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★ [MIS03-02] Can soil properties alone predict ecosystem processes?
Soils are a reservoir of nutrients, which supplies plants with essential elements. Soil chemical properties can change spatially and temporary in relation to parent materials, climate and pedogenesis as a function of time. In places, the concentrations of essential elements in soils are extremely impoverished, which can eventually limit the net primary productivity of terrestrial ecosystems. Accounts by Elser et al. (2007), Vitousek et al. (2010) and many others indicate that available P is depleted in deeply weathered soils in the tropics due to a long process of geochemical occlusion and losses without substrate rejuvenation and that P limits the productivity of many tropical rain forests.
Colleagues and I have been extensively studying soil P fractions and productivity in Bornean tropical rain forests. Indeed, the concentrations of soil total P or labile P fraction are generally impoverished in comparison to temperate ecosystems but can still vary greatly reflecting parent materials or the status of pedogenesis. For instance, the concentrations of soil total P in seven tropical rain forests below 700 m asl in north Borneo are generally low, but range from 66 to 512 (μg/g). That of Bray-1 extractable P ranges from 0.2 to 4.2 (μg/g). Yet, above-ground net primary productivities of these forests are remarkably high and converge to a narrow range. Therefore, soil chemical properties do not correlate with productivity in plausibly P-limited tropical rain forests. Why soil P fractions cannot explain productivity?
We ecologists consider net primary productivity as a fundamental ecosystem process, which is expressed as the rate of net carbon fixation in an ecosystem context. Trees invest P for carbon fixation and the efficiency of the carbon fixation is expressed by the ratio of C flux to P flux, i.e. P-use efficiency in productivity. In these forests, P-use efficiencies greatly vary by 5-fold from 800 to 4000 (gC/gP) indicating that trees can adjust to the magnitude of P deficiency to maintain productivity. This is the major reason why soil P fractions cannot explain productivity.
Biological mechanisms to explain the enhancement of P-use efficiencies are two fold, one is the efficiency of photosynthetic C fixation per unit P in leaves and the other is the residence time of P in tree bodies. Colleagues and I investigated relative importance of these two mechanisms and found that the variation of residence time of P can much better explain the variation of P-use efficiency. Tropical trees increase the residence time of P in their bodies by increasing leaf life span and also by increasing P resorption from senescing leaves when facing to increasing P deficiency. We consider these are the two important plant traits which plants have acquired as adaptation. Our studies imply that understanding adaptive mechanisms as well as soil chemical properties is essential to understand ecosystem processes.
Colleagues and I have been extensively studying soil P fractions and productivity in Bornean tropical rain forests. Indeed, the concentrations of soil total P or labile P fraction are generally impoverished in comparison to temperate ecosystems but can still vary greatly reflecting parent materials or the status of pedogenesis. For instance, the concentrations of soil total P in seven tropical rain forests below 700 m asl in north Borneo are generally low, but range from 66 to 512 (μg/g). That of Bray-1 extractable P ranges from 0.2 to 4.2 (μg/g). Yet, above-ground net primary productivities of these forests are remarkably high and converge to a narrow range. Therefore, soil chemical properties do not correlate with productivity in plausibly P-limited tropical rain forests. Why soil P fractions cannot explain productivity?
We ecologists consider net primary productivity as a fundamental ecosystem process, which is expressed as the rate of net carbon fixation in an ecosystem context. Trees invest P for carbon fixation and the efficiency of the carbon fixation is expressed by the ratio of C flux to P flux, i.e. P-use efficiency in productivity. In these forests, P-use efficiencies greatly vary by 5-fold from 800 to 4000 (gC/gP) indicating that trees can adjust to the magnitude of P deficiency to maintain productivity. This is the major reason why soil P fractions cannot explain productivity.
Biological mechanisms to explain the enhancement of P-use efficiencies are two fold, one is the efficiency of photosynthetic C fixation per unit P in leaves and the other is the residence time of P in tree bodies. Colleagues and I investigated relative importance of these two mechanisms and found that the variation of residence time of P can much better explain the variation of P-use efficiency. Tropical trees increase the residence time of P in their bodies by increasing leaf life span and also by increasing P resorption from senescing leaves when facing to increasing P deficiency. We consider these are the two important plant traits which plants have acquired as adaptation. Our studies imply that understanding adaptive mechanisms as well as soil chemical properties is essential to understand ecosystem processes.