Abstract
In karst ecosystems, a high level of CaCO3 enhances the stabilization of soil organic matter (SOM) and causes nitrogen (N) and/or phosphorus (P) limitation in plants. Oxalic acid has been suggested to be involved in the nutrient-acquisition strategy of plants because its addition can temporarily relieve nutrient limitation. Therefore, understanding how oxalic acid drives N availability may help support successful vegetation restoration in the karst ecosystems of southwest China. We tested a model suggested by Clarholm et al. (2015) where oxalate reacts with Ca bridges in SOM, thus exposing previously protected areas to enzymatic attacks in a way that releases N for local uptake. We studied the effects of oxalic acid, microbial biomass carbon (MBC), and β-1,4-N-acetylglucosaminidase (NAG) on potential N mineralization rates in rhizosphere soils of four plant species (two shrubs and two trees) in karst areas. The results showed that rhizosphere soils of shrubs grown on formerly deforested land had significantly lower oxalic acid concentrations and NAG activity than that of trees in a 200-year-old forest. The levels of MBC in rhizosphere soils of shrubs were significantly lower than those of trees in the growing season, but the measure of shrubs and trees were similar in the non-growing season; the potential N mineralization rates showed a reverse pattern. Positive relationships were found among oxalic acid, MBC, NAG activity, and potential N mineralization rates for both shrubs and trees. This indicated that oxalic acid, microbes, and NAG may enhance N availability for acquisition by plants. Path analysis showed that oxalic acid enhanced potential N mineralization rates indirectly through inducing microbes and NAG activities. We found that the exudation of oxalic acid clearly provides an important mechanism that allows plants to enhance nutrient acquisition in karst ecosystems.
Highlights
Karst landscapes are distributed worldwide and account for nearly 15% of Earth’s land surface
The amounts of microbial biomass carbon (MBC) in rhizosphere soils of shrubs were significantly lower than that of trees during the growing season, but the measure of shrubs and trees were similar in the non-growing season
A high CaCO3 concentration in soil has a strong effect on stabilizing soil organic matter (SOM) (Hu et al, 2012; Clarholm et al, 2015), which leads to low amounts of C, N, and P being released from SOM
Summary
Karst landscapes are distributed worldwide and account for nearly 15% of Earth’s land surface. The karst region of southwestern China, one of the world’s largest karst regions, encompasses 540,000 km (Yuan, 1994; Qi et al, 2013). Characteristics of this region include a high ratio of rocky exposure, alkaline calcareous soils, and specialized regional vegetation. The availability of soil nitrogen (N) and phosphorus (P) are alternatively limiting elements for primary productivity in karst ecosystems (Niinemets and Kull, 2005; Pan et al, 2015; Zhang et al, 2015). Newly dissolved nutrients are leached into underground drainage networks caused by the shallow soil cover of karst ecosystems and the highly developed epikarst system (Zhang et al, 2011), which exacerbate nutrient limitation in these systems
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