陆地生态系统土壤铁结合态有机碳:含量、分布与调控

Soil organic carbon associated with iron oxides in terrestrial ecosystems:Content,distribution and control

土壤是陆地生态系统最大的有机碳库,其有机碳储量超过植被和大气碳库的总和.铁氧化物的矿物保护被认为是土壤有机碳长期稳定性的关键机制之一.铁氧化物具有比表面积大、吸附能力强的特点,且在热带和亚热带地区含量丰富.然而,目前关于陆地生态系统土壤铁结合态有机碳占土壤总有机碳的比例(f_(Fe-OC))及其分布格局和调控机制仍不明晰.本文整理了已报道的陆地生态系统351组土壤f_(Fe-OC)数据,分析了其在不同土层、生态系统、气候带的分布格局和受气候、土壤、矿物因子的调控机制.结果表明:(1)陆地生态系统土壤f_(Fe-OC)平均为21.9%,且深层土f_(Fe-OC)(37.5%)显著高于表层土(15.4%,P<0.01).(2)土壤平均f_(Fe-OC)在不同生态系统表现为:湿地(24.5%)>草地(16.2%)>森林(14.9%)>农田(14.8%),贫氧生态系统(24.2%)显著高于有氧生态系统(15.7%,P<0.01).土壤平均f_(Fe-OC)在不同气候带呈现出热带/亚热带(23.7%)>寒带(21.9%)>温带(20.2%)>高原带(16.6%)的规律.(3)陆地生态系统土壤f_(Fe-OC)主要受土壤深度的影响,且不同土层的调控机制不同:表层土f_(Fe-OC)主要受无定形态铁和有机络合态铁含量的影响,而深层土f_(Fe-OC)主要受土壤pH和土壤质地(黏粒、粉粒和砂粒含量)的影响.本研究结果为进一步认识陆地生态系统黏土矿物对土壤有机碳的保护机制提供了理论依据.

Soils are the largest terrestrial organic carbon pool,containing more organic carbon than vegetation and the atmosphere combined.Therefore,a small change in soil organic carbon(SOC)can have a dramatic influence on the concentration of atmospheric carbon dioxide.Mineral-association(e.g.,interaction with iron-(Fe-)oxides)has been demonstrated to be a key mechanism underlying long-term SOC persistence.Moreover,increasing evidence suggests that Fe oxides are more widely distributed than aluminum oxides(particularly in tropical and subtropical regions)in all kinds of soils worldwide and have the advantages of a higher surface area and a larger adsorption capacity.However,the contribution of Fe-bound organic carbon to the total SOC pool(f_(Fe-OC))in terrestrial ecosystems and its geographical pattern and underlying mechanisms are not fully understood.We hypothesized that(1)soil depth was the primary driver of the f_(Fe-OC)in terrestrial ecosystems,with higher values observed in subsoil than in topsoil,and that(2)the soils in oxygen-deficient ecosystems would have a higher f_(Fe-OC)than those in aerobic ecosystems.Here,we compiled 351 observations of f_(Fe-OC)across terrestrial ecosystems from 24 published articles to assess the f_(Fe-OC)content across different soil depths,ecosystem types and climatic zones using Kruskal-Wallis and Wilcoxon rank-sum tests.We also used a partial least squares(PLS)model to distinguish the relative importance of climatic,pedological,and mineral factors in explaining the geographical distribution of f_(Fe-OC)in bulk soil,topsoil,and subsoil,respectively.We found that the f_(Fe-OC)was,on average,21.9% across terrestrial ecosystems but varied greatly at different soil depths,with average values of 15.4%in topsoil and 37.5% in subsoil.The f_(Fe-OC)varied greatly in different terrestrial ecosystems,following the order of wetlands(24.5%)>grasslands(16.2%)>forests(14.9%)>croplands(14.8%),with higher values observed in oxygen-deficient ecosystems(24.2%)than in aerobic ecosystems(15.7%).Moreover,the f_(Fe-OC)across climatic zones showed the order of tropical and subtropical(23.7%)>boreal(21.9%)>temperate(20.2%)>plateau(16.6%).Soil depth was a primary control of the f_(Fe-OC)in terrestrial ecosystems,and the underlying mechanisms were much different:The f_(Fe-OC)in topsoil was strongly controlled by the concentrations of amorphous and complex Fe oxides,but that in subsoil was primarily controlled by soil pH,texture(clay,silt and sand contents)and bulk density.Therefore,the f_(Fe-OC)in topsoil could be the result of the direct accumulation of Fe oxides in different forms,but that in subsoil was mainly derived from the complexation and coprecipitation of Fe oxides and SOC.Our findings provide fundamental support for the mineral protection of Fe oxides in long-term SOC persistence in terrestrial ecosystems and distinguish the differential mechanisms underlying Fe oxide-associated SOC between topsoil and subsoil.Future research should focus on the coupling of Fe oxides and other metals(e.g.,aluminum,calcium and manganese)in controlling SOC stabilization under the fluctuation of environmental change.