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[1] Changes in land-use and climate are likely to alter moisture and substrate availability in tropical forest soils, but quantitative assessment of the role of resource constraints as regulators of soil trace gas fluxes is rather limited. The primary objective of this study was to quantify the effects of moisture and substrate availability on soil trace gas fluxes in an Amazonian regrowth forest. We measured the efflux of carbon dioxide (CO 2 ), nitric oxide (NO), nitrous oxide (N 2 O), and methane (CH 4 ) from soil in response to two experimental manipulations. In the first, we increased soil moisture availability during the dry season by irrigation; in the second, we decreased substrate availability by continuous removal of aboveground litter. In the absence of irrigation, soil CO 2 efflux decreased during the dry season while irrigation maintained soil CO 2 efflux levels similar to the wet season. Large variations in soil CO 2 efflux consistent with a significant moisture constraint on respiration were observed in response to soil wet-up and dry-down events. Annual soil C efflux for irrigated plots was 27 and 13% higher than for control plots in 2001 and 2002, respectively. Litter removal significantly reduced soil CO 2 efflux; annual soil C efflux in 2002 was 28% lower for litter removal plots compared to control plots. The annual soil C efflux:litterfall C ratio for the control treatment (4.0-5.2) was consistent with previously reported values for regrowth forests that indicate a relatively large belowground C allocation. In general, fluxes of N 2 O and CH 4 were higher during the wet season and both fluxes increased during dry-season irrigation. There was no seasonal effect on NO fluxes. Litter removal had no significant impact on N oxide or CH 4 emissions. Net soil nitrification did not respond to dry-season irrigation, but was somewhat reduced by litter removal. Overall, these results demonstrate significant soil moisture and substrate constraints on soil trace gas emissions, particularly for CO 2 , and suggest that climate and land-use changes that alter moisture and substrate availability are therefore likely to have an impact on atmosphere chemistry.

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Resumo -O uso de água salina, na irrigação, tem sido um desafio para os pesquisadores e produtores rurais, sendo constantemente desenvolvidos estudos que possibilitem o uso de água, de qualidade inferior, sem afetar o rendimento e qualidade dos produtos agrícolas. Este trabalho foi realizado com o objetivo de avaliar o efeito de diferentes níveis de salinidade da água de irrigação e doses de fósforo na cultura do rabanete. O delineamento experimental adotado foi inteiramente ao acaso, arranjados em esquema fatorial 4 x 4 com três repetições. Os tratamentos resultaram da combinação de quatro níveis de salinidade da água de irrigação (S 1 -0,5; S 2 -2,0; S 3 -3,5 e S 4 -5,0 dS m -1 ) com quatro doses de fósforo (0; 100; 200 e 300 mg dm -3 ). As variáveis avaliadas foram: número de folhas, área foliar, massa fresca e seca das raízes. A cultura do rabanete respondeu significativamente a interação entre salinidade e fósforo para área foliar, massa fresca e seca das raízes; e de forma isolada para número de folhas. Doses crescentes de fósforo reduziram à sensibilidade da cultura do rabanete à salinidade de até 3,5 dS m -1 . O manejo da adubação fosfatada é uma alternativa para amenizar o efeito da salinidade no desenvolvimento da cultura do rabanete. Palavras-chave -Raphanus sativus L.. Salinização. Plantas-efeito do fósforo.Abstract -The use of saline water for irrigation has been a challenge for the researchers and farmers, studies continually being developed to enable the use of lower quality water without interfering with the yield and quality of products. This work was conducted to evaluate the effect of different salinity levels water irrigation and phosphorus doses in radish. It was used an entirely statistical randomized design, in factorial scheme 4 x 4, with three replications. The treatments resulted from the combination of four salinity levels of irrigation water (S 1 -0.5; S 2 -2.0; S 3 -3.5 and S 4 -5.0 dS m -1 ) with four phosphorus levels (0; 100; 200 and 300 mg dm -3 ). The variables evaluated were: number of leaves, leaf area, root fresh and dry weight of roots. The radish responded significantly the interaction between salinity and phosphorus for leaf area, fresh and dry weight of roots, and isolated the factors for number of leaves. Increasing doses of phosphorus reduced the sensitivity of radish culture to salinity up to 3,5 dS m -1 . The management of phosphorus is an alternative to alleviate the effect of salinity in the development of radish.

Abstract:Litterfall quantity and quality may respond to alterations in resource availability expected with ongoing land-use and climate changes. Here, we quantify the effects of altered resource availability on non-woody litterfall quantity and quality (nitrogen and phosphorus concentrations) in eastern Amazonian forest regrowth (Brazil) through two multi-year experimental manipulations: (1) daily irrigation (5 mm d−1) during the dry season; and (2) fortnightly litter removal. Consistent with other tropical forest data litterfall exhibited seasonal patterns, increasing with the onset of the dry season and declining with the onset of the rainy season. Irrigation did not affect litterfall mass and had little impact on nitrogen (N) or phosphorus (P) concentrations and return, except for decreasing litter P concentration at the end of two irrigation periods. Litter removal did not alter litterfall mass or P concentration, but progressively reduced litterfall N during the course of the experiment. Overall, these results suggest significant resistance to altered resource availability within the bounds of our experimental treatments; our findings may help to constrain carbon and nutrient cycling predictions for tropical forests in response to land-use and climate changes.

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