Changes in soil organic matter and phosphorus fractions under planted fallows and a crop rotation system on a Colombian volcanic-ash soil

Planted tree or shrub fallows can help increase the fertility of degraded t ropical soils. We investigated the effects of planted fallows of Indigofera (IND), Calliandra (CAL), and Tithonia (TTH); a natural, unmanaged fallow ( NAT); and a maize/bean rotation (ROT) on the dynamics and partitioning of s oil organic matter (SOM) and phosphorus (P). One year after treatment, samp les were collected from a fine-textured volcanic-ash soil (Oxic Dystropept) of a mid-altitude hillside in southwestern Colombia. The SOM in the sand-s ize fraction (150-2000 mum) was subdivided into light (LL), intermediate (L M), and heavy (LH) fractions. Total soil P was also fractionated into inorg anic (P-i) and organic (P-o). Of the planted fallows, TTH most increased an d NAT least increased plant-available P-i and P-o. The amounts of C, N, and P in the LL and LM fractions of SOM followed the order, TTH > CAL > NAT > ROT > IND and CAL > TTH > IND > NAT > ROT, respectively. Total amounts of N , P, K, Ca, and Mg in the soil were significantly (P < 0.05) highest under TTH and lowest under NAT. The fallow and ROT systems did not affect the C/N , C/P, and N/P ratios in the soil but significantly did so in the LL and LM fractions of SOM. Significant correlations indicated that the P content in the LL and LM fractions of SOM may help determine the amounts of NaHCO3-ex tractable P-i and P-o, which may therefore serve as sensitive indicators of 'readily available' and 'readily mineralizable' soil P pools, respectively , in the volcanic-ash soils of the Andes.