Contribution of carbonates and oxalates to the calcium cycle in three beech temperate forest ecosystems with contrasting soil calcium availability

Abstract

Calcium (Ca) is an essential macronutrient which plays a fundamental role in ecosystem structure and function. In temperate forest ecosystems, Ca oxalate is ubiquitous, existing in live and decomposing biomass pools. Calcium in minerals occurs principally as carbonate and silicate. Because of structural differences between its different forms, Ca speciation can influence the Ca biogeochemical cycle. In this study, we compared three beech temperate forests of contrasting soil Ca availability, i.e., dystric cambisol (DC), eutric cambisol, and rendzic leptosol (RL), but with similar climates, atmospheric depositions, species compositions and management. The pools and fluxes of total Ca (Cat) and Ca oxalate (Caox) were assessed in plant samples (aboveground and belowground tree tissues, forest floor). Pools and fluxes of Cat were also quantified in the atmospheric inputs (dry and wet deposits), in the soil (total and exchangeable fractions) and in the soil solutions at different depths. The soil solutions were measured monthly for 4 years to study the seasonal dynamics of Ca fluxes. A budget of dissolved Ca was also determined for the forest floor and soil layers. Regarding the global Ca cycle at the stand scale, our study highlighted: (i) the predominant role of biological recycling in the Ca cycle in forest ecosystems for all soil types; the turnover rates of fine roots and leaves were approximately 43 and 39% of the total Ca taken up by trees each year, revealing that only 18% accumulated in the perennial tissues; (ii) the vegetation was Ca-enriched in the high Ca soil (RL) compared to the low-Ca soil (DC); (iii) the existence of an unexpected available Ca source in DC due to upwelling by the diffusion of Ca issued from the dissolution of carbonates in-depth; (iv) a higher production of dissolved Ca (from carbonate dissolution) in RL compared to DC, resulting in an important loss of Ca by drainage (280 kg ha−1 year vs. 14 kg ha−1 year); (v) monthly measurements demonstrated that the seasonal dynamics was mainly linked to biological activity. In addition, our study showed that the distribution of Caox differs greatly between vegetation compartments, ranging from null (stem and branch wood) to up to 40% (stem and branch bark and fine roots) of the Cat in the biomass. Vegetation is a factory of Ca oxalate, and the return of this Caox enriched vegetation to the soil influences the Ca mobility in the forest floor, as demonstrated by a Caox residence time shorter than that of Cat in the DC.