Consequences of afforestation for soil nitrogen dynamics in central China

The effects of afforestation are of great importance for terrestrial nitrogen (N) cycling. However, the consequences of afforestation for soil nitrogen (N) dynamics remain poorly quantified. We investigated soil net N mineralization and nitrification rates as well as the inorganic N (NH4+-N and NO3−-N) concentration in the top soil (0–10 cm) in a woodland, shrubland and adjacent cropland in the Danjiangkou Reservoir region of central China using the in situ closed-top tube incubation technique over one year. Afforestation significantly decreased the soil net N mineralization rate and soil inorganic N concentration but increased the soil NH4+-N concentration and soil ammonification rate. The major form of soil inorganic N was NO3−-N in the cropland versus NH4+-N in the woodland. The soil net N mineralization and nitrification rates were more sensitive to soil moisture than to soil temperature and were positively correlated with soil moisture. In contrast, the soil net N mineralization and nitrification rates were negatively related to SOC and the C:N ratio. These results suggest that afforestation could decrease N mineralization and availability due to increasing the recalcitrant C input and plant N uptake, which might in turn cause progressive N limitation over the long term.