Differential Responses of Soil Ammonia-oxidizing Bacterial and Archaeal Communities to Land-use Changes in Zambia.

Soil nutrient loss from intensive farming is a critical issue in sub-Saharan Africa that affects food security. While soil microbial nitrification supplies available nitrogen, excessive nitrification leads to nitrogen loss. However, the species driving nitrification and their functions in this region remain largely unknown. Therefore, we investigated the responses of ammonia-oxidizing bacterial (AOB) and archaeal (AOA) communities to land-use changes in Zambia and their relationship with nitrification potential. Soil samples were collected from three sites in Zambia that all had neighboring natural and farmed (maize) lands. We measured nitrification potential, quantified AOB and AOA, and analyzed these communities by targeting the ammonia monooxygenase subunit A (amoA) gene, which encodes a key enzyme in nitrification. Nitrification potential was 1.51-fold higher in farmlands than in natural lands. AOB abundance tended to be greater in farmlands, whereas AOA abundance was smaller. Farming changed the AOB community structure, increasing Nitrosospira cluster 3a.2 at the three sites, while minor site-specific responses were also observed. In contrast, the AOA community structure was not significantly different between land uses, but varied among sites, with cluster NS-ζ being more prominent in one site with neutral soil (pH 7.64) than in the other sites (pH 5.70 and 5.71). These results suggest that AOA species were generally vulnerable to farming, decreasing in abundance without structural changes, while some AOB species increased, driving changes in their community structure. These insights are fundamental for understanding soil nitrogen depletion due to microbial changes under farming and are crucial for developing sustainable land-use practices in sub-Saharan Africa.