Changes In Soil Pool Capacity For Lead In Response To Conversion Of Rainforest To Rubber Plantations In Hainan Island, China

Land degradation caused by deforestation seriously affects the soil environmental capacity (SEC), with potential risks to soil health. However, conventional SEC theory is unable to quantitatively describe the saturation capacity of different soil fractions and environmental pollution levels of a heterogeneous system. Thus, a new concept, thesoil pool capacity(SPC), was introduced to fill this gap. We undertook space-for-time substitution to investigate the responses of SPCand soil safety capacity (SSC) to changes in the soil physiochemical properties, chemical species, and molecular characteristics of soil lead (Pb) in the conversion of tropical secondary rainforest (TSR) to rubber (Hevea brasiliensis) monoculture plantations with 15- and 60-year histories (RP15/60). Conversion of TSR to RP(15/60)caused adverse effects on soil properties (i.e., lower soil organic matter [SOM] and higher bulk density [BD]). The saturated SPC (SPCsat) for Pb in bulk soils under RP15(2,203.17 g m(-2)) and RP60(1,634.09 g m(-2)) decreased compared with that in TSR (2,227.10 g m(-2)). The contribution rates of SPC(sat)in the exchangeable (P1) and carbonate (P2) pools to the total capacity stabilized at approximately 94%. The contribution rate of potential SPC(sat)in P4 (OM-bound Pb) significantly decreased with increasing agricultural intensity. The SSC of labile fractions (P1 + P2) accounted for 81-85% (a critical level) of the total SSC when the soil loading capacity for Pb reached the SSC in bulk soil regardless of land-use type. Fourier transform infrared and X-ray diffraction spectra revealed the Pb-loaded species of SPC(sat)in TSR and RP15/60(i.e., TSR: Pb(2)SiO(4)in P2, (CH3COO)(2)Pb in P4, RP15/60: Pb(5)Si(8)O(21)and Pb-2(P4O12) in P2, and (CHOO)(2)Pb in P4). Correlation and factor analyzes showed that SOM and BD were key factors that had an opposite impact on SPC(sat)and SSC. These results are helpful to choose appropriate agronomic measurements to reduce land degradation and crop safety risks caused by deforestation.