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

Abstract

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 (δ15N) of leaves, soils, and different-sized aggregates at the three stages of secondary succession were analyzed. The δ15N compositions of soil organic nitrogen (SON) in aggregates and soil to plant 15N enrichment factor (EF = δ15Nleaf −δ15Nsoil), combined with soil inorganic N contents and δ15N 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 δ15N values of SON also significantly decreased, mainly affected by plant δ15N composition and N mineralization. SON content in macro-aggregates and soil NH4+ content significantly increased while δ15N values of NH4+ decreased, implying increases in SON stabilization and improved soil N availability following secondary succession. Leaf δ15N 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.