Conservation tillage and biochar improve soil water content and moderate soil temperature in a tropical Acrisol

Abstract

Projected climate change in Sub-Saharan Africa involves increased drought and elevated soil temperature. Conservation farming (CF), including minimum tillage, crop rotation and crop residue retention, is proposed as a climate smart soil management option to adapt to climate change through enhanced climate resilience. Here, we determine the effect on soil moisture and temperature of CF planting basins in a Zambian Acrisol. Construction of CF planting basins (40 cm x 15 cm, while 20 cm deep), using hand-hoes, is a commonly used minimum tillage practice among small holders in southern Africa, effectively requiring tillage of only 10 % of a field. The study included basins under regular CF and under CF with 4 t ha−1 pigeon pea biochar (CF + BC). Effects are compared with those in an adjacent soil under conventional tillage, where the entire land surface is ploughed. Soil moisture and temperature sensors were installed in the root zone, 10–12 cm deep, for continuous monitoring during two growing seasons. Soil moisture decreased in the order CF + BC > CF > conventional farming. Due to rainwater harvesting in the basins, maximum soil water retention under CF + BC and CF was greater than under conventional farming (+59 % to +107 % and +15 % to +65 %, respectively). Soil drying after free drainage until permanent wilting point lasted longer under CF + BC (18.4–22.3 days) than under both CF and conventional farming (13.3–18.4 days and 14.9–17.8 days, respectively). In situ soil maximum temperature and diurnal temperature range in the growing season increased in the order CF + BC < CF < conventional farming due to decreases in soil moisture. However, additional laboratory tests, with soil-BC mixtures at field capacity, revealed that BC addition to soil, which caused a decrease in bulk density, also resulted in a significant decline in soil thermal conductivity (p < 0.001). Thus, we hypothesize that BC-enhanced soil moisture in basins helped to reduce soil temperature and its fluctuations, due to both increased heat capacity and decreased thermal conductivity. This study shows that CF in combination with BC in an Acrisol, through enhancing plant-available water and moderating soil temperature, is important for crop productivity and has potential as an element of climate smart agriculture.