Louis Verchot

Effects of soil water content on soil respiration in forests and cattle pastures of eastern Amazonia

The effect of soil water content on efflux of CO2 from soils has been described by linear, logarithmic, quadratic, and parabolic functions of soil water expressed as matric potential, gravimetric and volumetric water content, water holding capacity, water-filled pore space, precipitation indices, and depth to water table. The effects of temperature and water content are often statistically confounded. The objectives of this study are: (1) to analyze seasonal variation in soil water content and soil respiration in the eastern Amazon Basin where seasonal temperature variation is minor; and (2) to examine differences in soil CO2 emissions among primary forests, secondary forests, active cattle pastures, and degraded cattle pastures. Rates of soil respiration decreased from wet to dry seasons in all land uses. Grasses in the active cattle pasture were productive in the wet season and senescent in the dry season, resulting in the largest seasonal amplitude of CO2 emissions, whereas deep-rooted forests maintained substantial soil respiration during the dry season. Annual emissions were 2.0, 1.8, 1.5, and 1.0 kg C m-2 yr-1 for primary forest, secondary forest, active pasture, and degraded pasture, respectively. Emissions of CO2 were correlated with the logarithm of matric potential and with the cube of volumetric water content, which are mechanistically appropriate functions for relating soil respiration at below-optimal water contents. The parameterization of these empirical functions was not consistent with those for a temperate forest. Relating rates of soil respiration to water and temperature measurements made at some arbitrarily chosen depth of the surface horizons is simplistic. Further progress in defining temperature and moisture functions may require measurements of temperature, water content and CO2 production for each soil horizon.

Testing a Conceptual Model of Soil Emissions of Nitrous and Nitric Oxides

n ve s ti ga tors from many diverse disciplines—agron om i s t s , a tm o s ph e ric ch em i s t s , eco l ogi s t s , geoch em i s t s , m eteoro l ogi s t s , and microbi o l ogi s t s — a ll stu dy em i s s i ons of n i trous ox i de (N 2 O) and nitric ox i de (NO) from soi l s . Th ei r com m on interest in soil em i s s i ons of n i trogen ox i des stem s f rom the attem pt to answer the fo ll owing qu e s ti on s : • Are em i s s i ons of N 2 O and NO from soils of su f f i c i en t m a gn i tu de to sign i f i c a n t ly affect the regi onal and gl ob a l bu d gets of these gases in the atm o s ph ere ,a n d , i f s o , do these em i s s i ons also sign i f i c a n t ly affect gl obal warm i n g and the ch emical processes of ozone (O 3 ) pr odu cti on in the tropo s ph ere and ozone de s tru cti on in the stra to sph ere ?

Estimation of global biogeochemical controls and seasonality in soil methane consumption

Uptake by soils is a relatively small flux in the global budget of atmospheric methane, but CH4 consumption rates in soils could be susceptible to changes in land use and climate. Global estimates of the soil sink for atmospheric CH4 are usually made by multiplying averages of small chamber measurements for various ecosystem types (or other strata) by estimates of the area covered by each stratum. Process-level models driven by gridded databases can also be used to make global flux estimates, to evaluate potential effects of changes in climate and land use, and to identify weaknesses in both data and mechanistic understanding. Methane uptake by soils is an appropriate process to model globally because the probable controls are simple relative to many other microbially mediated soil processes of trace gas production and consumption. Field experience suggests that diffusion of atmospheric CH4 into the soil is the primary factor limiting rates of CH4 oxidation in many soils. We have applied a modified version of Ficks first law based on theoretical computations for diffusivity in aggregated media, together with a soil water balance model run on a 1δ global grid, to make independent estimates of CH4 uptake by soils worldwide. Uptake rates were assumed to be zero in very dry desert soils that are mostly devoid of microbial activity, in frozen soils, and in wetlands that are usually CH4 sources. Our mechanistically-based model supports a reference case for global net consumption of CH4 sources. Our mechanistically-based model supports a reference case for global net consumption of CH4 in soils of 17–23 Tg yr−1, which is near the middle of previously reported ranges, and is close to our own mean estimate from extrapolation of flux means across ecosystem strata (21 Tg CH4 yr−1). A new inference of our modeling approach is that over 40% of the soil sink for CH4 occurs in warm and relatively dry ecosystems, such as semi-arid steppe, tropical savanna, tropical seasonal forest, and chaparral. This model prediction results from a favorable climate regime, high porosity in coarse-to-medium textured soils, and low moisture content that permits rapid gaseous diffusion in these semi-arid and seasonally dry tropical ecosystems. Very few data on CH4 fluxes exist from these areas that can be used to compare with model predictions. Because of this paucity of data where uptake rates may be relatively high, and because humans have altered these landscapes extensively, our results suggest that more study is needed in seasonally dry ecosystems in order to understand the impacts of land-use change on soil sinks for methane.

Land-Use Change and Biogeochemical Controls of Methane Fluxes in Soils of Eastern Amazonia

ABSTRACT Tropical soils account for 10%–20% of the 15–35 Tg of atmospheric methane (CH4) consumed annually by soils, although tropical deforestation could be changing the soil sink. The objectives of this study were (a) to quantify differences in soil CH4 fluxes among primary forest, secondary forest, active pasture, and degraded pasture in eastern Amazonia; and (b) to investigate controlling mechanisms of CH4 fluxes, including N availability, gas-phase transport, and soil respiration. At one ranch, Fazenda Vitória, annual uptake estimates (kg CH4ha−1 y−1) based on monthly measurements were: primary forest, 2.1; secondary forest, 1.0; active pasture, 1.3; degraded pasture, 3.1. The lower annual uptake in the active pasture compared with the primary forest was due to CH4 production during the wet season in the pasture soils, which is consistent with findings from other studies. In contrast, the degraded pasture was never a CH4 source. Expressing uptake as a negative flux and emission as a positive flux, CH4 fluxes were positively correlated with CO2 fluxes, indicating that root and microbial respiration in the productive pastures, and to a lesser extent in the primary forest, contributed to the formation of anaerobic microsites where CH4 was produced, whereas this productivity was absent in the degraded pasture. In all land uses, uptake rates of atmospheric CH4 were greater in the dry season than in the wet season, indicating the importance of soil water content and gas transport on CH4 fluxes. These clay soils had low annual uptake rates relative to reported rates on sandy soils, which also is consistent with gas transport within the soil being a limiting factor. Nitrogen availability indices did not correlate with CH4 fluxes, indicating that inhibition of CH4 oxidation was not an important mechanism explaining differences among land uses. At another ranch, Fazenda Agua Parada, no significant effect of pasture age was observed along a chronosequence of pasture ages. We conclude that land-use change can either increase or decrease the soil sink of CH4, depending on the duration of wet and dry seasons, the effects of seasonal precipitation on gas-phase transport, and the phenology and relative productivity of the vegetation in each land use.

Gross vs net rates of N mineralization and nitrification as indicators of functional differences between forest types

Abstract Floristic species composition and differences in litter quality between species are the primary factors controlling N mineralization in forest ecosystems. Generalizations about species effects on N cycling are based on measurements of net rates of mineralization and nitrification. However, there have been few tests on the ability of these measurements to reflect the mechanistic complexity underlying the species effects. The objectives of this study are to: (1) determine whether differences in net mineralization and net nitrification rates between stands of different species composition are due to differences in gross rates of mineralization, nitrification, and microbial consumption; (2) determine whether field and laboratory assays of net mineralization and nitrification are useful indicators of internal N dynamics; and (3) test the hypothesis that microbial immobilization increases with rates of mineralization and nitrification. We measured net rates of mineralization and nitrification in the field and in the laboratory, and gross rates of mineralization, nitrification and microbial consumption in different stands at two sites in eastern New York State. The results indicated that vegetation type was not always a robust indicator of N cycling differences between ecosystems. At one site there was no difference in net mineralization (P

Landscape versus ungulate control of gross mineralization and gross nitrification in semi-arid grasslands of Yellowstone National Park

Abstract Grazers have marked effects on decomposition and N cycling processes, generally resulting in increased net N mineralization. Within landscapes, topographic and edaphic gradients also affect these microbial processes. The objective of this study was to evaluate the effects of grazers on N cycling processes in a landscape context that encompassed a wide range of environmental conditions in order to increase our mechanistic understanding of these processes and provide a stronger basis for management and assessment of grassland ecosystems. This study was conducted on a series of 37–41-year-old grazing exclosures on the northern winter range of Yellowstone National Park. We measured gross and net mineralization, nitrification and immobilization in grazed and ungrazed plots in upland and bottomland landscape positions by 15 N pool dilution in a laboratory incubation with intact cores. There were no significant differences in either gross mineralization, immobilization, gross nitrification or NO 3 − immobilization between grazed and ungrazed plots in a paired t -test using all plots ( P =0.52, P =0.32, P =0.91 and P =0.93, respectively). These results were unexpected, because previous reports indicated that grazers increased soil N dynamics, including net N mineralization, in Yellowstone grassland, and suggest that herbivore regulation of N processes may accumulate over a longer time scale than was measured in this study (24xa0h). Instead, we found that landscape position was the dominant factor controlling both mineralization and immobilization rates, with higher rates located at the bottoms of the slopes (32.9 and 33.4xa0μgxa0Nxa0gxa0soil −1 xa0d −1 , respectively), compared to the upland sites (4.4 and 3.2xa0μgxa0Nxa0gxa0soil −1 xa0d −1 , respectively). Gross mineralization and immobilization rates were highly correlated with soil C and N content, while gross nitrification was not. Turn over times for NH 4 + and NO 3 − pools averaged 1.2 days for the NH 4 + pool and 2.25 days for the NO 3 − pool. There was no apparent effect of grazers on turnover times, nor were there any significant correlations between turnover times and soil C and N pools, slope position, or soil water content. Previous modeling to understand how herbivores affect N cycling suggested that stimulation of net mineralization was most likely due to grazers suppressing immobilization by reducing belowground primary productivity and organic matter inputs to the soil. Our results do not suggest that grazers suppress immobilization. If anything, they suggest that grazers enhance this process, especially in the more mesic components of the landscape. Landscape position effects on N cycle processes were much stronger than grazer effects in this study, and appear to be driven by soil C and N contents.

Leaf-cutting ant (Atta Sexdens) and nutrient cycling: deep soil inorganic nitrogen stocks, mineralization, and nitrification in Eastern Amazonia

Abstract Nest excavation and agricultural activities of the leaf-cutting ant Atta sexdens create complex belowground heterogeneity in secondary forests of Eastern Amazonia. We examined the effects of this heterogeneity on inorganic-N stocks, net mineralization, and net nitrification to test the hypothesis that the bulk soil of the nests has higher net rates of mineralization and nitrification than soil that was not affected by the influences of ant nests, throughout the profile. This study was conducted in a secondary forest at Fazenda Vitoria, near Paragominas in the Eastern Brazilian Amazon, where a previous study showed that the bulk soil of ant nests had elevated NO3−. The results of the inorganic-N measurements were consistent with the previous study, showing elevated NO3− deep in the soil profile of the nests. However, neither net mineralization nor net nitrification were significantly greater at depth in the mineral soil of the nests compared to soil that was not influenced by nests (P=0.05), although variability was higher in the nest soil. These results suggest that the NO3− may have diffused into the surrounding mineral from the N-rich organic matter buried by the ants in chambers within the deep soil.