Purpose

Soil nitrogen (N) availability plays an important role in determining ecosystem structure and function. However, the variation patterns of soil N availability under different vegetation restoration strategies along climatic gradients in the ecologically fragile karst region of China have been poorly understood.

Methods

Soil N variables, including total N (TN), inorganic N [ammonium (NH₄⁺) and nitrate (NO₃^-)], and rates of net N mineralization and nitrification were measured following managed (plantation forest) and natural (spontaneous regeneration) vegetation restoration along a climatic gradient in the karst region of southwestern China.

Results

Compared to that in the cropland, soil TN and NH₄⁺ concentrations significantly increased in both vegetation restoration strategies after 15 years of cropland abandonment. The net N mineralization and nitrification rates were lower in both vegetation restoration strategies compared with those in cropland and secondary forest. There were no significant differences in the soil N variables between the two vegetation restoration strategies. Furthermore, soil NO₃⁻ was positively correlated with mean annual temperature (MAT) following vegetation restoration, whereas the net nitrification rate was negatively correlated with MAT. Generally, soil pH was the main factor affecting soil inorganic N, whereas elevation, soil microbial abundance, SOC or C:N, and climatic factors mainly affected soil net N mineralization and nitrification rates.

Conclusion

Vegetation restoration was beneficial to soil N accumulation and is likely to decrease the risk of N loss in warmer regions given the reduced potential nitrification rate with increased MAT in the karst region of Southwest China.