Interacting effects of urea and water addition on soil mineral‐bound phosphorus dynamics in semi‐arid grasslands with different land‐use history

Abstract

AbstractNitrogen (N) addition and precipitation increment can greatly influence soil phosphorus (P) dynamics, with much emphasis on total and available P, yet little is known about their interactive effects on soil mineral‐bound inorganic P (Pi) fractions under different historical land uses of grasslands and old fields. Thus, we compared (i) plant readily available P, (ii) less available Pi (sum of NH4F‐extractable Pi, NaOH‐Na2CO3‐extractable Pi and NaHCO3‐extracted Pi), (iii) refractory forms of Pi (NH4Ac‐extractable Pi, H2SO4‐extractable Pi and Na3(citrate)‐dithionite‐extractable Pi) and (iv) organic P under the same urea and water treatments in an old‐field grassland with a semi‐arid steppe. Soil total P remained unchanged with 10‐year urea addition in both sites, with lower organic P but higher Pi concentrations in the old field. Urea addition promoted transformation of refractory Pi into less available Pi in both sites, which potentially was related to higher plant productivity and thus enhanced plant P accumulation. Specifically, urea addition increased less available Pi fractions by as much as 42%, but decreased refractory Pi fractions by as much as 24%, for two sites under ambient precipitation. Water addition decreased less available Pi in the steppe under higher urea addition rates, whereas it increased less available and refractory Pi but only in the control plot of the old field. Irrespective of urea and water treatments, the old field had a higher Pi pool, which could replenish plant readily available P in the longer term and be less prone to P‐limitation than the steppe. We conclude that conversion of semi‐arid steppe to farmland causes long‐term increases in the soil Pi pool, possibly by enhancing organic P mineralization and anthropogenic fertilization during cultivation. Urea addition accelerated soil P cycling via promoting refractory Pi, transforming into less available Pi, with the process being strongly mediated by water availability, whereas the projected precipitation increment could decrease less available Pi via promoting plant P uptake and P leaching out of the plant–soil system.Highlights Urea addition promoted transformation of refractory Pi into less available Pi fractions. Water addition counteracted the positive urea effect on NH4F‐Pi in both steppe and old field. Less responsive organic P suggested that Pi fractions were more essential in soil P cycling. Water addition decreased NH4F‐Pi in both sites, potentially due to higher plant uptake and leaching. Conversion of steppe to farmland increased the soil Pi pool by enhancing organic P mineralization.