Effect of tea plantation on nutrient status of oxisols in at the Tshivhase Estate, Limpopo Province

Abstract

Addressing soil fertility decline and acidification due to land use change from undisturbed forest to tea plantation is important in order to foster sustainable management of tea plantations. The establishment of tea plantations could markedly lead to accumulation or depletion of carbon (C) and nutrients in soils. So far less is known on how the conversion of undisturbed forest to tea plantation influences the vertical distribution of soil C content and stocks in oxisols along a topographic gradient. The edaphic factors controlling the distribution of topsoil and subsoil C content and stocks in oxisols are poorly understood. The objectives of this mini-dissertation were (1) to quantify and compare the vertical distribution of soil C content and stocks under tea plantation and undisturbed forest oxisols, and (2) to evaluate the edaphic factors controlling the vertical distribution of soil carbon and stocks under both land uses. In order to address these objectives, three topographical positions were selected (i.e. upslope, midslope and downslope) from the undisturbed forest (reference site) and adjacent the tea plantation. From each topographical position, three soil pits were dug to 100 cm soil depth. To determine the vertical distribution of C content and stocks, soil samples were collected from the 0-5 cm, 5-10 cm, 10-20 cm, 20-40 cm, 40-60 cm, 60-80 cm and 80-100 cm depth intervals. The soil samples were taken to the laboratory where they analysed for cha. Correlation analysis was carried out to determine relationships between soil carbon content and stocks as well as selected soil properties. Results showed that soil C content and stocks were greater in tea plantation compared to undisturbed forest soils across all topographic positions. In both land uses carbon content and stocks declined with depth. Correlation analysis revealed that in the topsoil (0-40 cm), soil C was positively correlated with heavy metals; manganese (r = 0.83; p<0.05) and copper (r = 0.69), effective cation exchange capacity (ECEC) (r=0.72), mean weight diameter (MWD), a measure of soil aggregate stability (r=0.73) and negatively correlated with pH (r = -0.51). In the subsoil (40-100 cm), soil C was positively correlated with copper (r = 0.92) and zinc (r = 0.86), ECEC (r = 0.69) and MWD (r = 0.48) and negatively correlated with pH (r = -0.57) and clay (r=-0.61). Furthermore, C stocks were positively correlated with acid saturation percentage (r =0.50) and negatively correlated with ECEC (r = -0.59), manganese (r = -0.60), exchangeable cations; potassium (r =- 0.56), calcium (r =-0.52) and magnesium (r = -0.65) in the topsoil. In the subsoil, C stocks are positively correlated with phosphorus (r = 0.79), potassium (r = 0.94), copper (r = 0.94), zinc (r =0.81) and MWD (r = 0.81) and negatively correlated with silt (r = -0.61), clay (r=-0.61), magnesium (r =- 0.50) and pH (r = -0.57). These findings suggest that soil C in both topsoil and subsoil of oxisols is chemically stabilized via complexation with polyvalent cations (heavy metals) and partly physically stabilized by soil aggregates. Overall, the results show that land use change from undisturbed forest to tea plantation increased soil carbon content and stock in oxisols along a topographic gradient. Such information is important in understanding the behaviour of soil C content and stocks in tea plantation oxisols. Improved understanding of the effects of land use change from undisturbed forest to tea plantation, and how it influences soil C storage at varying topographic positions can help in development of sustainable tea plantation management practices that will mitigate soil fertility degradation.