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  1. Abstract

    Deep tropical soils with net anion exchange capacity can adsorb nitrate and might delay the eutrophication of surface waters that is often associated with many temperate croplands. We investigated anion exchange capacity and soil nitrate pools in deep soils in the Southern Brazilian Amazon, where conversion of tropical forest and Cerrado to intensive fertilized soybean and soybean-maize cropping expanded rapidly in the 2000s. We found that mean soil nitrate pools in the top 8 m increased from 143 kg N ha−1in forest to 1,052 in soybean and 1,161 kg N ha−1in soybean-maize croplands. This nitrate accumulation in croplands aligned with the estimated N surpluses in the croplands. Soil anion exchange capacity explained the magnitude of nitrate accumulation. High nitrate retention in soils was consistent with current low levels of streamwater nitrate exported from croplands. Soil exchange sites were far from saturation, which suggests that nitrate accumulation can continue for longer under current cropping practices, although mechanisms such as competition with other anions and preferential water flowpaths that bypass exchange sites could reduce the time to saturation.

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  2. Abstract

    In seasonally dry ecosystems, which are common in sub‐Saharan Africa, precipitation after dry periods can cause large pulses of nitrous oxide (N2O), a greenhouse gas, and of nitric oxide (NO), a precursor to tropospheric ozone pollution. Agricultural practices can change soil characteristics, affecting trace N gas emissions. To evaluate the effects of land use on trace gas pulses at the start of the rainy season, we conducted laboratory measurements of N2O and NO fluxes from soils collected from four pairs of agricultural and natural savannah sites across the Sudano‐Sahelian zone. We also conducted in situ wetting experiments, measuring NO fluxes from fallow sandy soils in Tanzania and NO and N2O fluxes from clayey soils in Kenya with different histories of fertilizer use. In incubation studies, NO increased by a factor of 7 to 25 following wetting, and N2O fluxes shifted from negative to positive; cumulative NO fluxes were an order of magnitude larger than cumulative N2O fluxes. In Kenya and Tanzania, NO increased by 1 to 2 orders of magnitude after wetting, and N2O increased by a factor of roughly 5 to 10. Cumulative NO fluxes ranged from 87 to 115 g NO‐N ha−1across both countries—a substantial proportion of annual emissions—compared to roughly 1 g N2O‐N in Kenya. There were no effects of land use or fertilization history on the magnitude of NO or N2O pulses, though land use may have been confounded with differences in soil texture potentially limiting the ability to detect land use effects.

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  3. Abstract

    Fertilized temperate croplands export large amounts of reactive nitrogen (N), which degrades water and air quality and contributes to climate change. Fertilizer use is poised to increase in the tropics, where widespread food insecurity persists and increased agricultural productivity will be needed, but much less is known about the potential consequences of increased tropical N fertilizer application. We conducted a meta‐analysis of tropical field studies of nitrate leaching, nitrous oxide emissions, nitric oxide emissions, and ammonia volatilization totaling more than 1,000 observations. We found that the relationship between N inputs and losses differed little between temperate and tropical croplands, although total nitric oxide losses were higher in the tropics. Among the potential drivers we studied, the N input rate controlled all N losses, but soil texture and water inputs also controlled hydrological N losses. Irrigated systems had significantly higher losses of ammonia, and pasture agroecosystems had higher nitric oxide losses. Tripling of fertilizer N inputs to tropical croplands from 50 to 150 kg N ha−1 year−1would have substantial environmental implications and would lead to increases in nitrate leaching (+30%), nitrous oxide emissions (+30%), nitric oxide (+66%) emissions, and ammonia volatilization (+74%), bringing tropical agricultural nitrate, nitrous oxide, and ammonia losses in line with temperate losses and raising nitric oxide losses above them.

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