Quantifying Soil and Nutrients Losses under Different Soil Amendments and Cropping Systems on a Plinthic Vetic Lixisol in Ghana
Soil erosion coupled with soil nutrients depletion affect crop production in small-scale cropping systems of sub-Saharan Africa (SSA). Reducing both threats, based on sustainable practices is crucial to enhancing crop productivity in the region. The current study was designed to help address the twin problems based on the following objectives: (i) developing and validating a new numerical method for surface runoff assessment; (ii) determining the effect of crop and soil management practices on soil loss; (iii) analyzing soil nutrients loss due to soil erosion under different amendments and cropping systems; (iv) assessing the effect of soil amendments and cropping systems on soil properties; (v) assessing the effect of soil amendments on crop productivity. In achieving these objectives, a field experiment was carried out on runoff plots under different cropping systems (commonly practiced in Ghana) treated with soil amendments. The study was a two-factor experiment in split-plot arranged in a randomized complete block design for three consecutive cropping seasons (2016 major, 2016 minor and 2017 major seasons). The cropping systems (sole maize, maize intercropped with soybean, sole soybean and cowpea) constituted the main plots whereas the subplots comprised soil amendments (inorganic fertilizers (NPK), inorganic fertilizers combined with biochar (NPK+BC), sole biochar (BC) and control). For the model development and soil erosion characterisation, a total of 33 erosive rainfall events were observed. Different statistical parameters viz. p-values, R², RMSE, NSE and RSR were used to assess the quality of the model developed. Parameters on the effects of the soil and crop management practices were analyzed in ANOVA and regression models. P value < 0.001 and R² ranging from 0.88 to 0.94 showed good accuracy of the model prediction. The dispersion between the predicted and observed values was low with RMSE varying from 1.68 to 2.66 mm. Moreover, the low variability between parameters was confirmed with the low values of RSR which ranged from 0.38 to 0.46 (with 0.00 ≤ RSR ≤ 0.50 for good prediction). During the observation periods, NSE values ranged from 0.79 to 0.86 (≥0.75 being the threshold for excellent prediction). The sensitivity analysis showed that the model under high runoff generation (simulation including bare plots), was poorly adapted. Results for crop yield and soil properties showed positive impacts of the different interventions. Soil loss characteristics based on amount of soil loss, soil depth reduction and runoff coefficient were significant (P < 0.05). Among the different treatments, sole cowpea and inorganic fertilizers application were most effective in reducing soil erosion. Also, biochar, due to its multipurpose effect on soil properties, had positive effects on soil erosion reduction compared to the control. Cumulative nutrients loss, enrichment ratios and monetary values of soil nutrients loss varied significantly under the amendments and cropping systems. Soil nutrients loss was more pronounced on the bare and the control plots than on the treated plots due to less soil erosion from the latter. All the nutrients had enrichment ratios (ER) greater than unity showing off-site nutrients deposition due to soil erosion; and this was more pronounced during the minor season than in the major seasons. The soil particles had ER greater than unity, except for the sand with values ranging from 0.77 to 0.88 and from 0.65-0.70 in the major and minor seasons, respectively. The economic effect of soil erosion based on the monetary values of soil nutrients loss was high for the control plots for each cropping system followed by the sole biochar (BC) treatment. Monetary loss under NPK and NPK+BC treatments was lowest due to their positive impacts on soil erosion reduction. The physical soil properties (bulk density and volumetric moisture content) were improved by the different practices and best values were observed under sole cowpea and sole biochar with respect to the cropping systems and soil amendments. Soil acidity increased slightly over time except under biochar treatments where a slight decline was observed. The legume-based cropping systems as well as the inorganic fertilizers applications improved soil organic carbon, total nitrogen, available phosphorus and exchangeable potassium contents slightly than the other treatments. For all the three crops evaluated (maize, cowpea and soybean), the productivity (grain and biomass yields) was better under the inorganic based treatments followed by sole biochar. Land equivalent ratio (LER) was greater than 1 under all the amendments under the maize-based systems. This emphasized the positive effect of the intercrop compared to the sole systems. With respect to cost effectiveness, VCR was greater than 2 for only sole NPK treatments under all the cropping systems and also for sole biochar treatment during the third season (2017 major). However, for NPK+BC, VCR > 2 was observed under the intercropped system throughout the study period. Indeed, sustainable nutrients management systems reduced soil loss and enhanced crop productivity and are recommended for small-scale farming activities in SSA.
A thesis submitted to the Department of Crop and Soil Sciences, Faculty of Agriculture, College of Agriculture and Natural Resources, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana in partial fulfillment of the requirement for the award of degree of Doctor of Philosophy in Soil Science.