Exogenous phosphorus compounds interact with nitrogen availability to regulate dynamics of soil inorganic phosphorus fractions in a meadow steppe

Here we investigated the effects of P compounds (KH2PO4 and Ca(H2PO4)(2)) with different addition rates of 0, 20, 40, 60, 80, and 100 kg P ha(-1) yr(-1) and NH4NO3 addition (0 and 100 kg N ha(-1) yr(-1)) on soil labile inorganic phosphorus (IP) (dicalcium phosphate, Ca-2-P), moderate-cycling IP, and recalcitrant IP fractions in a calcareous grassland of northeastern China. Soil moderate-cycling IP fractions, not readily available to plants but transforming into soil-available P quickly, include variscite (Al-P), strengite (Fe-P) and octacalcium phosphate (Ca-8-P); recalcitrant IP fractions include hydroxylapatite (Ca-10-P) and occluded P (O-P). Soil labile and moderate-cycling IP fractions and total P significantly increased with increasing P addition rates, with higher concentrations detected for KH2PO4 than for Ca(H2PO4)(2) addition. Combined N and P treatments showed lower soil labile IP and moderate-cycling IP fractions compared to ambient N conditions, due to enhanced plant productivity. Moderate-cycling IP was mainly regulated by P addition and plant P uptake to further enhance labile IP and total P concentrations with KH2PO4 and Ca(H2PO4)(2) addition. Soil labile IP was also directly and negatively affected by soil pH and plant P uptake with Ca(H2PO4)(2) addition. Ca(H2PO4)(2) addition significantly increased the soil recalcitrant IP (Ca-10-P) fraction, while KH2PO4 addition showed no impact on it. A significant positive correlation was detected between soil labile IP, moderate-cycling IP fractions and soil Olsen-P which illustrated that labile IP and moderate-cycling IP fractions were important sources for soil-available P. Our results suggest that moderate-cycling IP fractions are essential for grassland P biogeochemical cycling and the chemical form of P fertilizer should be considered during fertilization management for maintaining soil-available P.