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 NO(3)(–) 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 (15)N natural abundance (δ(15)N) of leaves, soils, and different-sized aggregates at the three stages of secondary succession were analyzed. The δ(15)N compositions of soil organic nitrogen (SON) in aggregates and soil to plant (15)N enrichment factor (EF = δ(15)N(leaf) −δ(15)N(soil)), combined with soil inorganic N contents and δ(15)N compositions were used to indicate the alterations of soil N availability and NO(3)(–)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 δ(15)N values of SON also significantly decreased, mainly affected by plant δ(15)N composition and N mineralization. SON content in macro-aggregates and soil NH(4)(+) content significantly increased while δ(15)N values of NH(4)(+) decreased, implying increases in SON stabilization and improved soil N availability following secondary succession. Leaf δ(15)N values, the EF values, and the (NO(3)(–)-N)/(NH(4)(+)-N) ratio gradually decreased, indicating reduced NO(3)(–) loss following secondary succession. CONCLUSIONS: Soil N availability improves and NO(3)(–) leaching loss reduces following secondary succession after agricultural land abandonment in the KCZO.