO) and soil loss (SL). To measure Greenhouse Gas (GHGs) fluxes (methane (CH4), carbon dioxide (CO2), and Nitrous Oxide (N2O) static chamber frames were installed across the topographic zones and vegetation cover types. GHGs were measured every 7-10 days in the dry season, intermediate and rainy season between 0800hrs and 1200hr local time. TZ, VC, depth and TZ*VC significantly (p<0.05) influenced BD and SOCs. There was a significantly higher bulk at MS (1.03 g/cm3 and 1.00 g/cm3) but not significantly different from TS (1.02 and 0.92 gcm-3) with FS having the lowest value (0.97 and 0.88 gcm-3) for LRS and SRS respectively. Vegetation cover significantly (P <0.05) influence with highest bulk density recorded under BR (1.04 and 0.96 gcm-3) which was not significantly different from TR (1.01 and 0.92 gcm-3) and significantly higher than GR (0.97 and 0.92 gcm-3) for LRS and SRS respectively. The interaction of topography and vegetation significantly influence bulk density with highest value recorded under FS*BR (1.11 and 1.03 gcm-3) for LRS and SRS respectively. Highest soil organic carbon stocks were recorded at the TS (6.40 and 6.51 MgHa-1) as compared to other zones though not significantly different v v from MS (6.16 and 6.46 MgHa-1) but significantly different from FS (5.29 and 5.93 MgHa-1). SOCs under GR (6.31 and 6.53 MgHa-1) were slightly higher than other vegetation cover, the lowest was recorded under BR (5.76 and 6.02 MgHa-1) for LRS and SRS respectively. The upper soil depth (0-10) had (8.70 and 8.74 MgHa-1) compared to the lower depth (40-50) with (3.52 and 4.07 MgHa-1). There were significant [P<.001] differences in evapotranspiration, runoff and soil loss across the three topographical zones and vegetation cover types. The run off was significantly higher in mid slope*bare [175.90 and 168.75 mm] and mid slope *grass [172.00 and 164.85mm] compared to toe slope *bare [169.79 and 162.64 mm] and Toe Slope* Grass [165.89 and 158.74 mm] during the LRS and SRS. Whereas Soil water balance was highest at the toe slope*grass [279.46 and 119.49 mm] than Foot slope*Grass [273.51 and 113.54 mm] and Mid Slope*Grass [267.23 and 104.76 mm] during the LRS and SRS respectively. The Run off Coefficient was significantly lower in the Toe slope*Grass [0.30 and 0.45] than Foot slope*Grass [0.31 and 0.46] for LRS and SRS). During the wet months, CH4, N2O and CO2 emission were significantly higher than the dry season. Methane fluxes ranged from -0.32 mg.m-2.h-1 to 0.24 mg.m-2.h-1 with the lowest (-0.32 mg.m-2.h-1) recorded under TS*T whereas CO2 was highest under TS*G (47 mg.m-2.h-1) as compared to MS*G (19 mg.m-2.h-1). TZ*VC significantly influence N2O with MS*B recording the lowest (0.008) as compared to TS*B (2.228 mg.m-2.h-1). CO2, N2O and CH4 In the month of January and February emissions were low and it increased in March and April in all the TZ*VC. Topography and vegetation have an effect on soil organic carbon stocks and bulk density. Toe slope and grass significantly increased soil organic carbon stocks and reduced bulk density. Toe slope with grass significantly reduced both runoff and soil loss thus increased Soil water balance and improved runoff coefficient. Therefore, protection of slopes from raindrops can effectively reduce soil loss and runoff and enhance deposition in the mid slope and toe slope. The trends of soil CO2, N2O, and CH4 fluxes were principally controlled by topography and plant cover, with larger soil CH4 uptakes and CO2 emissions on the toe slopes and foot slopes than in MS. Keywords: bulk density, runoff, soil loss, soil water balance, soil organic carbon stocks, topographical zones, vegetation cover