Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved. Phosphorus Dynamics in a Highly Weathered Soil as Revealed by Isotopic Labeling Techniques E. K. Bu ¨ nemann, F. Steinebrunner, P. C. Smithson, E. Frossard, and A. Oberson* ABSTRACT cations of 50 kg P ha -1 yr -1 as triple superphosphate (Bu ¨ nemann et al., 2004b), but mineral P fertilizers are Isotopic labeling techniques have the potential to elucidate soil P often not economical or not available to small-scale dynamics and the fate of P sources added to the soil, but they have rarely been applied to highly weathered tropical soils. We collected farmers. Without any P fertilizer added, maize yield and soils from two crop rotations [continuous maize (COM; Zea mays P uptake can also be twice as great after a one-season L.) and maize-crotalaria (MCF; Crotalaria grahamiana Wight & planted fallow with legumes such as Crotalaria grahami- Arn.) fallow rotation] in a field experiment in Kenya and incubated ana (subsequently referred to as crotalaria) than after them for 9 wk after addition of a plant residue or inorganic phosphorus maize (Niang et al., 2002; Smestad et al., 2002; Bu ¨ ne- (P i ), both labeled with 33 P and added at 6 mg P kg -1 soil, or after mann et al., 2004b). This suggests improved P availabil- carrier-free labeling of isotopically exchangeable soil phosphorus (soil ity after incorporation of fallow biomass, which consists IEP). The amount of P and recovery of 33 P were determined in resin- of planted legume plus weed biomass. Measurable extractable P i (P resin ), microbial P (P hex ), and in a 0.1 M NaOH extract changes in the topsoil 1 yr after incorporation include of samples from which P resin and P hex had been removed. The P resin an increase in P o and P hex under maize-fallow rotations increased after addition of P i , while P hex increased after plant residue amendment, involving considerable microbial uptake of soil P. The compared with COM, while the availability of P i does recovery of 33 P in P resin followed the order added P i  soil IEP  not differ between crop rotations (Bu ¨ nemann et al., plant residue, and decreased steadily from 7 to 22% after 1 d to 3 to 2004b). In an incubation experiment with the same soils, 5% after 9 wk. The recovery of 33 P in P hex remained constant through- incorporation of crotalaria residues that added 14.2 mg out the incubation, being greater after plant residue amendment P kg -1 soil increased P hex by 5 to 8 mg P kg -1 soil within (15%) than in the other two treatments (4–7%). An additional 66 to the first week, while it decreased P resin by 0.3 to 0.4 mg 76% of 33 P was recovered in the NaOH extract, as much as 27% of P kg -1 soil (Bu ¨ nemann et al., 2004a). This suggests mi- which was in organic phosphorus (P o ) after plant residue amendment crobial immobilization of P from the plant residue as and 2 to 8% in the other two treatments. Similar to P dynamics well as from soil P pools, which in the absence of isotopic after plant residue amendment, the comparison of the two rotations labeling cannot be distinguished. indicated a shift toward P hex and P o with increasing microbial activity due to previous fallow biomass incorporation. Labeling of plant residues with the radioisotopes 32 P or 33 P has been used to investigate the recovery of P from plant residues in soil P pools and in growing plants (Dalal, 1979; McLaughlin and Alston, 1986; Friesen and L imited availability of P is often the main constraint Blair, 1988; McLaughlin et al., 1988a, 1988b; Daroub et for plant growth in highly weathered soils of the al., 2000). In the pot experiment of McLaughlin and tropics. A better understanding of soil P dynamics is Alston (1986), 65% of 33 P added with a labeled medic required to improve management practices in tropical (Medicago truncatula Gaertn.) residue was recovered agroecosystems. Soil P dynamics are characterized by in the microbial biomass, compared with 8% in the physicochemical (sorption–desorption) and biological aboveground wheat biomass after 34 d. Even the recov- processes (immobilization–mineralization). Walker and ery of simultaneously added calcium phosphate labeled Syers (1976) showed that the proportion of P o increases with 32 P was greater in the microbial biomass (29%) at later stages of soil development, especially in relation than in the shoots of wheat (10%). While such results to labile P i . Thus, the relative contribution of soil biolog- point to the importance of the microbial biomass in soil ical processes to deliver plant-available P may become P dynamics, these studies were restricted to temperate more important when the availability of P i is low. The soils. The partitioning of labeled P additions among soil decomposition of soil organic matter and plant residues P pools may differ substantially in highly weathered is indeed often the main source of plant nutrients in tropical soils with greater P sorption on sesquioxides. low-input small-scale farming systems of the tropics The radioisotopes 32 P or 33 P can also be added to soil (Gijsman et al., 2002). without simultaneous application of 31 P (i.e., carrier- On highly weathered soils in western Kenya, produc- tion of maize (Zea mays L.) is doubled by annual appli- Abbreviations: COM, continuous maize; ddH 2 O, double-distilled wa- ter; MCF, maize-crotalaria fallow rotation; NaOH-P i , inorganic phos- E.K. Bu ¨ nemann, Univ. of Adelaide, Soil and Land Systems, Glen phorus extracted with 0.1 M NaOH; NaOH-P o , organic phosphorus Osmond, SA 5064, Australia; F. Steinebrunner, E. Frossard, and A. extracted with 0.1 M NaOH; NaOH-P resin , phosphorus of 0.1 M NaOH Oberson, Inst. of Plant Sciences, Swiss Federal Inst. of Technology extracts recovered on resin membranes; NaOH-P t , total phosphorus Zurich (ETH), Eschikon 33, CH-8315 Lindau, Switzerland; P.C. extracted with 0.1 M NaOH; P fum , phosphorus extracted with resin Smithson, Berea College, CPO 2064, Berea, KY 40404, USA. Re- membranes in the presence of hexanol; P hex , hexanol-labile microbial ceived 8 Dec. 2003. *Corresponding author (astrid.oberson@ipw.agrl. phosphorus (difference between P fum and P resin corrected for sorption ethz.ch). of microbial P); P i , inorganic phosphorus; P o , organic phosphorus; P resin , inorganic phosphorus extractable with anion-exchange resin Published in Soil Sci. Soc. Am. J. 68:1645–1655 (2004).  Soil Science Society of America membranes; P t , total P; SA, specific activity; soil IEP, isotopically exchangeable soil phosphorus. 677 S. Segoe Rd., Madison, WI 53711 USA 1645 Published September, 2004