Biochar vs. clay: Comparison of their effects on nutrient retention of a tropical Arenosol

The use of biochar as a soil amendment is being widely studied, whereas clay addition to meliorate soils has only been considered in a few studies. However, there is a limited understanding of the impact of biochar and clay on nutrient retention under field conditions, especially in seasonally dry tropical forests in semi-arid regions. In this study, biochar and clay were added to a tropical Arenosol to quantify nutrient leaching as a measure of nutrient retention to test the potential of both amendments for soil amelioration. In a field experiment, planting holes were prepared with 5 vol% medium-temperature biochar made of Prosopis juliflora (SW) DC, produced locally in the traditional manner in a kiln, or 10 vol% clayey sediment from a temporarily dry lake. Both amendments and control were tested as fertilised and unfertilised treatments. Then seedlings of the endemic tree Spondias tuberosa Arr. were planted. Self -integrating accumulators (SIAs) were used to quantify nutrient leaching at a depth of 0.7 m for ammonium -N (NH4+ - N), nitrate -N (NO3- - N), and potassium (K+). Leaching was assessed for two consecutive periods of eight months each. In the fertilised treatments, biochar addition significantly reduced NO3- - N leaching by 46% in the first period; leaching of all other nutrients was reduced but remained non-significant. In contrast, clay addition significantly reduced NH4+ - N leaching by 790/a in the second period, K+ leaching by 51% in the first period, and by 45% in the second period, but significantly increased NO3- - N leaching by 57% in the second period. Both treatments showed pronouncedly different leaching patterns over time. Between the first and second period, the ability of biochar to reduce leaching relative to the control decreased by about half for the nutrients tested, whereas the ability of the clay treatment remained rather stable over 1.5 years for NH4+ - N and K+. The reason for the distinct decrease in nutrient retention of biochar in the second period may be the rapid decomposition of biochar carbon by 51% within 16 months. Under the given conditions, only clay can be recommended as a sustainable land management strategy to enhance NH4+ - N and K+ retention capacity of Arenosols in seasonally dry tropical forests.