Soil nitrogen (N) accounts for approximately 88% of the global plant N demand and has an impact on net primary productivity and soil respiration in terrestrial ecosystems. Any change in soil N availability can greatly affect plant growth and productivity and, thus, impact ecosystem functions.  

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. 

LI Ming, supervised by Professor CHEN Xiaoli from Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden 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 Resewoir 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.