Phosphorus Dynamics in a Highly Weathered Soil as Revealed by Isotopic Labeling Techniques

Isotopic labeling techniques have the potential to elucidate soil P dynamics and the fate of P sources added to the soil, but they have rarely been applied to highly weathered tropical soils. We collected soils from two crop rotations [continuous maize (COM; Zea mays L.) and maize‐crotalaria (MCF; Crotalaria grahamiana Wight & Arn.) fallow rotation] in a field experiment in Kenya and incubated them for 9 wk after addition of a plant residue or inorganic phosphorus (P i ), both labeled with 33 P and added at 6 mg P kg −1 soil, or after carrier‐free labeling of isotopically exchangeable soil phosphorus (soil IEP). The amount of P and recovery of 33 P were determined in resin‐extractable P i (P resin ), microbial P (P hex ), and in a 0.1 M NaOH extract of samples from which P resin and P hex had been removed. The P resin increased after addition of P i , while P hex increased after plant residue amendment, involving considerable microbial uptake of soil P. The recovery of 33 P in P resin followed the order added P i > soil IEP > plant residue, and decreased steadily from 7 to 22% after 1 d to 3 to 5% after 9 wk. The recovery of 33 P in P hex remained constant throughout the incubation, being greater after plant residue amendment (15%) than in the other two treatments (4–7%). An additional 66 to 76% of 33 P was recovered in the NaOH extract, as much as 27% of which was in organic phosphorus (P o ) after plant residue amendment and 2 to 8% in the other two treatments. Similar to P dynamics after plant residue amendment, the comparison of the two rotations indicated a shift toward P hex and P o with increasing microbial activity due to previous fallow biomass incorporation.