Serita D. Frey

Stoichiometry of soil enzyme activity at global scale

Extracellular enzymes are the proximate agents of organic matter decomposition and measures of these activities can be used as indicators of microbial nutrient demand. We conducted a global-scale meta-analysis of the seven-most widely measured soil enzyme activities, using data from 40 ecosystems. The activities of beta-1,4-glucosidase, cellobiohydrolase, beta-1,4-N-acetylglucosaminidase and phosphatase g(-1) soil increased with organic matter concentration; leucine aminopeptidase, phenol oxidase and peroxidase activities showed no relationship. All activities were significantly related to soil pH. Specific activities, i.e. activity g(-1) soil organic matter, also varied in relation to soil pH for all enzymes. Relationships with mean annual temperature (MAT) and precipitation (MAP) were generally weak. For hydrolases, ratios of specific C, N and P acquisition activities converged on 1 : 1 : 1 but across ecosystems, the ratio of C : P acquisition was inversely related to MAP and MAT while the ratio of C : N acquisition increased with MAP. Oxidative activities were more variable than hydrolytic activities and increased with soil pH. Our analyses indicate that the enzymatic potential for hydrolyzing the labile components of soil organic matter is tied to substrate availability, soil pH and the stoichiometry of microbial nutrient demand. The enzymatic potential for oxidizing the recalcitrant fractions of soil organic material, which is a proximate control on soil organic matter accumulation, is most strongly related to soil pH. These trends provide insight into the biogeochemical processes that create global patterns in ecological stoichiometry and organic matter storage.

NITROGEN ADDITIONS AND LITTER DECOMPOSITION: A META‐ANALYSIS

We conducted a meta-analysis of previously published empirical studies that have examined the effects of nitrogen (N) enrichment on litter decomposition. Our objective was to provide a synthesis of existing data that comprehensively and quantitatively evaluates how environmental and experimental factors interact with N additions to influence litter mass loss. Nitrogen enrichment, when averaged across all studies, had no statistically significant effect on litter decay. However, we observed significant effects of fertilization rate, site-specific ambient N-deposition level, and litter quality. Litter decomposition was inhibited by N additions when fertilization rates were 2-20 times the anthropogenic N- deposition level, when ambient N deposition was 5-10 kg N·ha 21 ·yr 21 , or when litter quality was low (typically high-lignin litters). Decomposition was stimulated at field sites exposed to low ambient N deposition (,5 kg N·ha 21 ·yr 21 ) and for high-quality (low-lignin) litters. Fertilizer type, litterbag mesh size, and climate did not influence the litter decay response to N additions.

Bacterial and fungal abundance and biomass in conventional and no-tillage agroecosystems along two climatic gradients

Microbial community composition may be an important determinant of soil organic matter (SOM) decomposition rates and nutrient turnover and availability in agricultural soils. Soil samples were collected from six long-term tillage comparison experiments located along two climatic gradients to examine the effects of no-tillage (NT) and conventional tillage (CT) management on bacterial and fungal abundance and biomass and to examine potential controls on the relative abundances of bacteria and fungi in these two systems. Samples were divided into 0–5 and 5–20 cm depth increments and analyzed for bacterial and fungal abundance and biomass, total C and N, particulate organic matter C and N (POM-C and N), soil water content, texture, pH, and water-stable aggregate distributions. Soil moisture, which varied by tillage treatment and geographically with climate, ranged from 0.05 to 0.35 g g−1 dry soil in the surface 0–5 cm and 0.15 to 0.28 g g−1 dry soil at 5–20 cm. Measured organic matter C and N fractions and mean weight diameter (MWD) of water-stable aggregates were significantly higher in NT relative to CT at three of the six sites. Fungal hyphal length ranged from 19 to 292 m g−1 soil and was 1.9 to 2.5 times higher in NT compared to CT surface soil across all sites. Few significant tillage treatment differences in soil physical and chemical properties or in fungal abundance and biomass were observed at 5–20 cm. Bacterial abundance and biomass were not consistently influenced by tillage treatment or site location at either depth. The proportion of the total biomass composed of fungi ranged from 10 to 60% and was significantly higher in NT compared to CT surface soil at five of six sites. Proportional fungal biomass was not strongly related to soil texture, pH, aggregation, or organic C and N fractions, but was positively related to soil moisture (r=0.67; P<0.001). The relationship between soil moisture and the degree of fungal dominance was due to the positive response of fungal biomass and the lack of response of bacterial biomass to increasing soil moisture across the range of measured soil water contents. Tillage treatment effects on fungal biomass and proportional fungal abundance were not significant when the data were analyzed by analysis of covariance with soil moisture as the covariate. These results suggest that observed tillage treatment and climate gradient effects on fungi are related to differences in soil moisture. Further research is needed, however, to determine how tillage-induced changes in the soil environment shape microbial community composition in agroecosystems.

Reciprocal transfer of carbon and nitrogen by decomposer fungi at the soil-litter interface

We have investigated whether decomposer fungi translocate litter-derived C into the underlying soil while simultaneously translocating soil-derived inorganic N up into the litter layer. We also located and quantified where the translocated C is deposited within the soil aggregate structure. When 13 C-labeled wheat straw was decomposed on the surface of soil amended with 15 N-labeled inorganic N, we found that C and N were reciprocally transferred by fungi, with a significant quantity (121 –151 m gC g 21 whole soil) of litter-derived C being deposited into newly formed macroaggregates (. 250 mm sized aggregates). Fungal inhibition reduced fungal biomass and the bidirectional C and N flux by approximately 50%. The amount of litter-derived C found in macroaggregates was positively correlated with litter-associated fungal biomass. This fungal-mediated litter-to-soil C transfer, which to our knowledge has not been demonstrated before for saprophytic fungi, may represent an important mechanism by which litter C enters the soil and becomes stabilized as soil organic matter within the macroaggregate structure. q 2003 Elsevier Science Ltd. All rights reserved.

Quantifying global soil carbon losses in response to warming

The majority of the Earth’s terrestrial carbon is stored in the soil. If anthropogenic warming stimulates the loss of this carbon to the atmosphere, it could drive further planetary warming. Despite evidence that warming enhances carbon fluxes to and from the soil, the net global balance between these responses remains uncertain. Here we present a comprehensive analysis of warming-induced changes in soil carbon stocks by assembling data from 49 field experiments located across North America, Europe and Asia. We find that the effects of warming are contingent on the size of the initial soil carbon stock, with considerable losses occurring in high-latitude areas. By extrapolating this empirical relationship to the global scale, we provide estimates of soil carbon sensitivity to warming that may help to constrain Earth system model projections. Our empirical relationship suggests that global soil carbon stocks in the upper soil horizons will fall by 30 ± 30 petagrams of carbon to 203 ± 161 petagrams of carbon under one degree of warming, depending on the rate at which the effects of warming are realized. Under the conservative assumption that the response of soil carbon to warming occurs within a year, a business-as-usual climate scenario would drive the loss of 55 ± 50 petagrams of carbon from the upper soil horizons by 2050. This value is around 12–17 per cent of the expected anthropogenic emissions over this period. Despite the considerable uncertainty in our estimates, the direction of the global soil carbon response is consistent across all scenarios. This provides strong empirical support for the idea that rising temperatures will stimulate the net loss of soil carbon to the atmosphere, driving a positive land carbon–climate feedback that could accelerate climate change.

Soil pH and organic C dynamics in tropical forest soils: Evidence from laboratory and simulation studies

Acidic soil pH may affect decomposition of added organic materials in humid tropical forest soils. Our objective was to determine the effects of soil pH on decomposition of added organic materials to tropical forest soils of different soil texture and clay mineralogy. Release of 14CO2 and microbial biomass 14C were measured during a 270-d incubation at 25°C after either [14C]glucose or 14C-labeled blue grama grass (Bouteloua gracilis) material had been added to 13 tropical forest smectitic, kaolinitic, oxidic or allophanic mineralogies. Initial soil pH ranged from 3.9 to 6.7. An additional investigation examined 14CO2 release from kaolinitic or oxidic forest soils to which either Ca(OH)2 or CaSO4 had been previously applied to obtain 5 soil pH values. Initial soil pH and cumulative 14CO2 release in glucose-amended soils were positively related only after 1 and 4 d. In contrast, plant-residue-amended soils had positive relationships between initial soil pH and cumulative 14CO2 release after 7 d and continued with that relationship up to 270 d. Microbial biomass 14C was reduced at lower pH values in both glucose-and plant-residue-amended soils after 270 d. Water-extractable 14C was also higher at pH > 5.5 in plant-residue-amended soils after 58 d. Differences in soil texture and clay mineralogy had no apparent effect on the relationship between soil pH and decomposition. Simulated results of the experiment using the CENTURY Soil Organic Matter Model diverged from observed results for soils with pH < 6.5. Further research is required to determine the effects of acidic soil pH on decomposition rates of stable C pools and to develop functions for simulation models to account for the short- and long-term effects of soil acidity on decomposition.

Comparison of laboratory and modeling simulation methods for estimating soil carbon pools in tropical forest soils

Abstract Availability of methods to determine kinetically-defined soil carbon pools may assist in better understanding soil organic matter turnover in tropical soils and its relationship with soil mineral fractions and nutrient cycling. Our objective was to compare three methods of estimating soil C pools for the top mineral soil horizon of 13 tropical forest soils with a wide range of clay content and differing soil mineralogies. The methods were: (i) regression analysis of CO 2 -C release from a 341 day incubation of unamended soils; (ii) results of C analysis procedures including determinations of soluble C, microbial biomass C and light fraction C; and (iii) CENTURY model simulations of equilibrium values of soil C pools. Soil mineralogy did not have a significant effect on CO 2 -C release, expressed as a proportion of total organic C, during incubation. However, allophanic soils had significantly higher total organic C, soluble C and light fraction C than soils of smectitic, kaolinitic or oxidic mineralogies. Clay and sand contents significantly correlated with cumulative proportional CO 2 -C release. The active C pool, as determined by the CENTURY equilibrium method and measurements of soluble plus microbial biomass C, were less than the active C pool estimated by the incubation-regression method. Measured light fraction C was smaller than estimates from the CENTURY equilibrium method and incubation-regression estimates. Total organic C, soluble plus microbial biomass C and light fraction C had the highest correlations with cumulative incubation CO 2 -C release. Of the CENTURY model C pool estimates, only the slow C pool estimate correlated with incubation CO 2 -C release. The use of C analyses as soil C pool estimates for model simulations of the long-term incubation resulted in an underestimation of actual incubation CO 2 -C release. This underestimate was caused by a smaller slow pool estimated by light fraction analysis. In addition, structural and metabolic C pools were not measured and they have a large short-term effect on CO 2 -C release. Use of CENTURY equilibrium estimates, including estimates of structural and metabolic C, resulted in simulated CO 2 -C release comparable to actual CO 2 -C release patterns. However, the use of the CENTURY equilibrium method may be limited by the difficulty of obtaining adequate soil, plant and climatic information to run model simulations and by the validity of CENTURY model assumptions for factors controlling soil C pools under tropical climatic conditions.

Modeling the Measurable or Measuring the Modelable: A Hierarchical Approach to Isolating Meaningful Soil Organic Matter Fractionations

The approaches of modeling the measurable and measuring the modelable are both valuable for advancing our understanding of soil organic matter (SOM). In the former case, we assume that the measurements we make are the best representation of nature, and that model structure should follow. While this may simplify model testing, it may not yield a particularly useful description of SOM dynamics. There is no question that most of our knowledge about SOM is derived from experimentation, but models are being increasingly used to further our understanding. In the latter case, model structure and parameters are modified in reasonable ways to obtain the best fit of simulation with observation, thus deducing the “correct” structure. In this case, it is conceivable that more than one structure can result in a good fit to the data. Good predictions may be produced, but an incorrect structure may be derived. In either case, there are no clear criteria for obtaining the “truth” except for repeatedly testing our methods/models against new information, preferably obtained from definitive experiments. A combination of approaches is best. We elaborate a particular approach which is mindful of the habitat in which microbes and their substrates reside and relate this theory to methods developed to separately isolate plant and microbially derived SOM, which may be physically protected from microbial attack in the soil.

Direct evidence for microbial-derived soil organic matter formation and its ecophysiological controls

Soil organic matter (SOM) and the carbon and nutrients therein drive fundamental submicron- to global-scale biogeochemical processes and influence carbon-climate feedbacks. Consensus is emerging that microbial materials are an important constituent of stable SOM, and new conceptual and quantitative SOM models are rapidly incorporating this view. However, direct evidence demonstrating that microbial residues account for the chemistry, stability and abundance of SOM is still lacking. Further, emerging models emphasize the stabilization of microbial-derived SOM by abiotic mechanisms, while the effects of microbial physiology on microbial residue production remain unclear. Here we provide the first direct evidence that soil microbes produce chemically diverse, stable SOM. We show that SOM accumulation is driven by distinct microbial communities more so than clay mineralogy, where microbial-derived SOM accumulation is greatest in soils with higher fungal abundances and more efficient microbial biomass production.

Responses and feedbacks of coupled biogeochemical cycles to climate change: examples from terrestrial ecosystems

The biogeochemical cycles of carbon (C), nitrogen (N), and phosphorus (P) are fundamental to life on Earth. Because organisms require these elements in strict proportions, the cycles of C, N, and P are coupled at molecular to global scales through their effects on the biochemical reactions controlling primary production, respiration, and decomposition. The coupling of the C, N, and P cycles constrains organismal responses to climatic and atmospheric change, suggesting that present-day estimates of climate warming through the year 2100 are conservative. N and P supplies constrain C uptake in the terrestrial biosphere, yet these constraints are often not incorporated into global-scale analyses of Earths climate. The inclusion of coupled biogeochemical cycles is critical to the development of next-generation, global-scale climate models.