Phosphorus fractions and phosphatase activities on sandy soils in a temperate savanna and in a neighboring pine plantation.
Zeng, Dehui1, Zhao, Qiong1, 2, 1 Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, Liaoning, China2 Graduate School, Chinese Academy of Sciences, Beijing, China
ABSTRACT- It is known that shifting land-use alters soil chemical properties, such as phosphorus (P) status. In this study, we investigated P fractions and phosphatase activities in two soil horizons (0-5 cm and 5-20 cm) under a savanna and an adjacent 30-year-old Mongolian pine (Pinus sylvestris L. var. mongolica Litv.) plantation which was established in a savanna in the semi-arid sandy soil of northern China. In two ecosystems, we found that soil total P concentration was 67.5-244.4 g g-1 across two soil horizons, of which the labile inorganic P and organic P accounted for only 0.3%-1.6% and 1.7%-4.5%, respectively; microbial biomass P less than 2.5%, and soil organic P 41.0%-70.7%. The ratio of soil organic carbon to organic phosphorus (C:TPo) ranged from 102.8-210.8. The concentration of inorganic P fractions followed the order: calcium bound P (Ca-P) > iron bound P (Fe-P) > aluminum bound P (Al-P) > occluded Fe/Al bounded P (O-P) at both sites. Phosphatase activities were very high, with the highest acid phosphatase activity of 40.1 mg p-NPP g-1 soil h-1. Concentrations of all soil P fractions and phosphatase activities in the surface soil layer in both sites were higher than those in the deeper soil layer except that labile organic P under plantation was constant. In contrast to savanna, we found lower soil phosphatase activities and lower concentrations of all P fractions (with an exception of Al-P), less vertical variation of these variables within soil profile, lower proportion of organic P and Ca-P in total P, and higher proportion of labile P, and Al-P and Fe-P in pine plantation. Taken together, our results suggested that surface mineral soil is the most active layer of P cycling and P recycled through litterfall is the most important supply of soil P. Net mineralization occurring at both sites and mediated by acid phosphatase mainly excreted by plant roots is essential for P cycling. Under the plantation, organic P and Ca-P are sources of bioavailable P, and AL-P and Fe-P are two main forms of labile inorganic P. Mongolian pine plantation increased the bioavailability of both organic P and Ca-P, but substantially reduced long-term P supply potential, which indicates that P fertilization is necessary to maintain the long-term functioning of Mongolian pine plantation.
Key words: P cycling, P bioavailability, sandy soil, afforestation
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