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

Nitrogen (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 (P-i) fractions under different historical land uses of grasslands and old fields. Thus, we compared (i) plant readily available P, (ii) less available P-i(sum of NH4F-extractable P-i, NaOH-Na2CO3-extractable P(i)and NaHCO3-extracted P-i), (iii) refractory forms of P-i(NH4Ac-extractable P-i, H2SO4-extractable P(i)and Na-3(citrate)-dithionite-extractable P-i) 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 P(i)concentrations in the old field. Urea addition promoted transformation of refractory P(i)into less available P(i)in both sites, which potentially was related to higher plant productivity and thus enhanced plant P accumulation. Specifically, urea addition increased less available P(i)fractions by as much as 42%, but decreased refractory P(i)fractions by as much as 24%, for two sites under ambient precipitation. Water addition decreased less available P(i)in the steppe under higher urea addition rates, whereas it increased less available and refractory P(i)but only in the control plot of the old field. Irrespective of urea and water treatments, the old field had a higher P(i)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 P(i)pool, possibly by enhancing organic P mineralization and anthropogenic fertilization during cultivation. Urea addition accelerated soil P cycling via promoting refractory P-i, transforming into less available P-i, with the process being strongly mediated by water availability, whereas the projected precipitation increment could decrease less available P(i)via promoting plant P uptake and P leaching out of the plant-soil system. Highlights Urea addition promoted transformation of refractory P(i)into less available P(i)fractions. Water addition counteracted the positive urea effect on NH4F-P(i)in both steppe and old field. Less responsive organic P suggested that P(i)fractions were more essential in soil P cycling. Water addition decreased NH4F-P(i)in both sites, potentially due to higher plant uptake and leaching. Conversion of steppe to farmland increased the soil P(i)pool by enhancing organic P mineralization.

Automated summary

Urea addition accelerates soil P cycling by promoting the transformation of refractory P(i) into less available P(i) fractions. Water addition counteracts the positive effect of urea on NH4F-P(i) in both the steppe and old field. The soil P(i) pool increases in farmland compared to steppe due to enhanced organic P mineralization.