Effects of vegetation and slope aspect on soil nitrogen mineralization during the growing season in sloping lands of the Loess Plateau

The presence of vegetation has significant effects on soil nitrogen (N) turnover. However, whether these effects vary with slope aspect, vegetation type and time of season were not understood in previous studies, precluding our ability to understand how vegetation affects soil N cycling in sloping land. In this study, we investigated the effects of vegetation presence (plots with vs. without vegetation), vegetation type (grassland vs. woodland) and slope aspect (east vs. west slope) on soil N turnover during the growing season in sloping lands of the Loess Plateau, China. Soil metrics measured included concentrations of ammonium (NH₄⁺), nitrate (NO₃⁻) and total mineral N (Min-N) in soil solution and rates of ammonification (Rₐ), nitrification (Rₙ) and net N mineralization (Rₘ). We hypothesized that the presence of vegetation would deplete the soil Min-N pool and decrease Rₘ and that these effects would be greater in east slopes and woodlands than in west slopes and grasslands. In partial support of these hypotheses, vegetation presence decreased soil Min-N concentration but did not affect Rₘ. NH₄⁺ and NO₃⁻ contributed similarly to Min-N, while Rₙ dominated Rₘ. The effects of vegetation presence on soil mineral N and N mineralization varied with the time of the season but were not related to the vegetation type and slope aspect. The soil Min-N and Rₘ were significantly higher in woodlands, east slopes and 0–10 cm depth than grasslands, west slopes and 10–20 cm depth, respectively. The Rₐ and NH₄⁺ increased, while the Rₙ, Rₘ, NO₃⁻ and Min-N decreased with increasing soil moisture. These results indicated that soil Min-N and Rₘ in the sloping lands of the Loess Plateau consistently respond to vegetation presence across slope aspect and vegetation type, and were regulated by soil moisture.