Soil texture modulates the response of ammonia-oxidizing prokaryotes to biochemical quality of organic inputs in tropical agricultural soils

Soil texture modulates the response of ammonia-oxidizing bacterial (AOB) and archaeal (AOA) communities to biochemical quality of organic inputs in tropical agricultural soils. To verify this assumption, we used the soil organic matter long-term field experiments in Kenya established on contrasting soils (i.e., clayey Humic Nitisol, sandy Ferric Alisol) in 2002. Since the start of the experiments, soils were continuously treated (4 Mg C ha−1 year−1) with biochemically different organic inputs including Tithonia diversifolia (TD; C/N ratio: 13, Lignin: 8.9%; Polyphenols: 1.7%), Calliandra calothyrsus (CC; 13; 13; 9.4) and Zea mays stover (ZM; 59; 5.4; 1.2). In 2013, soils (0-15 cm) were sampled at young growth (EC30) and flowering (EC60) stages of maize and subjected to DNA-based community analysis of AOB, AOA and 16S rRNA genes. Soil texture exerted stronger effects on the dynamics of the assayed genes than organic input quality with AOB being more responsive than AOA. Clayey soil with its predominant soil organic matter background and large surface area of clay minerals revealed generally higher AOB and AOA abundance than the sandy soil. In the sandy soil, N-rich TD and cellulose-rich ZM promoted AOB abundance, while it was decreased under CC due to N limitation induced by high contents of polyphenols and lignin. N limitation under ZM revealed a clear community differentiation of AOB between ZM versus TD and CC in the clayey soil. For the sandy soil, AOA community composition was distinct between CC versus TD and ZM. To provide a more detailed mechanistic understanding of active AOB and AOA in response to organic input quality, we recommend for prospective research RNA-based along with enzymatic analyses as complement to those shown in this study. This should be approached using controlled experiments to eliminate interfering environmental effects including rainfall and soil moisture, respectively.