Dynamics of fractionated P and P budget in soil under different land management in two Tanzanian croplands with contrasting soil textures

Phosphorus deficiency is one of the largest constraints to crop production in tropical African soils owing to low native soil P and the high P fixation capacity of highly weathered soils. To achieve better soil P management, it is necessary to evaluate the effects of land management on soil P dynamics and P budget. We conducted a 4-year cultivation experiment in two dry tropical croplands in Tanzania at two sites with contrasting soil textures, viz. clayey and sandy. We then evaluated the effect of different types of land management (control plot, plot treated with crop residue only, plot treated with fertilizer only, and plot treated with both crop residue and fertilizer) on soil fractionated P and P budget at both sites. At the end of the experiment, fertilizer application (50 kg P ha−1 year−1) clearly increased NaOH-extractable inorganic P by 66.0–82.6 kg P ha−1 (soil depth 0–15 cm) at the clayey site and was equivalent to 33.0–41.3% of applied P fertilizer. However, fertilizer application did not clearly increase NaOH-extractable inorganic P at the sandy site owing to its low content of Al and Fe oxides. On the other hand, the effect of crop residue application (7.5 Mg C ha−1 year−1) on fractionated soil P was scarce, even with fertilizer application. Thus, applied crop residue increased soil C but did not significantly increase any P fractions, even with substantial P fertilization. On the basis of the P budget, we found that P was substantially unaccounted for only in the fertilizer treatment plots, i.e., 59.5–89.1 and 121.3–138.2 kg P ha−1 (soil depth 0–15 cm) at clayey and sandy sites, respectively. Our results indicate that substantial P loss, possibly through leaching, seemed to have occurred at the sandy site in particular. Therefore, not only P fixation but also P loss from leaching of applied P should be reduced to achieve better P management in this region.