Why Was Disturbed Grassland More Efficient in Soil Carbon and Nitrogen Sequestration than Woodlands in a Karst Slope Ecosystem, Southwest China?

Abstract

The sequestration of soil carbon (C) and nitrogen (N) is very important for the recovery and sustainability of degraded ecosystems. However, the contributions of different processes to soil C and N sequestration following land use change in karst areas remain unclear, hindering our ability to recover degraded ecosystems. Here, we chose eight typical land use types (SR: spontaneous regeneration shrub; Woodland: Cornus wilsoniana wanger, Cyclobalanopsis glauca, Eriobotrya japonica, and Zenia insignis plantations; disturbed grassland (DG): prescribed burning grassland, aboveground vegetation removal grassland, and complete vegetation removal grassland) at a long-term karst restoration site. We measured the effects of leaf inputs, fine root inputs, and fine root protection on soil C and N sequestration with a structural equation model and analysed the decadal changes in soil C and N concentrations among the land use types. The results showed that the soil C concentration was higher in the DG than in the Woodland, and it was similar to that in the SR. Moreover, the dynamic changes in the soil C concentration in the SR and DG were not significant from 2006 to 2017. These results were mainly attributed to fine root protection for native soil C rather than newly sequestered aboveground or root C derived from replanting vegetation. Overall, the dominant processes for soil C and N sequestration were similar. However, the changes in TN concentration were weaker and lagged compared to SOC during land use change, which may be due to the intrinsic soil C/N, difference in cycling complexity, leaf N return, and introduction of N₂-fixing plants. Taken together, our results indicate that the strategy of primary soil C and N protection, combined with active N inputs, may be more propitious to land restoration in karst areas. This can be achieved by the introduction of root-developed N₂-fixing grass or shrubs.

Keywords: Karst Land Restoration, Land Use Change, Fine Root, C Sequestration Process, N Sequestration Process