Gross nitrogen transformation rates in shifting cultivation systems in northern Thailand: Controlling factors and implications for inorganic nitrogen availability

Abstract

Slash-and-burn practices and subsequent fallow periods affect soil nitrogen (N) availability. Although extensive studies have examined the global temporal patterns of inorganic N after fire, NO3− regulation remains unclear due to ecosystem-dependent responses. Despite inorganic N pool control by NH4+ and NO3− production and consumption, local-scale patterns and mechanisms of gross N transformations, particularly in tropical regions, remain insufficiently understood. Thus, we aimed to elucidate the controlling factors of inorganic N in the shifting cultivation cycle in Northern Thailand, using a 15N pool dilution method on soil samples at 0–10 cm depth collected across various post-burning stages (1, 4, 6, and 7 years after burning) and forest. We found that gross ammonification was enhanced by high soil organic matter and microbial biomass, while gross nitrification was enhanced by greater soil alkalinity. Ammonium immobilization increased with higher microbial biomass, while NO3− immobilization increased with lower NH4+ availability. With increasing fallow period, microbial biomass and consequently gross ammonification increased, boosting the NH4+ pool. Soil acidity, correlated with soil texture, influenced nitrification: coarser-textured soil intensified acidity and decreased gross nitrification, resulting in NH4+-dominance, while finer-textured soil alleviated acidity, promoting nitrification over NH4+ immobilization, leading to NO3−-dominance. Our findings confirm that the fallow stage, along with which microbial biomass increases, was an essential driving factor regulating ammonification, while soil acidity, unrelated to fallow but a function of sand/silt content, was an essential driving factor regulating nitrification under shifting cultivation in Northern Thailand.