Arrow indicates location of this research project near Tarapoto, Peru, at Instituto de Cultivos Tropicales (above). A local cacao-agroforester in the region (below) sells her harvest to a local chocolate factory. Background Information The soils of the Peruvian Amazon have undergone excessive weathering of primary mineral phosphorus (P), resulting in the depletion of the most plant available forms, hindering plant productivity. Local people are dependant on agricultural productivity for food and economic securities, and there is no access to chemical fertilizers. Alternatives to P fertilizers are sought. Long-term growth of cover crops are known to increase P solubility (1,2). I examined the effects of short term cover-crop cultivation on P solubility and the relationship between extracted soil P pools and cover-crop tissue P content. Objective To assess the effects of cover cropping on bioavailable and total soil P pools in a Peruvian cacao agroforestry system. Bioavailable soil P pools are here defined as those that significantly correlate with cover- crop tissue P content. Hypothesis Cultivation of leguminous cover crops result in an increase of bioavailable soil phosphorus pools. Experimental Design Three randomized blocks of six treatments embedded in a two year old cacao agroforestry system ( 45 m x 10 m per rep.). Cover crop species: 1. Arachis pintoi 2. Calopogonium mucunoides 3. Canavalia ensiformis 4. Centrosema macrocarpum 5. Fertilized fallow Control: 6. Unmanaged fallow Soil Sampling Soil samples were collected from random locations. Ten samples at four depths collected from each treatment, per block. Soils were air-dried, cleaned of roots, and passed through at 2 mm sieve prior to analysis. Effects of cover cropping on phosphorus solubility in a Peruvian cacao agroforestry system Results Discussion and Future Research Soil Phosphorus Pool Extraction Mehlich I extraction of bioavailable soil phosphorus was conducted on all soil samples (3). Soil P Fractionation methods established by Hieltjes and Lijklema were preceded by a deionized water extraction (4). P fractions were sequentially extracted by shaking soil in: H 2 O→ 1M NH 4 Cl → 0.1M NaOH → 0.5M HCl ↓ ↓ Kjeldahl Digest 6M HCl Digest Cover-crop tissue P was extracted by combustion of plant material followed by spectrophotometry. Extracted Inorganic P (P i ) methods by Murphy and Riley (5). Statistical Analysis Determinations of significant treatment affects were made using ANOVA. Significant F values (α=0.05) were compared using a post hoc Dunnett’s Test to determine treatment significance, compared to the Control. Box and Cox transformations were used to normalize data. Significance and assumption testing of linear regressions were performed using JMP ® statistical software. Power analysis was conducted using SigmaPlot ® statistical software. Hollie Hall 1, Yuncong Li 1, Nicholas Comerford 1, Hugh Popenoe 1, and Virupax Baligar 2 1 Soil & Water Sciences Department, University of Florida; 2 USDA-ARS Introduction Methods Discussion Cover crop effect on P pools. After two years of cover crop cultivation the 0.5M HCl extracted Pi pool significantly decreased in three of the cover crop treatments. Likely, the ‘lost’ Pi is currently bound up in the plant tissue. Correlations between cover crop and soil extracted Pi are useful in elucidating the soil Pi pools and chemical extractants that are represent that which is available for plant uptake. Future Research Continue monitoring of soil phosphorus pools to assess long term effects of cover cropping on P solubility. Use citric acid to repeatedly extract soil P until exhaustion. Regress the quantity of P extracted with citric acid and those extracted by the sequential fractionation to further clarify the plant available soil P pools. Analyze agroforestry species biomass for phosphorus content and use statistical analyses to determine significant correlations between plant and soil phosphorus pools. Acknowledgments Advisory Committee: Dr. Li, Dr. Comerford, Dr. Popenoe, Dr. Baligar. Funding: University of Florida, Latin American Studies Research Grant, Tinker Grant. Field Research Assistance: Everyone at Instituto De Cultivos Tropicales. Laboratory Assistance: Aja-Marie Stoppe, Yun Qian, Guingin Yu. Administrative Support: Jordan Mayor, Rhiannon Pollard, Dr. Dunne. Figure 1. Correlations between cover crop tissue [P] and extractable soil pools of P revealed significant relationships between cover crop P content and Mehlich I soil extractable Pi in the 0-15 cm depth (R 2 = 0.41, P = 0.026), as well as between cover crop tissue P content and H 2 O soil extractable Pi in the 0-5 cm depth (R 2 = 0.44, P = 0.018). Soil Phosphorus Pools Soil P Fractionation revealed significant decreases in the HCl extracted Pi pool for the C. mucunoides and C. ensiformis treatments relative to Control (11.50, 10.31, vs ug/g). NH 4 Cl extracted significantly more P i from the Fertilized treatment relative to Control (0.65 vs ug/g). Significant differences occurred in only the 0-5 cm soil horizon (Table 1). Linear Regressions of cover crop tissue P with Mehlich I and Water extractable soil Pi (0-15 cm and 0-5 cm depth respectably) revealed significant correlations. Linear regression of cover-crop [P] with Mehlich I and H 2 O extractable Pi were significant (Fig 1). 1. Dinesh, R., et al., Long-term influence of leguminous cover crops on the biochemical properties of a sandy clay loam Fluventic Sulfaquent in a humid tropical region of India. Soil and Tillage Research, : p Li, L., et al., Diversity enhances agricultural productivity via rhizosphere phosphorus facilitation on phosphorus-deficient soils. PNAS, (27): p Kuo, S., Extraction with Dilute Concentration of Strong Acids, in Methods in Soil Analysis. Part 3. Chemical Analysis. 1996, Soil Society of America and American Society of Agronomy: Madison. p Hieltjes, A.H.M. and L. Lijklema, Fractionation of Inorganic Phosphates in Calcareous Sediments. Journal of Environmental Quality, (3): p Kuo, S., Ascorbic Acid Method, in Methods of Soil Analysis. Part 3. Chemical Methods. 1996, Soil Science Society of America and American Society of Agronomy: Madison. p