Understanding the interactive effects of ecotype, seasonality, and rainfall reduction on the soil nutritional status of the Brazilian savanna

Climate models forecast a significant reduction in average precipitation and an extension of the dry period over the coming decades, which will likely influence the biogeochemical processes of the Brazilian savanna (i.e. Cerrado). However, studies about the fate of soil nutrient availability in this biome remain scarce. We conducted a field-based experimental rainfall reduction in three Cerrado ecotypes, namely an open savanna (OS), a woodland savanna (WS), and an intermediate savanna (IS), to assess its effects on soil organic matter, total phosphorus, microbial biomass nitrogen, dissolved organic nitrogen and inorganic nitrogen (NH₄⁺-N and NO₃^--N) pools in dry and rainy seasons for two consecutive years. Despite sharing common soil origins and climatic conditions, these ecotypes showed contrasting levels of soil organic matter as well as phosphorus and nitrogen pools. The WS showed higher soil fertility with organic matter levels of 11.19 %, P levels of 2.54 mg Kg⁻¹, and NO₃^--N levels of 2.15 mg Kg⁻¹, compared to the OS, which had organic matter levels of 7.04 %, P levels of 0.65 mg Kg⁻¹, and NO₃^--N levels of 1.49 mg Kg⁻¹. The IS showed intermediate values, with organic matter levels of 8.94 %, P levels of 0.65 mg Kg⁻¹, and NO₃^--N levels of 1.93 mg Kg⁻¹. Seasonal patterns considerably influenced soil nutrient content, with higher soil inorganic nitrogen levels typically found during the rainy season and higher soil phosphorous levels during the dry season, although these patterns varied among ecotypes. Soil organic matter, dissolved organic nitrogen, and microbial biomass nitrogen exhibited greater sensitivity to the experimental reduction in rainfall compared to soil phosphorus and inorganic nitrogen. However, this sensitivity varied across ecotypes and seasons, with a particularly strong effect observed in the IS and during the rainy season. Our findings suggest that the effects of climate change in the Cerrado might strongly vary among seasons and the different ecotypes, highlighting the need for more comprehensive experimental studies in the different ecotypes to fully understand future nutrient dynamics in the Cerrado.