Relationships between microbial biomass and dissipation of 2,4-D and dicamba in soil

A study was conducted to evaluate relationships between microbial biomass and the dissipation of 2,4-D (2,4-dichlorophenoxy acetic acid) and dicamba (2-methoxy-3,6-dichlorobenzoic acid) in soil. We hypothesized that the size of the microbial biomass should be a strong predictor of the pesticide degradation capacity of a particular soil. Soils with a high microbial biomass should have relatively high levels of general microbial activity and should support a diversity of degradation pathways. In this study, we quantified the degradation of 2,4-D and dicamba in a range of soils with different concentrations of microbial biomass. The herbicides 2,4-D and dicamba were added to similar soils collected from five different land use types (home lawn, cornfield, upland hardwood forest, wetland forest, and aquifer material) and incubated for 80 days under laboratory conditions. Herbicide residue and microbial biomass (C and N) analyses were performed 5, 10, 20, 40, and 80 days following herbicide application. Microbial biomass-C and -N and soil organic matter content were positively correlated with dissipation of 2,4-D and dicamba. The results suggest that there are relationships between the size of the soil microbial biomass and the herbicide degradation capacity of an ecosystem. These relationships may be useful for developing approaches for evaluating and predicting the fate of pesticides in different ecosystems.     

Experimental snowpack reduction alters organic matter and net N mineralization potential of soil macroaggregates in a northern hardwood forest

Climate change is predicted to reduce or delay annual wintertime snow pack formation in the forests of the northeastern US. Any delay in snowpack formation could increase soil freezing in winter and, thereby, alter soil characteristics and processes. We examined the hypothesis that delayed snowpack would disrupt soil structure and change organic matter bioavailability in an experimental snow removal study at the Hubbard Brook Experimental Forest (HBEF), NH, USA. Pairs of reference and snow removal treatment plots were studied in four different sites at HBEF. Snow was removed from November–January of two winters, inducing soil freezing throughout both winters. Size class distribution and organic matter concentration and content of aggregates, and carbon and nitrogen mineralization potential of size fractions were quantified for surface mineral soils in the spring of both years immediately after snowmelt. In the first year of sampling, the only significant effect of snow removal was an increase in the smallest (<53 μm) size fraction of mineral soil. In the second year, snow removal increased organic matter concentrations of macroaggregate (250–2,000 μm) and microaggregate (53–250 μm) size fractions. This change corresponded to an increase in net N mineralization potential and the ratio of N to C mineralized in the macroaggregate fraction, but there were no effects of snow removal on C mineralization. We propose that soil freezing increases the movement of organic matter from organic to mineral soil horizons and increases the N content of mineralizable substrates in mineral soil following years with delayed snowpack formation.     

Regional scale analysis of denitrification in north temperate forest soils

Large scale analyses of biogeochemical processes are necessary for understanding anthropogenic effects on global climate and environmental quality. Regional scale estimates of denitrification from forest soils in southern lower Michigan USA were produced by stratifying the region into landscape experimental units using soil texture and natural drainage classes, and extrapolating data to larger areas using a geographic information system (GIS). Previous landscape-scale research established relationships between soil texture and drainage and denitrification and quantified annual denitrification N loss in nine soil texture/drainage groups. All forest soils within the region (64 series) were assigned to one of these nine groups based on their texture and drainage characteristics and were assigned an annual denitrification N loss value. A regional estimate of denitrification was produced by multiplying the areal extent of each of the nine soil groups by their annual denitrification N loss value. Loam-textured soils underlie 47% of the regional forest and accounted for 73% of the forest denitrification. Sandy soils were found under 44% of the regional forest but produced only 5% of the regional denitrification. Clay loam soils underlie 9% of the regional forest and produced 22% of the denitrification. Annual denitrification N loss for the region was estimated as 1.4×10⁷ kg N/yr. We used denitrification enzyme activity (DEA) as a proxy for annual denitrification N loss to determine if the relationship between denitrification and soil texture and natural drainage that we observed at the landscape scale held up at the regional scale. DEA was measured in 22 soils across the region and was strongly related to soil texture and natural drainage (r²=0.61), suggesting that extrapolation of data from the landscape to the regional scale was justified.     

Soil nitrogen cycling under litter and coarse woody debris in a mixed forest in New York State

Coarse woody debris (CWD) could alter N availability and transformations in the underlying soil and therefore contribute to spatial heterogeneity and influence ecosystem loss of N. We measured soil N concentrations and transformations in soil beneath CWD and beneath a litter layer at a mixed forest in NY State. We found that total and microbial biomass N was lower and that microbial biomass C-to-N ratio was higher in soil beneath CWD. Rates of N₂O production and denitrification enzyme activity were reduced beneath CWD. These results suggest that CWD is an important controller of spatial heterogeneity in N dynamics and may influence the magnitude of N loss in temperate forests.     

Phosphate additions have no effect on microbial biomass and activity in a northern hardwood forest

High rates of atmospheric nitrogen (N) deposition have raised questions about shifting patterns of nutrient limitation in northern hardwood forests. Of particular interest is the idea that increased supply of N may induce phosphorus (P) limitation of plant and microbial processes, especially in acid soils where P sorption by Al is high. In this study, we established field plots and plant-free laboratory mesocosms with P and Ca additions to test the hypotheses that 1) microbial biomass and activity are limited by P in the northern hardwood forest soils at the Hubbard Brook Experimental Forest in NH USA; 2) elevated Ca increases inherent P availability and therefore reduces any effects of added P and 3) P effects are more marked in the more carbon (C) rich Oie compared to the Oa horizon. Treatments included P addition (50 kg P ha⁻¹), Ca addition (850 kg Ca ha⁻¹) and Ca + P addition (850 kg Ca ha⁻¹ and 50 kg P ha⁻¹). The P treatments increased resin-available P levels and reduced phosphatase activity, but had no effect on microbial biomass C, microbial respiration, C metabolizing enzymes, potential net N mineralization and nitrification in the Oie or Oa horizon of either field plots or plant free mesocosms, in either the presence or absence of Ca. Total, prokaryote, and eukaryote PLFA were reduced by P addition, possibly due to reductions in mycorrhizal fungal biomass. These results suggest that increased N deposition and acidification have not created P limitation of microbial biomass and activity in these soils.     

Relationships between microbial biomass and dissipation of 2,4-D and dicamba in soil

A study was conducted to evaluate relationships between microbial biomass and the dissipation of 2,4-D (2,4-dichlorophenoxy acetic acid) and dicamba (2-methoxy-3,6-dichlorobenzoic acid) in soil. We hypothesized that the size of the microbial biomass should be a strong predictor of the pesticide degradation capacity of a particular soil. Soils with a high microbial biomass should have relatively high levels of general microbial activity and should support a diversity of degradation pathways. In this study, we quantified the degradation of 2,4-D and dicamba in a range of soils with different concentrations of microbial biomass. The herbicides 2,4-D and dicamba were added to similar soils collected from five different land use types (home lawn, cornfield, upland hardwood forest, wetland forest, and aquifer material) and incubated for 80 days under laboratory conditions. Herbicide residue and microbial biomass (C and N) analyses were performed 5, 10, 20, 40, and 80 days following herbicide application. Microbial biomass-C and -N and soil organic matter content were positively correlated with dissipation of 2,4-D and dicamba. The results suggest that there are relationships between the size of the soil microbial biomass and the herbicide degradation capacity of an ecosystem. These relationships may be useful for developing approaches for evaluating and predicting the fate of pesticides in different ecosystems.     

Release and consumption of nitrogen by snail feces in Negev Desert soils

Snail grazing and feces production have been shown to be major components of the nitrogen (N) budget of Negev Desert ecosystems. However, the movement of N from feces into soil N cycling processes has not been studied. In this study, we measured immediate N release from different types of snail feces following wetting of dry desert soils, and characterized potential net N mineralization and nitrification and soil respiration over a 12-day incubation under laboratory conditions. The dynamics of morganic N exhibited two distinct phases during the 12-day incubation: (1) immediate release of inorganic N following wetting of the soil and (2) decline of inorganic N from day 1 today 12 of the incubation. The immediate pulse of N release from this one wetting event (6–25 mg N m^-2) was larger than annual atmospheric inputs of N to Negev Desert ecosystems (<2 mg N m^-2); however, from 50 to 80% of the N released upon wetting was consumed by the end of the incubation. There were differences in inorganic N release and respiration from feces from different kinds of snails, and from feces from the same species of snail fed different plants. The results suggest that while snail feces contribute significant amounts of plant available N to Negev ecosystems, plants must compete with other sinks for this N.     

Calcium and phosphorus interact to reduce mid-growing season net nitrogen mineralization potential in organic horizons in a northern hardwood forest

Acid deposition can deplete soil calcium (Ca) and be detrimental to the health of some forests. We examined effects of soil Ca and phosphorus (P) availability on microbial activity and nitrogen (N) transformations in a plot-scale nutrient addition experiment at the Hubbard Brook Experimental Forest in New Hampshire, USA. We tested the hypotheses that (1) microbial activity and N transformations respond to large but not small changes in soil Ca, (2) soil Ca availability influences net N mineralization via the immobilization of N, rather than via changes in microbial activity, and (3) the response to Ca is constrained by P availability. Seasonality was a primary influence on the microbial response to treatments; N cycling processes varied from May to October and treatment effects were only detectable in the mid-growing season, in July. Neither microbial activity (C mineralization) nor gross N mineralization responded to Ca or to P, in either horizon. In the Oa horizon in July net N mineralization was reduced by high Ca and by Ca + P, and gross nitrification was increased by P addition. In the Oe horizon in July net N mineralization was reduced by Ca + P. These results partially supported our hypotheses, suggesting that soil Ca depletion has the potential to increase mid-growing season N availability via subtle changes in N immobilization, and that this effect is sensitive to soil P chemistry. The horizon-specific nature of the responses that we detected suggests that the proportions of Oe and Oa horizons comprising the surface organic layer will influence the relative importance of these processes at the ecosystem scale. Our results highlight the need for further attention to seasonal changes in controls of microbial mineralization/immobilization processes, to functional differences between organic horizons, and to interactions between Ca and P in soils, in order to learn the specific mechanisms underlying the influence of Ca status on nutrient recycling in these northern hardwood ecosystems.     

Microbially available carbon in buried riparian soils in a glaciated landscape

Buried horizons and lenses in riparian soil profiles harbor large amounts of carbon relative to the surrounding soil horizons. Because these buried soil horizons, as well as deep surface horizons, frequently lie beneath the water table, their impact on nitrogen transport across the terrestrial–aquatic interface depends upon their frequency and spatial distribution, and upon the lability of associated organic matter. We collected samples of 51 soil horizons from 14 riparian zones Rhode Island, USA, where soil profiles are characterized by glacial outwash and alluvial deposits. These soil samples came from as deep as 2 m and ranged in carbon content from <1% to 44% in a buried O horizon 54–74 cm deep. We used these samples to: (1) determine the extent to which carbon in buried horizons, and deep surface horizons, is potentially microbially available; (2) identify spatial patterns of carbon mineralization associated with surface and buried horizons; and (3) evaluate likely relationships between soil horizon types, chemical characteristics and carbon mineralization. Carbon mineralization rates associated with buried horizons during anaerobic incubations ranged from 0.0001 to 0.0175 μmol C kg soil^?1 s^?1 and correlated positively with microbial biomass (R=0.89, P<0.0001, n=21). Excluding surface O horizons from the analysis, carbon mineralization varied systematically with horizon type (surface A, buried A, buried O, lenses, A/C, B, C) (P<0.05) but not with depth or depth x horizon interaction (overall R²=0.59, P<0.0005, n=47). In contrast to this result and to most published data sets, ¹³C-to-¹²C and ¹⁵N-to-¹⁴N ratios of organic matter declined with depth (¹³C?26.9 to ?29.3 per mil, ¹⁵N+5.6 to ?0.8 per mil). The absence of a relationship between horizon depth and C availability suggests that carbon availability in these buried horizons may be determined by the abundance and quality of organic matter at the time of horizon formation or burial, rather than by duration since burial, and implies that subsurface microbial activity is largely disconnected from surface ecosystems. Our results contribute to the emerging view that buried horizons harbor microbially available C in quantities relevant to ecosystem processes, and suggest that buried C-rich soil horizons need to be incorporated into assessments of the depth of the biologically active zone in near-stream subsurface soils.     

Plant productivity and nitrogen gas fluxes in a tallgrass prairie landscape

We explored relationships between plant productivity and annual fluxes of nitrogen (N₂) and nitrous oxide (N₂O) in a tallgrass prairie landscape in central Kansas. Our objective was to develop predictive relationships between these variables that could be used in conjunction with remote sensing information on plant productivity to produce large-area estimates of N gas fluxes. Our hypothesis was that there are inherent relationships between plant productivity and N gas fluxes in tallgrass prairie because both are controlled by water and N availability. The research was carried out as part of a multi-investigator project, the First ISLSCP Field Experiment (FIFE, ISLSCP = International Satellite Land Surface Climatology Program), directed toward the use of remote sensing to characterize land-atmosphere interactions. Fluxes of N₂ (denitrification) and N₂O were measured using soil core techniques. Estimates of annual flux were produced by temporal extrapolation of measured rates. Annual aboveground net primary productivity (ANPP) was estimated from measurements of the maximum standing crop of plant biomass. There were strong relationships between ANPP and N gas fluxes, and between a satellite remote sensing-based index of plant productivity (normalized difference vegetation index, NDVI) and gas fluxes. We used these relationships to convert images of NDVI into images of N gas fluxes for one 83 ha watershed and for the entire 15 by 15 km FIFE site. These images were used to compute mean landscape gas fluxes (0.62 g N m^-2 y^-1 for N₂, 0.66 g N m^-2 y^-1 for N₂O) and total N gas production for the two areas. Our flux and production values are useful for comparison with values produced by simulation models and site-specific studies, and for assessing the significance of N gas production to ecosystem and landscape scale processes related to nutrient cycling, water quality and atmospheric chemistry.