David Sotomayor-Ramírez

Short-term tillage practices on soil organic matter pools in a tropical Ultisol

In tropical regions, pasture establishment involves tillage operations. Adoption of conservation tillage practices could result in lower costs and in improved soil quality by decreasing soil organic carbon (SOC) losses. This study investigated the effects of 3 tillage practices on the establishment of Brachiaria decumbens and on the total SOC and soil organic nitrogen (SON) content and its fractions in an Ultisol from the humid mountain zone of Puerto Rico that was previously under pasture. The treatments evaluated were no-tillage, minimum tillage, and conventional tillage (CT). At 120 days after planting (DAP), plant cover and density was improved in the CT treatment compared with the other treatments. At 180 DAP, there were no significant differences in the SOC, SON, aggregate size distribution, distribution of C within aggregate size classes, and labile C physical fractions among tillage treatments. Approximately 60% of the total SOC associated with aggregates was found within macroaggregates. About an equal proportion of the particulate organic matter (POM) was associated within aggregates and nonaggregate-protected free light fraction, and these were not affected by tillage management. Lower amounts of C mineralised after disruption of macroaggregates containing POM with high C/N ratio was probably due to immobilisation of the more labile protected C (iPOM). Labile forms of C were greater in macroaggregates than in microaggregates, yet comprised a lower proportion of total SOC, suggesting that macroaggregates have a greater proportion of C physically protected from microbial attack. The results indicate that there are no short-term changes in the tendency of the soil to lose C and N as a result of tillage practices for the establishment of pastures in this soil.

Phosphorus soil tests for environmental assessment in subtropical soils

Abstract Soil test phosphorus (P) is used to evaluate the nutritional status of a soil in relation to a crops yield response. Recently, there is interest to calibrate agronomic soil tests to predict risk of P transport in runoff. An experiment was conducted to evaluate the relationships among three agronomic soil test P extraction methods (Olsen, Bray1, Mehlich3) and dissolved P in water (0.01 M CaCl2 extractable P) in five representative soils of tropical and subtropical areas in Puerto Rico and Florida. The soils were of the series Astatula (Entisol), San Antón (Mollisol), Caguabo (Inceptisol), Corozal (Ultisol), and Bayamón (Oxisol). Soils were amended with three inorganic P levels as triple super phosphate, three organic P levels as broiler litter, and their combinations in an incomplete factorial design for a total of 11 treatments. Soils were sampled during 46 weeks and soil test P was measured. Exploratory statistics were used to evaluate relationships among the variables. Relationships using the Astatula soil were studied separately as inherently high soil test P values influenced regressions greatly. A near 1:1 relationship between soil test P extracted using Bray1 and Mehlich3, and Mehlich3 and Olsen extractants was obtained. In general, improved relationships were obtained using individual soils as soil pH and mineralogy influenced the amount of P extracted. Critical soil test concentrations to reach dissolved P concentrations of 1 mg P L−1 were (95% confidence intervals in parenthesis) 179 (174–185), 197 (190–207), and 252 (243–266) ppm for Olsen, Bray1, and Mehlich3 extractants, respectively for all soils combined excluding Astatula. The critical values obtained in combination with soil-landscape information can be used to establish guidelines for P management in agricultural soils.

Limnological assessment of two reservoirs in Puerto Rico

Dissolved oxygen (DO) is essential to the metabolism of freshwater lentic ecosystems in all climates and latitudinal gradients (WETZEL 2001). Low aquatic DO concentrations and anoxia directly influence fish population dynamics, algal community structure, and biogeochemical cycling of nutrients and metals. While many general limnological principles related to DO dynamics are transferable among latitudes, management of tropical lakes for intended uses must be approached in a different manner than temperate counterparts (LEWIS 2000). Some features related to DO concentrations leading to specific management implications for tropical lakes are high hypolimnetic temperatures, long thermal stratification season, recurrent changes in mixed layer thickness, and shifts in nutrient limitations from phosphorus (P) to nitrogen (N) (TOWNSEND 1998, DOWNING et al. 1999, LEWIS 2000, 2002). Persistent thermal stability occurs in tropical lakes of moderate depth, but seasonal mixing occurs coincident with hemispheric winter and interannual mixing due to wind exposure, rainfall, and hydraulic inflow (LEWIS 1987). Alternate recurrent changes in the mixed layer promote the accelerated cycling of nutrients between the deeper nutrient rich waters and phytopolankton growth in the upper water column (LEWIS 2000). Hence, tropical lakes have high phytoplanktonic response to nutrients. Other factors that enhance nutrient and organic enrichment of deep waters are the duration of thermal stratification, decreased DO solubility, and high microbial reaction rates. The result is that most tropical reservoirs, even oligotrophic reservoirs, commonly experience temporal hypolimnion anoxia (MACKINNON & HERBERT 1996, TOWNSEND 1999, LEWIS 2000). The tropical river-lakes and reservoirs of the Caribbean are of prime economic and social importance for the annual flux of tourists and the 37 million inhabitants. Water chemistry and phytoplankton abundance surveys have been conducted in reservoirs of Puerto Rico (CANDELAS-REYES 1956, JOBIN et al. 1979, QUINONES-MARQUEZ 1980,GARCIA & TILLY 1982), but none have dealt with extent of stratification and relationship with DO. Low DO concentrations have been identified as the cause of impairment in 18 of the principal reservoirs of Puerto Rico, with sediments and nutrients identified as the sources (PREQB 2003). Surface water quality data suggest that the majority are eutrophic according to the Carlson trophic state index (MARTINEZ et al. 2005). The whole-column DO Puerto Rico criteria of 5 mg/L may need to be revised based on the natural functioning of tropical reservoirs. The objective of this study was to assess the limnological conditions of two “minimally impacted” reservoirs of Puerto Rico. The principles derived from the data can serve as baseline information for development of criteria levels and future lake management.

Fertilizer-nitrogen Management in an Onion and Tropical Pumpkin Rotation in Puerto Rico

Onion (Allium cepa) and tropical pumpkin (Cucurbita moschata) combined contribute 13% of the total gross agricultural income (GAI) for vegetable crops in Puerto Rico, which is estimated at

Phosphorus in runoff from two highly weathered soils of the tropics

54.5 million. Both crops are usually rotated on an annual basis. In this study, an onion-tropical pumpkin rotation was used to test the effect of fertilizer-nitrogen (N) on agronomic indicators of onion (plant height, number of leaves per plant, leaf color index, and leaf nutrient concentration), yield of both onion and tropical pumpkin, and inorganic N changes in the soil profile. Three fertilizer-N levels (140, 196, 252 kg ha) were applied to onion, followed by 112 and 280 kg ha of N applied to tropical pumpkin. For tropical pumpkin, N was applied in plots with the lowest and highest fertilizer-N levels from the previous onion crop. Changes in onion agronomic indicators with increasingN fertilizationwere either not significant or showed no clear trend. There was no increase in total andmarketable yields and number of onions with increasing fertilizer-N levels. Tropical pumpkin yields significantly increased with 280 kg ha compared with 112 kg ha of N. Using 112 kg ha as a baseline fertilizer-N application, the value/cost ratio for tropical pumpkin was

Phytoplankton dynamics of the Guajataca reservoir, Puerto Rico

12.70 per dollar of fertilizer-N. In low fertilizer-N plots, immediately available inorganic soil N (0 to 30 cm) did not change between the onion and tropical pumpkin crop, but then decreased at the end of the rotation. In high fertilizer-N plots, immediately available soil N greatly increased after onion, but then decreased at the end of the rotation. Potentially leachable soil N (30 to 100 cm) also increased after the onion crop and then decreased after pumpkin. However, in high fertilizer-N plots, potentially leachable soil N remained 44% higher at the end, compared with the beginning, of the rotation. The increased income attainable with the highest fertilizer-N in tropical pumpkinmay be offset by greater residual soil N in the lower part of the soil profile, and the potential for this N to have a negative environmental impact.

Impact of Coffee Management Practices on Earthworm Populations in Puerto Rico

Ramírez-Ávila, J. R., Sotomayor-Ramírez, D., Martínez-Rodríguez, G. A. and Pérez-Alegría, L. R. 2011. Phosphorus in runoff from two highly weathered soils of the tropics. Can. J. Soil Sci. 91: 267-277. Agricultural fields with high soil phosphorus (P) content are important contributors to surface water degradation. Two consecutive simulated rainfall events were conducted on two Ultisols previously amended with inorganic P fertilizer or broiler litter. Soil test P (Bray 1 and Olsen) levels evaluated ranged from 1 to 350 mg kg-1. Surface runoff concentrations of total P (TP) and dissolved P (DP) generated by a 30-min runoff event were quantified. Runoff DP concentrations ranged from 0.08 to 3.98 mg L-1 in fertilizer P-amended soils and from 0.08 to 4.93 mg L-1 in broiler litter-amended soils. A single exponential model adequately described the relationships between soil test P and DP concentrations in runoff. For each soil, the soil test P-DP concentration relationships were positively influenced by soil organic matter and negatively influenced by antecedent soil moisture (P<0.05). For both soils, the soil test P-DP concentration relationships were positively influenced by groundcover percentage and negatively influenced by slope. Environmental soil test P critical levels corresponding to a runoff threshold of 1 mg L-1 DP, ranged between 176 and 296 mg kg-1 (Olsen) and 143 to 276 mg kg-1 (Bray 1) in soils amended with fertilizer-P. In broiler litter-amended soils, threshold values were 88 and 111 mg kg-1 using Olsen and Bray 1, respectively. Differences in surface runoff-P concentrations due to amendment sources, antecedent soil moisture content, soil organic matter, groundcover and slope suggest that these factors need to be considered in P management decisions at the farm level.

SOIL FERTILITY EVALUATION OF COFFEE (Coffea spp.) PRODUCTION SYSTEMS AND MANAGEMENT RECOMMENDATIONS FOR THE BARAHONA PROVINCE, DOMINICAN REPUBLIC

Tropical reservoirs frequently suffer from high rates of sediment and nutrient loads (SALAS & MARTINO 200 l, WETZEL 200 l). Alloctonous nutrient inputs through runoff largely control the entrance o f nutrients and other dissolved and particulate materials; however, interna! cycling during mixis is a mechanism for the vertical transfer and exchange of nutrient through the water column. Seasonal changes in precipitation and thermal regimes are important driving factors for water column stratification and mixing pattems in the tropics (HORNE & ÜOLDMAN 1994). The excessive input of nutrients influences the structure and dynamics ofthe phytoplankton community by increasing primary production and altering species composition (CHALAR 2006). Hence, assessing the phytoplankton assemblage of a reservoir can be used as an indicator of its trophic state and overall ecological integrity (REYNOLDS 1986). There is limited information of phytoplankton diversity and temporal dynamics in tropical zones (LEWIS 1978, TuNDISI 1986, KALF & WATSON 1986, ÜONZÁLEZ & ÜRTAZ 1998), and in reservoirs ofPuerto Rico (CANDELAS-REYES 1956, JOBIN et al. 1979, ÜUINONES-MÁRQUEZ 1980, ÜARCÍA & TILLY 1982). The purpose of this research was to characterize the spatial, temporal, and vertical dynamics of the phytoplanktonic community and its relation with the water column physico-chemical characteristics.

Benthic algal diversity and biomass as indicators to establish reference streams in Puerto Rico

Abstract The conditions under which coffee is grown may affect its long-term sustainability through effects on soil fauna, including earthworms, which have a profound effect on abiotic and biotic properties and processes in natural and agricultural ecosystems. We examined differences in earthworm populations among sun (SUN) and shade (SHD) coffee and secondary forest (FOR) ecosystems, as well as soil order (Oxisols, Ultisols, Inceptisols) in western-central Puerto Rico. Ecosystem type significantly affected earthworm population density (individuals/m2), which followed the order: SUN (281) > SHD (125) = FOR (37). Earthworm biomass (g fresh weight/m2) was also significantly affected by ecosystem type, following the order: SUN (71) > SHD (34) = FOR (12). In general, neither earthworm biomass nor abundance was significantly related to litter quantity, litter composition or soil properties for any of the ecosystems studied. The absence of significant differences between SHD and FOR ecosystems suggests similarity in those environmental variables that constrain earthworm populations. The larger earthworm populations in coffee cultivated under sunlight may indicate that earthworms play a more important role in organic matter translocation and decomposition, nutrient cycling, and soil physical structure than in shade coffee or forest ecosystems.

Grass filter strips evaluation for reducing sediment and nutrient exportation from grasslands under manure applications in Puerto Rico

Reported yields in most coffee farms of the Barahona province in the Dominican Republic are relatively low (< 290 kg ha -1 parchment coffee). In general, coffee producers do not use diagnostic techniques such as soil testing. This fact prevents them from identifying the limiting factors (especially nutrients), complicates the work of coffee cultural management practices, and potentially reduces productivity and coffee quality. This study was designed to diagnose the fertility level of soils in coffee farms in the area of Barahona in 96 farms within an area of 637 hectares and design a nutrient management strategy. Soils from each farm were sampled and analyzed for soil fertility parameters. A survey was provided to farmers that permitted the collection of information regarding yields, management practices, and landscape features. Soils in the area were predominantly clayey. Soil pH varied between 4.61 and 7.69 and soil organic matter ranged between 3.29 and 10.9%. Exchangeable potassium levels were classified as deficient in all areas. The clustering of results identified two main components, which accounted for 76% of the variability of the data and the grouping into five communities by similarity of features. The results show that soil testing of this coffee coffee-growing region can be used as a tool to diagnose the soil fertility status and guide them in implementing management and fertilization recommendations.