Abstract: |
Acidic soils (pH < 5.5) cover roughly 30% of Sub-Saharan Africa. Low nitrogen fertilizer
application (15 kg N ha−1 yr−1) has no effect on soil acidification in Sub-Saharan Africa (SSA). However, the effect of optimized fertilizer use on soil acidification (H+) in SSA crops remains unknown.
This study intended to predict the spatial variation of H+ caused by optimized fertilizer use using
data from 5782 field trials in SSA cropland. We used ensemble machine learning to predict spatial
variation (H+) after measuring the inputs and outputs of major elements and their effect on H+ production. The results revealed that H+ ranged spatially from 0 to 16 keq H+ ha−1 yr−1. The most protons
(H+) were produced by cassava, banana, and Irish potatoes systems with 12.0, 9.8, and 8.9 keq H+
ha−1 yr−1 , respectively. The results of the 10-fold cross validation for the soil acidification model were
a coefficient of determination (R2) of 0.6, a root mean square error (RMSE) of 2.1, and a mean absolute error (MAE) of 1.4. Net basic cation loss drives soil acidification under optimized fertilizer application and climate covariates had a higher relative importance than other covariates. Digital soil
mapping can produce soil acidification maps for sustainable land use and management plans.
Keywords: digital soil mapping; ensemble machine learning; optimized fertilizer use;
soil acidification; soil pH
1. Introduction
More than a third of Sub-Saharan Africa has acid soils (pH < 5.5) [1] and soil acidity
is caused by the presence of hydrogen (H+) ions, which can be generated either naturally
by soil formation processes or by anthropogenic activities [2,3]. In regions where precipitation exceeds evapotranspiration, soil acidification is a persistent process that can be sped
up or slowed down by the actions of plants, animals, and humans [4,5]. Soil acidification
is primarily caused by the release of protons (H+) during the transformations and cycling
of carbon (C), nitrogen (N), and sulfur (S) in the soil–plant–animal system [6]. The most
proton (H+) and hydroxyl ion (OH¯) generating processes occur during the cycling of C,
N, and S (Table S1). Soil acidification in agricultural systems is typically attributed to an
imbalance in the carbon and nitrogen cycles, which leads to (1) net H+ excretion by plant
roots due to excess cation uptake over anions; (2) removal of alkalinity in farm products
(crop harvest and animal product); (3) accumulation of organic anions in soil organic matter; (4) mineralization of organic matter, nitrification of ammonium, and subsequent nitrate leaching; and (5) input of ammonium-based fertilizers [7].
Citation: Uwiragiye, Y.; Ngaba,
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