Phosphorus cycling within soil aggregate fractions of a highly weathered tropical soil: A conceptual model

Effective use of soil phosphorus (P) for crop production requires an understanding of how P pools are stabilized and cycled within soil aggregates, rather than assuming that P dynamics, particularly organic P, closely follow those of C. The main goal of this study was to compare C and N cycling with P dynamics in soil aggregate fractions under two distinct crop species, maize (Zea mays) and pigeon pea (Cajanus cajan) in a highly-weathered Lixisol. We found that while C and N follow an open cycle, whereby C and N are mineralized from microaggregates during macroaggregate turnover and partially exit the soil system as gas and leachate, P has a relatively closed cycle, where most of the mineralized and solubilized P from microaggregates is lost from the plant-available pool via sorption to the unaggregated silt and clay-sized particles (<53 μm). While the above postulated P cycling mechanisms were the same for maize and pigeon pea, P loss from microaggregates and subsequent enrichment of the silt and clay particles was significantly higher in soils under maize compared to pigeon pea (320 and 331 mg P kg−1 lost from occluded microaggregates and gained by free silt and clay particles, respectively, compared to 129 and 97 mg P kg−1 under pigeon pea). This is attributed to the significantly increased soil aggregation under pigeon pea, which led to greater accumulation of P, particularly organic P, in the free microaggregates (77 mg P kg−1 compared to 29 mg P kg−1) and slower rates of macroaggregate turnover. We conclude that increasing soil aggregation can substantially reduce organic P losses from aggregate occluded fractions and its subsequent sorption as inorganic P to silt and clay particles. Thus, P cycling can be improved in tropical cropping systems on highly-weathered soils by introducing crop species that enhance the occlusion of organic P into aggregates.