Alterations of ecosystem nitrogen status following agricultural land abandonment in the Karst Critical Zone Observatory (KCZO), Southwest China

Background. Secondary succession after agricultural land abandonment generally affects nitrogen (N) cycle processes and ecosystem N status. However, changes in soil N availability and NO3- loss potential following secondary succession are not well understood in karst ecosystems.Methods. In the Karst Critical Zone Observatory (KCZO) of Southwest China, croplands, shrub-grass lands, and secondary forest lands were selected to represent the three stages of secondary succession after agricultural land abandonment by using a space-for-time substitution approach. The contents and 15N natural abundance (815N) of leaves, soils, and different-sized aggregates at the three stages of secondary succession were analyzed. The 815N compositions of soil organic nitrogen (SON) in aggregates and soil to plant 15N enrichment factor (EF = 815Nleaf -815Nsoil), combined with soil inorganic N contents and 815N compositions were used to indicate the alterations of soil N availability and NO3-loss potential following secondary succession.Results. Leaf N content and SON content significantly increased following secondary succession, indicating N accumulation in the soil and plant. The 815N values of SON also significantly decreased, mainly affected by plant 815N composition and N mineralization. SON content in macro-aggregates and soil NH4+ content significantly increased while 815N values of NH4+ decreased, implying increases in SON stabilization and improved soil N availability following secondary succession. Leaf 815N values, the EF values, and the (NO3--N)/(NH4+-N) ratio gradually decreased, indicating reduced NO3- loss following secondary succession.Conclusions. Soil N availability improves and NO3- leaching loss reduces following secondary succession after agricultural land abandonment in the KCZO.