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Nitrogen (N) fertilizer use is rapidly intensifying on tropical croplands and has the potential to increase emissions of the greenhouse gas, nitrous oxide (N₂O). Since about 2005 Mato Grosso (MT), Brazil has shifted from single-cropped soybeans to double-cropping soybeans with maize, and now produces 1.5% of the world's maize. This production shift required an increase in N fertilization, but the effects on N₂O emissions are poorly known. We calibrated the process-oriented biogeochemical DeNitrification-DeComposition (DNDC) model to simulate N₂O emissions and crop production from soybean and soybean-maize cropping systems in MT. After model validation with field measurements and adjustments for hydrological properties of tropical soils, regional simulations suggested N₂O emissions from soybean-maize cropland increased almost fourfold during 2001–2010, from 1.1 ± 1.1 to 4.1 ± 3.2 Gg 1014 N-N₂O. Model sensitivity tests showed that emissions were spatially and seasonably variable and especially sensitive to soil bulk density and carbon content. Meeting future demand for maize using current soybean area in MT might require either (a) intensifying 3.0 million ha of existing single soybean to soybean-maize or (b) increasing N fertilization to ~180 kg N ha⁻¹on existing 2.3 million ha of soybean-maize area. The latter strategy would release ~35% more N₂O than the first. Our modifications of the DNDC model will improve estimates of N₂O emissions from agricultural production in MT and other tropical areas, but narrowing model uncertainty will depend on more detailed field measurements and spatial data on soil and cropping management. 

Abstract. The strong phosphorus (P) sorption capacity of iron (Fe)and aluminum (Al) minerals in highly weathered, acidic soils of humidtropical forests is generally assumed to be an important driver of Plimitation to plants and microbial activity in these ecosystems. Humidtropical forest soils often experience fluctuating redox conditions thatreduce Fe and raise pH. It is commonly thought that Fe reduction generallydecreases the capacity and strength of P sorption. Here we examined theeffects of 14 d oxic and anoxic incubations on soil P sorption dynamics inhumid tropical forest soils from Puerto Rico. Contrary to the conventionalbelief, soil P sorption capacity did not decrease under anoxic conditions,suggesting that soil minerals remain strong P sinks even under reducingconditions. Sorption of P occurred very rapidly in these soils, with atleast 60 % of the added P disappearing from the solution within 6 h.Estimated P sorption capacities were much higher, often by an order ofmagnitude, than the soil total P contents. However, the strength of Psorption under reducing conditions was weaker, as indicated by the increasedsolubility of sorbed P in NaHCO3 solution. Our results show that highlyweathered soil minerals can retain P even under anoxic conditions, where itmight otherwise be susceptible to leaching. Anoxic events can alsopotentially increase P bioavailability by decreasing the strength, ratherthan the capacity, of P sorption. These results improve our understanding ofthe redox effects on biogeochemical cycling in tropical forests. 

Abstract. Tropical ecosystems contribute significantly to global emissionsof methane (CH4), and landscape topography influences the rate ofCH4 emissions from wet tropical forest soils. However, extreme eventssuch as drought can alter normal topographic patterns of emissions. Here weexplain the dynamics of CH4 emissions during normal and droughtconditions across a catena in the Luquillo Experimental Forest, Puerto Rico.Valley soils served as the major source of CH4 emissions in a normalprecipitation year (2016), but drought recovery in 2015 resulted in dramaticpulses in CH4 emissions from all topographic positions. Geochemicalparameters including (i) dissolved organic carbon (C), acetate, and soil pH and (ii) hydrological parameters like soil moisture and oxygen (O2)concentrations varied across the catena. During the drought, soil moisturedecreased in the slope and ridge, and O2 concentrations increased in thevalley. We simulated the dynamics of CH4 emissions with theMicrobial Model for Methane Dynamics-Dual Arrhenius and Michaelis–Menten (M3D-DAMM), which couples a microbialfunctional group CH4 model with a diffusivity module for solute and gastransport within soil microsites. Contrasting patterns of soil moisture,O2, acetate, and associated changes in soil pH with topographyregulated simulated CH4 emissions, but emissions were also altered byrate-limited diffusion in soil microsites. Changes in simulated availablesubstrate for CH4 production (acetate, CO2, and H2) andoxidation (O2 and CH4) increased the predicted biomass ofmethanotrophs during the drought event and methanogens during droughtrecovery, which in turn affected net emissions of CH4. A variance-basedsensitivity analysis suggested that parameters related to aceticlasticmethanogenesis and methanotrophy were most critical to simulate net CH4emissions. This study enhanced the predictive capability for CH4emissions associated with complex topography and drought in wet tropicalforest soils.