Soil Carbon Dioxide dynamics and Nitrogen cycling in an Eastern Amazonian Rainforest, Caxiuana, Brazil

Abstract

The significance of climate changes and of increasing N deposition to the interaction of both carbon and nitrogen cycles in tropical soils is still unknown and deserves more attention. In this study I evaluate soil CO2 dynamics and N cycling in an old-growth forest. The study was divided in three parts: 1) measurement of soil CO2 efflux and its environmental controls from two Oxisol sites with contrasting soil texture and at different landscape positions, 2) observation of how a throughfall exclusion (TFE) experiment affected soil CO2 production in a deeply weathered sandy Oxisol of Caxiuanã, and 3) evaluation of the soil N status of two heavily weathered soils which contrast in texture (sand versus clay). Over the course of two years, soil CO2 efflux and soil CO2 concentrations, soil temperature and moisture in pits down to 3 m depth was measured. Using 15N pool dilution, for both soils rates of N cycling (gross rates of N transformation) and N retention (microbial N immobilization, dissimilatory NO3- reduction to NH4+, and abiotic N immobilization) were quantified. The most important results are:1) Average CO2 efflux was 21% higher for sand (3.93 ± 0.06 µmol CO2 m-2s-1) than for the clay (3.08 ± 0.07 µmol CO2 m-2s-1). No difference was detected for soil temperature between sites, while soil water content in sandy soil (23.2 ± 0.33 %) was much lower than the clay soil (34.5 ± 0.98 %), for the two-year period. Soil CO2 efflux did not differ between dry and wet season, but I detected a significant interaction between season and topographic position. Mean contribution of the litter layer to the CO2 efflux rates was 20 % and varied from 25 % during the wet season to close to 0 % during the dry season. The results of our study illustrate that soil moisture is an important driver of temporal variations in soil CO2 efflux in this old-growth forest. When extrapolating soil CO2 efflux to larger areas, a significant influences of soil texture, litter, and the interaction of topographical position and time illustrate that it is necessary to include some of the complexity of landscapes.2) TFE reduced soil CO2 efflux from 4.3 ± 0.1 µmol CO2 m-2 s-1 (control) to 3.2 ± 0.1 µmol CO2 m-2 s-1 (TFE). The contribution of the subsoil (below 0.5 m depth) to the total soil CO2 production was higher in the TFE plot (28 %) compared to the control plot (17 %), and it did not differ between years. I distinguished three phases of drying after the TFE was started. The first phase was characterized by a translocation of water uptake (and accompanying root activity) to deeper layers and not enough water stress to affect microbial activity and/or total root respiration. During the second phase a reduction in total soil CO2 efflux in the TFE plot was related to a reduction of soil- and litter decomposers activity. The third phase of drying, characterized by a continuing decrease in soil CO2 production was dominated by a water stress-induced decrease in total root respiration. These results strongly contrast to results of a drought experiment on clay Oxisols which I explain with differences in water holding capacity and depth of rooting zone. I conclude that Amazonian forest ecosystems located on soils with coarse texture are more sensitive to drought than forests located on heavier textures because they cannot compensate the relatively low water holding capacity in the top soil with water stored in deeper soil layers.3) The clay Oxisol had high gross rates of N mineralization, nitrification and 15N enrichment factor and hence high potential for N losses. The sand Oxisol had low gross rates of N cycling and 15N enrichment factor, and reacted more like a soil that is N-limited. Faster turnover rates of NH4+ compared to NO3- signified that NH4+ cycles faster through microorganisms than NO3-, possibly contributing to better retention of NH4+ than NO3-. However this was opposite to abiotic retention processes, which showed higher conversion of NO3- to the organic N pool than NH4+. The combined results suggest that some Amazonian forest soils have higher N availability than others which will have important consequences for soil N cycling and losses when projected increases in anthropogenic N deposition will occur.