Changes in soil inorganic phosphorous pools along a precipitation gradient in northern Iran

This study was undertaken to develop our knowledge relating to the soil inorganic phosphorous (P) pools along a precipitation gradient in calcareous forest soils. The precipitation gradient at Mazandaran province, northern Iran, reflected in the three soil moisture regimes (SMRs) e.g. xeric, ustic and udic SMRs that differed in their annual precipitation (550, 780, and 1250 mm y-1, respectively). The representative soil profiles on the same slope position were selected for the three studied SMRs and the soil samples were collected from the genetic horizons to determine some soil characteristics and soil inorganic P (Pi) pools namely, Ca2-P, Ca8-P, Ca10-P, Al-P, Fe-P, and Occluded-P. Increasing available water balance along the precipitation gradient induced greater weathering intensity across this climosequence leading to the pH and EC decline, organic matter accumulation, carbonate loss, and clay formation. Olsen-P and Ca2-P which are considered as readily available for plants, constitutes a small fraction of total soil Pi for all soils studied and its amounts decreased with depth within all sites. The distribution of soil P fractions across the gradient suggested decreasing Ca-bound P and relatively increasing amounts of occluded P with increasing rainfall. The total Pi was decreased appreciably at udic SMR compared with xeric SMR. The maximum total Pi and Ca-bound P occurred in the upper part of the secondary calcium carbonate horizons when such a zone was present in soils. With increasing precipitation the measured amounts of the Ca8-P, as pedogenic Ca, and Ca10-P, as primary mineral P (mainly apatite P), decreased about 20 and 2.5 times from the xeric to udic SMR, respectively. Overall, the results herein strengthen our understanding of Pi transformations during pedogenesis along precipitation gradient in northern Iran and provide important insight into P pools distributing within soil solum at different observable SMRs in these regions.